Many flowering plants form a single female gamete (egg), which is formed from the megaspore by meiosis from a somatic cell called the megaspore mother cell (MMC). Olmedo-Monfil et al. found that Arabidopsis deficient for ARGONAUTE 9 (AGO9), which is a member of a family of RNA-cleaving proteins involved in gene silencing by small RNAs, formed more than one MMC and had multiple enlarged subepidermal cells in the developing ovule. However, only one of the MMCs subsequently underwent meiosis. The enlarged epidermal cells expressed a marker that is normally only present in the megaspore in wild-type plants, suggesting that in the absence of functional AGO9, somatic cells can develop gamete-like characteristics without undergoing meiosis. Genes encoding two proteins involved in the generation of trans-acting silencing RNAs (ta-siRNAs), which can move out of the cell in which they are produced to silence gene expression in other cells, also showed a phenotype like that of the ago9 mutants. The authors immunoprecipitated AGO9 from the female reproductive organs of wild-type plants and found more than 2000 sRNAs, most of which were 24 nucleotides long and derived from retrotransposons, a type of transposable element (TE). By crossing plants expressing tagged specific TEs with plants heterozygous for ago9, the authors showed that AGO9 was necessary for TE inactivation in the egg and supporting cells. Thus, female gametogenesis appears to involve an RNA silencing pathway that targets TEs and is dependent on AGO9.
V. Olmedo-Monfil, N. Durán-Figueroa, M. Arteaga-Vázquez, E. Demesa-Arévalo, D. Autran, D. Grimanelli, R. K. Slotkin, R. A. Martienssen, J.-P. Vielle-Calzada, Control of female gamete formation by a small RNA pathway in Arabidopsis. Nature 464, 628–632 (2010). [PubMed]