The plant cell division cycle, a highly coordinated process, is continually regulated during the growth and development of plants. In this report, we demonstrate how two cell-cycle regulators act together to control cell proliferation in transgenic Arabidopsis. To identify potential cyclin dependent kinase regulators from Arabidopsis, we employed an two-hybrid screening system to isolate genes encoding G1 specific cyclin-interacting proteins. One of these, p22(ack1), which encodes a novel 22 kDa protein, binds to cyclin D1. Overexpression of p22(ack1) in transgenic Arabidopsis resulted in growth retardation due to a strong inhibition of cell division in the leaf primordial and meristematic tissue. The leaf shape of p22(ack1) transgenic Arabidopsis was altered from oval in wild-type to dentate. Wild-type phenotype was successfully restored in F1 hybrids by cross-hybridizing the p22(ack1) Arabidopsis mutants with cyclin D1. Taken together, these results suggest that p22(ack1) and cyclin D1, which act antagonistically, are major rate-limiting factors for cell division in the leaf meristem. (C) 2004 Elsevier Inc. All rights reserved.