The RNA dependent DNA methylation pathway is required to restrict <i>SPOROCYTELESS/NOZZLE</i> expression to specify a single female germ cell precursor in Arabidopsis
Marta Adelina Mendes, Rosanna Petrella, Mara Cucinotta, Edoardo Vignati, Stefano Gatti, Sara Cristina Pinto, Dayton C. Bird, Veronica Gregis, H. G. Dickinson, Matthew R. Tucker, Lucia Colombo
Abstract
In higher plants the female germline is formed from the megaspore mother cell (MMC), a single cell in the pre-meiotic ovule. Previously, it was reported that mutants in RNA dependent DNA methylation - RdDM - pathway might be involved in restricting female germline to a single nucellus cell. We show that the DRM methyltransferase double mutant drm1drm2 also presents also presents ectopic enlarged cells, consistent with supernumerary MMC-like cells. In wild-type ovules MMC differentiation required SPOROCYTELESS/NOZZLE (SPL/NZZ), as demonstrated by the spl/nzz mutant failing to develop an MMC. We address the poorly understood upstream regulation of SPL/NZZ in ovules, showing that the RdDM pathway is important to restrict SPL/NZZ expression. In ago9, rdr6 and drm1drm2 mutants, SPL/NZZ is ectopically expressed, suggesting that the mutiple MMC-like cells observed might be due to the ectopic expression of SPL/NZZ. We show that the ovule identity gene SEEDSTICK, directly regulates the AGO9 and RDR6 expression in the ovule and therefore indirectly SPL/NZZ expression. A model is presented describing the network required to restrict SPL/NZZ expression to specify a single MMC.