The cell division cycle in plants as in other eukaryotes is controlled by the cyclin-dependent kinase (CDK). This CDK paradigm determines that developmental cues and environmental signals need to impinge on the CDK complex to affect the cell cycle. An important part of understanding cell cycle regulation is to understand how CDK is regulated by various factors. In addition, there are features that set the cell cycle regulation in plants apart from that in other eukaryotes such as animals. Our knowledge of the molecular mechanisms that underlie the differences is poor. A family of plant CDK inhibitor proteins has been identified. The plant CDK inhibitors share similarity with a family of animal CDK inhibitors in a small region, while most of the sequence and the structural layout of the plant CDK inhibitors are different from the animal counterparts. Studies of plant CDK inhibitors have been performed mostly with the CDK inhibitors from Arabidopsis called ICKs (also referred to as KRPs). ICKs interact with D-type cyclins and A-type CDK. Overexpression of ICKs has been shown to affect cell division, plant growth, and morphogenesis. Studies of ICKs have also provided insightful information on the control of endoreduplication in plants. These aspects as well as cellular localization and protein regulation of ICKs are reviewed.
