A role for post-transcriptional gene silencing in vegetative phase change in Arabidopsis

Abstract

Higher plants undergo a transition from a juvenile to an adult phase of vegetative development prior to flowering. Screens for mutants that undergo this transition precociously resulted in the identification of a number of complementation groups with loss-of-function phenotypes similar to that of mutations in the Argonaute gene ZIPPY (ZIP). Two groups correspond to mutations in the SGS2/SDE1/RDR6 and SGS3 loci, genes originally identified in screens for mutants deficient in posttranscriptional gene silencing (PTGS). By positional cloning, another group was found to correspond to mutations in the previously uncharacterized At5g20320 (DCL4), one of the four Dicer-like family members in Arabidopsis. Double and triple mutant analyses suggest that ZIP, RDR6, SGS3, and DCL4 act in the same pathway, along with HST, the Arabidopsis exportin-5 homologue, a protein that has been shown to regulate the export of miRNAs in mammals, and which probably has the same function in Arabidopsis. Such observations suggested that RDR6 and SGS3 are important for both transgene silencing and the regulation of endogenous genes, and we hypothesized that DCL4 might provide the dicer activity necessary for the production of such a class of small regulatory RNAs. Indeed, genetic and molecular analysis showed that these genes operate as core components of a PTGS pathway in Arabidopsis where DCL4 is required for the biogenesis and function of a novel class of endogenous siRNAs, termed tans-acting siRNAs (ta-siRNAs). The work presented here, demonstrates a role for endogenous siRNAs in the normal regulation of gene expression, and suggest that this system plays a central role in the temporal control of shoot development in Arabidopsis