MODULATION OF CYCLIN TRANSCRIPT LEVELS IN CULTURED-CELLS OF ARABIDOPSIS-THALIANA

Previous studies on the cell cycle of Arabidopsis thaliana have been h indered by the lack of synchronous cell culture systems. We have used liquid callus cultures and a cycloheximide-synchronized suspension cul ture of Arabidopsis to investigate changes in cyclin transcript levels in response to exogenous auxin, cytokinin, and nutrients, and during the cell cycle. CYCD1 (delta 1) transcript was virtually undetectable in liquid-cultured callus or suspension-culture cells. CYCD2 (delta 2) transcript levels were largely unaffected by the readdition of phytoh ormones or nitrate to the growth medium, and remained constant through out the cell cycle in suspension-culture cells. CYCD3 (delta 3) transc ript levels were strongly dependent on nitrate, and were induced at th e G1/S transition following phytohormone readdition. In synchronized s uspension-culture cells, CYCD3 transcript accumulated during the S pha se, and remained constant thereafter. These results support the hypoth esis that D cyclins function as part of the cellular machinery that in tegrates diverse signals impinging upon commitment to cell division. I n synchronized cells transcripts of the mitotic cyclins CYC1, CYC2, an d CYC3 reached a maximum with peak mitotic index, but CYC3 transcript levels increased earlier than those of CYC1 or CYC2. The kinetics of a ccumulation of CYC transcript levels support their classification as A -type (CYC3) and B-type (CYC1 and CYC2) cyclins, respectively.