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

regulation and ta-siRNAs mediated regulation. These can be introduced

exogenously into the plants to regulate stress responses or sustain growth

and development under stressful situations (Guleria et al., 2011). The

ta-siRNAs bound to the AGO1 complex joins to specific sites in the target

mRNA and executes gene silencing either through mRNA degradation or

DNA methylation, or inhibition of synthesis of specific functional proteins

at the post-transcriptional levels (Dutta et al., 2020). To date almost 80% of

identified siRNAs are hc-siRNAs and more than thousands of them were

reported to present in Arabidopsis (Sanan-Mishra et al., 2021). Such siRNAs

are reported to participate both in the modification of histone proteins and

suppression of gene expression at the transcription level (Dutta et al., 2020).

Besides this, some siRNAs have also been found in several plants and are

effective in stress resistance. In spite of the recognition of the multiple

importance of siRNAs in stress response in plants as well as in growth

and development, a lot of things still remain undiscovered and unraveled,

especially the information regarding their biogenesis, functions, and mode of

action. Under stressful conditions, nat-siRNAs in the majority of plants have

been reported to generate nat-siRNA for the regulation of stress-responsive

genes. By conducting gradual experiments on Arabidopsis growing under

salinity, researchers found that nat-siRNAs, are generated from a specialized

cis-NAT gene duo, made up of SRO5 and P5CDH genes and further research

has shown that nat-siRNAs are important in protecting plants from osmotic

and oxidative stress (Borsani et al., 2005). Further findings concluded that

the pyrroline-5-carboxylate dehydrogenase (P5CDH) gene encodes an

enzyme that catalyzes proline degradation, while the salt stress-inducible

gene (SRO5) is involved in managing ROS production. Introduction of

the SRO5 gene generates dsRNA sequences in SRO5-P5CDH overlapping

juncture (Deuschle et al., 2004; Dutta et al., 2020). The hc-siRNAs also play

a key role in gene regulation when these are methylated by HEN1 and in all

cases, they act as a component of a complex structure similar to RISC, named

as RNA-induced transcriptional silencing complex (RITS) (Sanan-Mishra

et al., 2021). In Triticum, the pool of stress-related siRNAs was reported;

among them siRNA002061_0636_3054.1, siRNA005047_0654_1904.1

and siRNA007927_0100_2975.1 are regulated in a descending manner by

salt-induced stress (Yao et al., 2010). Another endogenous siRNA has been

reported in Craterostigma plantagineum which is induced during dehydra­

tion and similar stress conditions and can regulate desiccation tolerance

(Furini et al., 1997). The discovery of siRNAs and RNAi has opened a new

era that might introduce a novel tool for crop improvement, especially in the