Role of Arabidopsis AGO6 in siRNA accumulation, DNA methylation and transcriptional gene silencing

Argonautes (AGOs) are conserved proteins that contain an RNA‐binding PAZ domain and an RNase H‐like PIWI domain. In Arabidopsis , except for AGO1, AGO4 and AGO7, the roles of seven other AGOs in gene silencing are not known. We found that a mutation in AGO6 partially suppresses transcriptional gene silencing in the DNA demethylase mutant ros1‐1 . In ago6‐1ros1‐1 plants, RD29A promoter short interfering RNAs (siRNAs) are less abundant, and cytosine methylation at both transgenic and endogenous RD29A promoters is reduced, compared to that in ros1‐1 . Interestingly, the ago4‐1 mutation has a stronger suppression of the transcriptional silencing phenotype of ros1‐1 mutant. Analysis of cytosine methylation at the endogenous MEA‐ISR, AtREP2 and SIMPLEHAT2 loci revealed that the CpNpG and asymmetric methylation levels are lower in either of the ago6‐1 and ago4‐1 single mutants than those in the wild type, and the levels are the lowest in the ago6‐1ago4‐1 double mutant. These results suggest that AGO6 is important for the accumulation of specific heterochromatin‐related siRNAs, and for DNA methylation and transcriptional gene silencing, this function is partly redundant with AGO4.