Phytohormones-Assisted Management of Salinity Impacts in Plants

Abstract

The salinity of soils has been significantly limiting crop production in most arid and semi-arid regions of the world. Plant hormones (phytohormones), small molecules with versatile roles in plants can be a sustainable approach for minimizing the major salinity-impacts in plants. Most phytohormones are reported to regulate various signaling cascades interrelated with plant development and stress-resilience and -coping mechanisms. In addition to regulating photosynthesis and related variables, phytohormones also modulate nutrient homeostasis, source-sink capacity, osmoregulation, and antioxidant defense systems in plants under abiotic stresses including soil salinity. Molecular studies have confirmed the coordination between phytohormones and signaling networks, which in turn also maintains ionic homeostasis and plant-salinity tolerance. This chapter aims to appraise the literature available on the role of 10 well-characterized stress response hormones (abscisic acid, ABA; ethylene; salicylic acid, SA; jasmonic acid, JA; and nitric oxide, NO) and also other growth-promoting hormones (such as auxins, gibberellins, GA; cytokinins, CKs; brassinosteroids, BRs; and strigolactones, SLs) in the management of salinity impacts in plants. The discussion outcomes may help in devising and furthering the strategies aimed at sustainably strengthening plant-salinity tolerance.

1. Introduction

Human’s major three requirements, namely food, clothing, and shelter are mainly provided by agricultural activity. Unfortunately, the health and productivity of most agricultural crops are impacted by both biotic (insect pests and disease pathogens) and abiotic (temperature, drought, flooding, salinity, heavy metals, radiation, nutrient deficiency, and excess) stress factors. In turn, these stress factors, in isolation and/or combination, are significantly threatening global food security. In particular, abiotic stresses are responsible for about 51–82% of annual loss in yield of the major food crops in the world agriculture. Interestingly, abiotic stress impacts on agricultural crops and food security are further aggravated by the deteriorating agro-climatic conditions [1, 2, 3, 4]. According to the United Nations Population Division, the world population will reach 9 billion in 2037 and 10 billion in 2058 [5]. It would be far easier to feed the projected human population because most grain crops exhibit about 1% annual yield (much < world’s population growth rate), and direct feeding of people shares only 55% of the world’s crop calories [6, 7]. Among the major environmental challenges, the salinization of soils (soil salinity) has been inducing most land degradation, constituting a primary limit on crop health and productivity, and thus, is threatening agriculture across the world [8].

2. Soil salinity: causes, status, and major impacts

2.1 Causes and status

Soils exhibiting the saturation paste extracts (ECe) in the root zone as electrical conductivity (EC) > 4.0 dS m⁻¹ (≈40 mM NaCl) at 25°C and 15% exchangeable Na⁺ ion are considered saline. There can be five major classes of soil salinity: non-saline (EC = 0–2 dS m⁻¹; low saline (EC = 2–4 dS m⁻¹; moderately saline (EC = 4–8 dS m⁻¹; highly saline (EC = 8–16 dS m⁻¹; and extremely saline (EC = ≥ 16 dS m⁻¹) (Figure 1) [9, 10, 11, 12]. Most saline soils exhibit Na⁺ as an anion and Cl⁻ as a cation [13]. However, Na⁺, Ca^2+, and Mg²⁺ are the major cations component of total soluble salts in soils, whereas Cl⁻, SO₄²⁻, and carbonates (CO₃²⁻ and HCO₃⁻) are the major anions in total soluble salts in soils. Despite the aforementioned fact, most studies aimed at exploring plant-salinity responses and tolerance have considered Na⁺ and Cl⁻; and have largely ignored other cations (Ca²⁺ and Mg²⁺) and/or anions (SO₄²⁻, CO₃²⁻, and HCO₃⁻) [14]. Both Na⁺ and Cl⁻ ions are the most widespread causes of soil salinity, where Cl⁻ is more dangerous than Na⁺ since it is responsible for many physiological disorders in plants [15].

Figure 1.

Schematic representation of the major classes of soil salinity [9, 10]. dSm⁻¹, decisiemens per meter.

Notably, the soil salinity may be developed as natural (geological, hydrological, and pedological processes) or induced by human activity (human-caused factors). The long-term natural accumulation of salts (including Cl⁻ of Na⁺, Ca²⁺, and Mg²⁺and sometimes SO₄²⁻ and CO₃²⁻) in the soil or surface water contributes to the primary or natural salinity. On the other hand, the disruption of the hydrologic balance of the soil between water applied (irrigation or rainfall) and water used by crops (transpiration), as a result of anthropogenic activities, causes secondary soil salinity [16]. In fact, the secondary salinization (rising NaCl levels in groundwater) impacts irrigated land and eventually leads to the loss of agricultural soils [17]. About 1125 million hectares worldwide have already been impacted by the soil salinization [18]. Huge annual global loss in crop production (≈ US$27.3 billion) has been reported in saline soils in irrigated areas, representing about 20% of the total salinity-affected soils, mainly in North America, Oceania, and the Middle East. Interestingly, salinity may impact half of the world’s irrigated land by 2050 [19, 20, 21, 22].

2.2 Major impacts

In terms of the tolerance to salinity levels, plants are grouped into two categories, namely halophytes (salinity-tolerant and adapted to salinized environments) and glycophytes (salinity-sensitive plants). Unfortunately, most agricultural crop plants are glycophytes (salt-sensitive), where soil salinization adversely inhibits growth, metabolism, development, and productivity (yield). Thus, the salinity-sensitivity of most crop plants, increasing rate of land salinization, and salinity-caused serious loss in crop productivity are challenging food security. Soil salinity significantly limits the growth, metabolism, seed germination, flowering, fruiting, and productivity (crop yields) in most crop plants, mainly as a result of osmotic stress and ionic stress [3, 23].

2.2.1 Osmotic stress and ionic stress

High soil salinity is bound to cause osmotic stress, followed by ionic stress. These two high soil salinity-caused direct stresses (ionic and osmotic) induce secondary stresses such as oxidative stress. Increased Na⁺ ions in the soil mainly cause osmotic stress. Equilibrium in ion homeostasis is severely disturbed, which includes an increase in the Na⁺ levels, K⁺ efflux, K⁺ leakage, K⁺ deficiency in the cytosol, replacement of Ca²⁺ with Na^+, and eventual impaired Na⁺/K⁺ ratio, nutritional disbalance, and disrupted enzyme activity [11, 12, 15]. Significantly reduced water absorption, decreased osmotic potential, closure of the stomata, inhibited CO₂-influx, and impaired downstream processes were reported as a result of osmotic stress in plants [22, 23, 24].

2.2.2 Oxidative stress and antioxidant metabolism

Elevated salinity-induced ionic and osmotic stresses are bound to cause oxidative stress, which is a physiological condition of elevated cellular generation of reactive oxygen species (ROS) and or diminished scavenging (antioxidant-mediated) of ROS leading to impaired cellular redox homeostasis. In turn, elevation in the generation of ROS and impaired ROS metabolism/scavenging has been widely reported in plants under soil salinization. The list of major ROS includes O₂^•−, •OH, H₂O₂, and ₁O² are produced in different including chloroplast, mitochondria, apoplast, and peroxisome, under normal conditions of stress such as salinity. If not metabolized and/or scavenged, most elevated cellular ROS may lead to cytotoxicity and severe damage to biological molecules such as proteins, lipids, and nucleic acids. In order to avoid the harmful effects of oxidative stress, plants have triggered an antioxidant defense system based on two types of machinery [12, 25]. Fortunately, plants are endowed with an antioxidant defense system comprising (enzymes and non-enzymes/antioxidant metabolites), which in isolation and/or combination tend to scavenge/metabolize most ROS and thereby avert elevated ROS-caused consequences [26, 27]. Major enzymatic antioxidants comprise superoxide dismutase (SOD), catalase (CAT), guaiacol peroxidase (GPX), glutathione sulfotransferase (GST), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR) and glutathione reductase (GR). On the other hand, ascorbate (AsA), glutathione (GSH), carotenoids, tocopherols, and phenolics are among the major non-enzymatic antioxidants in the plant stress defense system [27, 28, 29, 30, 31, 32, 33, 34].

Given the rising demands on crop yield for keeping up with the burgeoning human population, strategies for sustainably maintaining an optimum crop plant health under increasing soil salinization and climate change and the development of salt-tolerant crop varieties are being exhaustively explored.

3. Phytohormones and plant-salinity tolerance

Phytohormones stand second to none among the regulatory compounds and chemical messengers in terms of their importance in almost every aspect of plant life. Although phytohormones are small molecules, they are widely known to play key roles in plant growth, development, and various plant physiological processes. These also regulate internal and external stimuli, stress-involved signal transduction pathways, oxidative stress-scavenging, and stress tolerance in plants [35, 36, 37]. Plants under salinity stress usually tend to accumulate a range of osmolytes (osmoprotectants/compatible solutes), namely proline, glycine betaine, polyamines, and sugars. Interestingly, the cellular levels of most of these osmolytes are significantly modulated by varied phytohormones [38].

Employing the protocols, based mainly on the use of various phytohormones, may help sustainably improve optimum growth, metabolism, photosynthesis and productivity (yield) under salinity-affected soils, thereby minimizing increasing strain on global food security. Notably, the list of well-characterized stress response hormones includes abscisic acid (ABA), ethylene, salicylic acid (SA), and jasmonic acid (JA). In contrast, phytohormones classified as growth promotion hormones are auxin, gibberellin (GA), cytokinins (CKs), brassinosteroids (BRs), nitric oxide (NO), and strigolactones (SLs) [39, 40].

Apart from presenting a brief overview, the following sections attempt to enlighten the major roles (and the basic mechanisms involved) of ABA, auxins, BRs, CKs, ethylene, GAs, JA, NO, SA, and SLs in plant-salinity tolerance (Figure 2).

Figure 2.

Schematic representation of the typical structures of phytohormones discussed in the chapter.

3.1 Abscisic acid

Chemically, a sesquiterpenoid, 15-C compound, abscisic acid (ABA) is a naturally occurring and enigmatic stress phytohormone widely known to play key roles in plant growth and development. Important processes including leaf abscission, seed dormancy, embryo morphogenesis, stomatal opening and cell turgor maintenance are known to varyingly involve ABA [37, 41, 42, 43]. ABA has also been argued as a modulator of plant’s adaptive stress responses via integrating various stress signals, controlling downstream responses, biosynthesizing dehydrins, osmolytes, and protective proteins, and regulating protein-encoding genes [43, 44, 45]. ABA-supply helped pepper (Capsicum annuum) seeds to exhibit high germination percentage, radicle emergence, and cotyledon expansion of seeds under NaCl stress mainly as a result of low expression in seeds of ABA signaling components such as CaABI, CaPYL2, CaPYL4, CaSnRK2.3, and CaSnRK2.6 [46]. ABA-nitrogen coordination alleviated salinity-inhibited photosynthetic potential in mustard (Brassica juncea) by improving proline accumulation and antioxidant activity [47]. Major physiological mechanisms underlying ABA-induced salinity tolerance in plants may also involve enhanced activity of antioxidant enzymes (CAT, APX, peroxidase, and POD) and the contents of antioxidant non-enzyme/metabolites (AsA and GSH) [48]. Additionally, significant reductions in Na⁺ content, increased contents of K⁺, Mg²⁺, and Ca²⁺; and that of hormones such as 1-aminocyclopropane carboxylic acid, trans-zeatin, N6-isopentenyladenosine, indole-3-acetic acid (IAA), and ABA were also observed in ABA-supplied plants under salinity stress [48]. Involvement of ABA signaling in reduction of transpiration flow, regulation of Na⁺ ion homeostasis and antioxidant enzyme activities was reported to induce salinity tolerance in wheat (Triticum aestivum) seedlings [49]. Earlier, ABA-mediated improved Indica rice (Oryza sativa)-tolerance to salinity stress involved the calmodulin signaling cascade and the ABA-mediated induction of OsP5CR gene expression in osmolyte (proline) accumulation [50].

3.2 Auxins

A chemical messenger involved in the light and gravity-stimulated shoot-to-root transport of a ‘growth stimulus’ was argued to be auxin, whose chemical nature was later identified as IAA [51, 52]. Chemically similar to the amino acid tryptophan, IAA is the representative and most studied auxin in plants [51, 52, 53, 54]. Auxins are mainly involved in cell division, cell elongation, and cell differentiation [45]. However, auxins can also regulate plant abiotic stress responses, where its homeostasis, distribution, and metabolism can be modulated by most abiotic stress factors [35, 45, 52]. Auxins can mediate the root growth plasticity in response to salinity stress [55, 56]. Earlier, a significant remodeling of root architecture was reported under high salinity, which was argued due to salinity-led altered auxin-accumulation and -redistribution [57, 58]. Pre- or post-treatment of seeds with IAA significantly alleviated salinity impacts and improved seed germination and early seedling establishments of T. aestivum under salinity stress [59]. Hence, an optimum concentration and timely exogenous application of auxins would be a promising approach for countering the salinity stress impacts in crop plants [36].

3.3 Brassinosteroids

Considered ubiquitous in the plant kingdom, polyhydroxy steroidal phytohormones, namely brassinosteroids (BRs), promote growth, seed germination, rhizogenesis, and senescence in plants, as well as their stress-tolerance capacity. Notably, among so far identified 60 BRs-related compounds, the list of bioactive BRs includes only three: brassinolide (BL), 28-homobrassinolide (28-HomoBL), and 24-epibrassinolide (24-EpiBL) [60, 61, 62]. Extensive reports are available on the role of BRs in salinity-impact control in different test plants [62, 63, 64, 65, 66, 67].

In several salinity-exposed test plants, 28-homoBL detoxified the NaCl-caused stress by elevating the activities of antioxidative enzymes including (SOD, CAT, GR, APX, and GPX) [68, 69]. Seed priming with BL can improve seed germination and seedling growth by significantly increasing POD, SOD, and CAT activity under salt stress [70]. The supply of polyhydroxylated spirostanic brassinosteroid analog (BB-16) can also enhance the activity of CAT, SOD, and GR and thereby mitigate the salinity impacts in plants [71]. 24-EpiBL-mediated improved tolerance of different test plants to varying salinity levels involved a 24-EpiBL-mediated decrease in oxidative stress via induction in the activity of ROS-metabolizing enzymatic antioxidants including APX, CAT, and POD [72, 73, 74, 75, 76, 77]. Significant decreases in the cellular levels of electrolyte leakage, O₂^•− production, MDA, H₂O₂, and improved growth, carbonic anhydrase activity, photosynthetic efficiency in epiBL-supplied salinity-treated test plants were corroborated with enhanced activity of SOD, POD, GPX, CAT and APX enzymes and the improved contents of AsA and GSH [78, 79]. Interestingly, BRs have been extensively reported to regulate plant-salinity tolerance via interacting with a number of plant hormones including auxins [80, 81], ethylene [63, 65], ABA [82, 83, 84, 85], and NO [64, 79, 82, 86, 87]. Moreover, BR signaling components can be directly regulated by salt stress signals at both transcriptional and post-translational levels [84, 85, 88, 89, 90].

3.4 Cytokinins

Cytokinins (CKs) are the derivatives of adenine (such as zeatin, kinetin, and N6-benzyladenine, BA) or of phenylurea (such as diphenylurea and thidiazuron) [45, 91]. Notably, the first naturally occurring CK was zeatin, which was identified and purified from immature maize (Zea mays) kernels [92]. Kinetin was the first CK discovered as an adenine (aminopurine) derivative [92]. CKs mainly regulate the major plant growth and developmental processes [93, 94]. However, literature also supports the immense roles (and underlying mechanisms) of CKs in plant abiotic stress tolerance [95, 96, 97]. Notably, the genetic engineering of CKs-metabolism was argued as one of the prospective ways to improve agricultural traits of crop plants [98]. CK signaling-mediated promotion in salt tolerance in Z. mays was argued to involve CK-mediated modulation of shoot Cl⁻ exclusion [99]. Soaking Z. mays seeds in zeatin-type cytokinin biostimulators (namely cis-zeatin-type CKs, c-Z-Ck; trans-zeatin, t-Z-Ck isomers) was reported to enhance antioxidant system and photosynthetic efficiency and thereby improve Z. mays salt tolerance [100]. Notable contradictory results yielded in several studies on the functional analyses of CK receptor mutants and the involvement of CK in ABA-mediated stress signaling in plants under osmotic/salinity stress warrant further molecular-genetic clarifications regarding the role of CKs in plant osmotic/salinity stress tolerance [101, 102, 103].

3.5 Ethylene

Ethylene is an important signaling molecule and a gaseous phytohormone. Its coordination with downstream signaling components has been reported to help plants in varyingly tolerating salinity stress [104, 105, 106]. Induction of ethylene generation in salinity-exposed plants is indicative of its significance as a downstream signal and modulation of gene expression [104]. Ethylene-homeostasis and ethylene signaling have been argued as an important factor required for plant-salinity tolerance [107, 108]. The maintenance of cellular ethylene (via endogenous production-induced accumulation and/or by exogenously supplied of ethylene precursor, 1-aminocyclopropane-1-carboxylic acid) has been reported to enhance Na⁺ and K⁺ homeostasis and induce downstream signaling for ROS-homeostasis; and eventually to improve plant salt tolerance [109, 110, 111]. Earlier, ethylene-mediated improvement in Arabidopsis salt tolerance mainly involved enhanced retention of K⁺ in shoots and roots rather than decrease in tissue Na⁺ content [112]. Ethylene (or its biochemical precursor, 1-aminocyclopropane-1-carboxylic acid) supplies improved plant tolerance to high salinity [110, 113, 114, 115]. Ethylene can trigger plant salt tolerance by modulating polyamine catabolism enzymes associated with H₂O₂ production [116]. S-nitrosylation of ACO homolog 4 (1-aminocyclopropane-1-carboxylate oxidase homolog 4; ACOh4) improved ethylene synthesis and improved salt tolerance in salinity-exposed tomato plants [117]. The roles of S-adenosylmethionine (SAM, involved in ethylene biosynthesis) and its derivatives in plant salt tolerance have also been recently discussed [118].

Molecular studies have unveiled the ‘MdNAC047-ETHYLENE RESPONSE FACTOR (MdERF3)-ethylene-salt tolerance’ regulatory pathway in apple [108]. Apple MdERF4 was reported to negatively regulate salt tolerance by inhibiting MdERF3 transcription [119]. MdMYB46 enhanced salt (and osmotic) stress tolerance in apple by directly activating stress-responsive signals [120]. Additionally, plant responses to salt stress may also involve EIN3/EIL1-dependent genes and other ROS scavenger-coding genes [110]. The outcomes of crosstalk between miR319 and ethylene contribute to plant-salinity tolerance. To this end, overexpression of Osa-MIR319b and targeting mimicry form of miR319 (MIM319) confirmed the role of miR319-mediated positive regulation of ethylene synthesis, and eventually improved salinity tolerance in switchgrass (Panicum virgatum) [121]. However, negative roles of ethylene have also been reported in salinity-exposed plants, where enhanced ethylene levels did not help plants in counteracting salinity stress impacts [122, 123, 124]. Thus, the reported few ambiguous roles of ethylene in plant-salinity stress responses require further explanations.

3.6 Gibberellins

Gibberellins (gibberellic acid, GA), a large family of tetracyclic di-terpenoid compounds, are classical plant hormones denoted largely by ‘gibberellin numbers’ (GA_n) in order of discovery, such as GA1, GA2, …, GA_n. In general, GAs are involved in growth and development [125, 126]. However, the literature is full on the involvement of GA in plant tolerance to a number of abiotic stresses [96, 126, 127]. In salinity-exposed barley, exogenous GA3 increased the shoot and root length of germinated barley seeds; significantly reduced ion-leakage, osmolyte (proline) accumulation; and thereby rescued the expression of the HvABI5, HvABA7, and HvKO1 by 3, 10, and 33 fold, respectively [128]. Exogenously applied GA enhanced growth and salinity stress tolerance in Z. mays by modulating the morpho-physiological, biochemical, and molecular attributes [129]. Moreover, GA3-supply improved pigment content, plant growth, and development, reduced Na⁺ concentration in shoots and roots, increased the water absorption and metabolic activities in seeds, uplifted the seed dormancy, modulated cell division, and cell elongation. In this way, GA3-supply increased the growth of root, shoot, and number of leaves; increased photosynthetic activities and the dry matter production; maintained a fine-tuning among AsA-GSH cycle components; improved the plant height, yield, and yield-related traits, Ca²⁺ and K⁺ concentrations, and transpiration rates; and decreased Na⁺ concentrations in different test plants under salinization [129, 130, 131, 132, 133].

As reported in plants under most abiotic stresses, a higher accumulation of DELLA proteins was reported in salinity-exposed plants, which in turn was argued to restrain growth and enhance stress tolerance through reducing GA signaling activity [134, 135]. Seed priming with GA was reported to induce high salinity tolerance in Pisum sativum, where the applied GA modulated antioxidants, secondary metabolites, and upregulated antiporter genes [136]. Moreover, pre-treating/soaking of seeds with GAs was widely evidenced to improve increased α-amylase; salinity-caused nutritional disorders; decreased Na⁺ content; enhanced ion uptake, photosynthesis, and redox homeostasis; improved coordination among CAT, APX, and SOD, as an adaptive mechanism to salt stress [137, 138, 139, 140].

3.7 Jasmonic acid

Important critical signaling molecule jasmonic acid (JA) is among the most abundant members of the jasmonate class of plant hormones. Derived from linolenic acid (as cyclopentanone) and lipids, JA is known to regulate plant growth, development, and stress responses [45, 141]. Extensive reports are available on the role (and underlying mechanisms) of JA-mediated plant-salinity tolerance. Methyl jasmonic acid supply shifted the endogenous fatty acid levels and supported O. sativa growth in saline soil [142]. Transcriptomic analysis has revealed methyl jasmonate-mediated salt tolerance in alfalfa (Medicago sativa) as a result of antioxidant activity regulation and ion homeostasis [99]. JA-supply mediated mitigation of the inhibitory effect of salt stress in T. aestivum by increasing the endogenous levels of CK and IAA, reducing ABA contents, increasing α-tocopherol, phenolics, and flavonoids levels, and triggering SOD and APX activity [143]. In salinity-exposed Anchusa italica, methyl jasmonate improved test plant-salinity tolerance by enhancing contents of photosynthetic pigment, soluble sugars, K⁺ and Ca²⁺, declining Na⁺ content, and eventually improving the major growth attributes [144]. Salinity stress-mediated induction in endogenous JA is also known in plants [145, 146]. Interaction outcomes of JA with ABA can also improve plant-salinity tolerance [141, 147]. JA-mediated saline stress tolerance in O. sativa involved autophagy and programmed cell death as critical pathways [148]. Jasmonate biosynthesis gene OsOPR7 was involved in the mitigation of salinity-induced mitochondrial oxidative stress [149]. Conferment of a greater cell elongation under salt stress was achieved with mutations of JA-receptor CORONATINE INSENSITIVE1 (COI1) and MYC2/3/4 along with the stabilized JASMONATE ZIM mutant jaz3-1 [150].

3.8 Nitric oxide

A highly versatile gaseous, free-radical, redox-signaling molecule, nitric oxide (NO), has been widely reported to perform diverse functions in plants [15, 151]. In a wide range of studies on salinity-exposed plants, exogenous supply of NO (or sodium nitroprusside, SNP; a NO donor) improved seed vigor, germination, and plant health and productivity through alleviating oxidative damage as a result of decreased levels of electrolyte leakage, MDA, and H₂O₂, improving antioxidant defense mechanism, decreasing methylglyoxal toxicity, and upregulating the glyoxalase system, adjusting the levels of osmolytes, and maintaining ionic balance [16, 152, 153, 154]. NO-supply can also reverse the glucose-mediated photosynthetic repression in plants under salinity exposure [154]. In NO (0.1 mM SNP)-mediated T. aestivum seed priming (for 20 h) helped improve germination rate, the weight of radical and coleoptile, and K⁺ ion and Na⁺ ion homeostasis [155, 156]. Exogenous application of NO (or its donor, SNP) can increase leaf area, plant dry mass, and the lengths of shoot and root in NaCl-stressed plants [157, 158]. Additionally, the maintenance of ion homeostasis (via enhanced K⁺ uptake and reduced Na⁺ uptake) and the modulation of the Na⁺/H⁺ antiporter enzyme were also reported in salinity-exposed and NO (or its donor, SNP)-supplemented test plants [159]. NO-mediated high salt tolerance in plants may also involve a NO-accrued increase in H⁺-ATPase activity and eventual reduced leakage-mediated maintenance of high cytosolic K⁺/Na⁺ ratio [156, 160]. NO-mediated improved defense against salt-induced stress also involves NO’s interaction with signaling molecules [15]. In earlier studies, NO (or its donor, SNP)-supply mediated improvements in the photosynthetic capacity involved NO-mediated protection of photosynthetic pigments; maintenance of normal shape of thylakoids and increase in chloroplast size; enhancement in the quenching of additional energy and quantum-yield of photosystem II; increase in ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) activity; induction in the influx and efflux of Ca²⁺; regulation of stomatal behavior and guard cells-ABA concentration; and efficient energy dissipation [158, 161, 162, 163].

The asA-GSH cycle is a central modulator of the plant stress responses and defense, ROS metabolism, and cellular redox balance [26, 164]. A plethora of reports supports the role of NO in the maintenance of the cellular redox balance via regulation of AsA-GSH cycle components (enzymatic and non-enzymatic) in salinity-impacted plants [123, 165, 166]. In different salinity-exposed plants, NO (or its donor, SNP)-supply resulted in significantly decreased cellular O₂^•− generation, H₂O₂, MDA content, and electrolyte leakage via maintaining a fine-tuning among antioxidant enzymes (including SOD, CAT, APX a H₂O₂-scavenging enzyme, MDHAR, DHAR, GR, GST, GPX, and CAT) and non-enzymatic antioxidants (including GSH and AsA) [15, 16, 151, 167].

3.9 Salicylic acid

Salicylic acid (SA), a phenolic plant hormone, is widely known for its involvement in plant growth and development and modulation of plant stress responses. Exogenous SA supply has strengthened salinity stress-tolerance mechanisms in extensive studies [168, 169]. Reports are also available on a high bio-stimulatory capacity of SA for salinity tolerance involving positive regulation of the AsA-GSH cycle, elevated accumulation of osmoprotectors, antioxidant enzyme activation, and increasing tolerance under ion toxicity and oxidative stress [34, 170, 171, 172]. In salinity-exposed Vigna radiata, the role of SA-induced accumulation of glycine betaine protected photosynthesis and growth against NaCl-accrued impacts in V. radiata as a result of the minimized accumulation of Na⁺ and Cl⁻ ions and oxidative stress and maintained high GSH level and eventually reduced cellular redox environment [25]. In many instances, SA-mediated plant-salinity tolerance has SA-dose dependency [25, 40, 173, 174]. In salinity (100 mM NaCl)-exposed pepper (Capsicum annuum) plants, an exogenous supply of SA (0.5 mM) reduced leaf Na⁺ content and oxidative stress-related traits [171]. SA-supply mediated regulation of ROS-metabolism and AsA-GSH cycle has also been reported in plants under salinity stress [175]. In earlier studies, supplied SA-assisted mitigation of salinity stress impacts in plants involved characteristic changes in the expression pattern of GST-gene family members such as SlGSTT2, SlGSTT3, and SlGSTF4 [176]; enhanced transcript level of antioxidant genes; GPX1, GPX2, DHAR, GR, GST1, GST2, MDHAR, and GS [177], and GORK channel-mediated control of K⁺ loss [178].

3.10 Strigolactones (SLs)

Synthesized in several plant species, strigolactones (SLs) are multifunctional β-carotene derivative molecules and are considered as an essential plant hormone in regulating plant functions. SLs have been considered an emerging growth regulator for developing resilience in plants [37, 41, 179]. In salinity-treated rapeseed (Brassica napus), the supplied SL increased plant growth, photosynthetic traits, and antioxidant enzyme activity [180]. Involvement of SL signal transduction and SL-biosynthetic mutants in more axillary growth3 (max3) and max4 was reported in SLs-mediated positive regulation of plant salt tolerance [181, 182]. In many instances, SL-mediated plant salt tolerance involved ABA accumulation [40, 183, 184]. In some studies on salinity-exposed plants, exogenous SLs improved ROS metabolism and decreased lipid peroxidation and cellular damage [185, 186]. The role of arbuscular mycorrhizal fungi and ABA in SL-assisted improvement in plant-salinity tolerance has also been reported [183, 184].

4. Conclusions and prospects

The salinity of soils is significantly limiting crop production in most arid and semi-arid regions of the world. Owing to their sensitivity to salinity stress, most agricultural crop plants exhibit soil salinization-accrued inhibition in growth, metabolism, development, and productivity (yield). Thus, salinity-caused serious impacts on crop health and productivity are challenging food security. Though very small molecules are produced in small quantities, phytohormones have versatile roles in plants as the major regulator of various signaling cascades interrelated with plant development, stress resilience, and coping mechanisms. Apart from presenting a brief overview, this chapter attempted to enlighten the major roles (and the basic mechanisms involved) of selected 10 phytohormones (ABA, auxins, BRs, CKs, ethylene, GAs, JA, NO, SA, and SLs) in plant-salinity tolerance. An optimum concentration and timely exogenous application of these phytohormones would be a promising approach for countering the salinity stress impacts in crop plants. There are still challenges to understanding how ABA, auxins, BRs, CKs, ethylene, GAs, JA, NO, SA, and SLs and associated close molecules can function at the molecular level and how the intimate mechanisms of interaction among these phytohormones and also with other emerging signaling molecules work. Additionally, a few contradictory results related to the involvement of CK in ABA-mediated stress signaling and the reported few ambiguous roles of ethylene in plant-salinity stress responses under osmotic/salinity stress warrant further molecular-genetic clarifications.