MicroRNA-dependent trans-acting siRNA production.

Less than a year has elapsed between the discovery of trans-acting small interfering RNAs (tasiRNAs) in plants and the elucidation of the major steps of the corresponding pathway. During tasiRNA biogenesis, polyadenylated RNAs transcribed from non-protein-coding TAS genes are cleaved by a microRNA (miRNA)-programmed RNA-induced silencing complex. In contrast to classical miRNA targets, RDR6 and SGS3 convert one of the TAS RNA cleavage products into double-stranded RNA, which is subsequently processed, in a phase determined by the initial miRNA cleavage site, by DICER-LIKE 4 to generate a 21-nucleotide tasiRNA population. tasiRNAs guide endogenous mRNA cleavage through the action of AGO1 or, perhaps in some cases, AGO7. Some of the tasiRNA targets probably regulate the juvenile-to-adult phase transition, but the roles of other tasiRNA targets remain to be determined.