Control of fertilization-independent endosperm development by the MEDEA polycomb gene Arabidopsis

Higher plant reproduction is unique because two cells are fertilized in the haploid female gametophyte, Egg and sperm nuclei fuse to form the embryo. A second sperm nucleus fuses with the central cell nucleus that replicates to generate the endosperm, a tissue that supports embryo development. To un derstand mechanisms that initiate reproduction, we isolated a mutation in A rabidopsis, f644, that allows for replication of the central cell and subse quent endosperm development without fertilization. When mutant f644 egg and central cells are fertilized by wild-type sperm, embryo development is inh ibited, and endosperm is overproduced. By using a map-based strategy, we cl oned and sequenced the F644 gene and showed that it encodes a SET-domain po lycomb protein. Subsequently, we found that F644 is identical to MEDEA (MEA ), a gene whose maternal-derived allele is required for embryogenesis [Gros sniklaus, U., Vielle-Calzada, J.-P,, Hoeppner, M. A. & Gagliano, W. B. (199 8) Science 280, 446-450], Together, these results reveal functions for plan t polycomb proteins in the suppression of central cell proliferation and en dosperm development. We discuss models to explain how polycomb proteins fun ction to suppress endosperm and promote embryo development.