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

place (Jaleel et al., 2008). The consequences of imminent climate change,

drought will have an intense impact on crop productivity in the future

(Shanker et al., 2014). Drought stress intensity and drought stress-responsive

genes have a complimentary connection (Kim et al., 2012). Some drought

stress-responsive genes that encode functional and regulatory transcription

factors in response to drought have been studied (Shinozaki & Yamaguchi-

Shinozaki, 2007). Histone modification and chromosome density occur

as a response to drought stress (Kim et al., 2012). Under severe drought

stress the upregulated genes, such as RD20 and RD29A shows histone

modifications H3K4me3 and H3K9ac compared to the average drought

situation (Kim, 2021). Additionally, remarkable nucleosome loss occurred

in the RD29A region of the gene, while a small loss of nucleosome from

the same region occurs under moderate drought conditions. These results

reveal that the intensity of drought stress affects epigenetic responsiveness.

Nucleosomal substitution and strong histone deacetylation are necessary to

complete repression of stress-upregulated genes and to reset the chromatin

structure under non-drought situation (Kim et al., 2012). Plants can regulate

the repeated cycles of stress by modifying the expression pattern of stress-

induced genes. The expression of “memory genes” takes place at extremely

increased or decreased through consequent dehydration, thus allowing plants

to respond quickly during future drought stress. This phenomenon is known

as “stress memory” which has several synonyms, i.e., imprinting, priming,

training, and acclimation. In mammals, the gained stress memory can be

rearranged upon healing, consequent development, and meiotic cell division.

How plants control the resetting process during meiosis and transmitted to

the next generation remains a question to be answered yet (Kim et al., 2020).

12.2.2 EXTREME TEMPERATURE STRESS

Heat stress and cold stress are terms used to describe when temperatures

increase or fall dramatically beyond or below a threshold level above a

prolonged time, causing serious harm to the plant’s growth and development.

Generally, a sudden increase, i.e., above 10–15C above the ambient tempera­

ture disturbs the homeostasis of the plant, which is regarded as heat shock or

heat stress. This heat stress act as an ultimatum to crop production globally.

In the last couple of years, several research on heat and cold stress response

mechanisms are demonstrated along with their epigenetic regulation (Ding et

al., 2019; Driedonks et al., 2015; Guo et al., 2018; Liu et al., 2018).