Involved in the control of the cell cycle at the G1/S (start) transition. Activates the G1/S phase transition in response to cytokinin hormone signal, but declines in response to sucrose starvation leading to G1 arrest. Involved in the induction of mitotic cell division. Plays an important role in the switch from cell proliferation to the final stages of differentiation during plant development. May not be involved in the activation of cell cycle in the root apical meristem (RAM) in the early phase of seed germination. Promotes divisions in the guard cells (GCs) after the guard mother cells (GMC) symmetric division (PubMed:24687979). Interacts with the C-terminal domain of CDKA-1. Interacts with KRP1/ICK1. Interacts with KRP6 (PubMed:23617622). Highly expressed in roots and at lower levels in leaves and flowers. Expressed in vegetative shoot meristem and inflorescence. Expressed 2 hours before the S phase and remains constant therafter. By cytokinin. Induction by cytokinin is blocked by auxin, but not by cycloheximide. Induced by sucrose and glucose. Induced by 24-epi-brassinolide. Down-regulated by sucrose starvation. Phosphorylated. Plants overexpressing CYCD3-1 show extensive leaf curling, disorganized meristems, increased leaf number, late flowering and delayed senescence. CYCD3-1 is a highly unstable protein whose proteolysis is mediated by a proteasome-dependent pathway, and whose levels are highly dependent on the rate of protein synthesis. Belongs to the cyclin family. Cyclin D subfamily.