The E2FC-DPB Transcription Factor Controls Cell Division, Endoreplication and Lateral Root Formation in a SCF-Dependent Manner.

Cell division is a highly regulated process that has to be coordinated with cell specification and differentiation for proper development and growth of the plants. Cell cycle regulation is carried out by key proteins that control cell cycle entry, progression and exit. This regulation is controlled at different stages such as gene expression, posttranslational modification of proteins and specific proteolysis. The G(1)/S and the G(2)/M transitions are critical checkpoints of the cell cycle that are controlled, among others, by the activity of cyclin-dependent kinases (CDK). Different CDK activities, still to be fully identified, impinge on the retinoblastoma (RBR)/E2F/DP pathway as well as on the programmed proteolysis pathway. The specific degradation of proteins through the ubiquitin pathway in plants, highly controlled in time and space, is emerging as a powerful mechanism to regulate the levels and the activity of several proteins, including many cell cycle regulators.

The balance between cell division and endoreplication depends on E2FC-DPB, transcription factors regulated by the ubiquitin-SCFSKP2A pathway in Arabidopsis.

The balance between cell proliferation, cell cycle arrest, and differentiation needed to maintain the organogenetic program depends on the coordination of gene expression, posttranslational modification, and specific proteolysis of cell cycle regulators. The G1/S and G2/M transitions are critical checkpoints controlled, in part, by cyclin-dependent kinases in the retinoblastoma (RBR)/E2F/DP pathway. Arabidopsis thaliana DPB is regulated by phosphorylation and targeted to proteasome-mediated proteolysis by the SCF(SKP2A) complex. In addition, DPB interacts in vivo with E2FC, because ectopic coexpression of E2FC and DPB produces severe developmental defects. To understand E2FC/DPB heterodimer function, we analyzed the effect of reducing E2FC mRNA levels with RNA interference. The e2fc-R plants developed organs with more but smaller cells and showed increased cell cycle marker gene expression and increased proliferative activity in developing leaves, meristems, and pericycle cells. This last feature produces plants with more lateral roots, consistent with an E2FC role in restricting lateral root initiation. The e2fc-R plants also show marked reductions in ploidy levels of mature leaves. These results indicate that the transition from cell division to the endocycle is sensitive to different pathways, E2FC/DPB being one of them. Our results show that E2FC/DPB is a key factor in controlling the balance between cell proliferation and the switch to the endocycle program.