Activation of CYCD7;1 in the central cell and early endosperm overcomes cell-cycle arrest in the Arabidopsis female gametophyte, and promotes early endosperm and embryo development

In angiosperms, double fertilization of the egg and central cell of the megagametophyte leads to development of the embryo and endosperm respectively. Control of cell cycle progression in the megagametophyte is essential for successful fertilization and development. Central cell-targeted expression of the D-type cyclin CYCD7;1 (CYCD7;1 OE) using the imprinted FWA promoter overcomes cycle arrest of the central cell in the Arabidopsis female gametophyte in the unfertilized ovule leading to multinucleate central cells at high frequency. Unlike FERTILIZATION-INDEPENDENT SEED (fis) mutants but similar to lethal RETINOBLASTOMA-RELATED (rbr) mutants, no seed coat development is triggered. Unlike loss of rbr, post-fertilization CYCD7;1 OE in the endosperm enhances the number of nuclei during syncytial endosperm development, and induces partial abortion of developing seeds, associated with enhanced size of surviving seeds. The frequency of lethality was less than the frequency of multi-nucleate central cells, indicating that these aspects are not causally linked. These larger seeds contain larger embryos comprised of more cells of wild type size, surrounded by a seed coat composed of more cells. Seedlings arising from these larger seeds displayed faster seedling establishment and early growth. Two different embryo lethal mutants similarly also conferred enlarged seed size in surviving siblings, consistent with seed size increase being a general response to sibling lethality although the cellular mechanisms were found to be distinct. Our data suggest that a tight control of CYCD activity in the central cell and in the developing endosperm is required for optimal seed formation