Meristematic plant cells require balanced auxin and cytokinin concentrations for both the initiation and the maintenance of the cell cycle. Highly synchronized tobacco BY-2 cells, as an ideal model of meristematic root cells, must be supplied with auxin, but are cytokinin-autonomous. Whereas the auxin concentration controls the extent of cell division and differentiation, oscillations in endogenous cytokinin concentration regulate the progression of the cell cycle phase transitions. The complexity and flexibility of cell cycle regulation necessitate sophisticated crosstalk between auxin and cytokinin. The results of the hormone can be observed at various levels: i.e. those of signal generation, signal perception and gene regulation. The project presented here is therefore focused on characterizing the crosstalk between auxin and cytokinin at these physiological levels. Crosstalk at the level of signal generation will be studied by measurements of both endogenous and external auxin and cytokinin concentrations, of the activity of the cytokinin-degrading enzyme CKX and of the distribution of auxin efflux facilitators (PIN ) under modified conditions of auxin and of cytokinin during the course of the cell cycle. Modified cytokinin conditions will be affected by using a transgenic CKX-overexpressing BY-2 line which exhibits a reduced endogenous background of cytokinins and is well suited to evaluate cytokinin-specific effects. The interaction of auxin and cytokinin at the level of signal perception will be analyzed by hormone-responsive promoter studies, whereas the interaction of the two hormones at the level of gene expression will be characterized by microarray analysis.