In vascular plants, the female germline derives from the megaspore mother cell (MMC). RNA-dependent DNA methylation (RdDM) is known to restrict the MMC to a single cell, whereas SPOROCYTELESS/NOZZLE (SPL/NZZ) is required for MMC development. In their latest study, Lucia Colombo and colleagues investigate the regulation of SPL/NZZ in Arabidopsis. Using drm1drm2 mutants, which lack DRM1/DRM2 methyltransferases, the authors show that loss of the RdDM pathway causes ectopic MMC-like cell development. Similarly, mutants for SEEDSTICK (STK), which encodes a master regulator of ovules, form additional MMC-like cells, suggesting that STK regulates RdDM. Indeed, gene expression and chromatin immunoprecipitation analyses reveal that STK directly regulates the expression of key RdDM genes, such as RDR6 and AGO9. By investigating marker gene expression and cell divisions, the researchers show that the additional MMC-like cells in these mutants lack MMC identity. Finally, they show ectopic expression of SPL/NZZ in RdDM mutants and genetic overexpression of SPL/NZZ is sufficient to produce additional MMC-like cells. Taken together, these data show that RdDM restricts expression of SPL/NZZ to specify a single MMC precursor; however, additional factors are required to drive MMC function.
