DNA Replication Licensing Affects Cell Proliferation or Endoreplication in a Cell Type–Specific Manner

Abstract

In eukaryotic cells, the function of DNA replication licensing components (Cdc6 and Cdt1, among others) is crucial for cell proliferation and genome stability. However, little is known about their role in whole organisms and whether licensing control interfaces with differentiation and developmental programs. Here, we study Arabidopsis thaliana CDT1, its regulation, and the consequences of overriding licensing control. The availability of AtCDT1 is strictly regulated at two levels:(1) at the transcription level, by E2F and growth-arresting signals, and (2) posttranscriptionally, by CDK phosphorylation, a step that is required for its proteasome-mediated degradation. We also show that CDC6 and CDT1 are key targets for the coordination of cell proliferation, differentiation, and development. Indeed, altered CDT1 or CDC6 levels have cell typespecific effects in developing Arabidopsis plants: in leaf cells competent to divide, cell proliferation is stimulated, whereas in cells programmed to undergo differentiation-associated endoreplication rounds, extra endocycles are triggered. Thus, we propose that DNA replication licensing control is critical for the proper maintenance of proliferative potential, developmental programs, and morphogenetic patterns.

E2F RegulatesFASCIATA1, a Chromatin Assembly Gene Whose Loss Switches on the Endocycle and Activates Gene Expression by Changing the Epigenetic Status

“…A characteristic feature of leaf development is that a developmentally regulated switch from cell division to the endocycle occurs (Boudolf et al, 2004;Castellano et al, 2004;Beemster et al, 2005). Thus, we carried out detailed analysis of ploidy levels during leaf development.…”

Regulating DNA Replication in Plants

“…Whether it is a direct effect or mediated by downstream RBR/E2F targets is not clear. For example, maintaining high levels of CDC6 and/or CDT1, which are regulated by RBR/E2F, during leaf development increases cellular ploidy (Castellano et al 2001(Castellano et al , 2004. Together, these data show a direct role of RBR/E2F, some of their targets, and CDK/cyclin in controlling endoreplication during development.…”

“…However, it is known that the MCM complex remains bound to chromatin throughout the cell cycle, except during mitosis (Shultz et al 2009). Also, increased levels of CDC6 and/or CDT1 trigger extra endoreplication cycles, which implies full-genome duplication, instead of limited rereplication (Castellano et al 2001(Castellano et al , 2004. These differences with other model organisms support the idea that plantspecific mechanisms of rereplication control may have evolved (Costas et al 2011a).…”

“…Plant CDC6 and CDT1 conserve the major domains of these proteins, although each of the two homologs of CDC6 and CDT1 proteins show significant divergence, in particular CDT1 (Castellano et al 2001(Castellano et al , 2004. Another major difference with the metazoan proteins is the absence in plants of a homolog of metazoan geminin (Gmnn), the CDT1-interacting protein that inhibits its activity Miotto and Struhl 2010).…”

“…Expression of both CDT1a and CDT1b depends on a complex regulation. Whereas both CDT1a and CDT1b are E2F targets (Castellano et al 2004), only CDT1a is activated by ORC1 (Sanchez and Gutierrez 2009). Both CDT1a and CDT1b genes are also repressed by ABAP1 (ARMADILLO BTB ARABIDOPSIS PROTEIN1), a novel factor that works in association with the TCP24 transcription factor (Masuda et al 2008).…”