Epigenetics for Combating Chromium Stress in Plants

The past decades have witnessed rapid growth in industries and an accelerated increase in heavy metal (HM) pollution of the biosphere. Even though HMs are present in all environmental compartments in trace levels, their excess presence can be a serious cause for concern as it may lead to severe eco-toxicity resulting in loss of biodiversity and can cause hazardous effects on human health. The long-term persistence of these metals is another serious threat to the environment, especially plants, as they turn out to be conservative pollutants. The extent of concern over the adverse effects of HMs may differ, as some are toxic, while others may be essential nutrients at various magnitudes. Among the various HMs, chromium (Cr) is one of the major HMs that causes extensive environmental contamination, due to its widespread applications in textile, steel, electroplating, tanning, polishing, painting, and catalytic manufacturing industries. Chromium exists in different oxidation states, of which the most toxic one is Cr(VI). Although Cr is non-essential and possesses no known biological function in plants, all plant species can absorb Cr from the soil solution along with other essential micronutrients. The hexavalent Cr has a significant adverse effect on the growth and development of plants, due to accumulation of Cr(VI) in plant parts, inhibited nutrient uptake, chlorosis, reduction in photosynthesis, change in soil microbial flora, and oxidative stress. Epigenetics plays a very significant role in combating Cr(VI) stress in plants. DNA and histone acetylation, methylation, or phosphorylation have a profound effect on chromatin structure and gene expression, improving a plant's response to stress. These modifications might lead to the activation of stress-responsive genes. Epigenome editing and mRNAs also increase the stress tolerance in plants. These epigenetic variations are inherited across generations, helping plants to deal with the ill effects of long-term heavy metal stress. However, a more profound overview of epigenetic manipulation is required to understand and develop strategies to combat Cr stress.