Abstract
In plants, microRNAs (miRNAs) control various biological processes by negatively regulating the expression of complementary target genes, either (1) post-transcriptionally by cleavage or translational inhibition of target mRNA, or (2) transcriptionally by methylation of target DNA. Besides their role in developmental processes, miRNAs are main players in stress responses, including metal stress responses. Exposure of plants to excess metal concentrations disturbs the cellular redox balance and enhances ROS accumulation, eventually leading to oxidative damage or signaling. Plants modify their gene expression by the activity of miRNAs in response to metal toxicity to regulate (1) complexation of excess metals, (2) defense against oxidative stress and (3) signal transduction for controlling various biological responses. This review focuses on the biogenesis, working mechanisms and functioning of miRNAs in plants. In a final part, our current knowledge on the regulatory roles of miRNAs in plant metal stress responses is highlighted, and whether stress-regulated miRNAs have specific roles or are secondary consequences is discussed.
Highlights
IntroductionSince late 19th century, metal pollution of the environment is of great concern all over the world
Since late 19th century, metal pollution of the environment is of great concern all over the world.Due to anthropogenic activities, such as the metal industry and agriculture, concentrations of metals in soil, air and water are exceeding the natural occurrence [1,2]
The effects of excess essential (Cu, Mn, Zn and Fe; blue boxes) and non-essential (Cd, Al, Hg and As; purple boxes) metals on the expression of miRNAs is shown. These metal-regulated miRNAs are obtained from several plant species, like Arabidopsis thaliana (At), Phaseolus vulgaris (Pv), Medicago truncatula (Mt), Brassica napus (Bn), Oryza sativa (Os) and Nicotiana tabacum (Nt) [14,21,71,72,73,74,75,76,77,78,79,80,81,82]
Summary
Since late 19th century, metal pollution of the environment is of great concern all over the world. Plants try to avoid free metal accumulation in the cells (1) by keeping the metals extracellularly through reducing the uptake and stimulating the efflux or (2) by metal complexation with ligands, such as glutathione, phytochelatins and metallothioneins [9,10,11,12] If these defenses are insufficient to prevent the build-up of free metal ions, plants have to cope with the oxidative stress that can be directly induced by redox active metals and indirectly by non-redox active metals. Antioxidative defense mechanisms are activated to reduce elevated ROS levels and limit oxidative damage They consist of metabolites, such as ascorbate, glutathione and vitamin E, as well as enzymes, like superoxide dismutases, peroxidases and catalases [13,14]. Current knowledge on miRNA induced transcriptional and post-transcriptional regulation of gene expression will be reviewed, similarities or differences of metal exposure with other stresses will be discussed and progress to be made towards a better insight in understanding plant metal stress responses will be indicated
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