The role of microRNAs in sensing nutrient stress.

Recent studies have demonstrated the novel functions of microRNAs (miRNAs) in regulating plant adaptive responses to nutrient stresses. Plant miRNAs usually down-regulate the abundance of their target mRNAs by post-transcriptional cleavage. miR395 and miR399 are up-regulated during sulphate and phosphate (Pi) deficiency, respectively. miR395 participates in sulphate assimilation and allocation via adjusting the expression of ATP sulphurylase (APS) and a sulphate transporter (AtSULTR2;1). Up-regulation of miR399 results in the down-regulation of UBC24 encoding a ubiquitin-conjugating E2 enzyme. Plants overexpressing miR399 or are defective in UBC24display Pi toxicity because of increased Pi uptake, enhanced root-to-shoot translocation and retention of Pi in the old leaves. This observation suggests that the miR399-mediated regulation of UBC24 expression is critical in Pi homeostasis. Moreover, the existence and conservation of miR395 and miR399 and their target genes among many plant species reveals the evolutionary importance of these miRNA-mediated nutrient stress responses.