Cell cycle-independent expression of the Arabidopsis cytokinesis-specific syntaxin KNOLLE results in mistargeting to the plasma membrane and is not sufficient for cytokinesis

The Arabidopsis KNOLLE gene encodes a cytokinesis-specific syntaxin that lo calises to the plane of division and mediates cell-plate formation. KNOLLE mRNA and protein expression is tightly regulated during the cell cycle. To explore the significance of this regulation, we expressed KNOLLE protein un der the control of two constitutive promoters, the flower-specific AP3 and the cauliflower mosaic virus 35S promoter. The transgenic plants developed normally, although KNOLLE mRNA and protein accumulated to high levels in no n-proliferating cells and protein was incorporated into membranes. Immunolo calisation studies in transgenic seedling roots revealed mistargeting of KN OLLE protein to the plasma membrane in tip-growing root hairs and in expand ing root cells, whereas no mislocalisation was observed in proliferating ce lls. By comparative in situ hybridisation to embryo sections, the 35S promo ter yielded, relative to the endogenous KNOLLE promoter, low levels of KNOL LE mRNA accumulation in proliferating cells that were insufficient to rescu e cytokinesis-defective knolle mutant embryos. Our results suggest that in wild type, strong expression of KNOLLE protein during M phase is necessary to ensure efficient vesicle fusion during cytokinesis.