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Plant Breeding Approaches in Developing Stress Tolerance

in view, the current and coming challenges in the future, a comprehensive

review on biotic and abiotic stress is composed by using breeding and genetic

approaches.

6.2 GENETIC MECHANISM OF STRESS TOLERANCE

6.2.1 SALINITY

Excess of salt leads to ionic as well as osmotic stress in plants. Salt overly

sensitive pathway is a crucial mechanism in salt stress which is responsible

for regulating ion homeostasis. The salt overly sensitive pathway consists

of three main genes SOS2, SOS3, and SOS1 (Ji et al., 2013). Regulation of

the ion homeostasis is a crucial way to tackle salt stress in which sodium ion

extrusion is a basic phenomenon to make the plant tolerant to salt stress. As

the level of sodium ion increases from a certain threshold level, all metabolic

pathways and enzymatic mechanisms are disrupted, leading to toxicity.

Calcium ions are responsible for stimulating salt overly sensitive pathway

in the cytoplasm of root cell. SOS3 activates the basic calcium ion sensor

and the attachment of the SOS3, and calcium ion is stimulation to activate

the SOS2. But SCaBP8 is an activator of SOS2 in shoots. As SOS2 is acti­

vated, it resides in the cell membrane where it activates downstream SOS1,

an antiporter of the sodium ion/hydrogen ion, and its activation results in

sodium-ion extrusion. Various SOS1 mutants are very effective and sensitive

to salinity (Ji et al., 2013). Highly reactive potassium transporter genes are

also responsible for exclusion ions. With the help of sodium ion exclusion,

HKT2 works efficiently to enhance salt tolerance in the plant (Roy et al.,

2014). In rice, the salt tolerance can be induced through sodium ion/hydrogen

ion antiporters such as OsNHX2, OsNHX4, OsNHX1, and OsNHX3 by the

sodium ion sequestration. Sodium ion/hydrogen ion antiporters activities are

very high when salt is high. When the expression of OsNHX1 is high, it

enhances tolerance owing to sodium ion accumulation in the vacuole of plant

cells and makes transgenic rice plant tolerant. In rice, the most effective and

efficient adaptation is gathering the sodium ion in old leaves of rice to tackle

salt stress (Reddy et al., 2017).

A high amount of salt promotes abscisic acid (ABA) synthesis, which

is responsible for closing stomata in leaves and makes plants tackle serious

osmotic stress during salinity. The expression of genes in rice plant such as

OsDREB1F, OsDREB2A, and OsDREB1A makes it tolerant to salt stress.