**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 [35–37]. 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**).

#### **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, 41–43]. 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 [43–45]. ABA-supply

*Phytohormones-Assisted Management of Salinity Impacts in Plants DOI: http://dx.doi.org/10.5772/intechopen.113734*

#### **Figure 2.**

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

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 C*aSnRK2.6* [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<sup>+</sup> content, increased contents of K<sup>+</sup> , 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<sup>+</sup> 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 [51–54]. 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) [60–62]. Extensive reports are available on the role of BRs in salinity-impact control in different test plants [62–67].

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 [72–77]. 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 [82–85], 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, 88–90].

### **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 [95–97]. 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 zeatintype 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 [101–103].
