Functional analysis of cyclin-dependent kinase inhibitors of Arabidopsis

Cyclin-dependent kinase inhibitors, such as the mammalian p27(Kip1) protein , regulate correct cell cycle progression and the integration of developmen tal signals with the core cell cycle machinery. These inhibitors have been described in plants, but their function remains unresolved. We have isolate d seven genes from Arabidopsis that encode proteins with distant sequence h omology with p27(Kip1), designated Kip-related proteins (KRPs). The KRPs we re characterized by their domain organization and transcript profiles, With the exception of KRP5, all presented the same cyclin-dependent kinase bind ing specificity. When overproduced, KRP2 dramatically inhibited cell cycle progression in leaf primordia cells without affecting the temporal pattern of cell division and differentiation. Mature transgenic leaves were serrate d and consisted of enlarged cells. Although the ploidy levels in young leav es were unaffected, endoreduplication was suppressed in older leaves. We co nclude that KRP2 exerts a plant growth inhibitory activity by reducing cell proliferation in leaves, but, in contrast to its mammalian counterparts, i t may not control the timing of cell cycle exit and differentiation.