Salicylic Acid: An All-Rounder in Regulating Abiotic Stress Responses in Plants

Abstract

Salicylic acid (SA) is an endogenous growth regulator of phenolic nature and also a signaling molecule, which participates in the regulation of physiological processes in plants such as growth, photosynthesis, and other metabolic processes. Several studies support a major role of SA in modulating the plant response to various abiotic stresses. It is a well-founded fact that SA potentially generates a wide array of metabolic responses in plants and also affects plant-water relations. This molecule also found to be very active in mitigating oxidative stress under adverse environmental conditions. Since abiotic stress remained the greatest constraints for crop production worldwide, finding effective approaches is an important task for plant biologists. Hence, understanding the physiological role of SA would help in developing abiotic stress tolerance in plants. In this chapter, we will shed light on the recent progress on the regulatory role of SA in mitigating abiotic stress.

1. Introduction

Abiotic stresses are a potential threat to agricultural productivity all over the world. Anthropogenic activities provoked the degradation of the agricultural system. Drought, excess soil salinity, extreme high, and low temperatures, metals/metalloids, ozone, UV-B radiation, nutrient (deficiency and excess) are the abiotic stresses which have increased many more times than previous due to anthropogenic activities [1, 2]. It has been projected that abiotic stresses may adversely affect 70% yield of staple food crops and decrease overall crop production by more than 50% [3, 4]. Thus, to improve plant performance and to reduce the loss of productivity caused by abiotic stress is vital. This can be implemented through various approaches and one of those is the application of exogenous phytoprotectant molecule.

Salicylic acid or orthohydroxy benzoic acid is ubiquitously distributed plant growth regulator [5]. Salicylic acid has positive effects on plant growth and developmental processes [5–7]. Research findings demonstrated its roles in seed germination, glycolysis, flowering, fruit yield [8], ion uptake and transport [9], photosynthetic rate, stomatal conductance (g_s), and in transpiration [10]. Salicylic acid can modulate antioxidant defense system thereby decreasing oxidative stress [11]. Photosynthesis, nitrogen metabolism, proline (Pro) metabolism, production of glycinebetaine (GB), and plant-water relations in abiotic stress affected plants were regulated by SA [12–14]. Induction of defense-related genes and stress resistance in biotic stressed plants have also been reported [15]. Moreover, exogenously applied SA showed putative positive effects on stressed plants [16–20]. Salicylic acid induced genes encoding chaperone, heat shock proteins (HSPs), antioxidants, and secondary metabolites of different types. Moreover, SA was involved in mitogen-activated protein kinase (MAPK) regulation, and in the expression [21]. There is no doubt about the vital roles of SA under abiotic stress condition. So, we will review and cover the area regarding the biosynthesis, involvement, and role of salicylic acid on abiotic stress affected plants.

2. Biosynthesis and metabolism of SA

Salicylic acid biosynthesis can occur through two distinct pathways viz. isochorismate (IC) pathway and phenylalanine ammonia-lyase (PAL) pathway ( Figure 1 ). Both IC and PAL pathways are started with chorismic acid. Chorismic acid is the end product derived from the shikimic acid pathway in plastid [22–24]. Chorismic acid is converted to IC by isochorismate synthase (ICS) as reported in several plant species [25–27]. Isochorismate pyruvate lyase (IPL) supposed to catalyze the conversion of IC to SA (but the mechanism is not clear) [28].

Figure 1.

Proposed pathways for SA biosynthesis and metabolism. Biosynthesis of SA is occurred by isochorismate (IC) or phenylalanine ammonia-lyase (PAL) pathways. Salicylic acid is also metabolized into different forms. Isochorismate synthase (ICS), BA2H (benzoicacid-2-hydroxylase), IPL (isochorismate pyruvate-lyase); MeSA (methylsalicylate), SA-Asp (salicyloyl-l-aspartic acid), SAG (salicylic acid 2-O-β-glucoside), SGE (salicyloyl glucose ester) are involved in either or biosynthesis or metabolic pathway of SA. Here, 1 indicates UDP-glucosyltransferase, 2 indicates SAM-dependent carboxyl methyltransferase, 3 indicates GH3-like phytohormone amino acid synthetase, and 4 indicates sulfotransferase.

In PAL pathway, deamination of phenylalanine is accomplished by the activity of PAL which generates trans-cinnamic acid. Trans-cinnamic acid is converted to intermediate product ortho-coumaric acid or producing the benzoic acid which later on produces SA [29–31].

After biosynthesis, SA can be modified into different other forms. Glucosylation of SA generates salicyloyl glucose ester (SGE) and salicylic acid 2-O-β-glucoside (SAG) where the activity of UDP-glucosyltrasferase is involved [32]. The SAG can be stored in the vacuole. Methylation of SA is occurred by SAM-dependent carboxyl methyltransferase to produce methyl salicylate (MeSA) [33]. After production, MeSA is transported to different parts of the plant. Through amino acid conjugation with SA, salicyloyl-l-aspartic acid (SA-Asp) generates (GH3-like phytohormone amino acid synthetase is proposed enzyme catalyze this reaction). SA-Asp can undergo through further catabolism [34]. The conversion of SA to SA-2-sulfonate is proposed to catalyze by sulfotransferase and this process is termed as sulfonation [35]. Hydroxylation of SA is responsible for the production of 2,5-dihydroxybenzoate (Gentisic acid) but the enzyme is unknown [36, 37].

Modifications of SA often render it inactive but these modifications are also related to accumulation, function, and/or mobility. Glucosylation inactivates SA and allows vacuolar storage. Methylation inactivates SA and increases its membrane permeability, volatility which is vital for long-distance transport of this defense signal. Amino acid conjugation of SA is involved in SA catabolism [37].

3. Salicylic acid and abiotic stress tolerance

As a phytohormone, the role of SA in regulating plant growth and development is well known. The role of SA in mitigating abiotic stress has widely been studied since last few decades ( Tables 1 – 4 ). A large volume of research reports indicate that both endogenous SA synthesis and exogenous application enhance plants tolerance to salinity [38–42], drought [43–45], extreme temperature [46–49], toxic metal and metalloids [50–53], and others [54–58]. Exogenous SA showed enhanced plant growth, photosynthesis, and decreased ROS production under various abiotic stresses ( Tables 1 – 4 and Figure 2 ).

Plant species	Salinity level	Effect of salinity	SA application	Protective effects	Reference
V. radiata L. cv. Pusa Vishal	100 mM NaCl, 20 d	TBARS and H2O2 contents increased 2.5 and 4-times respectively, compared to control Na^+ and Cl^− accumulation increased more than 3-times compared to control Activity of GR, contents of GSH and GSSG increased compared to control Net photosynthesis, gs, Ci, carboxylation efficiency, WUE, and plant dry mass decreased compared to control Content of methionine, GB, and ethylene evolution increased	Spraying 0.5 mM SA, 15 d	TBARS and H2O2 contents decreased 1.5-times and 2.5-times respectively, compared to salt-treated plant Reduced Cl^− and Na^+ contents by 50 and 39.8% compared to salt-treated plant GR activity and GSH content increased but GSSG content decreased compared to stressed plant Pn, gs, Ci, carboxylation efficiency, WUE, and plant dry mass decreased compared to salt-treated plant Content of methionine, GB and ethylene evolution decreased compared to stressed plant	Khan et al. [17]
T. aestivum cv. S-24 and MH-97	150 mM NaCl, 30 d	Reduced shoot fresh and dry mass, and leaf area in both cultivars Reduced grain yield plant^−1, 100-grain weight and number of spikelets in both cultivars Reduced the net CO2 assimilation rate (A), transpiration rate (E), gs, Ci and WUE in both cultivars	0.25, 0.50, 0.75 and 1 mM SA in growth media, 30 d	Increased fresh and dry masses of both shoot and root at 0.25 mM SA under saline condition 0.25 mM SA reduced salt-induced damage in grain yield, 100-grain weight and number of grains of S-24 but, in case of MH-97 grain yield slightly improved with 0.50 mM SA application SA increased A in S-24 at 0.25 mM concentration but in MH-97 at higher concentrations (0.75 and 1 mM)	Arfan et al. [59]
L. esculenta cv. DPL-62	100 mM NaCl, 10 d	Decreased germination percentage, shoot length, root length, FW and DW Free Pro and GB content increased in both shoot and root Activities of P-5-CR and γ-glutamyl kinase increased but Pro oxidase decreased	0.5 mM SA in growth media, 10 d	Improved germination percentage, shoot length, root length, FW and DW compared to salt stresses plants Increased rate of free Pr and GB content was higher in shoot than root under salt stress Activities of P-5-CR and γ-glutamyl kinase further increased and Pro oxidase decreased	Misra and Saxena [61]
G. jamesonii L. cv. Amaretto	100 mM NaCl, 15 d	EL and MDA content increased Activities of SOD, POD, CAT and APX increased Higher Pro content	0.5 mM SA pretreatment, 2 d	EL and MDA content decreased Activities of SOD, POD, CAT and APX further increased Lower Pro content	Kumara et al. [39]
B. juncea L. cv Pusa Jai Kisan	100 mM NaCl, 30 d	TBARS and H2O2 contents increased 2.5-times and 3.8-times respectively, compared to control Increased Na^+ and Cl^− contents in leaves Increased activities of DHAR, APX and GR by 30, 217 and 79%, respectively compared to control Activities of ATPS and Serine acetyl transferase (SAT) and cystein (Cys) contents increased by 30, 23, and 70%, respectively, but S content decreased by 32% compared to control Increased DHA, GSH, and GSSG contents but, decreased AsA content Reduced net photosynthesis, gs and Ci by 40.0, 26.4, and 41.3%, respectively, compared to control	0.5 mM SA spray, 15 d	TBARS and H2O2 contents were decreased significantly Reduced Na^+ content by 36% and Cl^− content by half compared to salt treated plants Activities of DHAR, APX and GR further increased by 54, 248 and 111%, respectively compared to control Activities of ATPS and SAT, and contents of Cys and S increased by 87, 76, 128, and 63% respectively, compared to control GSH content further increased while DHA and GSSG both were reduced, AsA content increased Limited the decreases in the above characteristics to 22, 19 and 25% respectively, compared to control	Nazar et al. [41]
S. lycopers-icum Mill. L. cv. Rio Fuego	100 mM NaCl	Higher accumulation of ABA in both leaves and root Ethylene production increased Reduced net CO2 fixation rate	Pretreated with 0.01 μM and 100 μM SA, 21 d	Lower accumulation of ABA compared to stressed plants Prevented higher production of ethylene Net CO2 fixation rate enhanced	Horváth et al. [62]
Hordeum vulgare L. cv. Gustoe	150 mM NaCl, 14 d	Shoot FW and height decreased by 30 and 36% respectively, compared to control Photosynthetic pigment contents decreased by 57% compared to control MDA content increased by 40% compared to control Na^+ content increased and K^+ content decreased compared to control	50 μM SA, 14 d	Shoot FW and height increased compared to NaCl treated plants Photosynthetic pigment contents decreased by only 39% compared to control MDA content was lower compared to salt-stressed plants Na^+ content decreased and K^+ content increased compared to plants exposed to salt stress	Fayez and Bazaid [63]
V. radiata L. cvs. Pusa Vishal and T44	50 mM NaCl, 30 d	Increased root and shoot Na^+ and Cl^− contents Decreased NR activity and N content Leaf ATPS activity, S, H2O2, MDA contents and EL increased Decreased water potential and osmotic potential by 69 and 52% in Pusa Vishal and 70 and 69% in T44, respectively compared to control SOD, APX, GR activities and GSH, GSSG contents increased gs, Ci and chl fluorescence decreased	0.5 mM SA spray, 15 d	Reduced root and shoot Na^+ and Cl^− contents Increased NR activity and N content Leaf ATPS activity and S content further increased, but H2O2, MDA contents, and EL decreased Enhanced the water and osmotic potential by 54 and 34% in Pusa Vishal and 33 and 15% in T44 in comparison to control SOD activity and GSSG content decreased, but APX and GR activities and GSH content further increased gs, Ci, and chl fluorescence increased	Nazar et al. [12]
Torreya grandis	0.4% NaCl, 60 d	Reduced the dry mass of shoots and roots by 29 and 25%, respectively RWC decreased by 16.70% but Pro content increased Pn, transpiration rate (Tr) and gs increased but Ci decreased Chlorophyll content decreased by 24% Activities of SOD, CAT, and POD decreased markedly EL and MDA content increased	0.5 mM SA, 30 d	Increased the dry mass of the shoots and roots by 16.8 and 18.2%, respectively Both RWC and Pro content increased compared to stressed plants Pn, Tr, and gs decreased and Ci increased Reduced the decrease of chl content by 17% Increased the activities of SOD, CAT, and POD by 13, 38, and 19%, respectively Reduced the EL rate and MDA content	Li et al. [64]
S. lycopersicum cv. Rio Fuego	100 mM NaCl, 10 d	Water potential decreased and root length increased Decreased Chl content and increased MDA content ABA accumulation decreased in shoot but increased in root	Pretreated with 0.1 and 10 μM SA, 21 d	Water potential increased and root length decreased Improved Chl content and reduced MDA content than stressed one ABA accumulation increased in shoot but decreased in root	Szepesi et al. [38]
A. thaliana	100 mM NaCl, 14 d	Reduced FW, DW and water content Increased shoot Na^+ and reduced K^+ contents	50 μM SA pretreatment, 1 h	Improved FW, DW and water content Decreased shoot Na^+ and enhanced K^+ contents	Jayakannan et al. [65]
Medicago sativa cv. Aragon	200 mM, 12 d	Reduced the F V /F M ratio by 15% Decreased the nodule mass about 60% Activities of CAT, APX and GR reduced but SOD, POX, and DHAR increased	Pretreated with 0.1 and 0.5 mM, 2 d	Reduced the F V /F M ratio by only 2% Reduced the decreasing of the nodule mass Only CAT activity further reduced, but the activities of APX, GR, SOD, POX and DHAR further increased	Palma et al. [20]
T. aestivum L. cv. Yumai 34	250 mM NaCl, 3 d	GSH content increased but AsA content decreased	0.5 mM SA, 3 d	GSH content further increased along with AsA content	Li et al. [66]
Z. mays L., Hamidiye F1	40 mM NaCl, 56 d	Membrane permeability and MDA content increased Higher Na^+ and Cl^− concentrations Decreased N and increased P concentrations	Soil incorporated with 0.5 mM SA, 56 d	Ameliorated the deterioration of membrane and reduced MDA content Lower accumulation of Na^+ And Cl^− ions Increased N and decreased P concentrations	Gunes et al. [60]
S. lycopersicum cv. Rio Fuego	100 mM NaCl, 7 d	Activities of APX, GR increased and SOD, CAT decreased Both total ascorbate (AsA) and GSH contents increased in leaves and roots	Pretreated with 0.1 μM or 100 μM SA, 21 d	Activities of APX, GR further increased along with SOD and CAT Further increase of total AsA and GSH	Tari et al. [42]

Table 1.

Salicylic acid–mediated tolerance of different plant species to salinity stress.

Plant species	Drought condition	Effect of drought	SA application	Protective effects	Reference
T. aestivum. cvs. Giza164 and Gemaza 1	Drainage (control, 1/3 field capacity, 2/3 field capacity), 14 d	Shoot and root DW and mineral content decreased Increase of sugar, protein in shoot and root of plants	Seed soaking with 0.5 mM SA	Higher accumulation of sugars, protein and mineral Increased DW	El Tayeb and Ahmed [77]
B. juncea L. cv. BARI Sharisha 11	10 and 20% PEG, 48 h	Increased MDA, H2O2 and Pro levels Decreased leaf RWC, chl content Decreased AsA content and increased in GSH and GSSG contents GR, APX, GST activities increased DHAR and Gly I activities decreased	Foliar spray with 50 μM SA	Increased leaf RWC, chl, AsA and GSH contents Decreased the GSSG content and maintained a higher ration of GSH/GSSG Increased the activities of MDHAR, DHAR, GR, GPX, CAT, Gly I, and Gly II Decreased H2O2 content and lipid peroxidation	Alam et al. [44]
Cymbopogn flexuosus Steud. Wats. cvs. Neema and Krishna	Water has been reduced to 75 and 50% of field capacity	Reduced growth Increased activities of nitrate reductase, carbonic anhydrase, and PEP carboxylase Increased EL, Pro and free amino acid contents	Foliar application of 10 μM SA	Improved growth parameters Modulated the activities of nitrate reductase, carbonic anhydrase, and PEP carboxylase Decreased EL, Pro and free amino acid contents	Idrees et al. [76]
T. aestivum	PEG 15%, 3 d	Decreased growth	0.5 mM SA, 3 d	Overexpressed proteins associated with signal transduction, stress defense, photosynthesis, carbohydrate metabolism, protein metabolism, and energy production Improved growth and drought tolerance in wheat seedlings	Kang et al. [43]
Satureja hortensis	1/3 and 2/3 of field capacity.	Decreased sugar and protein contents Increased Pro and lipid peroxidation	1.0 and 3.0 mM SA	Increased sugar, protein and Pro contents Decreased lipid peroxidation	Yazdanpanah et al. [78]
C. setosa	Kept in PEG, 4 h	Increased leaf rolling Decreased antioxidant enzyme activities	SA pretreatment, 14 h	Increased Pro, sugar, activities of SOD, APX, CAT, GPX and NOX (NADPH oxidase) SA conferred drought tolerance mediated by H2O2 Reduced leaf rolling	Demiralay et al. [45]
Z. mays	Withholding water, 5 d	Decreased RWC, leaf MSI, chl and K content	100, 150, 200 ppm	Increased RWC, leaf MSI, chl, and K contents	Rao et al. [79]
Musa acuminata cv. ‘Berangan’, AAA	1, 2 and 3% of PEG in vitro, 60 d	Slight increase in proliferation rate, FW Decreased RWC and chl content	1.0, 2.0, and 3.0 mM SA	Increased Pro content Reduction in H2O2 and MDA contents	Bidabadi et al. [80]
H. vulgare L. cv Nosrat	40% FC	Drought stress decreased the dry mass and net CO2 assimilation rate	Spraying with 500 μM SA, 15 d	Increased the dry mass, net CO2 assimilation rate and gs	Habibi [81]
O. sativa L. cv. Super-Basmati	Water was reduced to 50% of field capacity	Increased H2O2, MDA, and relative membrane permeability	100 mg L^−1 SA	Increased tissue water potential, increased synthesis of metabolites and enhanced capacity of the antioxidant system	Farooq et al. [82]
T. aestivum L.	20 % PEG, 24 h	Decreased MSI, increased total soluble sugar and soluble protein content but decreased yield	10 μM SA, 24 h	Increased MSI, increased total soluble sugar and soluble protein content Increased yield	Khan et al. [73]
T. aestivum L. cv. Hassawi	Reduced water to 60 and 30% field capacity	Decreased content of photosynthetic pigments Increased soluble carbohydrate, protein, and Pro Decreased insoluble carbohydrates and proteins Free amino acids were significantly increased in roots, while it was decreased in shoots	50 ppm SA	Stimulated growth, photosynthetic pigments and accumulation of soluble and insoluble carbohydrates and proteins	Azooz and Youssef [83]

Table 2.

Salicylic acid–mediated tolerance of different plant species to drought stress.

Plant species	Temperature and duration	Damaging effects	SA dose and duration	Protective effects	References
T. aestivum	HT (30°C, 2 h)	Decreased FW Increased total soluble protein and total RNA contents Decreased soluble starch synthase activity	100 mM, foliar spraying, 3 times at 4 h interval	Improved FW No significant differences in the total soluble protein content Further increased total RNA content Increased soluble starch synthase activity	Kumar et al. [46]
Z. mays	HT (40 ± 1°C, 2 h)	Reduced dry biomass Increased MDA, H2O2 but decreased Pro contents APX and GR activities increased but CAT and SOD activities decreased	10–800 μM, foliar spraying, 2 h	Improved dry biomass content Reduced MDA, H2O2 but increased Pro contents Further improved CAT, SOD and POX activities	Khanna et al. [48]
L. lycopersicum	HT (32/26°C, 12/12 h, day/night)	Severely reduced germination and plant growth Negatively affected reproductive parameters Decreased TSS (total soluble solid), vitamin and lycopene contents	0.25, 0.5, and 0.75 mM, seed priming	Reduced germination time and increased germination percentage Improved reproductive parameters. Increased TSS, vitamin and lycopene contents	Singh and Singh [49]
T. aestivum	HT (40°C, 6 h)	Increased Pro and γ-glutamyl kinase (GK) but reduced Pro oxidase contents Increased TBARS and H2O2 contents Negatively affected the net photosynthesis, Rubisco activity, chl and WUE of the plant	0.5 mM, foliar spraying	Further increased Pro and GK and reduced Pro oxidase contents Reduced TBARS and H2O2 contents Counteracted the negative effects on net photosynthesis, Rubisco activity, chl and WUE of the plant	Khan et al. [1]
Digitalis trojana Ivanina	HT (45°C, 2 or 4 h)	Lowered CAT and SOD activities Increased accumulation of Pro Increased total phenolic and flavonoid contents	150 μM, pretreatment, 4 h	Increased CAT and SOD activities Further increased Pro content Further increased phenolic and flavonoid contents	Cingoz and Gurel [47]
Vitis vinifera	HT (43°C, 5 h)	Sharply declined Pn and gs Lowered Rubisco activity	100 μM SA, pretreatment, 24 h	Alleviated the harmful effect on Pn Counteracted the negative effect on Rubisco activation state	Wang et al. [95]
Matricaria chamomila	HT, Min (10.1–28.2°C), Max (21–44°C), 8 months	Reduced plant height, capitol diameter, fresh flower weight, dried flower weight etc. Decreased total chl content Essential oil content was not significantly affected	1, 10, 25 and 100 mg L^−1, foliar spraying	Improved plant height, capitol diameter, fresh flower weight, dried flower weight etc. Increased total Chl content Improved essential oil content	Ghasemi et al. [96]
Cucumis sativa	HT (40°C, 36 h)	Highly increased EL Increased H2O2 and TBARS contents Improved SOD, CAT, DHAR, GPX, APX and GR activities	1 mM SA, foliar spraying, 12 h	Decreased EL Reduced H2O2 and TBARS contents Improved SOD, DHAR, GPX, APX and GR activities but inhibited CAT activity	Shi et al. [97]
B. juncea	HT (30 or 40°C, 24 h)	Decreased root length, shoot length, FW and DM of the plant Decreased Pn, gs, Ci, WUE and SPAD value Increased activities of CAT, POD SOD, and Pro accumulation Decreased N, P, K contents in leaves	10 μM, foliar spraying	Root length, shoot length, DW and FW of plant increased Increased Pn, gs, Ci, WUE and SPAD value Further enhancement of CAT, POD SOD, and Pro accumulation Improved N P, K contents in leaves	Hayat et al. [98]
Musa acuminata	Chilling (5°C, 3 d)	Reduced SOD, CAT, APX activities but improved POX activity Increased accumulation of H2O2	0.5 mM, pretreatment, 1 d	Increased SOD, CAT and APX activities but unaffected POX activity Decreased overproduced H2O2	Kang et al. [99]
T. aestivum	Chilling (3 °C, 48–72 h)	Reduced chl, CO2 assimilation and rate of respiration RuBisCO activity decreased Decreased SOD Increased glycolate oxidase (GO) and CAT activities Highest MDA content found	500 μM, foliar spraying, 24 h	Improved Chl content and rubisco activity Enhanced CAT, APX, POX, and glycolate oxidase (GO) activities but GR activity found not affected Reduced MDA content	Yordanova and Popova [100]
V. vinifera	HT (38 ± 0.5°C, 12 h)	Reduced H^+ and Ca^2+-ATPase activities The appearance of cerium phosphate grain found	100 μM, pretreatment, 6 h	Increased H^+ and Ca^2+-ATPase activities Remained cerium phosphate grain	Liu et al. [101]
V. vinifera	Freezing (−3°C, 3 h) and HT (44°C, 3 h)	Increased EL and TBARS content Decreased activities of GR, APX, MDHAR and DHAR Increased AsA/GSH ratio Reduced cytosolic Ca^2+ homeostasis	2 μM, pretreatment, 1, 3, 6, or 12 h	Reduced EL and TBARS content Increased activities of GR, APX, MDHAR but reduced DHAR and ratio of AsA/GSH pool Increased cytosolic Ca^2+ homeostasis	Wang and Li [102]
Poa pratensis	HT (46°C, 72 h)	Overproduction of O2 ^•− and H2O2 in a time-dependent manner Enhanced CAT and SOD activities in a time-dependent manner	0.1, 0.25, 0.5, 1.0, and 1.5 mM, foliar spraying	Reduced O2 ^•− H2O2 generation in a time-dependent manner Significantly increased CAT and SOD activities in a time-dependent manner	He et al. [103]
O. sativa	HT (27–32°C, night ambient temp)	Spikelet fertility and grain size reduced Damaged membrane and resulted oxidative damage	1 mM, foliar spraying	Improved grain characteristics and spikelet fertility Counteracted oxidative damage Enhanced antioxidant enzymes activities	Mohammed and Tarpley [104]
Prunus persica	Chilling (0°C, 28 d)	Increased TBARS content, chilling injury index and decay index Lowered APX and GR activities and heat shock protein 101 (HSP101) expression	0.35, 0.7, and 1 mM, 5 min	Maintained optimum level of TBARS content, chilling injury index and decay index Enhanced reduced to oxidized AsA and GSH ratios Higher APX and GR activities and heat shock protein 101 (HSP101) expressions	Wang et al. [105]
Rhododendron	HT (38/30°C (day/night, 6 d)	Resulted in brown, withered and defoliated leaves Decreased chl and total soluble protein contents Decreased activity of POX and SOD MDA and H2O2 contents increased	0.5, 1.0, and 2.0 mM spraying of leaves, 3 d	Lowered the leaf damage rate Highly declined Chl and total soluble protein contents Increased POX and SOD activities Reduced MDA and H2O2 contents	Shen et al. [106]
V. radiata	HT (50°C, 3 h)	Increased lipid peroxidation, H2O2, and EL Reduced CAT, APX, POD and GSH contents Increased endogenous SOD and SA activities	0.5 and 1 mM, foliar spraying	Decreased lipid peroxidation, H2O2, and EL Increased CAT, APX, POD and GSH contents Enhanced defense system and radical scavenging mechanism	Saleh et al. [107]
Z. mays	Chilling (15°C)	Disturbed seedling emergence, root and shoot growth, FW, and DW Lowered RWC and increased membrane permeability	50, 100, and 150 mg L^−1, seed priming, 24 h	Improved seedling emergence, root and shoot growth, FW and DW Increased RWC and decreased membrane permeability Highly activated CAT, SOD and APX activities	Farooq et al. [108]
Brassica sp.	HT (40–55°C, 3 h)	Hampered seedling growth Increased EL Increased CAT, POX enzyme activities	10 and 20 μM, pretreatment, 2 h	Conferred protection of membrane Reduced EL Increased FW, DW, and total soluble sugar Increased CAT, POX, and appearance of heat shock proteins	Kaur et al. [3]

Table 3.

Summary of the protective roles of exogenous SA in mitigating extreme temperature-induced damages in different crop plants.

Plant species	Toxic metals/metalloids	Doses and duration	Toxic effects	SA doses and duration	Protective effects	References
O. sativa	As	25 and 50 μM Na2HAsO4, 7 d	Decreased growth and biomass production Enhanced MDA and H2O2 contents	100 μM, pretreatment, 7d	Reverted the growth inhibition Reduced oxidative stress by reducing MDA and H2O2 contents	Singh et al. [50]
A. thaliana	As	100 μM Na2HAsO4, 14 d	Decreased plant biomass and chl contents Increased lipid peroxidation and antioxidant enzymes (APX, CAT, SOD) activities	250 μM, pretreatment, 14 d	Alleviated the toxic effects of As by restoring growth and chl content Decreased lipid peroxidation and downregulated activities of APX, CAT, and SOD	Odjegba [115]
T. aestivum	Cd	0.01, 0.1 and 1 mM Cd-(OOH)2, 24 h	Increased MDA and EL percentage Reduced FW and DW Enhanced accumulation of ABA and dehydrins	50 μM, 24 h, pretreatment	Declined MDA content and EL Recovered growth Activated phenylalanine ammonia-lyase (PAL) enzyme activity Further increased low molecular weight dehydrins by 1.5 folds	Shakirova et al. [53]
H. vulgare	Cd	15 μM CdCl2, 30 min	Reduced root growth markedly Stimulated activity of LOX, GPX and enhanced accumulation of IAA Increased generation of ROS in the root apex	0.25 or 0.5 mM SA, pretreatment, 10 min	Alleviated Cd-induced root growth inhibition Reduced IAA-induced LOX and GPX activity Inhibited ROS generation in roots	Tamás et al. [51]
Lolium perenne	Cd	100 μM CdCl2, 14 d	SOD, APX, and CAT activity decreased drastically in both shoots and roots. Increased accumulation of O2 ^•−, H2O2 and increased MDA content	100, 200, 300, and 400 μM, 14 d	Increased antioxidant enzyme activities and chl content Increased mineral uptake but decreased Cd uptake Decreased accumulation of MDA and H2O2	Bai et al. [116]
Phaseolus vulgaris	Cd	0.25, or 0.50 mM CdCl2, 2–3 d	Increased Pro and Cd^2+ ion accumulation Enhanced EL and lipid peroxidation Increased activities of SOD, CAT, APX, and GR	1 mM, foliar spraying,50, 36, and 22 d	Decreased EL and lipid peroxidation Reduced activities of antioxidant enzymes, such as SOD, CAT, APX, and GR	Wael et al. [117]
T. aestivum	Cd	100, 400 and 1000 μM CdCl2 . 2.5 H2O, 30 d	Inhibited root growth and enhanced Cd accumulation in roots Decreased RWC, chl content, and CO2 fixation Increased MDA, H2O2 and Pro contents Altered root and chloroplast ultrastructure	500 μM, pretreatment, 20 h	Reversed root growth inhibition Ameliorated the adverse effects on RWC, chl content, and CO2 fixation Reduced MDA, H2O2 and Pro contents Recovered chloroplast and root ultrastructures	Moussa and El-Gamal [118]
Linum usitatissimum	Cd	50 and100 mM CdCl2, 4 d	Inhibited growth and nutrient absorption Enhanced MDA content and EL Reduced total lipid and chl contents	250 and1000 μM, presoaking of grains, 8 h	Alleviated growth inhibition and nutrient absorption Ameliorated the enhanced MDA content and EL Alleviated the harmful effect on total lipid and Chl contents	Belkhadi et al. [119]
Phaseolus aureus	Cd	50 and 100 μM CdCl2, 3 or 6 d	Enhanced H2O2 and O2 ^•− production in the root Increased TBARS content and relative EL rate Increased antioxidant enzymes such as SOD, CAT and APX activities	100 μM SA, seed soaking, 16 h	Significantly decreased H2O2, O2 ^•− production in the root Enhanced TBARS content and relative EL Further increased SOD, CAT and APX activities	Zhang et al. [120]
T. aestivum	Cd	500 and 1000 μM CdCl2, 3 d	Reduced growth, chl content, RWC and SOD, CAT, POX activities Enhanced Pro, EL and Cd contents	500 μM, seed soaking, 12 h	Mitigated adverse effects of Cd on chl, RWC and SOD, CAT and POX activities Alleviated damaging effects on EL Improved leaf anatomy and reduced uptake of Cd	Agami and Mohamed [121]
Z. mays	Cr	50 mg L^−1 K2Cr2O7, 7 d	Decreased growth, photosynthetic pigments, CHO metabolism Increased MDA, H2O2, Pro and Cr contents Decreased CAT but increased SOD and POD activities	100 μM L^−1, foliar application, 15 d	A significant decline in MDA, H2O2, Pro and Cr contents Improved plant growth, photosynthetic pigments Reduced oxidative stress by upregulating antioxidant enzymes (CAT, SOD, POD) activities	Islam et al. [52]
Catharanth roseus	Ni	50, 100, and 150 mg kg^−1 NiSO4⋅6H2O	Significantly reduced growth, photosynthetic pigments and activities of carbonic anhydrase and nitrate reductase. Increased SOD, CAT, POX activities, EL and Pro contents	10 μM, foliar spraying, 4 sprays at 10 d intervals from 30 DAS	Restored plant growth processes Further increase of antioxidant enzymes (SOD, CAT, POD) activities	Idrees et al. [122]
B. napus	Ni	0.5 mM NiCl2⋅6H2O, 10 d	Resulted leaf chlorosis and necrosis Decreased growth and leaf chl content LOX, MDA, H2O2, and Pro content increased Reduced CAT, APX and GPX activity	0.2 mM SA, cotreatment, 10 d	Decreased leaf chlorosis and necrosis Improved growth and leaf chl content A significant decline in LOX, MDA, H2O2 and Pro contents Increased CAT, APX and GPX activities	Kazemi et al. [123]
O. sativa	Cu	75 or 150 μM CuSO4, 48 h	Highly increased MDA, O2 ^•− H2O2 contents and LOX activity Reduced Chl and leaf RWC Reduced AsA GSH, nonprotein thiol and pro contents in roots	100 μM SA, pretreatment, 24 h	Curtailed Cu-induced enhancement of MDA, O2 ^•−, H2O2 contents and LOX activity Increased Chl, leaf RWC, AsA, GSH, nonprotein thiol contents Enhanced antioxidant enzymes (SOD, CAT, APX, GR, GPX, DHAR and GST) activities	Mostofa and Fujita [124]
Zygophyllum fabago	Pb	0.75 mM Pb(NO3)2,7 d	Downregulated CAT, APX, GR but upregulated DHAR activity Increased MDA and EL	0.5 mM SA, pretreatment, 24 h	Increased SOD activity Reduced accumulation of Pb and chl contents	Lopez-Orenes et al. [125]
T. aestivum	Pb	10, 50, 100, 200 mg L^−1 Pb^2+, 7 d	Decreased chl content in a dose-dependent manner Inhibited APX, CAT, MDA and Pro contents POD, SOD, and soluble sugar contents were affected	200 mg L^−1, cotreatment, 7 d	Alleviated the inhibitory effects on Chl and antioxidant enzymes activities. Lowered MDA and Pro contents Reduced the adverse effects of Pb on POD, SOD, and soluble sugar contents	Song et al. [126]
P. pratensis	Cd	5,10, or 50 mM CdCl2, 7 d	Reduced growth, chl and nutrient (K, Ca, Fe, Mg) elements Elevated MDA and H2O2 contents Increased Cd uptake and accumulation	500 mM SA, pretreatment, 12 h	Increased growth, chl, and nutrient (K, Ca, Fe, Mg) elements Decreased MDA and H2O2 contents and Cd uptake Marked increase in SOD, APX, and POD but decreased CAT activity	Guo et al. [127]
Spinacia oleracea	B	50 mg kg^−1, H3BO3	Enhanced B Accumulation in plants Decreased chl and anthocyanin contents Increased MDA, H2O2 and stomatal resistance	0.5 mM kg^−1, cotreatment	Decreased B accumulation Improved chl and anthocyanin contents Influenced antioxidant enzymes activity and stomatal resistance	Eraslan et al. [128]
Daucus carota	B	25 mg kg^−1, H3BO3	Increased storage root diameter Increased oxidative damage as indicated by increased MDA content Lowered chl content	0.5 mM kg^−1, cotreatment	Enhanced storage root DW Increased anthocyanin and carotenoid contents Controlled metal toxicity and pro accumulation in roots and shoots	Eraslan et al. [129]
B. juncea	Mn	3.0, 6.0, or 9.0 mM MnCl2, 3 d	Decreased growth, photosynthetic pigments, carbonic anhydrase activity, and water relations Increased MDA, H2O2, Pro accumulation Elevated antioxidant enzymes (CAT, SOD, POD) activity in a dose-dependent manner	10 μM, 14 d, foliar application	Improved growth, photosynthetic pigments, carbonic anhydrase activity, and water relations Lowered MDA, H2O2, and EL in a dose-dependent manner Further accelerated activity of antioxidant enzymes (CAT, SOD, POD)	Parashar et al. [130]
C. sativus	Mn	600 μM MnSO4, 11 d	Caused stunted growth, severe chlorosis, a marked increase in Mn accumulation Inhibited nutrients (Ca, Mg, Zn) absorption Increased ROS production and lipid peroxidation	100 μM SA, co-treatment, 11 d	Promoted growth and reduced Mn toxicity Alleviated the effects on nutrients (Ca, Mg, Zn) absorption Reduced Increased ROS production and lipid peroxidation Increased SOD, POD, DHAR, GR, ASA and GSH activities but decreased CAT and POX activities	Shi and Zhu [131]
Glycine max	Al	30 μM AlCl3, 12 h	Inhibited root elongation Accelerated SOD, POD, and APX activities in roots Upregulated the CaM-like protein genes	10 μM, cotreatment, 12 h	Restored root growth Further enhanced SOD and POD activities Upregulated CaM-like protein genes Increased cytosolic Ca^2+ Concentration	Lan et al. [132]
Vallisneria natans	Pb	50 μM Pb(NO3)2, 4 d	Decreased total chl content Increased MDA, O2 ^•− H2O2 contents Increased CAT but no significant effect on APX and POD	10 or 100 μM, cotreatment, 4 d	Decreased Chl and carotenoid contents Inhibited the overproduction of MDA, O2 ^•− and H2O2 Decreased APX but increased CAT, DHAR and POD activities Decreased NADPH oxidase, nonprotein thiols, AsA	Wang et al. [133]
Medicago sativa	Hg	10 μM HgCl2, 24 h	Increased Lipid peroxidation and TBARS accumulation Slight increase in NADH oxidase, SOD and POD activities Decreased APX and GR activities Decreased AsA, GSH, and Pro accumulation	0.2 mM, pretreatment, 12 h	Increased activities of NADH oxidase, APX, POD, GR but decreased SOD activity Elevated AsA, GSH and Pro accumulation	Zhou et al. [134]
B. oleracea var. botrytis	Co, Ni, Cd, Cr, and Pb	0.25 M	Retarded growth Increased MDA, nonprotein thiol, EL percentage and Pro contents Increased POD and SOD activities	50 and 100 mM, cotreatment	Reversed all the toxic effects caused by HM except for Cr In the case of Cr, SA accelerated the toxic effects and the plats died	Sinha et al. [135]
Pisum sativum	Cd	0.5, 1, 2, and 5 μM CdCl2	Decreased FW, CO2 fixation, chl content and RuBPC activity Increased MDA, Pro content, and EL percentage	500 μM, seed pretreatment, 6 h	Restored FW, CO2 fixation, chl content and RuBPC activity Alleviated the effects on MDA, Pro contents, and EL percentage	Popova et al. [136]
Cannabis sativa	Cd	CdCl2. 2.5 H2O at 0, 25, 50, and 100 mg kg^−1 sands	Inhibited plant growth Slightly reduced photosynthetic capacity Increased Cd uptake	500 μM, seed soaking, 6 h	Counteracted growth inhibition Improved photosynthetic capacity Reduced Cd uptake Enhanced SOD and POD activities	Shi et al. [137]
H. vulgare	Zn^2+, Cu^2+, Mn^2, Cd^2+, Hg^2+, and Pb^2+, 7 d	0.1, 0.2, 0.5, and 1 mM Zn^2+, Cu^2+, Mn^2, Cd^2+, Hg^2+, and Pb^2+	Inhibitory effects on SOD and CAT at higher dose	2 mM, cotreatment, 7 d	Alleviated the harmful effects on antioxidant enzymes (CAT, SOD) activities	Song et al. [138]

Table 4.

Summary of the protective roles of SA in mitigating toxic metal/metalloid–induced damages in different crop plants.

Figure 2.

Some possible ways of SA-induced oxidative stress tolerance to plants.

3.1. Salinity

Among the prevailing catastrophic abiotic stresses, salinity or salt stress can be considered as the most devastating one. It shows enormous negative effects, both direct and indirect, on morphological, physiological and biochemical attributes of plants. When plants are exposed to salt, they not only face osmotic and ionic stresses, but also water stress and other subsequent stresses may emerge. These ultimately reduce the quality and quantity of the desired yield. However, good news is there are certain species that show some tolerance mechanisms and also some protectants that can help plants to develop tolerance against the salt stress. In the recent era where global warming and rising of sea level are the most alarming issues, these can be promising facts to be considered for further research. There are a number of studies that prove the protective roles of SA against salt stress in many plant species ( Table 1 ).

Salicylic acid has been proved to have effective roles on enhancing the germination percentage, shoot and root length, fresh weight (FW) and dry weight (DW) of both shoot and root of plants, uptake of beneficial ions, and also some antioxidant enzyme activities. It also has been proved to reduce the toxic ions uptake and accumulation in plants, membrane damage and transpiration rate, etc. Photosynthesis, growth, and yield were improved and oxidative damages were ameliorated with the application of effective concentrations of SA in different plant species. To demonstrate the role of SA in alleviating the salt stress-induced damage, an experiment was conducted by Arfan et al. [59] with two Triticum aestivum varieties, of which one is salt-tolerant (S-24) and another one is salt-sensitive (MH-97). They applied different levels of SA starting from 0.25 to 1.00 mM and created salt stress with 150 mM NaCl in the growing media. Both SA and NaCl were exposed to plants from the very beginning and data were taken at 30 days after sowing (DAS). Their results showed that at lower concentration (0.25 mM) SA could reduce the negative effects of salt on the shoot and root FW and DW in tolerant variety, but this was true at higher concentration (0.75 mM) for the sensitive one. However, yield attributes such as grain yield and 100-grain weight were increased in both the varieties at lower concentrations (0.25 and 0.50 mM) SA under salt stress. Similarly, application of SA also increased the water use efficiency (WUE) of those varieties. Similarly, Nazar et al. [12] also chose two such varieties of Vigna radiata cvs. Pusa Vishal (salt-tolerant) and T44 (salt-sensitive). They also used different concentrations of SA (0.1, 0.5, and 1.0 mM) against 50 mM NaCl stress and 0.5 mM was concluded as the most suitable concentration for both varieties irrespective of their tolerance ability. At this concentration, V. radiata seedlings could reduce the accumulation of toxic Na⁺ and Cl⁻ ions, and increase S and N uptake and nitrate reductase activity. Salicylic acid application also enhanced the water and osmotic potential which was higher in the tolerant one. Stomatal conductance (g_s), intercellular CO₂ concentration (C_i), and chlorophyll (chl) fluorescence were increased along with leaf ATP-sulfurylase activity. However, mainly the reduction of electrolyte leakage (EL), malondialdehyde (MDA), H₂O₂, oxidized glutathione (GSSG) contents, superoxide dismutase (SOD) activity and enhancement of reduced glutathione (GSH) content, and ascorbate peroxidase (APX) and glutathione reductase (GR) activities prove the role of SA in reducing salt-stress damages. Similar results were observed with V. radiata seedlings. These seedlings were exposed to 100 mM NaCl at 10 DAS and then was spraying SA (0.5 mM) at 15 DAS. At 30 DAS some parameters were monitored related to gas exchange e.g., net photosynthesis (Pn), gs, and Ci; and also carboxylation efficiency, WUE, and plant dry mass [17]. Application of SA increased P_n, g_s, and C_i by 17.9, 19.2, and 23.5%, respectively, under salt stress condition. It also enhanced carboxylation efficiency and WUE compared to salt-stressed plants. Plant dry mass was increased by 25.2% under salt condition compared to control plants. Meanwhile, Zea mays was tested with different levels of SA under salt stress, and positive roles of SA was demonstrated in ameliorating the membrane damage by reducing MDA content [60]. It can also decrease the accumulation of Na⁺ and Cl⁻ ions and increase uptake of N and P and thus render tolerance to plants against salt stress. Another experiment was conducted with Lens esculenta, which included only four with Lens esculenta, which included only four treatments: nonsaline control (I), 0.5 mM SA (II), 100 mM NaCl (III), and the combination of 100 mM NaCl + 0.5 mM SA (IV). The results showed that growth parameters: germination (%), shoot and root length, FW and DW were improved in treatment (IV) compared to treatment (III). In addition, SA increased the free Pro and GB content in shoot and also the activities of pyrroline-5-carboxylate reductase (P-5-CR) and γ-glutamyl kinase which are the enzymes related to Pro anabolism. But, in contrast, it reduced the activity of Pro oxidase [61]. In the case of pretreatment with SA, it also showed some positive results. Solanum lycopersicum seeds pretreated with 10 μM SA improved the chl content and reduced MDA content under salt stress (100 mM NaCl) [38]. Higher accumulation of abscisic acid (ABA) in shoot and enhancement of water potential of SA-treated seedlings compared to the seedlings exposed to salt alone were also observed [38]. These results were supported by Horváth et al. [62] in the same plants with the equal concentrations of SA. Similarly, when pretreatment with SA (0.5 and 1.0 mM) was done, Gerbera jamesonii seedlings also showed positive results in salt stress (100 mM). Salicylic acid application reduced the EL, MDA and Pro contents and increased the activities of SOD, catalase (CAT), peroxidase (POD), and APX compared to salt stressed seedlings [39]. But, these effects were more acceptable in case of lower (0.5 mM) concentration of SA. Recently, Nazar et al. [41] again used SA (0.5 mM) to demonstrate the preventive role of it in Brassica juncea seedlings exposed to 100 mM of NaCl stress for 30 consecutive days. Application of SA reduced thiobarbituric acid reactive substances (TBARS) and H₂O₂ contents, also dehydroascorbate (DHA) and GSSG contents. It was found to increase the activities of dehydroascorbate reductase (DHAR), APX, and GR to a remarkable content. And most importantly, it reduced the toxic Na⁺ and Cl⁻ uptake to almost half of the salt-stressed plants [41].

From the above-mentioned studies, the role of SA in alleviating salt stress can be considered as clear and concise. But, there are also some points to be considered as higher concentrations of SA may itself cause damage to plants [12, 59] and very lower concentrations may have a minimum effect [42, 62] against salt stress. So, the concentration of SA, application method and time, duration of salt stress and plant age are some of the important points to be considered while using SA against salt stress.

3.2. Drought

Drought stress is one of the most devastating abiotic stresses adversely affecting growth and developmental processes of the plant. Drought stress affects the physiological processes, brings biochemical changes, leads to the formation of secondary metabolites, significantly accumulates endogenous reactive oxygen species (ROS) and increases toxins (such as methylglyoxal). Drought stress hampering the reproductive development drastically reduces yield or productivity of plants [67].

Several studies demonstrated and proved the pivotal roles of SA in alleviating drought damage and improving drought stress tolerance in plants ( Table 2 ). Salicylic acid pretreatment (0.5 mM) alleviated substantial water loss and its damaging effects on wheat seedlings that enhanced drought tolerance [43]. Pretreatment with SA upregulated 37 protein spots under drought stress which has been investigated through proteomics. Glutathione S-transferases, APX, and 2-cysteine peroxiredoxin were enhanced under drought stress. Enhancement of antioxidant defense system worked against the oxidative damage [43]. Proteins involved in ATP synthesis are also upregulated by SA under drought stress. Salicylic acid supplementation with drought also upregulated 21 protein spots, including RuBisCo and related enzymes [43]. In their other experiment, influential role of SA was also demonstrated on AsA-GSH cycle [68]. Exogenous SA supplementation enhanced the transcription of GST1, GST2, GR, and MDHAR genes during almost the entire drought period. The increase of DHAR was noticed at 12 h, GPX1 at 48 h, phospholipid hydroperoxide glutathione peroxidase (GPX2) at 12 and 24 h, and glutathione synthetase (GSHS) at 12, 24, and 48 h of drought stress. Upregulation of transcription level of AsA-GSH cycle enzymes contributed to drought tolerance [68]. SA-accumulating (siz1 and cpr5) genes were highly expressed in guard cells of drought which modulated movement of stomatal aperture in Arabidopsis plants. The generation of ROS was also modulated in this plant [69]. In tomato (Lycopersicon esculentum), SA treatment with drought has been demonstrated to protect the activity of nitrate reductase which helps to maintain the protein and nitrogen contents of the leaves, compared to the drought affected plant without SA addition. Photosynthetic parameters, membrane stability, water potential and activity of carbonic anhydrase were maintained by SA which also contributed to drought stress tolerance [70]. In sunflower, water stress-induced decrease in the yield and oil content. Salicylic acid (0.724 mM) application increased the Pro content, head diameter, number of achene, 1000-achene weight, achene yield, and oil yield of sunflower, compared to drought treatment alone [71]. The addition of acetyl SA in (0.1–1.0 mM) also improved drought tolerance of muskmelon seedlings [72]. Two wheat varieties viz. Wafaq-2001 and Punjab-96 were subjected to drought stress. Drought stress significantly decreased membrane stability index (MSI) and yield. Salicylic acid supplementation caused 37% increase in soluble sugars in Wafaq-2001 cultivar which was higher, compared to Punjab-96 cultivar. Salicylic acid also increased protein content and MSI in both cultivars with a higher increase in Wafaq-2001. The overall drought tolerance was higher in Wafaq-2001 after SA application which is evident from higher yield [73]. Exogenous addition of SA increased the activity of antioxidant enzymes which helped to alleviate the drought stress damage in Ctenanthe setosa [74]. Mustard (B. juncea L. cv. BARI Sharisha 11) seedlings were subjected to two different levels of drought with 10 and 20% polyethylene glycol (PEG) for 48 h. Leaf relative water content (RWC), chl b and chl (a + b) decreased but Pro content increased. Disrupting the antioxidant defense system, drought stress increased oxidative damage which was indicated by high MDA and H₂O₂ levels. Supplementation of SA in drought-stressed seedlings increased the leaf RWC and chl content, increased the AsA and GSH, decreased the GSSG content, and maintained a higher ratio of GSH/GSSG. Salicylic acid increased the activities of monodehydroascorbate reductase (MDHAR), DHAR, GR, glutathione peroxidase (GPX), CAT, glyoxalase I (Gly I), and glyoxalase II (Gly II) in drought affected seedlings as compared to the drought-stressed plants without SA supplementation, with a concomitant decrease in H₂O₂ and lipid peroxidation level [44]. Methyl-SA (at 0.1 mM) spray promoted drought-induced leaf senescence in Salvia officinalis [75]. Drought stress adversely affected growth performance of winter wheat, Cheyenne. Application of SA analogue 4-hydroxybenzoic acid (4-HBA) increased drought tolerance of winter wheat Cheyenne [16]. Foliar application of SA (10 μM) protects lemongrass (Cymbopogon flexuosus Steud. Wats.) varieties (Neema and Krishna) from drought stress by improving growth parameters, modulating the activities of nitrate reductase, carbonic anhydrase, and EL, Pro content, free amino acid, and in PEP carboxylase activity [76].

3.3. Extreme temperatures

Temperature is one of the vital factors that determine plants establishment, growth, development, and productivity. Due to climate change, global average temperature is fluctuating very rapidly and threatening the survival of living beings. Thus, among the various abiotic stresses, extreme temperature has become the talk of the topic in recent decades because of its devastating and damaging effects on plants [84]. Extreme temperature includes both high temperature (HT) and low temperature (LT) that can injure plants. The high temperature is the increasing of temperature beyond the critical threshold level that can deplete plant growth and metabolism depending on the sufficient time period [85]. Heat stress often becomes worse because of its combination with other stresses including drought [86]. High temperature severely alters the plant physiological processes including germination, photosynthesis, respiration, transpiration, partitioning of dry matter, etc. [87]. In addition, HT results in enzyme inactivation, protein denaturation, disruption of proteins and membranes which ultimately affects plant growth [88, 89]. Low-temperature consists of both freezing (<0°C) and chilling (0–15°C) temperatures. In chilling stress plant faces injury without formation of ice whereas, in freezing stress, the formation of ice occurs in plant tissues. Chilling and freezing stresses are together called cold stress or LT stress. Low-temperature stress shows various damaging symptoms in plants including faster senescence and decay [90, 91], interference with germination, cell membrane disruption, photosynthesis, water and nutrients uptake, reproductive development as well as growth and yield [92]. Either HT or LT conditions, at molecular level, leads to the overproduction of ROS which ultimately gives rise to the oxidative stress [84, 93]. Nowadays, to develop temperature-stress tolerance, the use of exogenous SA is one of the common approaches. Salicylic acid being the endogenous growth regulator or phytohormone acts as an important signaling molecule and develops abiotic stress tolerance in plants [94]. Recent advances on SA-mediated temperature stress tolerance have been listed in Table 3 .

High temperature (30°C) resulted in a significant reduction in FW and soluble starch synthase activity of T. aestivum [46]. Foliar application of SA improved the FW, total RNA, and soluble starch synthase activity. In Z. mays, HT (40 ± 1°C) induced oxidative damage and reductions in dry biomass were reversed by exogenous SA treatment. SA developed HT tolerance by improving CAT, SOD, and POX activities [48]. The effects of SA on seed germination and physiological attributes of heat stressed (32/26°C, 12/12 h, day/night) L. lycopersicum were investigated. It has been revealed that SA reduced the germination time and increased the germination percentage together with the increased vitamin, lycopene, total soluble solid (TSS) and titratable acidity (TA) contents [49]. Temperature above 40°C, increased TBARS and H₂O₂ contents but decreased the net photosynthesis, RuBisCo activity, chl, and WUE of T. aestivum plant. Negative HT effects were counteracted significantly by exogenous SA supplementation [13]. Heat stress (45°C) in Digitalis trojana Ivanina, compared to its normal temperature, lowered the important antioxidant enzymes (CAT and SOD) activities. Pretreatment with SA significantly increased CAT and SOD activities with increased Pro, phenolic, and flavonoid contents [47]. The sharp decline of photosynthetic apparatus was found in Vitis vinifera in response to heat (43°C) stress. Alleviation of photosynthetic rate and RuBisCo activity were found when pretreated with SA [95]. Cannabis sativa induced thermotolerance against (40°C) temperature, when supplied with exogenous SA, were studied. Improved activities of antioxidant enzymes (SOD, DHAR, GPX, APX, and GR) were documented with decreased CAT activity. Decreased EL percentage with reduced H₂O₂ and TBARS contents were also evident [97]. Salicylic acid involved in various protective functions in B. juncea after HT (30 or 40°C) exposure. However, increased growth, g_s and CO₂ fixation along with improved defense system were found with SA treatment [98]. Chilling (5°C) stressed Musa acuminate when treated with exogenous SA increased SOD, CAT, and APX activities with decreased H₂O₂ accumulation [99]. Low temperature (3°C) disrupted the RuBPC and PEPC activities with decreased rate of CO₂ assimilation and respiration. Treatment with SA improved the activity and ameliorated the chilling effects [100]. Performance of V. vinifera was investigated upon HT (38 − 0.5°C) [101]. Pretreatment with SA increased H⁺ and Ca²⁺-ATPase activities with cerium phosphate grain appearance and thus gave higher stress tolerance. Wang and Li [102] noted that upon freezing (−3°C) stress in V. vinifera, besides upregulating antioxidants, increased maintenance of AsA-GSH pool and cytosolic Ca²⁺ homeostasis caused improved heat stress tolerance. High-temperature stress mediated increased ROS generation and oxidative stresses have been reported in several other plant species. Exogenous SA treatment resulted in the reduced ROS generation and oxidative stress in Pratylenchus pratensis [103], Arabidopsis thaliana [109]. Night ambient temperature ranges from 27 to 32°C in Oryza sativa caused the significant reduction in spikelet fertility and grain size. Exogenous SA treatment improved the rice grain fertility and hence, increased yield [104]. Prunus persica fruits were pretreated with SA before imposition of chilling injury (0°C). Reduced chilling injury was observed due to higher activities of antioxidants and heat shock protein 101 (HSP101) expression [105]. Leaves of heat (38/30°C, day/night) stressed Rhododendron became withered, defoliated, and brown. Total soluble protein and Chl contents were also reduced. Lower damage rate of leaves with higher chl and soluble protein were observed when supplemented with SA [106]. In a study with HT stressed (50°C) V. radiata, SA treatment increased the CAT, APX, POD, and GSH contents with enhanced defense system and radical scavenging mechanism [107].

3.4. Toxic metal/metalloids

In the industrial era, the most important and potential threat for crop production is the abiotic stress. Among them, toxic metal stress is one of the major concerns. Growing population and fast industrialization coincide together, results in the generation and dissemination of huge amount of toxic metals in the environment [110]. Toxic metal consists of a set of harmful elements having no biological role in organisms such as Cd, Pb, Hg, St, Al, etc. Although toxic metals and heavy metals (HMs) are often thought to be synonymous, some lighter metals such as Al may also cause toxicity. Toxic and HMs are differed in the case of their biological role. Some HMs having a biological role in plants also considered toxic when they are used in high concentrations, viz. Ni, Cu, Zn, etc. On the other hand, metalloid includes those elements that show behavior both like metals and nonmetals including B, Si, Ge, St, As, Sb, etc. The underlying parent material and atmosphere are the two main sources of toxic metals. Metals are uptaken and accumulated easily by plants and causes toxicity within the plant tissue. They directly interact with the proteins, enzymes, and causes phytotoxicity. The inhibition of growth rate is the most certain consequences of metal toxicity [111]. Leaf rolling, chlorosis, necrosis, stunted growth, stomatal dysfunctioning, cation efflux, reduced water potential, alterations in the membrane, photosynthesis, metabolism, and various key enzymes are some other toxic metal effects in plants [111, 112]. Toxic metals also manipulate the nutrient homeostasis, water uptake, transport, transpiration, respiration, and ultimately may lead to plant death [113, 114]. Metal toxicity at the cellular level results in the overproduction of ROS [110]. To mitigate metal induced stresses in plants, plant biologists are trying to develop new strategies. Salicylic acid is a very important molecule that induces defense responses against various toxic metal/metalloids stresses ( Table 4 ).

Several research findings demonstrated that exogenously applied SA improved the growth and photosynthetic traits in different plants by reducing the damaging effects of toxic metals. O. sativa exposed to As (25 and 50 μM) [50], A. thaliana exposed to As (100 μM) [115], T. aestivum exposed to Cd (500 and 1000 μM) [121], and Z. mays exposed to Cr (500 ppm) [52] grown well under SA supplementation. In a recent study, SA pretreatment reduced the oxidative stress in T. aestivum after Cd (0.01, 0.1, and 1 mM) exposure. Cd stress increased the lipid peroxidation and EL percentage. But the exogenous application of SA significantly declined the MDA content and EL percentage [53]. Salicylic acid also evidenced to alleviate the oxidative stress induced by metal toxicity in several other plant species by decreasing the toxic effects of overproduced ROS and lipid peroxidation. In O. sativa, enhanced MDA and H₂O₂ contents induced by As (25 and 50 μM) were reduced by the exogenous SA pretreatment [50]. Similarly, adverse oxidative stress was also demonstrated in Lolium perenne as induced by Cd (100 μM). Results showed that increased accumulation of O₂ ^•−, H₂O_2, and higher MDA were decreased by SA application. Some other research findings also supported that the SA mitigates metal-induced oxidative damage in Pisum sativum [136], Brassica oleracea var. botrytis [135], Medicago sativa [134], etc. It was reported that treatment with SA alleviated the Cd (15 μM) induced root growth inhibition and improved the antioxidant activities thus reduced the Cd-induced oxidative stress in Hordeum vulgare [51]. Salicylic acid supplementation in As (100 μM) stressed A. thaliana showed improved performance in terms of antioxidant enzymes (APX, CAT, SOD) activities and enhanced tolerance to metal stress. In other experiment it was demonstrated that when SA was exogenously applied against Cd (0.25, or 0.50 mM) stress, increased amelioration of metal stress was observed with increasing activities of defense responsive genes and upregulating the antioxidant (SOD, CAT, APX, and GR) enzymes [117]. Decreased root growth, RWC, and increased oxidative stress were decreased by seed priming with SA [118]. Under Cd (50 and 100 mM) stress, plant growth, chl, total lipid contents, and nutrient absorption became decreased which were further increased by soaking seeds with SA. Increased stress tolerance with reduction of oxidative damage was also evident [119]. Increased ROS production, TBARS content, and EL with increased SOD, CAT, and APX activities were found after Cd (50 and 100 μM) exposure. Seed priming with SA significantly decreased the oxidative damage and increased the antioxidant enzymes activities. Agami and Mohamed [121] reported that SA efficiently alleviated the adverse Cd (500 and 1000 μM) stress by restoring the growth parameters and increasing the antioxidant defense system. Exogenous SA developed Cr (mg L⁻¹) stress tolerance by improving growth, photosynthetic pigments and oxidative stress reduction by upregulating antioxidant defense system [52]. Inhibited antioxidant enzymes (APX, CAT, POX, and SOD), soluble sugars and chl contents were showed when T. aestivum exposed to Pb (10, 50, 100, 200 mg L⁻¹). Alleviated inhibitory effects were found after SA supplementation as cotreatment [126]. In Zygophyllum fabago, increased Pb accumulation after Pb (0.75 mM) exposure was reduced by SA pretreatment. Upregulated antioxidants and downregulated oxidative damage together induced stress tolerance [125]. Salicylic acid treatment against Cd (5, 10 or 50 mM) stress increased tolerance in P. pratensis by controlling uncontrolled absorption of Cd and maintaining nutrients (K, Ca, Fe, Mg) homeostasis [127]. Effects of B was investigated (25 and 50 mg kg⁻¹) toxity in Daucus carota and S. oleracea. In both plants, the growth, physiology, and antioxidant enzymes activity were affected by B toxicity. But exogenous SA application showed some protective effects in the alleviation of the metal toxicity [128, 129]. A similar finding was also demonstrated by Shi and Zhu [131] in Crocus sativus plant. Exposure to a toxic level of Mn (600 μM) with SA in C. sativus plant, maintained nutrient (Ca, Mg, Zn) homeostasis, reduced metal stress, and improved tolerance. Recent findings in B. juncea against Mn (3, 6, or 9 mM) toxicity revealed that SA is an important regulator of photosynthetic enzymes including carbonic anhydrase (CA). It together with the upregulated defense system and reduced oxidative damage-regulated the photosynthesis in a concentration-dependent manner [130]. Besides upregulation of antioxidant enzymes, SA involved in the activation of CaM-like protein genes and cytosolic Ca²⁺ in Al (30 μM) stressed Glycine max. It has been showed that increased metal stress tolerance resulted from exogenous SA treatment [132]. Effect of exogenous SA was investigated upon Pb (50 μM) stressed Vallisneria natans. Decreased chl, carotenoid, NADPH oxidase, nonprotein thiols, AsA but increased CAT, DHAR, and POD activities were observed. Pretreatment with SA increased the activities of NADH oxidase, AsA, and GSH in Hg (10 μM) stressed M. sativa and thus developed better tolerance to stress [133]. Recent evidences suggested that SA develops stress tolerance by involving in the regulation of photosynthetic pigments, activities of CA, NR, and anticancer alkaloids (vincristine and vinblastine) upon Ni (50, 100, and 150 mg kg⁻¹) exposure in Catharanth roseus [122]. Toxic level of Ni (0.5 mM) impacts on Brasicca napus also suggested that application of SA decreased the leaves’ toxicity symptoms (chlorosis, necrosis, etc.) and improved growth and survival [123]. Recently a combined effect of metals (Co, Ni, Cd, Cr, and Pb) (0.25 M) on B. oleracea var. botrytis has been investigated. It has been found that SA alleviated all the toxic effects of metals except for Cr. In the case of Cr, SA accelerated the toxic effects and the plant died [135]. Improved CO₂ fixation, RuBPC activity, and chl content were found in Cd (0.5, 1, 2, and 5 μM) stressed P. sativum when supplied with exogenous SA.

3.5. Ozone and ultraviolet radiation

Due to the gradual increase in atmospheric ozone (O₃) concentration, it has become a major threat for plant species mainly because of its pollutant and photochemical oxidant affects [139]. Significant crop losses due to O₃ damage is predicted to be increased by 25% in background O₃ concentration over the next 30–50 years [140]. High concentrations of ozone induce oxidative stress, which activates programmed cell death and significantly inhibits plant growth, causing plant death and loss of quality [141]. It is the most noteworthy atmospheric pollutant in terms of phytotoxicity. It is to be noted that in the concentration of O₃ has been decreased by 5% in the past 50 years due to the release of anthropogenic pollutants and, a larger proportion of the UV radiation (especially (UV-B) spectrum reaches the Earth’s surface [142]. Although sunlight plays an integral role in harvesting light energy through photosynthesis, high light, especially ultraviolet (UV) radiation, resulted in stress to plants, which include damage to DNA, proteins, and other cellular components [143]. This episode is unavoidable as 7% of the electromagnetic radiation emitted from the sun is in the UV range (200–400 nm). UV radiation also leads to oxidative stress by photooxidation and excess generation of ROS [84]. To cope up the adverse effects of both O₃ and UV radiation needs some adaptive mechanisms. In few plant species, SA was found to take part in enhancing the tolerance to O₃ and UV radiation mainly by enhancing antioxidant defense and improving plant growth.

In A. thaliana, UV-C light stress activated the transition to flowering through SA. SA could regulate the time of flowering by inducing photoperiod and autonomous pathways which are evident by late flowering in SA-deficient plants. While investigating the genes responsible for flowering induction viz. constans (CO), flowering locus T (FT), suppressor of overexpression of constans 1 (SOC1), and flowering locus C (FLC) it was observed that the expression of CO, FT, and SOC1 transcripts decreased to around 50% in long day-grown SA-deficient plants when compared to contron plants [54]. In short day plants, only the levels of FT transcripts were reduced compared to CO. Thus, it indicated that SA might play important role in flowering under UV radiation [54]. The effect of UV was investigated (UV-A: 320–390 nm, UV-B: 312 nm, and UV-C: 254 nm radiation with a density of 6.1, 5.8, and 5.7 W m⁻²) on Capsicum annuum plants and found that activities of antioxidant enzymes were enhanced in leaves in response to UV-B and UV-C radiation. Moreover, SA treatment showed further enhancement in the activities of POD, APX, CAT, and GR while some other enzymes were modulated [55]. In another report, a clear decline was reported in photosynthetic pigments (chl a, chl b, and carotenoid) under UV-A, UV-B, and UV-C, while a foliar spray of SA recovered this decline. The level of anthocyanins, flavonoids, and rutin in SA-treated plants was also higher than in a UV-exposed plant grown without SA [56]. V. radiata, exposed to UV-B radiation (ambient + 4–8 kJ m⁻²) showed declined growth, photosynthetic pigments and photosynthesis (F _v /F _m and qP except NPQ) which were accompanied by significant decrease in SA level [57]. UV radiation also causes overproduction of ROS and concomitantly damaging effects on lipids, proteins, and membrane stability. However, SA pretreatment significantly alleviated the adverse effects. They also revealed that UV-B altered SA biosynthesis and SA-pretreatment might act as a signal that reduces oxidative stress by triggering upregulation of antioxidant defense and subsequent improvement of growth and photosynthesis [57]. In Satureja hortensis, both UV-B and UV-C exhibited decreased plant growth (plant height, root length, shoot DW, and leaf area), node number, internode distance and chl content, while stem diameter, leaf thickness, flavonoid content, phenolic content, and antioxidant activity were increased [58]. The increase in secondary metabolite such as flavonoid content, phenolics might be able to protect cells against free radicals but this level was not well enough under severe stress. On the other hand, plants treated with 1 mM SA exhibited higher growth and improved physiology compared to nontreated one and subsequently showed better appearance under UV radiation [58].

4. Salicylic acid and ROS detoxification

Salicylic acid is the most studied phytohormone regarding its role in oxidative stress. Due to its multifarious actions, it has been found very effective in detoxifying ROS in plant cells ( Figure 2 ). According to Janda and Ruelland [144], SA-induced tolerance to abiotic stresses such as chilling, heat, heavy metals, osmotic stress, and salinity is involved in activation of the stress-induced antioxidant system. It has been demonstrated that SA could significantly improve both photosynthesis parameters and antioxidant defense system in conferring salt stress tolerance in V. radiata [17]. In O. sativa, exogenous SA significantly reduced the oxidative burst by reducing H₂O₂ and O₂ ^•− contents under herbicide exposure which was mainly due to the SA-mediated upregulation of antioxidant defense enzymes (SOD, POD, CAT, APX, GR, and GST) and efficient GSH pool. The positive role of SA is mostly dependent on their dose and application methods [145]. Other study also proved that SA-mediated antioxidative defense system was dependent on the concentration used and the method of application [146]. In T. aestivum, 0.25 mM SA resulted in marked increase in the antioxidant enzyme activities (SOD, CAT, POD, GPX, APX, and GR), while the treatment with 2.5 mM SA resulted in a decrease in the activities under water stress [146]. A lower dose of SA also maintains higher AsA pool which in turns significantly scavenged the ROS. This ROS detoxification induced by SA also associated with improved photosystem II (PSII) efficiency. Belkadhi et al. [147] showed that Linum usitatissimum plants showed a lower amount of lipid and protein oxidation and membrane oxidation under Cd stress when pretreated with SA. This protection was availed by the enhanced activities of SOD, GPX, and APX as well as the 2,2′-diphenyl-1-picrylhydrazyl (DPPH) scavenging activity and ferric-reducing antioxidant power (FRAP). Increases in MDHAR, DHAR, GR, GPX, and CAT activities 53, 64, 49, 82, and 65% were noticed in PEG-treated B. juncea seedlings when sprayed with 50 μM SA [44]. SA-induced upregulation of antioxidant enzymes caused 32 and 26% decrease in MDA and H₂O₂ content compared to drought (20% PEG, 48 h) alone. One of our research results showed that exogenous SA enhanced the activities of antioxidant enzymes and nonenzymatic antioxidants under salt stress (200 mM NaCl, 48 h). Compared to salt stress alone, NaCl + SA resulted in 41, 107, 25, 37, 44, and 59% increases in MDHAR, DHAR, GR, GST, GPX, and CAT activities [40]. Salicylic acid supplementation also increased AsA and GSH content by 48 and 39%, respectively and enhanced GSH/GSSG ratio by 47% compared to salt stress alone. As a result, MDA and H₂O₂ contents decreased by 39 and 31% [40]. In Nitraria tangutorum SA mitigated salt-induced oxidative stress (evidenced by a marked reduction in MDA and H₂O₂ content) by upregulating the activities of SOD, POD, and CAT. The content of MDA in the 1.5 mM SA treated seedlings under 100–400 mM NaCl treatments declined to 2.27–3.59 fold of the control which was a clear sign of the reduction of oxidative stress [148]. However, some of the enzymes like APX activity was inhibited at higher concentrations (1.0 and 1.5 mM) of SAMDA content was measured with 1.5 mM SA applied, and the contents of MDA in the leaves of SA-treated seedlings under 100–400 mM NaCl treatments declined to only 2.27–3.59 fold of the control.

5. Interaction of SA with other signaling molecules

Salicylic acid not only exerts its positive effect independently but also interacts with other signaling molecules, phytohormones, and other phytoprotectants. These interactions show different signaling events and ultimate protection to plants from stress-induced damages ( Figure 3 ). In grapevines, Wang and Li [102] showed improved Ca²⁺ homeostasis and associated antioxidant defenses under heat and cold stress-regulated by SA. When plants were treated with exogenous SA, they showed enhanced PM-Ca²⁺ATPases and V-Ca²⁺ ATPases activities. Moreover, Ca²⁺ precipitates were shown on the inner side of the plasma membrane, and less were in intercellular spaces and the vacuole. However, in SA treated plants Ca²⁺ precipitates were in vacuoles, and few were on the inner side of the plasma membrane. Ca²⁺ precipitates in chloroplasts were bigger even after heat or cold stress. Importantly, SA treatment caused enhancement of the activities of APX, GR, and MDHAR, which efficiently reduced the lipid peroxidation and relative EL (REL), which concluded that exogenous SA could mitigate oxidative stress by maintaining Ca²⁺ homeostasis under extreme temperature stress [102]. In O. sativa, Wang et al. [145] reported that SA treatment downregulated ABA genes more in cultivar XS 134, which correlated with the enhanced tolerance to quinclorac-induced oxidative stress. Application of SA had obvious effects on all of the ABA-related genes and inhibited the expression of OsABA8ox1, OsABA8ox2, OsABA8ox3, OsNCED1, OsNCED2, and OsNCED3 as compared to quinclorac stress alone. Since overproduction of ABA and ROS is highly associated this downregulation protected the plants from herbicide-induced damages. Wang et al. [145] also reported SA-induced inhibition of ABA synthesizing enzymes. Leslie and Romani [149] reported that SA inhibited ethylene formation which triggered biosynthesis of ABA under stress conditions [150]. In the adventitious roots of Panax ginseng, SA-induced enhancement of the activities of NADPH oxidase, SOD, CAT, POD, and APX was evident while no significant effect on AsA and GSH content were observed [151]. These effects were mostly NO-dependent and it was also observed that SA-induced the generation of NO. They revealed that at lower concentration (100 μM) SA was highly effective in inducing the accumulation of NO, O₂ ^•− and it took part in stress signaling. Interactive effects of SA and NO were studied in mitigating osmotic stress (−0.4 MPa) in T. aestivum. It was observed that osmotic stress induced chl degradation and membrane instability, and H₂O₂ generation and lipid peroxidation were effectively reduced by exogenous application of SA or SNP, which was associated with the enhancement of antioxidant defense. However, pretreatment of plants with methylene blue (MB; as a guanylate cyclase inhibitor) reversed or reduced the protective effects of SA and SNP suggesting that the protective effects were likely attributed to NO signaling. They also concluded that NO may act as downstream of SA signaling in the reduction of induced oxidative damage [152]. SA-mediated H₂O₂ signaling and subsequent Cd stress tolerance was revealed in L. usitatissimum [147]. Seedlings pretreated with 250 or 1000 μM SA resulted in enhanced production of H₂O₂ because of inhibited CAT activity. Although the control plants with SA pretreatment showed significant (1.2 fold) increase in H₂O_2, this level is remarkably lower when compared with Cd alone and Cd+SA. These results indicated that SA could regulate the Cd-induced oxidative stress because Cd-treated seedlings primed with SA exhibited a higher level of total antioxidant capacities and increased activities of H₂O₂-detoxifying enzymes [147]. Exogenous SA application was found to activate GSH synthesis in B. juncea and B. napus and showed enhanced protection against drought- and salt-induced oxidative damages [44, 40].

Figure 3.

Interaction of SA with other signaling molecules to elicit defense responses in plants.

6. Conclusions and perspectives

Salicylic acid plays an important role in the regulation of growth and physiology in relation to the abiotic stress responses of plants. The SA was found to be effective in the different form of application foliar spray/incorporation with growing media depending upon plant species. The low concentration of SA showed advantageous effects in abiotic stress tolerance of plants. In contrast, the high concentration of SA showed toxic effects. Thus, both the concentration and application method of SA are critical to obtaining its best effect on different plant species. In the biosynthesis pathway of SA, there are unknown steps and enzymes which should be discovered. The catabolism and the further fate of transformed product of SA are not known clearly. How SA interacts and being regulated by the cross-talk in harmony with other phytohormones and plant growth regulators working (auxins, cytokinins, gibberellins, ethylene, jasmonates, brassinosteroids, etc.) and other signaling molecules (NO, H₂O₂) were not studied extensively. SA-mediated defense networks and insights into the cross-talk of SA with other defense-signaling pathways should be revealed. An integrated approach combining the knowledge of genetics, molecular biology, biochemistry, genomics, and bioinformatics techniques is a useful tool to study the functioning of SA in plants. Clear understanding of the biosynthesis and catabolic pathway and other unanswered question are vital to exploit SA as a potent phytoprotectant molecule to improve abiotic stress tolerances.

Acknowledgments

We are highly thankful to Mazhar Ul Alam, Laboratory of Plant Stress Responses, Faculty of Agriculture, Kagawa University, Japan, for his critical reading and formatting of the manuscript draft. The first author acknowledges Japan Society for the Promotion of Science (JSPS) for funding in his research. We are also highly thankful to Mr. Md. Mosfeq-Ul-Hasan, Zhejiang University, Hangzhou, China, for providing us several supporting articles. As page limitation precluded us from citing a large number of studies, we apologize to those whose original publications are therefore not directly referenced in this chapter.

Abbreviations

4-HBA	4-hydroxybenzoic acid
ABA	Abscisic acid
APX	Ascorbate peroxiddase
AsA	Ascorbate/ascorbic acid
BA2H	Benzoicacid-2-hydroxylase
CAT	Catalase
chl	Chlorophyll
Ci	Intercellular CO2 concentration
DAS	Days after sowing
DHA	Dehydroascorbate
DHAR	Dehydroascorbate reductase
DPPH	2,2′-diphenyl-1-picrylhydrazyl
DW	Dry weight
EL	Electrolyte leakage
FRAP	Ferric reducing antioxidant power
FW	Fresh weight
GB	Glycinebetaine
Gly I	Glyoxalase I
Gly II	Glyoxalase II
GO	Glycolate oxidase
GR	Glutathione reductase
gs	Substomatal conductance
GSH	Reduced glutathione
GSSG	Oxidized glutathione
GST	Glutathione S-tranferase
HSPs	Heat shock proteins
HT	High temperature
IC	Isochorismate
ICS	Isochorismate synthase
IPL	Isochorismate pyruvate lyase
LOX	Lipoxygenase
LT	Low temperature
MAPK	Mitogen-activated protein kinase
MB	Methylene blue
MDA	Malondialdehyde
MDHAR	Monodehydroascorbate reductase
MeSA	Methyl salicylate
MSI	Membrane stability index
NADH	Nicotinamide adenine dinucleotide phosphate
NO	Nitric oxide
PAL	Phenylalanine ammonia-lyase
PEG	Polyethylene glycol
Pn	Net photosynthesis
POD	Peroxidase
POX	Peroxidases
Pro	Proline
ROS	Reactive oxygen species
RWC	Relative water content
SA	Salicylic acid
SA-Asp	Salicyloyl-l-aspartic acid
SAG	Salicylic acid 2-O-β-glucoside
SAT	Serine acetyl transferase
SGE	Salicyloyl glucose ester
SNP	Sodium nitroprusside
SOD	Superoxide dismutase
TBARS	Thiobarbituric acid reactive substances
Tr	Transpiration rate
TSS	Total soluble solid
WUE	Water use efficiency