PART OF A SPECIAL ISSUE ON THE PLANT CELL CYCLE Analyses of phylogeny, evolution, conserved sequences and genome-wide expression of the ICK/KRP family of plant CDK inhibitors

†Background and Aims The cell cycle is controlled by cyclin-dependent kinases (CDKs), and CDK inhibitors are major regulators of their activities. The ICK/KRP family of CDK inhibitors has been reported in several plants, with seven members in arabidopsis; however, the phylogenetic relationship among members in different species is unknown. Also, there is a need to understand how these genes and proteins are regulated. Furthermore, little information is available on the functional differences among ICK/KRP family members. †Methods We searched publicly available databases and identified over 120 unique ICK/KRP protein sequences from more than 60 plant species. Phylogenetic analysis was performed using 101 full-length sequences from 40 species and intron–exon organization of ICK/KRP genes in model species. Conserved sequences and motifs were analysed using ICK/KRP protein sequences from arabidopsis (Arabidopsis thaliana), rice (Orysa sativa) and poplar (Populus trichocarpa). In addition, gene expression was examined using microarray data from arabidopsis, rice and poplar, and further analysed by RT-PCR for arabidopsis. †Key Results and Conclusions Phylogenetic analysis showed that plant ICK/KRP proteins can be grouped into three major classes. Whereas the C-class contains sequences from dicotyledons, monocotyledons and gymnos-perms, the A- and B-classes contain only sequences from dicotyledons or monocotyledons, respectively, suggesting that the A- and B-classes might have evolved from the C-class. This classification is also supporte