SIAMESE, a gene controlling the endoreduplication cell cycle in Arabidopsis thaliana trichomes

Cell differentiation is generally tightly coordinated with the cell cycle, typically resulting in a nondividing cell with a unique differentiated morp hology. The unicellular trichomes of Arabidopsis are a well-established mod el for the study of plant cell differentiation. Here, we describe a new gen etic locus, SIAMESE (SIM), required for coordinating cell division and cell differentiation during the development of Arabidopsis trichomes (epidermal hairs). A recessive mutation in the sim locus on chromosome 5 results in c lusters of adjacent trichomes that appeared to be morphologically identical 'twins'. Upon closer inspection, the sim mutant was found to produce multi cellular trichomes in contrast to the unicellular trichomes produced by wil d-type (WT) plants. Mutant trichomes consisting of up to 15 cells have been observed. Scanning electron microscopy of developing sim trichomes suggest s that the cell divisions occur very early in the development of mutant tri chomes. WT trichome nuclei continue to replicate their DNA after mitosis an d cytokinesis have ceased, and as a consequence have a DNA content much gre ater than 2C. This phenomenon is known as endoreduplication. Individual nuc lei of sim trichomes have a reduced level of endoreduplication relative to WT trichome nuclei. Endoreduplication is also reduced in dark-grown sim hyp ocotyls relative to WT, but not in light-grown hypocotyls. Double mutants o f sim with either of two other mutants affecting endoreduplication, triptyc hon (try) and glabra3 (gl3) are consistent with a function for SIM in endor eduplication. SIM may function as a repressor of mitosis in the endoredupli cation cell cycle. Additionally, the relatively normal morphology of multic ellular sim trichomes indicates that trichome morphogenesis can occur relat ively normally even when the trichome precursor cell continues to divide. T he sim mutant phenotype also has implications for the evolution of multicel lular trichomes.