Abstract
RNA-directed DNA methylation (RdDM) is a small interfering RNA (siRNA)-mediated epigenetic modification that contributes to transposon silencing in plants. RdDM requires a complex transcriptional machinery that includes specialized RNA polymerases, named Pol IV and Pol V, as well as chromatin remodelling proteins, transcription factors, RNA binding proteins, and other plant-specific proteins whose functions are not yet clarified. In Arabidopsis thaliana, DICER-LIKE3 and members of the ARGONAUTE4 group of ARGONAUTE (AGO) proteins are involved, respectively, in generating and using 24-nt siRNAs that trigger methylation and transcriptional gene silencing of homologous promoter sequences. AGO4 is the main AGO protein implicated in the RdDM pathway. Here we report the identification of the related AGO6 in a forward genetic screen for mutants defective in RdDM and transcriptional gene silencing in shoot and root apical meristems in Arabidopsis thaliana. The identification of AGO6, and not AGO4, in our screen is consistent with the primary expression of AGO6 in shoot and root growing points.
Highlights
RNA-directed DNA methylation is a small interfering RNA-mediated epigenetic modification that is highly developed in flowering plants
We have identified AGO6 in a screen for mutants defective in RNA-directed DNA methylation (RdDM) and transcriptional gene silencing in root and shoot meristem regions in Arabidopsis thaliana ecotype Col-0
AGO6 belongs to the AGO4 group of AGO proteins that is specialized for siRNAmediated chromatin modifications [3,15]
Summary
RNA-directed DNA methylation is a small interfering RNA (siRNA)-mediated epigenetic modification that is highly developed in flowering plants. RdDM is characterized by de novo methylation of cytosines in all sequence contexts (CG, CHG and CHH, where H is A, T or C) within the region of siRNA-DNA sequence homology. Pol IV is involved in producing the siRNA trigger for RdDM whereas Pol V acts downstream of siRNA production to facilitate de novo methylation of homologous DNA sequences. Pol V is thought to synthesize scaffold transcripts that interact with the complementary siRNA trigger, leading to recruitment of the methylation machinery at the target DNA site [4]. Two independent forward genetic screens have recently confirmed that DRM2 is the major DNA cytosine methyltransferase acting in the RdDM pathway [5,6]
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