Involved in RNA-mediated post-transcriptional gene silencing (PTGS). Main component of the RNA-induced silencing complex (RISC) that binds to a short guide RNA such as a microRNA (miRNA) or small interfering RNA (siRNA). RISC uses the mature miRNA or siRNA as a guide for slicer-directed cleavage of homologous mRNAs to repress gene expression. Required for reliable formation of primary and axillary shoot apical meristems. Specifies leaf adaxial identity by repressing the miR165 and miR166 microRNAs in the embryonic shoot apex, in the shoot apical meristem (SAM) and leaf. Represses the microRNA miR398 which targets CCS1 chaperone mRNAs for translational inhibition. Acts as a negative regulator of AGO1 protein level. Like AGO1, is required for stem cell function and organ polarity. Unlike AGO1, is not subjected to small RNA-mediated repression itself. Essential for multiple processes in development. Coregulates, with GATA18/HAN, the shoot apical meristem (SAM) organization