Transgenically expressed viral RNA silencing suppressors interfere with microRNA methylation in Arabidopsis

HEN1-dependent methylation of the 3′-terminal nucleotide is a crucial step in plant microRNA (miRNA) biogenesis. Here we report that several viral RNA silencing suppressors (P1/HC-Pro, p21 and p19) inhibit miRNA methylation. These suppressors have distinct effects on different miRNAs. We also show that miRNA∗ is methylated in vivo in a suppressor-sensitive manner, suggesting that the viral proteins interfere with miRNA/miRNA∗ duplexes. p19 and p21 bind both methylated and unmethylated miRNA/miRNA∗ duplexes in vivo. These findings suggest miRNA/miRNA∗ as the in vivo substrates for the HEN1 miRNA methyltransferase and raise intriguing possibilities regarding the cellular location of miRNA methylation.