Plant Vacuolar Trafficking Occurs through Distinctly Regulated Pathways

The multifunctional vacuole is the largest organelle in plant cells, and many proteins are transported to and stored in this organelle; thus, the vacuole has great physiological and agronomical importance. However, the molecular mechanism and regulation of plant vacuolar traffic remain largely unknown. In this study, we demonstrate that multiple vacuolar trafficking pathways operate in plants. RAB5 and RAB7 are evolutionarily conserved subfamilies of Rab GTPase [1-3], whose animal and yeast counterparts regulate vacuolar/endosomal trafficking in a sequential manner [4-7]. Functional analyses of a putative activating complex for RAB7 indicated that this complex is responsible for maturation from RAB5- to RAB7-positive endosomes in plant cells. Moreover, these machinery components are recruited to a more complex trafficking network. Mutations in RAB5 and RAB7 conferred counteracting effects on the vti11 mutant. Furthermore, impairment of RAB5- and RAB7-dependent pathways differentially affected the transport of distinctive cargos. These results indicate that plants have developed a complex vacuolar transport system distinct from that of nonplant systems by assigning evolutionarily conserved machinery to unique trafficking pathways. These pathways provide a fundamental basis for plant development at the cellular and higher-ordered levels.