The continuous mode of plant growth and the relative immobility of plant cells due to their semi-rigid walls have long been assumed to require a close integration of plant growth and development with the control of the mitotic cell cycle. The primary control of the cell cycle is the decision in G1 phase to move to S phase with the initiation of DNA replication, a step controlled in plants as in all higher eukaryotes by the retinoblastoma-related (RBR protein), itself regulated by cyclin-CDK phosphorylation. Also conserved in all higher eukaryotes is the involvement of D-type cyclins (CYCD) in the phosphorylation of RBR and the subsequent progression into the cell cycle. This phosphorylation is directed by a specific motif present in D-type cyclins and absent from other cyclin classes. Plants however are distinct from other higher eukaryotes in having multiple sub-groups of CYCD, with six distinct sub-groups conserved across all higher plants, as compared to three cyclin D genes in mammals. Regulation of some CYCD genes by hormones, particularly cytokinin, has been known for some time, but the functional links between CYCDs and patterning processes have been less clear. Using inducible expression in a mutant background, we have found that in Arabidopsis the CYCD3 subclass appear to have a role in the patterning of growth, promoting shoot formation over roots in callus, and influencing the localization of PIN proteins involved in auxin transport. This suggests a mechanism for the morphogenetic influence of CYCD gene activity, and emphasizes the close links between the cell cycle in general and CYCD genes in particular with hte processes of development in plants.