Conserved CDC20 cell cycle functions are carried out by two of the five isoforms in Arabidopsis thaliana

New insights into the dynamics of plant cell nuclei and chromosomes

The plant lamin-like protein NMCP/AtLINC and orthologues of the SUN-KASH complex across the nuclear envelope (NE) show the universality of nuclear structure in eukaryotes. However, depletion of components in the connection complex of the NE in plants does not induce severe defects, unlike in animals. Appearance of the Rabl configuration is not dependent on genome size in plant species. Topoisomerase II and condensin II are not essential for plant chromosome condensation. Plant endoreduplication shares several common characteristics with animals, including involvement of cyclin-dependent kinases and E2F transcription factors. Recent finding regarding endomitosis regulator GIG1 shed light on the suppression mechanism of endomitosis in plants. The robustness of plants, compared with animals, is reflected in their genome redundancy. Spatiotemporal functional analyses using chromophore-assisted light inactivation, super-resolution microscopy, and 4D (3D plus time) imaging will reveal new insights into plant nuclear and chromosomal dynamics.

The anaphase-promoting complex/cyclosome in control of plant development

Temporal controlled degradation of key cell division proteins ensures a correct onset of the different cell cycle phases and exit from the cell division program. In light of the cell cycle, the Anaphase-Promoting Complex/Cyclosome (APC/C) is an important conserved multi-subunit ubiquitin ligase, marking targets for degradation by the 26S proteasome. However, whereas the APC/C has been studied extensively in yeast and mammals, only in the last decade has the plant APC/C started to unveil its secrets. Research results have shown the importance of the APC/C core complex and its activators during gametogenesis, growth, hormone signaling, symbiotic interactions, and endoreduplication onset. In addition, recently, the first plant APC/C inhibitors have been reported, allowing a fine-tuning of APC/C activity during the cell cycle. Together with the identification of the first APC/C targets, a picture emerges of APC/C activity being essential for many different developmental processes.

OSD1 promotes meiotic progression via APC/C inhibition and forms a regulatory network with TDM and CYCA1;2/TAM

Cell cycle control is modified at meiosis compared to mitosis, because two divisions follow a single DNA replication event. Cyclin-dependent kinases (CDKs) promote progression through both meiosis and mitosis, and a central regulator of their activity is the APC/C (Anaphase Promoting Complex/Cyclosome) that is especially required for exit from mitosis. We have shown previously that OSD1 is involved in entry into both meiosis I and meiosis II in Arabidopsis thaliana; however, the molecular mechanism by which OSD1 controls these transitions has remained unclear. Here we show that OSD1 promotes meiotic progression through APC/C inhibition. Next, we explored the functional relationships between OSD1 and the genes known to control meiotic cell cycle transitions in Arabidopsis. Like osd1, cyca1;2/tam mutation leads to a premature exit from meiosis after the first division, while tdm mutants perform an aberrant third meiotic division after normal meiosis I and II. Remarkably, while tdm is epistatic to tam, osd1 is epistatic to tdm. We further show that the expression of a non-destructible CYCA1;2/TAM provokes, like tdm, the entry into a third meiotic division. Finally, we show that CYCA1;2/TAM forms an active complex with CDKA;1 that can phosphorylate OSD1 in vitro. We thus propose that a functional network composed of OSD1, CYCA1;2/TAM, and TDM controls three key steps of meiotic progression, in which OSD1 is a meiotic APC/C inhibitor.

In the life cycle of sexual organisms, a specialized cell division—meiosis—reduces the number of chromosomes from two sets (2n, diploid) to one set (n, haploid), while fertilization restores the original chromosome number. Meiosis reduces ploidy because it consists of two cellular divisions following a single DNA replication. In this study, we analyze the function of a group of genes that collectively controls the entry into the first meiotic division, the entry into the second meiotic division, and the exit from meiosis in the model plant Arabidopsis thaliana. We revealed a complex regulation network that controls these three key transitions.

Control of the plant cell cycle by developmental and environmental cues

Plant morphogenesis relies on cell proliferation and differentiation strictly controlled in space and time. As in other eukaryotes, progression through the plant cell cycle is governed by cyclin-dependent kinases (CDKs) that associate with their activator proteins called cyclins (CYCs), and the activity of CYC-CDK is modulated at both transcriptional and post-translational levels. Compared with animals and yeasts, plants generally possess many more genes encoding core cell cycle regulators and it has been puzzling how their functions are specified or overlapped in development or in response to various environmental changes. Thanks to the recent advances in high-throughput, genome-wide transcriptome and proteomic technologies, we are finally beginning to see how core regulators are assembled during the cell cycle and how their activities are modified by developmental and environmental cues. In this review we will summarize the latest progress in plant cell cycle research and provide an overview of some of the emerging molecular interfaces that link upstream signaling cascades and cell cycle regulation.

APC/C-mediated degradation of dsRNA-binding protein 4 (DRB4) involved in RNA silencing

Background

Selective protein degradation via the ubiquitin-26S proteasome is a major mechanism underlying DNA replication and cell division in all Eukaryotes. In particular, the APC/C (Anaphase Promoting Complex or Cyclosome) is a master ubiquitin protein ligase (E3) that targets regulatory proteins for degradation allowing sister chromatid separation and exit from mitosis. Interestingly, recent work also indicates that the APC/C remains active in differentiated animal and plant cells. However, its role in post-mitotic cells remains elusive and only a few substrates have been characterized.

Methodology/Principal Findings

In order to identify novel APC/C substrates, we performed a yeast two-hybrid screen using as the bait Arabidopsis APC10/DOC1, one core subunit of the APC/C, which is required for substrate recruitment. This screen identified DRB4, a double-stranded RNA binding protein involved in the biogenesis of different classes of small RNA (sRNA). This protein interaction was further confirmed in vitro and in plant cells. Moreover, APC10 interacts with DRB4 through the second dsRNA binding motif (dsRBD2) of DRB4, which is also required for its homodimerization and binding to its Dicer partner DCL4. We further showed that DRB4 protein accumulates when the proteasome is inactivated and, most importantly, we found that DRB4 stability depends on APC/C activity. Hence, depletion of Arabidopsis APC/C activity by RNAi leads to a strong accumulation of endogenous DRB4, far beyond its normal level of accumulation. However, we could not detect any defects in sRNA production in lines where DRB4 was overexpressed.

Conclusions/Significance

Our work identified a first plant substrate of the APC/C, which is not a regulator of the cell cycle. Though we cannot exclude that APC/C-dependent degradation of DRB4 has some regulatory roles under specific growth conditions, our work rather points to a housekeeping function of APC/C in maintaining precise cellular-protein concentrations and homeostasis of DRB4.

A putative homologue of CDC20/CDH1 in the malaria parasite is essential for male gamete development

Cell-cycle progression is governed by a series of essential regulatory proteins. Two major regulators are cell-division cycle protein 20 (CDC20) and its homologue, CDC20 homologue 1 (CDH1), which activate the anaphase-promoting complex/cyclosome (APC/C) in mitosis, and facilitate degradation of mitotic APC/C substrates. The malaria parasite, Plasmodium, is a haploid organism which, during its life-cycle undergoes two stages of mitosis; one associated with asexual multiplication and the other with male gametogenesis. Cell-cycle regulation and DNA replication in Plasmodium was recently shown to be dependent on the activity of a number of protein kinases. However, the function of cell division cycle proteins that are also involved in this process, such as CDC20 and CDH1 is totally unknown. Here we examine the role of a putative CDC20/CDH1 in the rodent malaria Plasmodium berghei (Pb) using reverse genetics. Phylogenetic analysis identified a single putative Plasmodium CDC20/CDH1 homologue (termed CDC20 for simplicity) suggesting that Plasmodium APC/C has only one regulator. In our genetic approach to delete the endogenous cdc20 gene of P. berghei, we demonstrate that PbCDC20 plays a vital role in male gametogenesis, but is not essential for mitosis in the asexual blood stage. Furthermore, qRT-PCR analysis in parasite lines with deletions of two kinase genes involved in male sexual development (map2 and cdpk4), showed a significant increase in cdc20 transcription in activated gametocytes. DNA replication and ultra structural analyses of cdc20 and map2 mutants showed similar blockage of nuclear division at the nuclear spindle/kinetochore stage. CDC20 was phosphorylated in asexual and sexual stages, but the level of modification was higher in activated gametocytes and ookinetes. Changes in global protein phosphorylation patterns in the Δcdc20 mutant parasites were largely different from those observed in the Δmap2 mutant. This suggests that CDC20 and MAP2 are both likely to play independent but vital roles in male gametogenesis.

GIGAS CELL1, a novel negative regulator of the anaphase-promoting complex/cyclosome, is required for proper mitotic progression and cell fate determination in Arabidopsis

Increased cellular ploidy is widespread during developmental processes of multicellular organisms, especially in plants. Elevated ploidy levels are typically achieved either by endoreplication or endomitosis, which are often regarded as modified cell cycles that lack an M phase either entirely or partially. We identified GIGAS CELL1 (GIG1)/OMISSION OF SECOND DIVISION1 (OSD1) and established that mutation of this gene triggered ectopic endomitosis. On the other hand, it has been reported that a paralog of GIG1/OSD1, UV-INSENSITIVE4 (UVI4), negatively regulates endoreplication onset in Arabidopsis thaliana. We showed that GIG1/OSD1 and UVI4 encode novel plant-specific inhibitors of the anaphase-promoting complex/cyclosome (APC/C) ubiquitin ligase. These proteins physically interact with APC/C activators, CDC20/FZY and CDH1/FZR, in yeast two-hybrid assays. Overexpression of CDC20.1 and CCS52B/FZR3 differentially promoted ectopic endomitosis in gig1/osd1 and premature occurrence of endoreplication in uvi4. Our data suggest that GIG1/OSD1 and UVI4 may prevent an unscheduled increase in cellular ploidy by preferentially inhibiting APC/C(CDC20) and APC/C(FZR), respectively. Generation of cells with a mixed identity in gig1/osd1 further suggested that the APC/C may have an unexpected role for cell fate determination in addition to its role for proper mitotic progression.