Distinct Cyclin D Genes Show Mitotic Accumulation or Constant Levels of Transcripts in Tobacco Bright Yellow-2 Cells

The commitment of eukaryotic cells to division normally occurs during the G1 phase of the cell cycle. In mammals D-type cyclins regulate the progression of cells through G1 and therefore are important for both proliferative and developmental controls. Plant CycDs (D-type cyclin homologs) have been identified, but their precise function during the plant cell cycle is unknown. We have isolated three tobacco ( Nicotiana tabacum ) CycD cyclin cDNAs: two belong to the CycD3 class (Nicta; CycD3 ;1 and Nicta; CycD3 ;2) and the third to the CycD2 class (Nicta; CycD2 ;1). To uncouple their cell-cycle regulation from developmental control, we have used the highly synchronizable tobacco cultivar Bright Yellow-2 in a cell-suspension culture to characterize changes in CycD transcript levels during the cell cycle. In cells re-entering the cell cycle from stationary phase, CycD3 ;2 was induced in G1 but subsequently remained at a constant level in synchronous cells. This expression pattern is consistent with a role for CycD3 ;2, similar to mammalian D-type cyclins. In contrast, CycD2 ;1 and CycD3 ;1 transcripts accumulated during mitosis in synchronous cells, a pattern of expression not normally associated with D-type cyclins. This could suggest a novel role for plant D-type cyclins during mitosis.