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Biology and Biotechnology of Environmental Stress Tolerance in Plants, Volume 3

as saline stress. This ultimately results in the inhibition of plant growth and

leads to death, thereby resulting in a reduced yield of crop plants. Plants

may be categorized into two major classes: (i) halophytes; and (ii) glyco­

phytes. Halophytes are the plants that can tolerate salinity and survive on

the other hand, glycophytes are the group of plants that cannot withstand

saline stress, and as a result, they die eventually. Glycophytic plants account

for a large portion of agricultural plants (Gupta & Huang, 2014). Thus,

salinity is among the most deleterious environmental stressors that hinder

agricultural yield across the world (Flowers, 2004; Munns & Tester, 2008).

Salinity stress contributes to the alteration of different metabolic as well

as cellular activities, the intensity and period of the stress, hamper crop

production (James et al., 2011; Rozema & Flowers, 2008). Abnormal intake

of sodium (Na⁺) and chloride (Cl^–) ions causing cytotoxicity and creates

an imbalance of nutrition, which results in a decrease in plant growth as

well as development through water stress. Salinity goes hand in hand with

oxidative stress since reactive oxygen species (ROS) are formed along with

increased salinity (Isayenkov & Maathuis, 2019). The effect of salinity in

plants has been divided into two phases of plant growth. Stomatal openings

close and the leaf expansion is repressed in phase I, which takes place in a

short time, i.e., minutes or few days. While in phase II, cytotoxic ions build

up especially in mature foliages producing premature senescence, which

downtrend metabolic processes, decrease production eventually kills the

plant (Kotagiri & Kolluru, 2017). The growth and development in agricul­

tural plants get hindered due to toxicity caused by ions (especially Na⁺ ions),

hyper osmotic stress and oxidative damage in a highly saline environment.

Cellular Na⁺ ion homeostasis is maintained by the secondary messenger

Ca²⁺ along with Salt Overly Sensitive proteins. Ca²⁺ signals are decoded

by Ca²⁺ sensors. The signal is interpreted into physiological, metabolic,

and molecular adaptations by calcineurin B-like protein 10 (CBL10) (Zhu,

2016). Na⁺ influx is mediated by high‐affinity K⁺ transporters 1 (HKT1). In

Arabidopsis, considerable methylation is shown at 2.6 kb upstream of the

start site of small RNA which represses the expression of AtHKT1 in shoots

(Baek et al., 2011). The expression of HKT1 is increased due to a decrease

of DNA methylation level in RdDM mutant rdr2; indicating AtHKT1 gene

expression is negatively regulated by RdDM, this control process is even

shown for wheat (Kumar et al., 2017). In the roots and shoots of salt-tolerant

and salt-sensitive plants, the expression of TaHKT2;1 and TaHKT2;3 gets

repressed. This repression is due to cytosine methylation as a result of salt

stress, although the modulation of DNA methylation is not the reason for