Root knot and cyst nematodes induce large multinucleated cells, designated giant cells and syncytia, respectively, in plant roots. We have used molecular markers to study cell cycle progression in these specialized feeding cells. In situ hybridization with two cyclin-dependent kinases and two cyclins showed that these genes were induced very early in galls and syncytia and that the feeding cells progressed through the G2 phase. By using cell cycle blockers, DNA synthesis and progression through the G2 phase, or mitosis, were shown to be essential for gall and syncytium establishment. When mitosis was blocked, further gall development was arrested. This result demonstrates that cycles of endoreduplication or other methods of DNA amplification are insufficient to drive giant cell expansion. On the other hand, syncytium development was much less affected by a mitotic block; however, syncytium expansion was inhibited.

Hyperosmotic stress severely affects plant growth and development. To examine the effect of salt stress on cell cycle activity in Arabidopsis thaliana (L.) Heynh., the transcriptional regulation of a cyclin-dependent kinase, CDC2aAt, and two mitotic cyclins, Arath;CycB1;1 and Arath;CycA2;1, was studied by using the beta-glucuronidase (gus) reporter gene. Moreover, the mRNA abundance of these cell cycle genes as well as CDC2bAt were monitored during salt stress. Upon NaCl treatment, the promoter activities and transcript levels of all cell cycle genes diminished initially in the shoot apex and were subsequently induced during salt-stress adaptation. Additionally, the promoter activities of CDC2aAt and CycA2;1 decreased in the vascular cylinder of the root in correlation with reduced lateral root formation. In the root tips, a regression of CDC2aAt, CycA2;1, and CycB1;1:gus expression was observed, concomitant with a shrinkage of the root meristem and inhibition of root growth. Our data indicate that salt stress interferes with cell cycle regulation at the transcriptional level, resulting in an adaptive growth response.

Cyclins are cell cycle regulators whose proteins oscillate dramatically during the cell cycle. Cyclin steady-state mRNA levels also fluctuate, and there are indications that both their rate of transcription and mRNA stability are under cell cycle control. Here, we demonstrate the transcriptional regulation of higher eukaryote cyclins throughout the whole cell cycle with a high temporal resolution. The promoters of two Arabidopsis cyclins, cyc3aAt and cyclAt, mediated transcriptional oscillation of the 8-glucuronidase (gus) reporter gene in stably transformed tobacco BY-2 cell lines. The rate of transcription driven by the cyc3aAt promoter was very low during G1, slowly increased during the S phase, peaked at the G2 phase and G2-to-M transition, and was down-regulated before early metaphase. In contrast, the rate of the cyclAt-related transcription increased upon exit of the S phase, peaked at the G2-to-M transition and during mitosis, and decreased upon exit from the M phase. This study indicates that transcription mechanisms that seem to be conserved among species play a significant role in regulating the mRNA abundance of the plant cyclins. Furthermore, the transcription patterns of cyc3aAt and cyclAt were coherent with their slightly higher sequence similarity to the A and B groups of animal cyclins, respectively, suggesting that they may fulfill comparable roles during the cell cycle.Cyclins are activators of specific serine/threonine protein kinases, termed CDKs, which drive progression of the eukaryotic cell cycle (reviewed in ref. 1). Based upon sequence analyses, most plant cyclins identified so far can be divided into those showing slightly higher sequence similarity to either the A or B groups of animal cyclins, but these similarities are not sufficient to exclusively assign them to either group (2, 3). Animal A-and B-type cyclins have distinct patterns of expression and fulfill different roles throughout the cell cycle (reviewed in ref. 4). As the roles of cyclins are far more understood in animals than in plants, further affiliation of a plant cyclin to a certain group of animal cyclins based on a similar expression pattern may give a clue to its function. The Arabidopsis cyclAt and cyc3aAt cyclin genes represent plant cyclins with slightly higher homology to the B and A groups of animal cyclins, respectively (2). Whole-mount in situ hybridization of Arabidopsis root tips treated with cell cycle blockers indicated that steady-state mRNA levels of cyclAt are high at early metaphase and low at early S phase, while the opposite was true for cyc3aAt (2). These data indicated that mRNA levels of cyc3aAt are increased in advance to that of cyclAt, but were not sufficient to give a complete picture of the expression pattern and transcriptional regulation of these cyclins throughout the whole cell cycle.Fluctuation in the steady-state mRNA level of a cell cycle gene may be regulated by a change in the rate of transcription, or in mRNA stability, or both. Transcriptional regulation of cell cycle genes...