MicroRNAs with macro-effects: their role in copper and cadmium-induced responses in Arabidopsis thaliana

Abstract

Mainly due to anthropogenic activities, many soils got contaminated with metals like copper (Cu) and cadmium (Cd) posing a threat for all organisms including human health. In plants, exposure to Cd and Cu results in growth retardation and disturbs physiological processes such as photosynthesis and respiration. A major phytotoxicity mechanism of Cd and Cu is the generation of increased amounts of reactive oxygen species (ROS) resulting in an oxidative challenge. Plants respond to Cd and Cu stress through (1) metal chelation by increasing phytochelatin (PC) and metallothionein production, (2) the induction of antioxidative defence pathways and (3) the activation of signalling pathways triggered by among others ROS and the stress hormone ethylene. An accurate regulation of these responses upon Cd and Cu stress is important for acclimation and plant survival. During the last ten years it became clear that microRNAs (miRNAs) play a major role in the post-transcriptional regulation of gene expression, also in case of metal stress. The first identified metal-responsive miRNA in Arabidopsis thaliana is miR398 being downregulated after Cu and iron exposure. However, under Cu deprivation miR398 is upregulated together with increased expression of miR397a, miR408 and miR857, the so-called cupro-miRNAs since they target Cu-containing proteins. Expression of these cupro-miRNAs under Cu deficiency is controlled by the transcription factor squamosa promoter binding protein-like 7 (SPL7), which is assumed to be the master regulator of Cu homeostasis. Despite the identification of metal-responsive miRNAs in several species, their role and regulation upon metal stress remains scarce.