Seed maturation in plants involves a morphogenic phase, in which cells actively proliferate, and a maturation phase, during which seed storage reserves build up. Cell proliferation is driven by the transcription factors E2FA and E2FB, which promote the expression of cell cycle genes. Now, Zoltán Magyar and co-workers investigate the contribution of E2FA and E2FB to seed and embryo development in Arabidopsis. They find that, surprisingly, mutation of both E2FA and E2FB (e2fab) has very little effect on the expression of cell cycle genes in the proliferative phase and, accordingly, the number of embryonic cells in mature seeds is similar in wild-type and e2fab plants. Conversely, proliferation genes are upregulated during the maturation phase in e2fab embryos compared with wild-type embryos. Moreover, the authors show that E2FA and E2FB together repress AFL genes, which are key regulators of seed maturation, at various stages in seed development. They also find that the seed storage proteins 12S globulin and 2S albumin accumulate prematurely during the morphogenic phase in mutants of E2FA and E2FB. Thus, this study reveals that E2FA and E2FB coordinate the switch between the morphogenic and the maturation phase of seed development by preventing the untimely expression of genes specific to each phase.
