In Medicago nodules, endoreduplication cycles and ploidy-dependent cell enlargement occur during the differentiation of bacteroid-containing nitrogen-fixing symbiotic cells. These events are accompanied by the expression of ccs52A , a plant ortholog of the yeast and animal cdh1/srw1/fzr genes, acting as a substrate-specific activator of the anaphase-promoting complex (APC) ubiquitin ligase. Because CCS52A is involved in the transition of mitotic cycles to endoreduplication cycles, we investigated the importance of somatic endoploidy and the role of the M. truncatula ccs52A gene in symbiotic cell differentiation. Transcription analysis and ccs52A promoter-driven ␤ -glucuronidase activity in transgenic plants showed that ccs52A was dispensable for the mitotic cycles and nodule primordium formation, whereas it was induced before nodule differentiation. The CCS52A protein was present in the nucleus of endoreduplication-competent cells, indicating that it may activate APC constitutively during the endoreduplication cycles. Downregulation of ccs52A in transgenic M. truncatula plants drastically affected nodule development, resulting in lower ploidy, reduced cell size, inefficient invasion, and the maturation of symbiotic cells, accompanied by early senescence and finally the death of both the bacterium and plant cells. Thus, ccs52A expression is essential for the formation of large highly polyploid symbiotic cells, and endoreduplication is an integral part of normal nodule development.

The anaphase-promoting complex (APC), a multisubunit E3 ubiquitin ligase, is an essential regulator of the cell cycle from metaphase until S phase in yeast and metazoans. APC mediates degradation of numerous cell cycle-related proteins, including mitotic cyclins and its activation and substrate-specificity are determined by two adaptor proteins, Cdc20 and Cdh1. Plants have multiple APC activators and the Cdh1-type proteins, in addition, are represented by two subclasses, known as Ccs52A and Ccs52B. The Arabidopsis genome contains five cdc20 genes as well as ccs52A1, ccs52A2 and ccs52B. In Schizosaccharomyces pombe, expression of the three Atccs52 genes elicited distinct phenotypes supporting nonredundant function of the AtCcs52 proteins. Consistent with these activities, the AtCcs52 proteins were able to bind both to the yeast and the Arabidopsis APCs. In synchronized Arabidopsis cell cultures the cdc20 transcripts were present from early G 2 until the M-phase exit, ccs52B from G 2 /M to M while ccs52A1 and ccs52A2 were from late M until early G 2 , suggesting consecutive action of these APC activators in the plant cell cycle. The AtCcs52 proteins interacted with different subsets of mitotic cyclins, in accordance with their expression profiles, either in free-or CDK-bound forms. Expression of most APC subunits was constitutive, whereas cdc27a and cdc27b, corresponding to two forms of apc3, and ubc19 and ubc20 encoding E2-C type ubiquitin-conjugating enzymes displayed differences in their cell cycle regulation. These data indicate the existence of numerous APC Cdc20/Ccs52/Cdc27 forms in Arabidopsis, which in conjunction with different E2 enzymes might have distinct or complementary functions at distinct stages of the cell cycle.

The Cdc20 and Cdh1/Fzr proteins are the substrate-specific activators of the anaphase-promoting complex (APC). In Medicago truncatula, the MtCcs52A and MtCcs52B proteins represent two subgroups of the Cdh1-type activators, which display differences in their cell cycle regulation, structure, and function. The ccs52A transcripts are present in all phases of the cell cycle. By contrast, expression of ccs52B is restricted to late G2-phase and M-phase, and its induced overexpression in BY2 cells inhibited mitosis. MtCcs52A is active in Schizosaccharomyces pombe and binds to the S. pombe APC, whereas MtCcs52B does not because of differences in the N-terminal region. We identified a new functional domain, the Cdh1-specific motif conserved in the Cdh1 proteins that, in addition to the C-box and the terminal Ile and Arg residues, was essential for the activity and required for efficient binding to the APC. Moreover, we demonstrate that cyclin-dependent kinase phosphorylation sites adjacent to the C-box may regulate the interaction with the APC. In the different plant organs, the expression of Mtccs52A and Mtccs52B displayed differences and indicated the involvement of the APC in differentiation processes.

SummaryThe dimension of organs depends on the number and the size of their component cells. Formation of polyploid cells by endoreduplication cycles is predominantly associated with increases in the cell size and implicated in organ growth. In plants, the CCS52A proteins play a major role in the switch from mitotic to endoreduplication cycles controlling thus the number of mitotic cells and the endoreduplication events in the differentiating cells.Arabidopsis has two CCS52A isoforms; AtCCS52A1 and AtCCS52A2. Here we focused on their roles in endoreduplication and cell size control during plant development.We demonstrate their complementary and dose-dependent actions that are dependent on their expression patterns. Moreover, the impact of CCS52A overexpression on organ size in transgenic plants was dependent on the expression level; while enhanced expression of the CCS52A genes positively correlated with the ploidy levels, organ sizes were negatively affected by strong overexpression whereas milder overexpression resulted in a significant increase in the organ sizes.Taken together, these finding support both complementary and dose-dependent actions for the Arabidopsis CCS52A isoforms in plant development and demonstrate that elevated ectopic CCS52A expression positively correlates with organ size, opening a route to higher biomass production.

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