E2FA and E2FB transcription factors coordinate cell proliferation with seed maturation

The E2F transcription factors and the RETINOBLASTOMA RELATED (RBR) repressor protein are principal regulators coordinating cell proliferation with differentiation, but their role during seed development is little understood. We show that in the fully developed embryos, cell number was not affected either in single or double mutants for the activator-type E2FA and E2FB. Accordingly, these E2Fs are only partially required for the expression of cell cycle genes. In contrast, the expression of key seed maturation genes; LEAFY COTYLEDON 1-2 (LECJ-2), ABSCISIC ACID INSENSITIVE 3 (ABI3), FUSCA 3 (FUS3) and WRINKLED 1 (WRil) are upregulated in the e2fab double mutant embryo. In accordance, E2FA directly regulates LEC2, and mutation at the consensus E2F-binding site in LEC2 promoter de-represses its activity during the proliferative stage of seed development. Additionally, the major seed storage reserve proteins, 12S globulin and 2S albumin became prematurely accumulated at the proliferating phase of seed development in the e2fab double mutant. Our findings reveal a repressor function of the activator E2Fs to restrict the seed maturation program until the cell proliferation phase is completed.