XVI International Botanical Congess

Plant homologs of D-type cyclins (CycD) respond to extracellular signals and are proposed to control G1 exit. In support of this view, transgenic Arabidopsis expressing high levels of the cytokinin-responsive gene CycD3 are able to form callus on media containing only auxin. CycD3 expression can therefore replace the requirement for added cytokinin for callus induction and growth. CycD2 in contrast is not regulated by phytohormones, and when this gene is overexpressed in tobacco, the rate of cell division in meristems is increased, the G1 phase of the cell cycle is shortened, and plant growth rate is accelerated. We therefore propose CycD2 is rate-limiting for growth. Finally, we have shown that the in vivo kinase partner of CycD2 and CycD3 is the Cdc2a protein.