The GRV2/RME‐8 protein of Arabidopsis functions in the late endocytic pathway and is required for vacuolar membrane flow

Summary The gravitropism defective 2 ( grv2 ) mutants of Arabidopsis thaliana were previously characterized as exhibiting shoot agravitropism resulting from mutations in a homolog of the Caenorhabditis elegans RECEPTOR‐MEDIATED ENDOCYTOSIS‐8 ( RME‐8 ) gene, which is required in C. elegans for endocytosis. A fluorescent protein fusion to the GRV2 protein localized to endosomes in transgenic plants, and vacuolar morphology was altered in grv2 mutants. A defect in vacuolar membrane dynamics provides a mechanistic explanation for the gravitropic defect, and may also account for the presence of an enlarged vacuole in early embryos, together with a nutrient requirement during seedling establishment. The GRV2‐positive endosomes were sensitive to Wortmannin but not brefeldin A (BFA), consistent with GRV2 operating late in the endocytic pathway, prior to delivery of vesicles to the central vacuole. The specific enlargement of GRV2:YFP structures by Wortmannin, together with biochemical data showing that GRV2 co‐fractionates with pre‐vacuolar markers such as PEP12/SYP21, leads us to conclude that in plants GRV2/RME‐8 functions in vesicle trafficking from the multivesicular body/pre‐vacuolar compartment to the lytic vacuole.