Minisymposium 10: Cell Division

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Abs # M1003: Isolation and characterization of SIAMESE, a plant-specific cell cycle regulator controlling endoreplication onset

Presenter:	Larkin, John C.       Contact Presenter
Authors	Larkin, John C. (A)   Speckhart, Michelle  (A)   Brown, Matthew  (A)   Kato, Naohiro  (A)   Gwin, Taylor  (A)   Kyryk, Viktor  (B)   Huelskamp, Martin  (B)   De Veylder, Lieven  (C)   Inze, Dirk  (C)
Affiliations:	(A): Louisiana State University, Dept. Biological Sciences (B): University of Cologne, Dept. of Botany (C): Ghent University, Dept. of Plant Systems Biology

Cell differentiation is closely coordinated with cell cycle progression. In many cell differentiation pathways, an altered cell cycle occurs along with differentiation. One example, common in both plants and animals, is the amplification of nuclear DNA by endocycles that continue during differentiation. The coordination of these modified cell cycles with cell differentiation remains poorly understood. Arabidopsis trichomes are an endoreplicated cell type that provides an excellent system in which to study the integration of endoreplication and development. Wild-type leaf trichomes are single-celled and have a DNA content of 16C-32C. In contrast, trichomes of siamese (sim) mutants are multicellular and have a reduced DNA content, suggesting that SIM plays a role in switching from the mitosis to the endocycle during trichome development. We have identified the SIM gene, and shown that it encodes a protein of previously unknown function that is a member of a small Arabidopsis gene family of at least four members, with homologs in other plants, but not in mammals or fungi. These SIM-Like proteins share a putative cyclin-binding motif with the KRP cell cycle inhibitors. YFP fusions to SIM and its homologs localize to the nucleus. Fluorescence Resonance Energy Transfer (FRET) experiments indicate that SIM interacts with Cyclin-Dependent Kinase A and with several cyclins. Plants overexpressing SIM are slow-growing and have narrow leaves and enlarged epidermal cells with an increased DNA content resulting from additional endocycles. Taken together, these results suggest that SIM mediates the switch from the mitotic cell cycle to the endocycle during trichome development by directly interacting with the cell cycle machinery.