The D-type alfalfa cyclin gene cycMs4 complements G₁ cyclin-deficient yeast and is induced in the G₁ phase of the cell cycle

Cyclins are key regulators of the cell cycle in all eukaryotes. In alfalfa, we have previously isolated three B-type cyclins. The closely related cycMs1 and cycMs2 genes are expressed primarily during the G₂ and M phases and are most likely mitotic cyclins; expression of the cycMs3 gene is induced in the G₀-to-G₁ transition, when cells reenter the cell cycle. By complementation of G₁ cyclin-deficient yeast cells, a novel alfalfa cyclin, designated cycMs4, was isolated. The predicted amino acid sequence of the cycMs4 gene is most similar to that of the Arabidopsis cyclin δ3 gene. CycMs4 and cyclin δ3 belong to the class of D-type cyclins and contain PEST-rich regions and a retinoblastoma binding motif. When comparing expression levels in different organs, cycMs4 transcripts were present predominantly in roots. Whereas expression of the cycMs4 gene was cell cycle-regulated in suspension-cultured cells, transcription in roots was observed to depend also on the positional context of the cell. When differentiated G₀-arrested leaf cells were induced to resume cell division by treatment with plant hormones, cycMs4 transcription was induced before the onset of DNA synthesis. Whereas this induction was preceded by that of the cycMs3 gene, cycMs2 expression occurred later and at the same time as mitotic activity. These data suggest that cycMs4 plays a role in the G₁-to-S transition and provide a model to investigate the plant cell cycle at the molecular level.