**3. Salinity impacts-minimization in plants**

The life of plants under salinity can be made easier considering both plant- and soil-focused strategies, namely adopting approaches aimed at improving plant growth, metabolism, and productivity and employing the management approaches for managing the health (physico-chemical and biological traits) of saline soils.

*Introductory Chapter: Making Plant Life Easier and Productive under Salinity – Updates… DOI: http://dx.doi.org/10.5772/intechopen.113407*

#### **3.1 Plant health-improving approaches**

Plants are endowed with inherent mechanisms for salt stress-impact mitigation, which include hormonal stimulation, ion exchange, antioxidant enzymes and nonenzymes (metabolites) and activation of signaling cascades. However, efficiency of most of these inherent mechanisms can be further improved by employing approaches comprising the optimum and timely supply of mineral nutrients, compatible solutes, bio-stimulants, nanomaterials, phytohormones, phenolic compounds, microorganisms and considering mineral nutrients-phytohormones crosstalk and molecular-genetic approaches.

#### *3.1.1 Mineral nutrients*

The role of different mineral nutrients (such as N, P, K and S) in both minimization of salinity impacts and strengthening plant salinity tolerance has been widely studied. The major mechanism involved in the improved health of salinity-treated and N-supplied plants included diminished accumulation of Na+ , efficient N-uptake and assimilation processes, controlled K+ /Na+ homeostasis, and improved plant-K status [53–55]. S-supply improved plant health and salinity tolerance via maintaining the improved status of S and S-containing compounds (including cysteine, Cys; glutathione, GSH); improved cellular redox homeostasis; efficient ROS-metabolism; decreased oxidative stress; decreased Na+ /K+ ratio and Na+ accumulation, increased K+ and Ca2+ ; improved uptake of K and P; and reduced electrolyte leakage [22, 48, 56, 57].

#### *3.1.2 Phytohormones*

Sustainable improvement of plant growth, metabolism, photosynthesis and productivity (yield) under salinity-affected soils can be possible with the judicious and timely use of various phytohormones and signaling molecules, and thereby minimizing increasing strain on the global food security. The major role (and underlying basic mechanisms) of phytohormones (namely abscisic acid, ABA: a sesquiterpenoid, 15-C compound; auxins: endogenous plant growth regulators; brassinosteroids, BRs: polyhydroxy steroidal phytohormones; cytokinins, CKs: derivatives of adenine or that of phenylurea; ethylene: an unsaturated hydrocarbon gas; and gaseous hormone; gibberellins, gibberellic acid (GA): a large family of tetracyclic di-terpenoid compounds; jasmonic acid, JA: a cyclopentane fatty acid; nitric oxide, NO: a highly versatile gaseous, free-radical, redox-signaling molecule; salicylic acid, SA: a phenolic plant hormone; and strigolactones, SLs: carotenoid derived phytohormone) in improving plant salinity tolerance are briefly highlighted hereunder.

Under saline condition, ABA supply improved plant health by modulating ABA signaling components; reducing Na+ content, increasing K+ , Mg2+ and Ca2+ content; improving coordination among antioxidant defense system components; improving cellular level of AsA and GSH; significant reduction in Na+ content, increasing the contents of hormones such as 1-aminocclopropane carboxylic acid, trans-zeatin, N6-isopentyladenosine, indole-3-acetic acid (IAA); reduction of transpiration flow, regulation of Na+ ion homeostasis; involving calmodulin signaling cascade; and induction of osmolytes accumulation [58–60]. The supply of auxins has been widely reported to improve plant salinity tolerance involving various mechanisms [61–63]. BRs-mediated improvements in plant salinity tolerance and growth and development mainly involved BRs-supply-induced activity of ROS-metabolizing enzymatic antioxidants (including

APX and CAT), and the cellular levels of non-enzymatic antioxidants (such as AsA and GSH); and decreased electrolyte leakage and membrane lipid peroxidation [24, 64, 65]. Cytokinins-supply can improve plant salinity tolerance via modulating shoot Cl− exclusion and enhancing antioxidant system and photosynthetic efficiency [66, 67].

Ethylene-induced plant salinity tolerance was argued earlier as a result of ethylenemediated maintenance of the homeostasis of ions; up-regulation of antioxidant enzymes; improved endogenous ethylene-overproduction; increased activity of seed α-amylase; decreased H2O2 and lipid peroxidation; decrease in osmotic stress; PSI cyclic electron flow-mediated controlled non-photochemical quenching [68–70]. Gibberellins (GA)-mediated improvement in plant health involved decreased ion leakage; improved osmolyte accumulation and proline content; elevated Ca2+ and K+ concentrations, and transpiration rates; modulated antioxidants and secondary metabolites; and improved redox homeostasis and coordination among antioxidant enzymes [71–73]. Jasmonic acid (JA)-supply can mediate the salinity-impact mitigation in plants involving increased endogenous levels of cytokinins and auxins, increased α-tocopherol, phenolics, and flavonoids levels; enhanced activity of SOD and APX; increased K+ and Ca2+; declined Na+ content; and crosstalk on JA and ABA [74, 75]. Nitric oxide (NO) is the smallest diatomic gas and a gaseous signaling molecule in plants [76, 77]. NO-supply protected plants against salinity impacts by enhancing mineral absorption, maintaining hormone equilibrium, improving osmolyte accumulation, strengthening antioxidative defense systems, mitigating H+ -ATPase inhibition, and maintaining oxidative homeostasis in plants under salt stress [78, 79]. Alleviation of salinity stress in plants has also been achieved through crosstalk of NO with other signaling compounds and phytohormone signaling pathways [80, 81]. In salinityexposed plants, salicylic acid (SA)-supply reduced Na+ and Cl− ions; maintained high GSH level; improved cellular redox environment; regulated AsA-GSH cycle, elevated osmolyte accumulation; and involved characteristic changes in the expression pattern of major GST-gene family members [82–84]. Strigolactones (SLs)-mediated improved salinity tolerance in plants was argued to involve enhanced antioxidant enzyme activity; improved ROS-metabolism, decreased lipid peroxidation and cellular damage; and SLs-ABA-arbuscular mycorrhizal fungi crosstalk [85–88].

### *3.1.3 Mineral nutrients-phytohormones crosstalk*

The major outcomes of the crosstalk of most mineral nutrients with phytohormones have improved plant salinity tolerance. To this end, in several instances, the crosstalk between the major phytohormones (such as indole-3-acetic acid (IAA), gibberellic acid (GA), kinetin (CK), ethylene, 24-epibrassinolide, SA, ABA, brassinosteroid (BR)), with N and S resulted in differential decreasing the content of Na+ and Cl− ions; modulation of key enzymes of N and S metabolism; cellular homeostasis, photosynthesis; lowering oxidative damage, improving photosynthetic efficiency, assimilation of N and S, proline content and antioxidant defense system and decreased generation of oxidative stress markers; maintain osmotic balance via controlling cellular osmolytes; modulating the photosynthetic N-use-efficiency and antioxidant metabolism; increased levels of free amino acids and soluble proteins [22, 44, 60, 68].

### *3.1.4 Osmolytes*

Exhibition of almost ceased water flow from soil into roots and cellular dehydration as a result of decreased cellular turgor pressure are common in plants under

*Introductory Chapter: Making Plant Life Easier and Productive under Salinity – Updates… DOI: http://dx.doi.org/10.5772/intechopen.113407*

salinity stress. To counter this, plants employ the mechanism of osmoregulation, which involves the accumulation of varied small water-soluble organic non-toxic solutes (osmolytes/compatible solutes). Osmolytes/compatible solutes such as polyamines (PAs), glycine betaine, b-alanine betaine, dimethyl-sulfonio propionate, and choline-O-sulfate are 'ammonium compounds'; whereas fructan, trehalose, mannitol, D-ononitol and sorbitol are grouped as the 'sugars and sugar alcohols'. On the other, the list of major amino acids acting as osmolytes includes proline and ectoine. The accumulation of most of these solutes in the cytoplasm largely leads to the maintenance of low cytosolic and cytosolic concentrations within narrow limits (100–150 mM) across a broad range of external and vacuolar concentrations of NaCl [89–91]. In addition to performing cytoplasmic osmoregulation/osmotic adjustment, osmolytes have been considered as efficient oxidative stress-busters in plants under varied abiotic stresses (**Figure 6**) [92].

#### **Figure 6.**

*Schematic representation of the major osmolytes involved in osmotic adjustment in plants under salinity stress [52].*

Salinity-exposed plants have been extensively reported to accumulate polyamines (PAs) which are low-molecular-weight aliphatic amines/polycations [91, 93, 94]. Notably, the list of most common natural PAs present in plants includes Spermidine (Spd), spermine (Spm) and putrescine (Put). These PAs are known to act as signaling molecules in plant-environmental stresses [95]. PAs-supply mediated strengthening of antioxidant defense system has been reported in plants [36, 96]. The synergistic effect of PAs (Put + Spd) was reported to confer salinity tolerance in foxtail millet (*Setaria italica*), by inducing antioxidant enzymes and osmoprotectants, and maintaining coordination among the complex physiological and biochemical processes [97]. Elevation in the cellular level of proline is involved plant salt stress tolerance mechanisms. Beside delivering C, N, and energy during stress condition, proline scavenges varied ROS and also stabilizes DNA, proteins and membranes, and reduces NaCl-induced enzyme denaturation [91, 98]. Osmotic adjustment in salinityimpacted plants can also be accomplished with the accumulation of total soluble sugars, namely glucose, sucrose, dextrins, and maltose, where these sugars provide osmoprotection and also act as C-storage [99]. A non-reducing storage disaccharide, trehalose is mainly involved in the regulation of carbohydrate metabolism [100]. Interestingly, trehalose-mediated improvement in plant salinity tolerance involves the maintenance of K+ :Na+ ratio, ROS-scavenging ROS, and increased soluble sugar concentration [101–103]. Mainly acting as important osmolytes in plant vacuoles, organic acids can also significantly contribute in plant tolerance to salt stress [104].

### *3.1.5 Microorganisms*

Numerous plant growth-promoting microorganisms have been identified, which have become a useful tool for achieving sustainable agricultural production [105, 106]. Interestingly, rhizosphere is the home to numerous plant growth-promoting bacteria (PGPB) (or plant growth-promoting rhizobacteria; PGPR) (**Figure 7**). Most of these PGPB/PGPR are cheap and easily available resources; hence, they are generally used as an inoculant for bio-stimulation, biocontrol and biofertilization for the mitigation of salinity impacts in plants. Therefore, PGPB/PGPR has been argued as an alternative strategy for salt tolerance in plants [106–108]. The role of plant growthpromoting rhizobacteria (PGPR) in plant salinity tolerance and immunity is worth mentioning, where most PGPR protect plants by colonizing within the rhizosphere and producing antimicrobial metabolites (antagonistic) and producing regulatory hormones [109]. Plant inoculation with selected PGPRs (including *Bacillus pumilus* and *Pseudomonas pseudoalcaligenes*) under saline conditions resulted in increased uptake of N, P, K, decreased uptake of Na and Ca; and improved growth traits [110]. Thus, the use of selected PGPR makes the solubility and bioavailability of major mineral nutrients (such as soil-P) feasible under saline condition. PGPR strain-mediated induction of antioxidant enzymes can also be promising in improving plant salinity tolerance.

The role of arbuscular mycorrhizal fungi (AMF), a unique group of root obligate endophytic fungal symbionts (reported in about 90% of terrestrial plants), in plant salinity tolerance has also been found significant [17, 91]. AMF-mediated improvements in plant health involved enhanced ability of plants to replace K<sup>+</sup> with Na<sup>+</sup> using various transporters [111]; maintenance of high K<sup>+</sup> in roots and shoots [112, 113]; efficient regulation of K<sup>+</sup> /Na<sup>+</sup> ratio required for cytoplasmic ion-balance maintenance [18]; strengthening of antioxidant defense mechanism [114]; improving water-use-efficiency, and compartmentalization of Na<sup>+</sup> within plant tissues

*Introductory Chapter: Making Plant Life Easier and Productive under Salinity – Updates… DOI: http://dx.doi.org/10.5772/intechopen.113407*

#### **Figure 7.**

*Schematic representation of the major plant growth-promoting rhizobacteria inhabiting the plant rhizosphere [106–108].*

[115, 116]; and inducing the osmolytic solutes (including proline, glycine betaine, or soluble sugars) [117].

#### *3.1.6 Phenolic compounds*

Elevated accumulation of phenolic compounds has been reported in salinityimpacted plants [118–120]. Considered significant among the most widely distributed secondary metabolites in the plant kingdom, phenolic compounds (including polyphenols and flavonoids) are the low-molecular-weight non-enzymatic antioxidants generated in plant cells. Most phenolic compounds act as signaling molecules and mediate auxin transport [121]. These phenolic compounds improve plant salinity tolerance via acting as reactive oxygen species (ROS)-scavengers, thereby maintaining a fine cellular redox homeostasis [118, 119].

#### *3.1.7 Bio-stimulants*

Bio-stimulants (also termed as bio-effectors) are viable microorganisms or active natural compounds, and can fall within four prime groups: acids (*humic acid*, *fulvic acid*, *humins*, *amino acids*, *fatty acids*, and *organic acids)*, microbes *(plant growthpromoting rhizobacteria*, *PGPR*; *arbuscular mycorrhizal fungi*, *AMF*; *Trichoderma* spp.), plant-derived bioactive substances *(polyphenols* and *allelochemicals, etc*.), and others (*beneficial elements: Al*, *Si*, *Na*, *Se*, *Co*, etc.). Most bio-stimulants are considered as the regulators of both ROS-metabolism and also stress metabolites involved in enhancing plant tolerance to major stresses (including salinity) [122, 123]. Bio-stimulants-mediated improved salinity tolerance in plants involved biostimulants-induced maintenance of reduced non-enzymatic antioxidants (such as GSH and AsA); improved tissue water status, ionic and nutrient homeostasis, and osmotic tolerance; stabilized membrane properties; maintenance of a fine-tuning among antioxidant enzymes; and decreased ROS generation electrolyte leakage and lipid peroxidation) [7, 124–126].

#### *3.1.8 Nanomaterials*

Nanomaterials (NMs) are materials with a basic structure of 1–100nm in at least one dimension. Nanomaterials (in terms of nano-fertilizer and nano-pesticides, and other plant protection nano-agents) have shown great potential in agriculture [127]. The major mechanisms underlying NMs-mediated improved plant salinity tolerance included NMs-mediated alleviation of osmotic and ionic stress, enabling the better ability to maintain cytosolic K+ /Na+ ratio; enhancing leaf mesophyll K+ retention; efficient scavenging of ROS; maintaining a fine-tuning among the components of antioxidant defense system; and protection of photosynthesis [128–131]. The role of nanozymes in salinity-exposed plants has also been reported, where the supplied nanozymes with ROS-scavenging ability in plants helped to improve plant salt tolerance via maintaining ROS homeostasis and alleviating ROS-accumulation in plant organs [132, 133].

### *3.1.9 Molecular-genetic approach*

Deep understanding of molecular insights into plant salinity stress tolerance has come to light mainly due to exhaustive studies on omics techniques. The list of the mentioned techniques comprises transcriptomics, genomics, proteomics, and metabolomics, where crucial cell signaling compounds crosstalk and integrative multi-omics techniques could be employed for improving salinity tolerance in plants [134, 135]. Notably, plants are of multi-genetic nature, which makes actual understanding of their responses to salinity very difficult. To this end, genomics (studies on a certain genome aimed at unveiling insights into organism's biology) has greatly helped identify and characterize the salinity stress response gene [135, 136]. Notably, understanding plants' salinity tolerance and also the development of salinity tolerant plants have been enabled to a great extent by employing high-throughput approaches including forward genetics, serial analysis of gene expression, expression sequence tag, next-generation sequencing), targeting-induced local lesion in genomes, RNA interference, and genome-wide association study [135, 137–139]. Additionally, plant salinity tolerance mechanisms are very complicated and involve polygenic traits [15]. To this end, dealing mainly with the RNA expression profile of organisms at temporal and spatial bases, transcriptomics has helped in the identification of transcripts/genes essential in controlling transcription and translation machinery in several studies on plants under salinity stress [20, 140–142].

Critical studies on the protein profiles (*which is actually the expression-reflection of the salinity-caused genes*) employing the proteomics approach have helped to identify proteins, and get their expression profile, post-translational modifications, and protein-protein interactions in both agricultural [143–145] and non-agricultural [146, 147] plants under salinity stress. Clear and reliable information about the major metabolites (such as most organic acids, hormones, amino acids, ketones, vitamins, and steroids) in salinity-exposed plants has been obtained employing metabolomics [47, 148, 149]. The collection of minerals and elements of an organism is considered as 'ionome', which has helped in understanding the role of adoption of controlled ion uptake, distribution system (homeostasis), and detoxification as plant's major strategy for adjusting high salinity [142, 143, 150]. The major impacts of salt stress on photosynthesis and related variables, ionic relationships, plant senescence, and yield can be assessed well by employing high-throughput phenotyping [151, 152].

### *Introductory Chapter: Making Plant Life Easier and Productive under Salinity – Updates… DOI: http://dx.doi.org/10.5772/intechopen.113407*

The manipulation of N-dynamics through genetic engineering has great potential to improve plant life under salinity stress [153]. As also mentioned above that, ethylene is an important gaseous phytohormone involved in the regulation of plants growth, development, and senescence [154]. Among the small transcription factor gene families in higher plants, ethylene-insensitive 3 (EIN3)/ ethylene-insensitive 3-like (EIL) gene family is very important. All members of the EIN3/EIL gene family are key genes in the ethylene signaling pathway [155–157]. Ethylene-mediated downstream transcriptional cascade has been reported to involve EIN3/EILs as the major key elements and positive factors [158]. Ethylenemediated enhanced salt tolerance in *Arabidopsis* involved EIN3/EIL1, which promoted EBF1/EBF2 proteasomal degradation and modulated several EIN3/ EIL1-regulated genes. Further, overexpression of EIN3 target genes (e.g., ERFs and SALT INDUCED EIN3/EIL1-DEPENDENT1 (SIED1)) can also improve plant salinity tolerance [159].

#### **3.2 Saline soil health-management approaches**

Appropriate, effective, cheap and environment-friendly management/amelioration approaches can be applied to improve the health (physicochemical and biological traits) of saline soils, and thereby providing the soil conditions favorable for establishment, and optimum growth and development of plants, which in turn can contribute in securing foods for future generations [160, 161]. Reclamation (salt removal from the plant-root zone) stands at the top of the saline soil healthmanagement approaches, which is mainly comprised of reducing salinity by leaching; salt scraping; and phytoremediation of accumulated salt. Approaches such as the addition of organic amendments as ameliorant (such as crop residue, compost, farm yard manure, cattle manure, poultry manure, clover hay and wheat straw; aimed at improving soil physical conditions; stabilization of soil aggregates and improving water holding capacity); addition of chemical amendments (flushing out of the Na<sup>+</sup> from the root zone; can be done using gypsum, CaSO4; lime, CaCO3; sulfuric acid, H2SO4; hydrochloric acid, HCl; nitric acid, HNO3); and soil ripping (reduce the compaction and assist with salt leaching; normally considered in sodic soils with compaction problem) may be adopted to reduce salt leaching from the upper layers of the soil. Salt scraping approach involves the physical removal of the salt crust, followed by leaching for salts (including Na<sup>+</sup> ) removal from the root zone [162, 163].

Phytoremediation is a plant-assisted approach alternative and efficient technique that has the potential to replace the abovementioned costly physical and chemical methods for reclaiming sodic and saline-sodic soils. Interestingly, the phytoremediation approach is based on the ability of plant roots to enhance the dissolution rate of native calcite and the removal of Na<sup>+</sup> [161, 164]. Plant species exhibiting hyperaccumulation of elevated soil salts and strong salinity tolerance can be considered in the phytoremediation-based management of saline soils. In particular, halophytes are the plants reported to exhibit salt resistance or salt tolerance to soils with >200 mM NaCl [165]. Notably, most halophytes (including grasses, shrubs, and trees) exhibit their extraordinary capacity of salt exclusion, excretion or salt accumulating at cellular, organelle and whole body levels. Given this, several halophytes have been considered a panacea in the remediation of salt-affected problematic soils (**Figure 8**). The use of phytoremediation approach

**Figure 8.**

*Schematic representation of the major halophytes used in saline soil management [161, 164].*

in salinity-loaded soils helped in improving physical, chemical, and microbiological properties; increasing soil fertility, organic matter accumulation, erodibility reduction, and increasing soil water retention [166–168].
