Expression of the plant cyclin-dependent kinase inhibitor ICK1 affects cell division, plant growth and morphology

The plant CDK inhibitor ICK1 was identified previously from Arabidopis thal iana with its inhibitory activity characterized in vitro. ICK1 displayed se veral structural and functional features that are distinct from known anima l CDK inhibitors. Despite the initial characterization, there is no informa tion on the functions of any plant CDK inhibitor in plants. To gain insight into ICK1 functions in vivo and the role of cell division during plant gro wth and development, transgenic plants were generated expressing ICK1 drive n by the cauliflower mosaic virus 35S promoter. In comparison to control pl ants, growth was significantly inhibited in transgenic 35S-ICK1 plants, wit h some plants weighing <10% of wild-type plants at the 3 week stage. Most o rgans of 35S-ICK1 plants were smaller. There were also modifications in pla nt morphology such as shape and serration of leaves and petals. The changes were so drastic that 35S-ICK1 plants with strong phenotype no longer resem bled wild-type plants morphologically. Analyses showed that increased ICK1 expression resulted in reduced CDK activity and reduced the number of cells in these plants. Cells in 35S-ICK1 plants were larger than corresponding c ells in control plants. These results demonstrate that ICK1 acts as a CDK i nhibitor in the plant, and the inhibition of cell division by ICK1 expressi on has profound effects on plant growth and development. They also suggest that alterations of plant organ shape can be achieved by restriction of cel l division.