MicroRNAs and Their Exploration for Developing Heavy Metal-tolerant Plants

Heavy metals (HMs), in particular the toxic/carcinogenic non-essential ones including cadmium (Cd), arsenic (As), aluminum (Al), mercury (Hg), and lead (Pb) are known to exert severe impacts on plant growth and yields. HM contamination and/or toxicity is seen a major threat for global food production, quality, and security. Plants use intricate molecular mechanisms for responding and adapting to HM stress, both at transcriptional and post-transcriptional levels, and microRNA (miRNA) have emerged as key post-transcriptional regulators. These tiny (19–25 nucleotide) non-coding RNA species found abundantly in plants are pivotal in tight regulation of gene expression via miRNA-directed mRNA cleavage, translational repression, chromatin remodeling, or through epigenetic modification. MiRNAs are reported to be involved in regulation of HM uptake and transport, besides their chelation and homeostasis, as well as in HM-induced oxidative stress and antioxidative defense. There are also reports of involvement of miRNAs in metallic cross- and co-tolerance. Technological advents in small RNA sequencing coupled with computational tools and databases have resulted into the identification, characterization, and validation of several HM-responsive miRNAs along with their respective target genes. Through his review, we present and discuss current understandings on miRNAs, their biosynthesis, and functions in plants, emphasizing on HM stress responses and adaptations. The main aim of this review is to discuss the possible exploration of plant miRNAs as potential targets for engineering plants (via loss-/gain-of-function approaches) to confer HM tolerance. Successful case studies, current challenges, and future directions are also discussed.