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Perspectives of Phytohormones Application to Enhance Salinity Tolerance in Plants

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Imran Khan, Muhammad Umer Chattha, Rizwan Maqbool, Muqarrab Ali, Muhammad Asif, Muhammad Umair Hassan and Muhammad Talha Aslam

Submitted: 03 October 2023 Reviewed: 07 October 2023 Published: 02 February 2024

DOI: 10.5772/intechopen.1003714

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New Insights into Phytohormones Edited by Basharat Ali

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New Insights Into Phytohormones

Basharat Ali and Javed Iqbal

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Abstract

Plants undergo a wide range of morphological, cellular, anatomical, and physiological changes in response to salinity stress. However, plants produce some signaling molecules, usually known as phytohormones, to combat stress conditions. Salinity tolerance is a complex mechanism, whereas phytohormones have a central role in it. Phytohormone-mediated plant responses improve nutrient uptake, the source-sink relationship, and plant growth and development. Phytohormones triggers the specific gene expressions which are responsible for the modification of various plant mechanisms under salinity stress. This review summarized the most recent research findings about plant responses to salinity stress at physiological and molecular levels and discussed the probable function of several (abscisic acid, indole acetic acid, cytokinins, gibberellic acid, salicylic acid, brassinosteroids, ethylene, and triazoles) phytohormones and their interaction in modulating salinity stress. Further, the understanding of specific genes involved in phytohormonal regulation toward salinity tolerance is a key to developing breeding and transgenic approaches for meeting food demand under sustainable crop production.

Keywords

  • salt stress
  • plant growth
  • phytohormones
  • salinity tolerance
  • future perspectives
  • abiotic stress
  • hormonal regulation

1. Introduction

The world’s population is continuously increasing, and it is expected to reach 9.1 billion in 2050 that would require 70% more food to feed to world [1, 2, 3, 4]. The changing climate is endangering the productivity and sustainability of agricultural production systems [5]. It would be the biggest challenge to meet the food requirement of growing population in near future [5]. Climate change exerts a range of severe abiotic stressors including high salt, drought, cold, and heat stress, which significantly impede crop yields [6]. Among these, salt stress is the most widely spread abiotic stress that covered the 1/3rd area of arable land globally [7] and is increasing by 10% annually due to global climate change [8]. Salt stress lowers seed germination, seedling establishment, enzymatic production, and protein synthesis in plants [9]. Salt stress is detrimental to crop growth, yield, and overall agricultural productivity [10]. Salinity develops osmotic stress and ion toxicity that affect the metabolic processes of plants [11]. The antagonism in essential nutrient uptake by plant roots from saline soil leads to the deficiency of Ca2+, K+, Fe++, and Zn++ ions and water scarcity in plant tissues [12]. Paliwal et al. [13] noticed shrinkage of leaf area, less stomatal conductance, and chlorophyll concentration, alongside the ROS development, which lowers the seed germination, plant height, and biomass [13]. Due to this, the average agricultural yield was reduced by 50% [2, 14].

However, crop plants exhibit complex alterations to mitigate the negative effect of salt stress for survival [15]. These alterations may include both morphological and developmental patterns (growth plasticity) and physiological and biochemical adaptations [16]. Further, plants build up bioactive endogenous compounds, also known as phytohormones, which accelerate the plant growth and development under stress conditions [17]. Phytohormones produce various organic solutes such as sugars, polyols, betaines, and proline to protect the cell structure, ionic balance, reactive oxygen species (ROS) scavenging, protein regulation, and expression of related genes to enhance crop growth and developmental processes under stress conditions [17, 18]. Further, enhancing stress tolerance has emerged as a significant area of focus in the agricultural sector that improves the crop performance under stress, environmental sustainability, horticulture, and economic viability.

The primary emphasis of this chapter is to develop a comprehensive understanding of phytohormones application in modulating the biochemical and physiological processes of plants exposed to salinity-induced stress. Several studies highlighted various detrimental effects of salinity on plants [19]. Contrarily, the salinity tolerance can be regulated through various kinds of phytohormones including auxin, cytokinin, gibberellic acid, abscisic acid, and ethylene [20]. Further, salicylic acid, brassinosteroids, and triazoles are also found imperative to enhance salinity stress tolerance in plants [20]. Plants produce different proteins under stress condition that act as phytohormones to regulate the plant’s normal functions [21]. However, poor hormonal regulation can lead to reduced germination and emergence and limited plant growth and development under salinity stress [22]. Moreover, the exogenous application of phytohormones found a promising practical strategy to deal with salt stress. The exogenous application of phytohormones exhibited a rapid increase in plant growth under stress conditions [17, 23, 24, 25]. Therefore, understanding the function of specific phytohormones will provide essential insights of plant mechanism to adapt salinity tolerance.

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2. Soil salinization: a threat to global food production

Soil salinization is the most significant global abiotic stress that reduced the soil fertility of approximately 1100 Mha (7% of total Earth land) area [26]. Soil salinization is increased with natural geochemical processes (primary salinity) and different human activities (secondary salinity) [27]. Primary salinization is caused by various atmospheric depositions, sea-level rise, saltwater intrusion into freshwater aquifers, and rising temperatures, while excessive fertilizer use, poorly managed practices, and intensified agriculture activities increase the secondary salinization that lowered the productivity of 30% cultivated land [28, 29]. The limited availability of freshwater resources for irrigation purposes, coupled with the continuous deterioration of agricultural fields due to salt stress, leads to significant reductions in agricultural productivity of arid and semiarid regions.

Soil salinization began from hundreds to thousands of years ago that degraded the arable land globally [30]. About 30% of irrigated land and 6% of the total land is degraded by soil salinity that caused an estimated $12 billion loss to agricultural production [15, 31]. The rising salinity in arable areas highlights the necessity for the understanding of the plant-salinity-tolerance mechanisms to maintain crop productivity. A significant reduction in plant growth and development, photosynthesis, respiration, and protein synthesis is reported due to salinity stress [32, 33]. The production of reactive oxygen species (ROS), including superoxide anion (O2•–), hydrogen peroxide (H2O2), and hydroxyl radicals (HO•) injuring the chloroplasts and mitochondria, is a significant indicator of oxidative damage in salinity [34]. These ROS disrupt the membranous structure and its cell permeability; thus, nutrient availability is reduced under salinity stress [35]. Therefore, plants use antioxidant enzymes as a defensive mechanism to safeguard nucleic acids, proteins, and membrane lipids to avoid the detrimental impacts of ROS [36].

2.1 Causes of soil salinization

The buildup of water-soluble salts (Na+, K+, Cl−, and SO42−) inside the root zone raises the salinity. This accumulation leads to osmotic fluctuations that impede the capacity of plant roots to uptake water [37, 38, 39]. The presence of high salt ions develops hyper-ionic salt stress. Soluble sodium and chloride ions are the primary ions responsible for developing soil salinity. In addition, the high Na+ ions develop a sodicity problem in soil [40]. Ancient cells were built to survive salinity since the early evolution of life originated in primeval oceans with similar or even more salt than contemporary oceans [41]. Therefore, numerous terrestrial plants have the ability to endure low to moderate levels of salinity. In contrast, naturally existing salt-tolerant plants (halophytes) exhibit a stringent salinity tolerance mechanism. However, a majority of growing crop species belong to the glycophyte group and therefore exhibit a poorly adapted mechanism to thrive in saline soils. Rice and tomato cultivars are highly susceptible to developing phytotoxic ions due to osmotic damage in saline stress conditions [42]. Salt-induced osmotic stress is observed during the early phase of salt exposure as a result of the gradual absorption of salts and consequent decrease in water potential in the vicinity of the root zone. This reduction in water conductivity within plant cells predominantly hampers plant growth [43, 44]. However, prolonged salinity stress leads to the accumulation of Na+, Cl−, and SO42− ions, which cause ion toxicity and hinder nutrient absorption, hence being detrimental to the growing plant cells and tissues [45]. The adverse effects of salt stress on plants can be observed in their morphology, physiology, and biochemical properties. Morphologically, plants experience stunted growth, chlorosis, and impaired seed germination. Physiologically, salt stress inhibits photosynthesis and disrupts nutrient balance. Further, plants undergo various oxidative stresses, electrolyte leakage, and membrane disorganization during biochemical processes [46, 47]. Moreover, salinity negatively affected the reproductive stage of the crop [48]. Therefore, a comprehensive understanding of salinity-induced injuries should be known, highlighting the need for salinity tolerance in plants for sustainable food production around the world.

2.2 Effects of salinity on plants: a challenge to agriculture

2.2.1 Plant growth and development

Salt stress decreases plant growth, and the rate of the decreasing trend depends on the plant’s growth stage and the intensity of the stress [49]. Researchers discovered that the stunted growth of plants acts as an adaptive mechanism for their survival against salt stress [50]. Salinity stress inhibits gene expression (cyclin and cyclin-dependent kinase) necessary for cell growth and number in the plant meristem, nutrient and water absorption, and plant stability. Numerous plants react quickly to stress and consequently stop their growth. Contrarily, some plants face salinity stress while maintaining their normal growth and start dying [51]. In moderate salinity, plants with quick responses activate the dehydration that results in cell shrinkage that recovers later [52, 53, 54]. However, salinity caused cell injury and decreased cell elongation and division, which resulted in less root and shoot growth in plants [55]. Under salinity, soil water component alteration in the rhizosphere disturbs the cell-water relationship of plants that develop osmotic injury [56]. Further, the osmotic injury lowers water uptake, causing inhibition or reduction in photosynthesis and respiration and carbohydrate and protein synthesis, leading to decreased plant growth and development [57, 58].

2.2.2 Photosynthesis

Photosynthesis is one of the most fundamental physiological attributes of plants, wherein solar energy is transformed into chemical energy. There may be a drop in photosynthesis caused by salt because of poor biosynthesis of chlorophyll [59], changes in enzyme activity [60], the closing of stomata [61], less carbon dioxide supplementation [62], and a damaged photosystem [63]. A significant decrease in chlorophyll levels in plants exposed to high salt concentrations could be due to the enhanced oxidation and degradation of chlorophyll owing to high ROS accumulation [63]. Further, ROS production under salinity stress inhibits the electron transport chain (ETC) that produces pseudocyclic electron transport chains [64]. As a result, modifications in photosynthetic proteins and disruptions in the assembly of photosystems occur [65]. Furthermore, the swelling of the thylakoid membrane under high salinity stress was observed to destroy the chloroplast ultrastructure [66]. Consequently, substantial photosynthesis resulted in less plant growth, which led to a decrease in plant production of Jatropha curcas, O. sativa, P. oleracea, and Solanum melongena in saline conditions [47, 67, 68, 69].

2.2.3 Nutrient balance

Plant growth and development in optimum soil conditions is commonly represented by the “generalized dose-response curve” [70]. Either nutrient-induced deficiency or nutrient toxicity can hinder plant growth due to suboptimal nutrient uptake [26]. The primary cause of less mineral acquisition under salt stress is the interactive effect of Na+ and Cl− with calcium (Ca2+), potassium (K+), and magnesium (Mg2+) ions [71]. Many researchers have reported a complex interaction between salt ions and vital mineral nutrients like potassium, phosphorus, and nitrogen [72, 73, 74]. Nitrogen is a vital mineral element that serves as a fundamental part of various cellular components within plants. Due to Cl−/NO3− antagonism, greater Cl− uptake and accumulation under saline conditions reduce total nitrogen uptake [75]. Phosphorous is needed for photosynthesis, storage, and energy transfer, whereas its uptake reduces in the presence of high Cl− and SO42− concentrations and due to the low solubility of the Ca ± P minerals [76]. Though potassium is essential for protein synthesis and water balance, high Na+ ions reduce K+ availability in saline environments [77]. Thus, the maintenance of cellular equilibrium between sodium and potassium is crucial for the survival of plants in saline soil. Although sodium cannot replace potassium in cellular functions, they both share a chemical resemblance that leads to potassium substitution by sodium. Na+ and K+ compete for root uptake sites and lower the K+ and Ca2+ intake during salinity stress [78]. Similarly, a significant decrease in K+ absorption by plants is due to high Na+ ions, regardless of whether Na+, chloride, or sulfate ions predominate in soil solutions [71]. Thus, salinity creates nutritional imbalances and lowers crop productivity in a saline environment [79, 80].

2.2.4 Water relations

The rapid uptake of ions results in the buildup of ions within the cellular structures of plants, hence exerting a detrimental impact on plant water relations [81]. The osmotic potential of plant cells decreased under salinity conditions that created an osmotic gradient; consequently, water moved out, causing a loss in plant cell turgidity [82]. Less water intake and transpiration rate of C. olitorius is noticed from saline soil [83, 84]. Further, these aforementioned findings are supported by recent investigations [85, 86]. However, the maintenance of turgor pressure at a constant level in plants is achieved by decreasing their osmotic potential relative to the overall water potential during salt stress [87]. Moreover, the hydrostatic potential gradient has immense importance as it controls the water transport during transpiration from soil through root xylem cells in an apoplastic pathway. However, the saline stress alters the water intake primarily through the cell-to-cell pathway, thus affecting the water relations more significantly [88, 89].

2.2.5 Yield

Salinity affects energy metabolism, cell signaling, and the synthesis of proteins necessary for plant growth, development, and achieving high yields. Hence, it ultimately impedes agricultural production by impeding plant growth and plant adaptation to stress responses, resulting in an overall reduction in yield [90]. The rate of salt absorption and salt-induced osmotic stress showed cellular and membrane injury leads to a significant loss in crop biomass [91, 92]. Different researchers reported the yield loss of different field crops is affected by salt stress [93]. They suggested that a thorough comprehension of the salinity of plants has the potential to help further enhance agricultural crop productivity in salt-affected areas [26].

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3. Role of phytohomones to alleviate salt stress on plants

Plants produce various kinds of phytohormones through biosynthetic pathways that can act locally or be transported to any part of the plant to maintain normal functioning in normal or stressful conditions [94]. These phytohormones, including auxins (IAA), cytokinins, gibberellins (GA), ethylene, abscisic acid (ABA), and brassinosteroids (BRs), regulate numerous physiological and biochemical processes to improve the growth and development of plants in saline conditions [19]. Moreover, strigolactone, nitric oxide (NO), and polyamines also act as phytohormones and regulate responses to environmental stimuli; therefore, phytohormones play a crucial role in salinity stress [95]. Salt stress-induced plants’ survival and avoidance mechanisms hinder their growth and normal functioning [96]. However, phytohormones regulate morphological, physiological, and other biochemical processes under stressful conditions [97]. Eyidogan et al. [98] observed continued changes in the synthesis, distribution, and signal transduction of phytohormones used for plant defensive mechanisms. Further, the salt stress signal transduction triggers the release of different phytohormones that act as baseline transducers [99]. Moreover, the details of the different phytohormones acting against salt stress are given below:

3.1 Abscisic acid (ABA)

ABA is a primitively important phytohormone that regulates the plant stress response through the expression of salt-responsive genes under salinity stress [100]. ABA showed promising results at different plant developmental stages of the crops under stress conditions [101]. Therefore, it is also known as stress hormone as internal ABA signaling activates under stress conditions to survive the plant through various adaptations [102]. Many scientists observed ABA production under various abiotic stresses [103]. ABA production regulates the water potential that tends to decrease in salinity stress [104]. Plants under salinity stress caused ABA generation that influenced the movement of guard cell and leaf water potential and thus considered as an important growth hormone under stress conditions [105]. However, a different response of ABA was observed for plant roots that increase their growth under stress conditions [106]. A contributory effect of soil pH was observed for the redistribution of ABA under stress conditions that control the stomatal opening and closure during salinity stress [107].

ABA and other phytohormones regulate root growth, stress-induced gene and protein expression (dehydrins and late embryogenesis abundant proteins), and compatible solute accumulation [15]. Salinity accelerates ABA buildup, which protects plants [108] and activates salt-induced genes in roots [109]. Another study found that salt-induced ABA helps leaves to restrict Na+ and Cl− accumulation [110]. The ABA regulates stomatal closure to reduce transpiration and water loss [111]. Salinity causes stomatal closure due to ABA-induced cytoplasmic Ca++ increases. Further, succeeding plasmalemma ion channel activation and guard cell turgor losses are also linked to ABA-induced H2O2 generation, an intermediate signal of ABA in stomatal closure [112].

The synthesis and accumulation of osmoprotectants (proline) and dehydrins in response to ROS formation during salt-stress-induced dehydration depend on phytohormone [19]. ABA-mediated H2O2 buildup generates NO, which activates MAPK and upregulates ROS-scavenging antioxidant enzyme genes [113]. ABA reduces Na+ accumulation and is translocation in shoot to augment salinity adaptation [114]. ABA increased barley (Hordeum vulgare L.) root vacuolar Na+ while inhibiting xylem transport and plasmalemma influx [115]. Exogenous ABA prevented harmful Cl−1 ions from accumulating in citrus leaves, decreasing ethylene release and leaf abscission under salt stress [116]. Exogenously applied ABA increased K+/Na+ ratio that enhanced the salinity tolerance in rice [117]. Similarly, [118] found that exogenous ABA prevented Na+ and Cl− ions and high Na+/K+ ratio in rice grains. Thus, ABA boosted rice grain production via boosting proline, soluble sugars, and K+ and Ca++ homeostasis [118]. Moreover, [109] noticed high accumulation of Ca2+ ions and membrane stability in plants due to ABA concentration under salt stress conditions. High ABA contents were recorded from salt-resistant maize hybrid leaves [119]. Further, a significant amount of ABA contents were noticed from tomato genotypes showing salinity tolerance [24].

In addition, the exogenously applied ABA at 100 μM increased the expression of OsP5CS1 responsible for proline accumulation that augment 20% more survival ratio under salt stress [120]. Mahajan et al. [103] indicated that multiple transcription factors regulate ABA-responsive gene expression. Salinity stress increases ABA and induces salt and osmotic relief genes [121]. ABA and salt stress regulate AtNHX1 expression and tissue distribution as ABA influences the HVP1 and HVP10 for vacuolar H+ inorganic pyrophosphatase and HvVHA-A for the catalytic subunit (subunit A) of ATPase [122]. They quantified the transcript levels and identified that ABA is responsible for salt-stress-adaptable gene expression in barley. Keskin et al. [102] found that ABA treatment induced TIP1 and GLP1 genes expression faster in wheat [123].

3.2 Auxin (IAA)

IAA is commonly known as the first plant hormone based on its discovery [2]. IAA controls cell elongation, vascular tissue development, and apical dominance [121]. Although IAA has been widely recognized for its effects on plant growth and development, it can regulate stress response or coordinate growth under stress [98]. Many researchers found a contributory role of IAA in plants during salinity stress [19, 124]. They proposed that a membrane-bound transcription factor (NTM2) includes auxin signal and modulates seed germination under salt stress. Further, NTM2 IAA30 gene overexpression mediates salt signaling pathway [124]. However, the mechanism of IAA to control salt stress is still unclear [123]. Further, salt alters the IAA metabolism and distribution, homeostasis, and its response against ROS production [125]. Many studies showed that IAA contents varied similar to ABA under salinity [126, 127]. High production of IAA lowers plant growth suggested the imbalance in stress-induced hormonal regulation plants [128]. However, the exogenous application of auxin augments plant growth and development.

IAA as a seed-priming agent also reduced the deleterious effects of salt stress on salt-sensitive wheat cultivar via regulating ionic homeostasis and auxin-induced leaf salicylic acid production [129, 130]. Auxin-response genes increase transcription of many genes in soybean, Arabidopsis, and rice [131]. Auxin/indoleacetic acid (Aux/IAA), GH3, and SAUR gene families contain these sensitive genes. Auxin reduces rice tiller bud growth by decreasing node OsIPT expression and cytokinin production [132]. However, the discovery of novel salt stress genes allows researchers to develop genetic engineering tools for stress tolerance [133]. From the perspectives of phytohormones, it is interesting to know that salinity considerably lowered the IAA production level in maize, while salicylic acid application increased IAA production. These findings show that hormonal homeostasis and cross talk are crucial for stress response signal perception, transduction, and mediation [126].

3.3 Cytokinins

Plants produce N6-substituted adenine derivatives commonly known as cytokinins that have either an aromatic or an isoprenoid side chain. Cytokinin affects cell division, chloroplast production, apical dominance, leaf senescence, vascular differentiation, nutrient mobilization, shoot differentiation, anthocyanin production, and photomorphogenic development in plants. Cytokinin is important to reduce the negative effects of salt stress on plant growth [134]. Cytokinin seed priming increased salinity stress tolerance by lowering ABA production in wheat [135]. Cytokinin also counteracts water stress-induced leaf and fruit abscission and seed dormancy under stress conditions. They observed cytokinin as ABA antagonists and IAA antagonists/synergists in plant processes [19]. The increased regulation in plant growth as salt stress adaption was noticed due to the cytokinin production [136].

Ref. [137] found that exogenous cytokinin boosted proline content and salt resistance in egg plants. Cytokinin is an intermediary in the protective action of epibrassinolide and methyl jasmonate in wheat under salt stress [109]. Cytokinin production increased the K-shuttle under salinity. Interestingly, cytokinin production decreased in the salt-resistant cultivar of barley under 65 mM NaCl. However, salinity negatively affected the growth of salt-sensitive cultivars [138]. Salinity lowers the contents of zeatin, zeatin riboside, isopentenyl adenine, and isopentenyl adenine in root and shoot of salt-sensitive barley genotype [139]. Further, the benzyl adenine production during salt stress hinders the growth parameters of barley, while the high cytokinin production improved growth rate and shoot/root ratio [139]. Kinetin acts as a direct free radical scavenger or part of the antioxidant process that protects purine degradation in stress conditions [140].

Moreover, functional investigations of cytokinin receptor mutants showed that all three Arabidopsis cytokinin receptors negatively regulate ABA signaling and osmotic stress responses due to cytokinin-dependent CRE1/AHK4 [141]. Cytokinin receptor genes are regulated by osmotic conditions, suggesting that their function in the osmotic stress response may be similar but poorly known [2].

3.4 Gibberellic acid (GA)

Gibberellic acid concentration increases under salinity stress that helps plants to regulate their mechanism through sugar production and other antioxidant enzyme metabolism [19]. GA substantially affects seed germination, leaf expansion, stem elongation, flower and trichome initiation, and fruit development [142]. The release of photosynthetic enzymes to improve plant photosynthetic efficiency by enlarging the leaf-area index and light interception [143]. GA increases photosynthate source potential and redistribution that helps in food storage [143]. GA3 reverses the morphological and stress-protective effects of triazoles (TR), demonstrating a close link between GA3 and plant stress protection [144]. Gibberellic acid improves plant water interactions and water usage efficiency (WUE) under salinity stress [142]. Maggio et al. [145] found that GA reduced stomatal resistance and improved WUE in tomatoes. The exogenously applied GA restores metabolic activity of plants [19].

Further, GA also regulates other phytohormones that helps in soybean development and higher yield production under salinity stress [146]. This improvement is due to the increased level of bioactive GA1 and GA4 that decreased the ABA, and SA; RNA maintenance and higher protein synthesis in soybean and mustard crops [146, 147]. Salt stress reduced the enzymatic activity, whereas GA signaling improves source-sink interaction under adverse environmental conditions [19]. GA enlarged the leaf area, root growth by increasing the nitrogen and magnesium uptake under salinity stress [148]. Moreover, GA3 boosted reducing sugars, antioxidant production, and protein synthesis and lowered ribonuclease and polyphenol oxidase in salt-stressed mung bean seedlings [149].

GA3-priming reduces ions (Na+, Cl−) uptake and their partitioning in plant root and shoot under salinity stress that improved wheat germination [150]. Salt stress disturbs the hormonal balance that impairs plant development, whereas the use of phytohormone provides an interesting stress-reduction strategy in barley cultivar [151]. Achard et al. [152] noticed the function of DELLA protein for plant survival in salt toxicity. GA production is responsible for salt-inducible DDF1 (dwarf and delayed flowering 1) gene that increases seed germination and growth responses under high saline environments. Further, GA production is stimulated by the influence of IAA in different plant species [153]. Additionally, GA increases ABA catabolism and enhances ethylene that affects its signaling mechanism, suggesting a cross talk between these phytohormones to improve salinity tolerance in crops [154].

3.5 Salicylic acid (SA)

Salicylic acid regulates the plant growth, development, and defense mechanisms against abiotic stresses [101]. SA improves stomatal conductance, transpiration, photosynthetic rate, fruit production, glycolysis, and ion uptake and transport in plants [155].

Though, SA is well-known for its response to biotic stresses, new studies suggested a significant effect of SA to regulate plant functions in salt stress conditions [2]. SA boosts photosynthetic capability by increasing Rubisco activity and pigments [156]. Moreover, SA treatment improved soybean pigments, photosynthesis, and glucose metabolism, thus improving salt tolerance in plants [157].

SA enhanced IAA and lowered ABA in maize that resulted in higher root growth and decreased antioxidant enzyme production while under SS conditions [158]. Bastam et al. [155] found that exogenous SA application enhanced salt tolerance in pistachio seedlings, beans [159], wheat [160], barley [161], mung bean, and mustard [162, 163]. The soil application of SA reduced the salt ion accumulation in maize and mustard crops [164]. SA improves the proline and glycine betaine accumulation in plants, which produce more antioxidants that helps to tolerate the salt stress [164]. Further, the decreased lipid peroxidation and membrane permeability improves during salinity stress in plants [165]. Nazar et al. [163] recorded the antioxidant metabolism and ATP sulfurylase and nitrate reductase activity with SA treatment and improved photosynthesis in mustard growing under saline soils. Additionally, SA pre-treatment of Arabidopsis prevented salt-induced membrane depolarization and guard cell outward rectifying channel K+ loss [166].

The use of SA as a stress management hormone provides a significant improvement in salinity tolerance to agricultural crops and resulted in higher yields. However, SA interaction with other phytohormones is important to open new avenues in the field of crop science [167].

3.6 Brassinosteroids (BRs)

Brassinosteroids are the novel phytohormone that promote plant growth under stress conditions [168]. Plants modify their response in the presence of BRs during stress [169]. The study showed that Arabidopsis’ CPD gene encodes CYP90, which was a steroid hydroxylase-like cytochrome P450 protein [123]. BRs regulate stress response by activating or suppressing important enzymatic pathways, inducing protein synthesis, and producing other hormones [170]. BRs increased the ear number, length, kernel number, and weight of maize crop [171]. Exogenously applied BRs augment the pod formation and seed yield in legumes [172]. Moreover, cotton and rapeseed growth and seed yield also increased with BR application [173]. Additionally, BRs restored chlorophyll and enhanced nitrate reductase that is essential for nitrogen uptake; thus, seed germination and seedling growth of rice (Oryza sativa L.) improved under salinity stress [170]. The experiment showed that 0.5 M NaCl damages the cellular nuclei and chloroplast, whereas the use of BRs protects the cellular structure of barley [174]. For rice, the addition of BRs in salt solution (150 nM NaCl) reduced the Na+ and Cl− ion uptake, which promotes seed germination that might be due to the high soluble protein synthesis [175]. The previous studies suggested the positive response of BRs to cope salinity, however, salt stress tolerance mechanism still demands a further study.

3.7 Ethylene

Unlike other phytohormones, ethylene is found in a gaseous form and controls plant growth and development [176, 177]. Researchers called ethylene as a stress hormone that is generated by several stimuli during stress conditions [178]. Plants with high salinity tolerance produce less ethylene level [179]. Contrarily, [180] reported the high ethylene production as a marker of salt tolerance in rice. Pierik et al. [181] observed larger crop growth stimulated with ethylene production. Salinity tolerance in Arabidopsis is due to the ethylene production [152]. In addition, Cao et al. [182] reported ethylene is vital for salt tolerance as the disruption in ethylene receptor activity caused salt sensitivity in plants.

The interaction and/or balance between the receptor and ethylene determines how a plant responds to salt stress. The plant exhibits large rosette with late flowering when receptor signaling is prominent in absence of ethylene signaling, whereas the abundant ethylene signaling triggers the early flowering of tiny rosette. In this, the plant faces two different extreme conditions and needs an adjustment [182]. Fine-tuning at different levels with active homeostasis can aid in plant survival and normal growth against stress conditions [15].

The mutant analysis of triple responses of etiolated seedlings treated with ethylene revealed an Arabidopsis ethylene signal transduction pathway involved in ethylene receptors, CTR1, EIN2, and EIN3, and other components [183]. Based on the structure, the five receptor genes of Arabidopsis are divided into two subfamilies as ETR1 and ERS1 in subfamily I and ETR2, EIN4, and ERS2 in subfamily II. Ethylene binds to all these receptors, and ETR1 kinases ethylene [184]. Both the ethylene receptors and signaling regulate plant development and stress responses. In Arabidopsis, salt stress lowered the ethylene receptor ETR1 expression, which mediates plant responses to abiotic stressors. Further, Khan et al. [20] observed a complex relationship between ethylene genes with GB-mediated salinity tolerance in T. aestivum.

3.8 Triazoles (TRs)

Triazoles are the plant growth regulators that can also be used as a fungicides [15]. Fletcher et al. [185] reported a noteworthy role of TRs against biotic and abiotic stressors in plants. Uniconazole was the most efficient TR that helps to boost the salinity tolerance, but its residual action in plant tissues and soil limits its usage in agriculture to prevent salinity [15]. TR increased net photosynthesis, intercellular CO2 concentration, and dry biomass of radish [185] and pigeon pea growing in saline soils [186]. The seed priming of wheat with paclobutrazol showed less Na+ accumulation and more water-soluble carbohydrates and reducing sugars under salt stress [187]. TR application increased nitrate reductase, protease, POD, SOD, and polyphenol oxidase activity under salt stress [186]. Propiconazole (TR-containing chemical) improved root development (length and biomass) and enzymatic antioxidant activity in Madagascar periwinkle plants under NaCl stress [188]. Though TR has a significant effect on plants, few studies have examined its role in relieving salt stress in diverse crops [185].

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4. Conclusion

The global impact of salinity and the trend of a growing population are alarming for food security. Crop yield and productivity is hindered in saline soil due to osmotic and ionic damage. Salt-responsive gene regulation, the antioxidant defense system, nutrient transport, osmolyte synthesis, and salt compartmentalization are badly affected under salinity stress. Plants regulate antioxidant enzymes, calcium-mediated responses, and hormonal signaling against ROS production during salinity stress to maintain homeostasis for optimal growth. However, the production of phytohormones during salinity stress regulates plant growth, seed germination, metabolism, and physiological activities. Plants use phytohormones as phytoprotectants against salinity stress. Exogenously applied phytohormones improved the stomata regulation, gas exchange mechanism that positively affected plant–water relations, and nutrient uptake, and subsequently, plant growth and productivity increased. Moreover, the hormonal priming of seeds that improves the seed germination in saline soil that results in utmost plant population. Plants integrate exogenous and endogenous cues during stressful and non-stressful situations, linking their stress response by hormonal route. Thus, hormones and their interactions with other plant components are vital to develop appropriate management strategies and phytoprotectants against salinity stress.

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Acknowledgments

Our heartfelt gratitude goes out to everyone who has contributed to this book chapter, whether via their skill or unwavering encouragement. We hope to leave a lasting impression on the scientific community, and we look forward to continuing this adventure of research and discovery with you all. We are also grateful to this publishing group for their supportive policy to spread knowledge in the form of this book.

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Written By

Imran Khan, Muhammad Umer Chattha, Rizwan Maqbool, Muqarrab Ali, Muhammad Asif, Muhammad Umair Hassan and Muhammad Talha Aslam

Submitted: 03 October 2023 Reviewed: 07 October 2023 Published: 02 February 2024

© The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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