Search

The coordination of cell division with stress response is essential for maintaining genome stability in plant meristems. Proteins involved in pre-mRNA splicing are important for these processes in animal and human cells. Based on its homology to the splicing factor SART1, which is implicated in the control of cell division and genome stability in human cells, we analyzed if MDF has similar functions in plants. We found that MDF associates with U4/U6.U5 tri-snRNP proteins and is essential for correct splicing of 2,037 transcripts. Loss of MDF function leads to cell division defects and cell death in meristems and was associated with up-regulation of stress-induced genes and down-regulation of mitotic regulators. In addition, the mdf-1 mutant is hypersensitive to DNA damage treatment supporting its role in coordinating stress response with cell division. Our analysis of a dephosphomutant of MDF suggested how its protein activity might be controlled. Our work uncovers the conserved function of a plant splicing factor and provides novel insight into the interplay of pre-mRNA processing and genome stability in plants.

The RETINOBLASTOMA–RELATED (RBR) is a key regulator of cell proliferation and differentiation in plants, and plays an important role in maintenance of the stem cell niche in the root., The RETINOBLASTOMA–RELATED (RBR) is a key regulator of cell proliferation and differentiation in plants, and plays an important role in maintenance of the stem cell niche in the root. We used microarray analysis to characterize the transcriptional response of Arabidopsis thaliana root tips from rRBr mutant (7 samples) against Col-0 wild type (6 samples) after 4, 6 and 10 das.

Mitotic progression is timely regulated by the accumulation and degradation of A- and B-type cyclins. In plants, there are three classes of A-, and two classes of B-type cyclins, but their specific roles are not known. We have generated transgenic tobacco plants in which the ectopic expression of a plant cyclin B2 gene is under the control of a tetracycline-inducible promoter. We show that the induction of cyclin B2 expression in cultured cells during G2 phase accelerates the entry into mitosis and allows cells to override the replication checkpoint induced by hydroxyurea in the simultaneous presence of caffeine or okadaic acid, drugs that are known to alleviate checkpoint control. These results indicate that in plants, a B2-type cyclin is a rate-limiting regulator for the entry into mitosis and a cyclin B2-CDK complex might be a target for checkpoint control pathways. The cyclin B2 localization and the timing of its degradation during mitosis corroborate these conclusions: cyclin B2 protein is confined to the nucleus and during mitosis it is only present during a short time window until mid prophase, but it is effectively degraded from this timepoint onwards. Although cyclin B2 is not present in cells arrested by the spindle checkpoint in metaphase, cyclin B1 is accumulating in these cells. Ectopic expression of cyclin B2 in developing plants interferes with differentiation events and specifically blocks root regeneration, indicating the importance of control mechanisms at the G2- to M-phase transition during plant developmental processes.

DNA topoisomerase II is required for mitotic chromosome condensation and segregation. Here we characterize the effects of inhibiting DNA topoisomerase II activity in plant cells using the non-DNA damaging topoisomerase II inhibitor ICRF-193. We report that ICRF-193 abrogated chromosome condensation in cultured alfalfa (Medicago sativa L.) and tobacco (Nicotiana tabaccum L.) mitoses and led to bridged chromosomes at anaphase. Moreover, ICRF-193 treatment delayed entry into mitosis, increasing the frequency of cells having a pre-prophase band of microtubules, a marker of late G2 and prophase, and delaying the activation of cyclin-dependent kinase. These data suggest the existence of a late G2 checkpoint in plant cells that is activated in the absence of topoisomerase II activity. To determine whether the checkpoint-induced delay was a result of reduced cyclindependent kinase activity, mitotic cyclin B2 was ectopically expressed. Cyclin B2 bypassed the ICRF-193-induced delay before mitosis, and correspondingly, reduced the frequency of interphase cells with a pre-prophase band. These data provide evidence that plant cells possess a topoisomerase II-dependent G2 cell cycle checkpoint that transiently inhibits mitotic CDK activation and entry into mitosis, and that is overridden by raising the level of CDK activity through the ectopic expression of a plant mitotic cyclin.