Plants under toxic element stress: microRNA-mediated regulatory pathways and emerging perspectives

Environmental toxicity from metal/metalloid pollution threatens the sustainability of crop species by impeding growth and yield. The acquisition of toxic elements (TEs) is manifested in tissues mostly through water relations and oxidative stress. Tolerant species can thrive through chelation, sequestration, and induction of antioxidative genes. The cellular-level regulations are controlled by non-coding small RNAs, microRNAs (miRNAs), which induce gene silencing and cause metal toxicity. With the advent of next-generation sequencing (NGS), miRNAs play a role in interpreting TE sensitivity via signaling pathways for stress tolerance. Recently, NGS technologies have identified more miRNAs with various roles in TEs. The regulation targets post-transcriptional modification following translational inhibition of specific genes. With another regulatory web, miRNAs are associated with genes concerned with transcription factors (TFs), membrane transporters, chaperons, growth regulators, signaling residues, etc. The redox balance in sensitive tissues, linking metabolic cycles and biogenesis, is influenced by feedback regulation of oxidative stress. Epigenetic alterations, including DNA methylation, following post-translational modification of regulatory proteins, play roles in modulating tolerance to TEs. The identified miRNAs synchronize signaling pathways that regulate cellular turgidity, the integrity of wall proteins, ion trafficking or sequestration, redox equilibrium, and metal transporter-like functions. Moreover, miRNA target sequences encoding TFs imply the involvement of major metabolic flux in the regulation of biogenesis for tolerance to TEs. In this review, we highlighted TE-mediated miRNA regulation and their associated roles in oxidative stress, signaling pathways, and coordinated physiological responses to TEs in plants.