Arabidopsis HEN1:: A genetic link between endogenous miRNA controlling development and siRNA controlling transgene silencing and virus resistance

In animals, double-stranded short interfering RNA (siRNA) and single-stranded microRNA (miRNA) regulate gene expression by targeting homologous mRNA for cleavage or by interfering with their translation, respectively [1-3]. siRNAs are processed from injected or transgene-derived, long, perfect double-stranded RNA (dsRNA), while miRNAs are processed from short, imperfect dsRNA precursors transcribed from endogenous intergenic regions [4-9]. In plants, both siRNAs and miRNAs activate cleavage of homologous RNA targets [10-12], but little is known about the genes controlling their production or action. The SGS2/SDE1 protein contributes to produce transgene siRNA [10], while DCL1 and HEN1 contribute to endogenous miRNA accumulation [8, 9]. Here, we show that: I) SGS2, SGS3 [13], AGO1 [14,15], and HEN1 contribute to produce transgene siRNA involved in sense post-transcriptional gene silencing (S-PTGS); ii) HEN1, but not SGS2, SGS3, or AGO1, contributes to the accumulation of the endogenous miR171 miRNA and to the cleavage of Scarecrow target mRNA by miR171 [11]; iii) SGS2, SGS3, AGO1, and HEN I contribute to resistance against cucumber mosaic virus [13, 16], but not to siRNA and IR-PTGS triggered by hairpin transgenes directly producing perfect dsRNA [16]; and iv) the actions of HEN1 in MiRNA/development and siRNA/ S-PTGS can be uncoupled by single-point mutations at different positions in the protein.