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

both of which are necessary due to stable inhibition in transposable element

(TE) silencing and gene activity modulation. Glechoma longituba (Nakai)

Kuprian clonal plants can remember prior UV-B stress. The parental ramets

were exposed to UV-B stress once or many times (20 and 40 repetitions). It

was observed that UV-B stress reduced the degree of DNA methylation in

parental ramets, and the reduction was boosted by increasing the number of

UV-B treatments. Recurrently stressed parents’ epigenetic variation lasted a

prolonged period, whereas singly stressed parents’ epigenetic heterogeneity

was just temporary. The research result indicated that the growth of progeny

and genets was affected by parents’ exposure to UV stress. This effect could

be influenced by parental epigenetic imprinting and the transgenerational

effect (Zhang et al., 2021).

A vast variety of microRNAs have been found in response to UV-B

exposure, including miR156, miR159, miR164, miR167a, miR171, and

miR395. When plants are exposed to UV-B radiation at various intervals of

time, miR159, miR167a, and miR171 are elevated, while miR156, miR164,

and miR395 are repressed. In a study, when wheat was exposed to UV-B

radiation a new wheat miRNA, Tae-miR6000 was identified, including their

diverse expression was confirmed (Wang et al., 2013).

12.2.4 HEAVY METAL (HM) STRESS

Natural elements are known as “heavy metals” have a relatively large atomic

mass as well as a high density. HM stress developed as a serious issue in

several terrestrial ecosystems around the world. Due to vast industrializa­

tion and HM accumulation in soil has negative consequences on soil and

crop yield (Shahid et al., 2015). Several natural factors such as volcanic

eruption, weathering of rocks is the reason for HM accumulation in the soil

along with these anthropogenic activities like mining, chemicals used in the

agricultural field and industrial pollution are the major factor of heavy metal

concentration in soil. Generally, HMs has a molecular weight over 20 g/mol

and/or density of at least 5 g/cm³ to distinguish them from, “light” metals.

An increase in heavy metal’s concentration in soil results in genotoxicity,

cytotoxicity, and mutagenic effect on both plants and animals. Based on

physiological conditions heavy metals are classified into two classes: (i)

essential elements; and (ii) non-essential elements. Although essential

elements are required in a small quantity in plants, a high concentration

of both types of results in toxicity and act as a barrier to plant growth and