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SIAMESE, a Plant-Specific Cell Cycle Regulator, Controls Endoreplication Onset in Arabidopsis thaliana^[W]

^a Department of Biological Sciences, Louisiana State University, Baton Rouge, Louisiana 70803
^b University of Köln, Botanical Institute III, 50931 Köln, Germany
^c Department of Plant Systems Biology, Flanders Interuniversitaru Instititut for Biotechnology, Ghent University, B-9000 Ghent, Belgium
^d Department of Botany, Genetics Institute and Plant Molecular and Cell Biology Program, University of Florida, Gainesville, Florida 32611

⁴ To whom correspondence should be addressed. E-mail jlarkin{at}lsu.edu; fax 225-578-2597.

Recessive mutations in the SIAMESE (SIM) gene of Arabidopsis thaliana result in multicellular trichomes harboring individual nuclei with a low ploidy level, a phenotype strikingly different from that of wild-type trichomes, which are single cells with a nuclear DNA content of 16C to 32C. These observations suggested that SIM is required to suppress mitosis as part of the switch to endoreplication in trichomes. Here, we demonstrate that SIM encodes a nuclear-localized 14-kD protein containing a cyclin binding motif and a motif found in ICK/KRP (for Interactors of Cdc2 kinase/Kip-related protein) cell cycle inhibitor proteins. Accordingly, SIM was found to associate with D-type cyclins and CDKA;1. Homologs of SIM were detected in other dicots and in monocots but not in mammals or fungi. SIM proteins are expressed throughout the shoot apical meristem, in leaf primordia, and in the elongation zone of the root and are localized to the nucleus. Plants overexpressing SIM are slow-growing and have narrow leaves and enlarged epidermal cells with an increased DNA content resulting from additional endocycles. We hypothesize that SIM encodes a plant-specific CDK inhibitor with a key function in the mitosis-to-endoreplication transition.

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