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Cytokinesis in eukaryotes involves specific arrays of microtubules (MTs), which are known as the \"central spindle\" in animals, the \"anaphase spindle\" in yeasts, and the \"phragmoplast\" in plants. Control of these arrays, which are composed mainly of bundled nonkinetochore MTs, is critically important during cytokinesis. In plants, an MAPK cascade stimulates the turnover of phragmoplast MTs, and a crucial aspect of the activation of this cascade is the interaction between the MAPKKK, nucleus- and phragmoplast-localized protein kinase 1 (NPK1) and the NPK1-activating kinesin-like protein 1 (NACK1), a key regulator of plant cytokinesis. However, little is known about the control of this interaction at the molecular level during progression through the M phase. We demonstrated that cyclin-dependent kinases (CDKs) phosphorylate both NPK1 and NACK1 before metaphase in tobacco cells, thereby inhibiting the interaction between these proteins, suggesting that such phosphorylation prevents the transition to cytokinesis. Failure to inactivate CDKs after metaphase prevents dephosphorylation of these two proteins, causing incomplete mitosis. Experiments with Arabidopsis NACK1 (AtNACK1/HINKEL) revealed that phosphorylated NACK1 fails to mediate cytokinesis. Thus, timely and coordinated phosphorylation by CDKs and dephosphorylation of cytokinetic regulators from prophase to anaphase appear to be critical for the appropriate onset and/or progression of cytokinesis.

Transgenic Arabidopsis thaliana plants overproducing the E2Fa-DPa transcription factor have two distinct cell-specific phenotypes: some cells divide ectopically and others are stimulated to endocycle. The decision of cells to undergo extra mitotic divisions has been postulated to depend on the presence of a mitosis-inducing factor (MIF). Plants possess a unique class of cyclin-dependent kinases (CDKs; B-type) for which no ortholog is found in other kingdoms. The peak of CDKB1;1 activity around the G2-M boundary suggested that it might be part of the MIF. Plants that overexpressed a dominant negative allele of CDKB1;1 underwent enhanced endoreduplication, demonstrating that CDKB1;1 activity was required to inhibit the endocycle. Moreover, when the mutant CDKB1;1 allele was overexpressed in an E2Fa-DPa-overproducing background, it enhanced the endoreduplication phenotype, whereas the extra mitotic cell divisions normally induced by E2Fa-DPa were repressed. Surprisingly, CDKB1;1 transcription was controlled by the E2F pathway, as shown by its upregulation in E2Fa-DPa-overproducing plants and mutational analysis of the E2F binding site in the CDKB1;1 promoter. These findings illustrate a cross talking mechanism between the G1-S and G2-M transition points.

The SUC1/CKS1 proteins interact with cyclin‐dependent kinases (CDKs) and play an essential, but yet not entirely resolved, role in the regulation of the cell cycle. With the Arabidopsis thaliana CKS1At protein as bait in a two‐hybrid screen, two novel Arabidopsis CDKs, Arath;CDKB1;2 and Arath;CDKB2;1, were isolated. A closely related homologue of Arath;CDKB2;1 was discovered in the databases and was nominated Arath;CDKB2;2. Transcript analysis of the five known Arath;CDKA and Arath;CDKB genes revealed that they all had the highest expression in flowers and cell suspensions. Differences in the expression patterns in roots, leaves and stems suggest unique roles for each CDK.

Cyclin-dependent kinases (CDKs) are key regulators of the cell cycle. In yeasts, only one CDK is sufficient to drive cells through the cell cycle, whereas higher eukaryotes developed a family of related CDKs. Curiously, plants contain a unique class of CDKs (B-type CDKs), whose function is still unclear. We show that the CDKB1;1 gene of Arabidopsis (Arabidopsis thaliana) is highly expressed in guard cells and stomatal precursor cells of cotyledons, suggesting a prominent role for B-type CDKs in stomatal development. In accordance, transgenic Arabidopsis plants with reduced B-type CDK activity had a decreased stomatal index because of an early block of meristemoid division and inhibition of satellite meristemoid formation. Many aberrant stomatal cells were observed, all of them blocked in the G2 phase of the cell cycle. Although division of stomatal precursors was inhibited, cells still acquired stomatal identity, illustrating that stomatal cell differentiation is independent of cellular and nuclear division.

The endocycle represents an alternative cell cycle that is activated in various developmental processes, including placental formation, Drosophila oogenesis, and leaf development. In endocycling cells, mitotic cell cycle exit is followed by successive doublings of the DNA content, resulting in polyploidy. The timing of endocycle onset is crucial for correct development, because polyploidization is linked with cessation of cell division and initiation of terminal differentiation. The anaphase-promoting complex/cyclosome (APC/C) activator genes CDH1, FZR, and CCS52 are known to promote endocycle onset in human, Drosophila, and Medicago species cells, respectively; however, the genetic pathways governing development-dependent APC/CCDH¹/FZR/CCS⁵² activity remain unknown. We report that the atypical E2F transcription factor E2Fe/DEL1 controls the expression of the CDH1/FZR orthologous CCS52A2 gene from Arabidopsis thaliana. E2Fe/DEL1 misregulation resulted in untimely CCS52A2 transcription, affecting the timing of endocycle onset. Correspondingly, ectopic CCS52A2 expression drove cells into the endocycle prematurely. Dynamic simulation illustrated that E2Fe/DEL1 accounted for the onset of the endocycle by regulating the temporal expression of CCS52A2 during the cell cycle in a development-dependent manner. Analogously, the atypical mammalian E2F7 protein was associated with the promoter of the APC/C-activating CDH1 gene, indicating that the transcriptional control of APC/C activator genes by atypical E2Fs might be evolutionarily conserved.