The E2FD/DEL2 factor is a component of a regulatory networkcontrolling cell proliferation and development in Arabidopsis

An emerging view of plant cell cycle regulators, including the E2F transcription factors, implicates them in the integration of cell proliferation and development. Arabidopsis encodes six E2F proteins that can act as activators or repressors of E2F-responsive genes. E2FA, E2FB and E2FC interact with the retinoblastoma-like RBR protein and bind to DNA together with their DP partners. In contrast, E2FD, E2FE and E2FF (also known as DEL2, DEL1 and DEL3) are atypical E2Fs that possess duplicated DNA binding regions, lack trans-activating and RBRbinding domains and are believed to act as transcriptional inhibitors/repressors. E2FE/DEL1 has been shown to inhibit the endocycle and E2FF/DEL3 appears to control cell expansion but the role of E2FD/DEL2 has not been reported so far. In this study, we investigated the expression of E2FD/DEL2 and analysed the accumulation of its product. These studies revealed that E2FD/DEL2 accumulation is subject to negative post-translational regulation mediated by the plant hormone auxin. Moreover, the analysis of mutant and transgenic plants characterized by altered expression of E2FD/DEL2 has revealed that this atypical E2F can affect plant growth by promoting cell proliferation and repressing cell elongation. Overexpression of E2FD/DEL2 increased the expression of E2FA, E2FB and E2FE/DEL1 whereas its inactivation led to the up-regulation of genes encoding repressors of cell division. These results suggest that E2FD/DEL2 is part of a regulatory network that controls the balance between cell proliferation and development in Arabidopsis