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. Associates preferentially with small RNAs of 24 nucleotide in length with a 5' terminal adenosine. Interacts with 24 nucleotide sRNAs derived from transposable elements (TEs). Required to silence pericentrometric-located TEs in female gametes and their accessory cells. Necessary to inactivate a significant proportion of long terminal repeat retrotransposons (LTRs) in the ovule. Required to specify cell fate in ovule. Involved in the control of female gamete formation by restricting the specification of gametophyte precursors in a dosage-dependent, non-cell-autonomous manner. Targeted by turnip yellows virus (TuYV) protein P0 (via F-box-like domain) for probable proteasome degradation and thereby inactivating AGO9 function in RNA silencing. Expressed in embryonic shoot apex region, pollen and developing ovules. Expressed in all cells of the young ovule primordium at pre-meiotic stages, including the L1 layer. At early stages of female gametogenesis, expression is restricted to the distal (micropylar) portion of the developing ovule, but is absent from the 1-nuclear female gametophyte. In fully differentiated ovules, expressed at the proximal and distal poles but not in the female gametophyte. Differentiation of multiple gametic cells able to initiate gametogenesis. Abnormally enlarged sub-epidermal cells in developing ovules. Belongs to the argonaute family. Ago subfamily.