Plant development requires stringent controls between cell proliferation and cell differentiation. Proliferation is positively regulated by cyclin dependent kinases (CDKs). Acting in opposition to CDKs are CDK inhibitors (CKIs). The first tobacco CKI (NtKIS1a) identified was shown to inhibit in vitro the kinase activity of CDK/cyclin complexes and to interact with CDK and D-cyclins. However, these features, which are common to other plant and animal CKIs already characterised, did not provide information about the function of NtKIS1a in plants. Thus, to gain insight into the role of NtKIS1a and especially its involvement in cell proliferation during plant development, we generated transgenic Arabidopsis thaliana plants that overexpress NtKIS1a. These plants showed reduced growth with smaller organs that contained larger cells. Moreover, these plants displayed modifications in plant morphology. These results demonstrated that plant organ size and shape, as well as organ cell number and cell size, might be controlled by modulation of the single NtKIS1a gene activity. Since in mammals, D-cyclins control cell cycle progression in a CDK-dependent manner but also play a CDK independent role by sequestering the CKIs p27(Kip1) and p21(Cip1), we tested the significance of cyclin D-CKI interaction within a living plant. With this aim, NtKIS1a and AtCycD3;1 were overexpressed simultaneously in plants by two different methods. Our results demonstrated that overexpression of the CKI NtKIS1a restores essentially normal development in plants overexpressing AtCycD3;1, providing the first evidence of cyclin D-CKI co-operation within the context of a living plant.