| Plants homozygous for recessive alleles of gravitropism defective 2 (grv2) exhibit abnormal embryo development as well as several pleiotropic defects as a seedling and as an adult plant. The grv2-1 embryo is characterized by an enlarged cell at the embryo apex, visible as early as the four cell stage and persisting through the late heart stage. In addition, the pattern of cell divisions is slightly altered, and the overall morphology is more variable than wild type. By the torpedo stage, grv2-1 embryos recover and more closely resemble wild type.; While grv2 seedlings are viable, they do not respond to light and gravity to the same degree as wild type. In addition, the curvature of the apical hook is reduced and, in some individuals, non-existent. Shoots of the adult plant also have a reduced gravitropic response, but surprisingly, have an enhanced phototropic response.; GRV2 encodes a large protein with homology to proteins involved in endocytosis in Caenorhabditis elegans, Drosophila, and mammalian cells. This gene is expressed at low levels in all organs of the plant. At the tissue level, promoter GUS fusions are more highly expressed in the vascular tissue than in other tissue types.; Green fluorescent protein fusions and antibody studies indicate that GRV2 is a peripheral membrane protein localized to small vesicles within the cell. The rate of endocytosis is increased in the grv2-1 mutant compared to wild type. In addition, the internal cellular membranes of the grv2 mutant appear to be different from wild type. Specifically, in wild type the vacuole in the small apical hook cells appears to be one large compartment with several folds, while in the mutant the vacuole appears to be composed of one medium sized vacuole and five or six small vacuoles with reduced folds. In addition, the mutant cells contain aggregates of small vesicles that contain tonoplast proteins, but have not merged with the large vacuole. These data indicate that GRV2 is involved in vesicle trafficking. |
