Emerging roles of microRNAs in plant heavy metal tolerance and homeostasis

Heavy metal stress is a major growth and yield-limiting factor for plants. Heavy metals include essential metals (copper, iron, zinc, and manganese) and nonessential metals (cadmium, mercury, aluminum, arsenic, lead). Plants use complex mechanisms of gene regulation under heavy metal stress. MicroRNAs are 21-nucleotide noncoding small RNAs as important modulators of gene expression post-transcriptionally. Recently, high-throughput sequencing has led to the identification of an increasing number of heavy metal-responsive microRNAs in plants. Metal-regulated microRNAs and their target genes are part of a complex regulatory network that controls various biological processes, including heavy metal uptake and transport, protein folding and assembly, metal chelation, scavenging of reactive oxygen species, hormone signaling, and microRNA biogenesis. In this review, we summarize the recent molecular studies that identify heavy metal-regulated microRNAs and their roles in the regulation of target genes as part of ...