CMA33/XCT Regulates Small RNA Production through Modulating the Transcription of Dicer-Like Genes in Arabidopsis

Small RNAs （sRNAs） play important regulatory roles in various aspects of plant biology. They are processed from double-stranded RNA precursors by Dicer-like （DCL） proteins. There are three major classes of sRNAs in Arabidopsis： DCLl-dependent microRNA （miRNA）, DCL3-dependent heterochromatic siRNA （hc-siRNA）, and DCL4-dependent trans-acting siRNA （ta-siRNA）. We have previously isolated a mutant with com- promised miRNA activity, cma33. Here we show that CMA33 encodes a nuclear localized protein, XAP5 CIRCADIAN TIMEKEEPER （XCT）. The cma33/xct mutation led to reduced accumulation of not only miRNAs but also hc-siRNAs and ta-siRNAs. Intriguingly, we found that the expression of DCL1, DCL3, and DCL4, but not other genes in the sRNA biogenesis pathways, was decreased in cma33/xct. Consistent with this, the occupancy of Pol II at DCL1, DCL3, and DCL4 genes was reduced upon the loss of CMA33/XCT. Collectively, our data suggest that CMA33/XCT modulates sRNA production through regulating the transcription of DCLs.