Heat stress induction of miR398 triggers a regulatory loop that is critical for thermotolerance in Arabidopsis

Summary micro RNA s (mi RNA s) play important roles in plant growth and development. Previous studies have shown that down‐regulation of mi R 398 in response to oxidative stress permits up‐regulation of one of its target genes, CSD 2 (copper/zinc superoxide dismutase), and thereby helps plants to cope with oxidative stress. We report here that heat stress rapidly induces miR398 and reduces transcripts of its target genes CSD 1 , CSD 2 and CCS (a gene encoding a copper chaperone for both CSD 1 and CSD 2). Transgenic plants expressing mi R 398 ‐resistant forms of CSD 1 , CSD 2 and CCS under the control of their native promoters are more sensitive to heat stress (as indicated by increased damage at the whole‐plant level and to flowers) than transgenic plants expressing normal coding sequences of CSD 1 , CSD 2 or CCS under the control of their native promoters. In contrast, csd1 , csd2 and ccs mutant plants are more heat‐tolerant (as indicated by less damage to flowers) than the wild‐type. Expression of genes encoding heat stress transcription factors ( HSF genes) and heat shock proteins ( HSP genes) is reduced in heat‐sensitive transgenic plants expressing mi R 398 ‐resistant forms of CSD 1 , CSD 2 or CCS but is enhanced in the heat‐tolerant csd1 , csd2 and ccs plants. Chromatin immunoprecipitation assays revealed that HSFA 1b and HSFA 7b are the two HSF s responsible for heat induction of mi R 398 . Together, our results suggest that plants use a previously unrecognized strategy to achieve thermotolerance, especially for the protection of reproductive tissues. This strategy involves the down‐regulation of CSD genes and their copper chaperone CCS through heat‐inducible miR398 .