Posttranscriptional induction of two Cu/Zn superoxide dismutase genes in Arabidopsis is mediated by downregulation of miR398 and important for oxidative stress tolerance

MicroRNAs (miRNAs) are a classof regulatoryRNAsof;21nucleotides that posttranscriptionally regulate gene expressionby directing mRNA cleavage or translational inhibition. Increasing evidence points to a potential role of miRNAs in diverse physiological processes. miR398 targets two closely related Cu/Zn superoxide dismutases (cytosolic CSD1 and chloroplastic CSD2) that can detoxify superoxide radicals. CSD1 and CSD2 transcripts are induced in response to oxidative stress, but the regulatorymechanism of the induction is unknown. Here, we show thatmiR398 expression is downregulated transcriptionally by oxidative stresses, and this downregulation is important for posttranscriptional CSD1 and CSD2mRNA accumulation and oxidative stress tolerance. We also provide evidence for an important role of miR398 in specifying the spatial and temporal expression patterns ofCSD1 andCSD2mRNAs. Our results suggest thatCSD1 andCSD2 expression is fine-tuned bymiR398-directed mRNA cleavage. Additionally, we show that transgenic Arabidopsis thaliana plants overexpressing a miR398-resistant form ofCSD2 accumulatemoreCSD2mRNA than plants overexpressing a regularCSD2 and are consequently much more tolerant to high light, heavy metals, and other oxidative stresses. Thus, relievingmiR398-guided suppression ofCSD2 in transgenic plants is an effective new approach to improving plant productivity under oxidative stress conditions