Epigenetics – The Molecular Tool in Understanding Abiotic Stress Response in Plants

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development (Rascio & Navari-Izzo, 2011). HMs in acidic soil is solubilized

and the phytotoxic cation species, e.g., magnesium (Mg²⁺), calcium (Ca²⁺),

phosphorus (P), sodium (Na⁺) as well as aluminum (Al³⁺) imparts a negative

effect on plant’s physiology and leading to a loss in crop yield (Fryzova et

al., 2017; Samac & Tesfaye, 2003). Approximately 30% of the fertile land

of the world is acidic soil, which is classified as ultisol or oxisol with pH

lower than 5.5 and 13% of staple food crops are obtained from this land

(Bojórquez-Quintal et al., 2017; Rahman et al., 2018). The toxic effect of

aluminum (Al) in the soil is a major bottleneck in agricultural productivity

(Kochian et al., 2015; Zheng, 2010).

The importance of epigenetic mechanisms in plant stress response is being

highlighted by an increasing number of research studies. Plants’ responses to

HM stressors are influenced by epigenetic mechanisms. The major epigenetic

approaches include: (i) plants are guarded against probable DNA damage

induced by metal ions by spontaneous DNA methylation around the genome

using epigenetic inscriptions; as well as (ii) transposon and stress-responsive

genes are regulated by epigenetic alterations (Gallo-Franco et al., 2020).

HMs stimulates DNA methylation alterations, which are primarily related

to hypomethylation. Heavy metal stress affects DNA methylation of metal-

tolerant Hemp (Cannabis sativa L.) and metal-sensitive Clover (Trifolium

repens L.) plants. When plants maintained in soils polluted with various

amounts of Ni²⁺, Cd²⁺, and Cr⁶⁺ revealed different levels of 5-methylcytosine

(5mC) in the root DNA. For the same quantity of root DNA, the methylation

level in hemp is roughly three times greater than in clover. Heavy metal

administrations resulted in a substantially dose-related drop in 5mC in both

hemp and clover ranging from 20 to 40%. Dose-dependent hypermethylation

occurs in both clover and hemp due to heavy metal treatment which suggests

that natural methylation level is the main reason for metal stress sufferance

in hemp (Aina et al., 2004). When the nuclei extracted from foliage cells

of Noccaea caerulescens plants (a Ni hyperaccumulator species) cultivated

in a habitat with high Ni were found to be intact in comet assay at basic

pH, but the nuclei of non-tolerant Arabidopsis thaliana subjected to Ni

were significantly degraded. This was due to considerable hypermethylated

DNA in Noccaea caerulescens plants compared to Arabidopsis thaliana at

the genome level. Furthermore, the qRT-PCR results demonstrated that in

the case of excess amount of Ni in foliages, the genes MET1, DRM2, and

HDA8 of N. caerulescens are involved in the up-regulation of epigenetic

DNA and histone modification. These epigenetic alterations could be a

defense mechanism that allows plants to survive in harsh environments by