Isolation and Characterization of the SIAMESE Family: A Novel Class of Cell Cycle Regulators in Plants

• Main Author: Churchman, Michelle Lynn
• Other Authors: John C. Larkin
• Format: Others
• Language: en
• Published: LSU 2007
• Subjects: Biochemistry (Biological Sciences)
• Online Access: http://etd.lsu.edu/docs/available/etd-06052007-093854/

Recessive mutations in the SIAMESE (SIM) gene of Arabidopsis 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 approximately 16-32C. These observations suggested that SIM is required to suppress mitosis as part of the switch to endoreplication in trichomes. We demonstrate that SIM encodes a novel 14kD protein that is part of a small Arabidopsis gene family comprised of four members. Homologs exist in other dicots and in monocots, although no obvious animal homologs have been identified. SIM and the SIAMESE-RELATED (SMR) proteins contain two putative cyclin-binding motifs: one found in the Kip-related (KRP) class of plant CDK inhibitors, and a "Cy" motif found in CDK inhibitors, E2F and Retinoblastoma in animals. Accordingly, SIM was found to associate with D-type cyclins as well as CDKA;1 in vivo. Although no interactions were detected between SIM and mitotic B-type cyclins, CYCB1;1 is ectopically expressed in sim mutant trichomes. These findings suggest a role for the SIM family in a pathway controlling the G2/M transition via interaction with specific CYCD/CDKA;1 complexes that are integral in regulation of B-type cyclin expression. 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. Overexpression of SIM, as well as the SMRs, in Arabidopsis results in slow-growing, severely dwarfed plants with greatly enlarged epidermal cells. Nuclei of the enlarged cells have drastically increased DNA contents resulting from additional endocycles in the absence of mitosis. Both SMR1 and SMR2 overexpressing plants fail to produce an inflorescence before senescence. Preliminary evidence implicates the SMRs in the conserved TERMINAL FLOWER1/FLOWERING LOCUS T/SELF-PRUNING signaling system that regulates maintenance of the inflorescence apical meristem. We hypothesize that the SIM family encodes a novel class of plant-specific CDK inhibitors with a key function in inhibiting mitosis.