Safeguard DCL2-Dependent 22-nt siRNA generation by DCL1.

MicroRNAs (miRNAs) and small interfering RNAs (siRNAs) are crucial for plant growth and development via mediating post-transcriptional gene silencing. In wild-type Arabidopsis, DICER-LIKE 2 (DCL2)-dependent 22-nt siRNAs are rare, whereas DCL1 and DCL4-dependent 21-nt miRNAs and siRNAs are highly abundant. DCL4 naturally inhibits DCL2 in producing abundant 22-nt siRNAs from endogenous transcripts, but whether DCL1 suppresses endogenous 22-nt siRNA production and the extent of repression are still unknown. Here, we report that DCL1 and DCL2 cleaved both miRNA precursors and coding transcript-derived double-stranded RNAs. In a dcl1 dcl4 double mutant, massive 22-nt siRNAs were produced from endogenous protein-coding genes (genic siRNAs). Compared with wild-type, the 22-nt genic siRNAs derived from the Nitrate Reductase 1 (NIA1), NIA2, DIACYLGLYCEROL ACYLTRANSFERASES 3 (DGAT3), SUPPRESSOR OF MAX2 1-LIKE 5 (SMXL5), and SMXL4 in dcl1 dcl4 increased up to 95%. Our analysis further indicated that the 22-nt genic siRNAs in dcl1 dcl4 were mainly loaded into ARGONAUTE 1 (AGO1) or AGO2. Thus, our results demonstrated that both DCL1 and DCL4 safeguard post-transcriptional gene silencing, preventing the production of DCL2-dependent 22-nt genic siRNAs from disrupting plant growth and development.