RAB GTPases are key regulators of membrane traffic. Among them, RAB11, a widely conserved subgroup, has evolved in a unique way in plants; plant RAB11 has notable diversity, whereas yeast and animals possess only a few RAB11 members. In Arabidopsis thaliana, 57 RAB GTPases are encoded in its genome, 26 of which are classified in the RAB11 group (further divided into RABA1-RABA6 subgroups). Although several plant RAB11 members have been shown to play pivotal roles in plant-unique developmental processes, including cytokinesis and tip growth, molecular and physiological functions of the majority of RAB11 members remain unknown. To reveal precise functions of plant RAB11, we investigated subcellular localization and dynamics of the largest subgroup of Arabidopsis RAB11, RABA1, which consists of 9 members. RABA1 members resided on mobile punctate structures beside the trans-Golgi network (TGN) and colocalized with VAMP721/722, R-SNARE proteins operating in the secretory pathway. In addition, the constitutive-active mutant of the RABA1 representative, RABA1bQ72L, was observed on the plasma membrane. The RABA1b compartment showed actin-dependent dynamic motion. Vesicles labeled by GFP-RABA1b moved dynamically forming queues along actin filaments. Interestingly, the Arabidopsis plants whose four major RABA1 members were knocked out and those expressing the dominant-negative mutant ofRABA1B exhibited hypersensitivity to salinity stress. Altogether, these results indicate that RABA1 members mediate transport between the TGN and the plasma membrane and are required for salinity stress tolerance.
© 2012 The Authors. The Plant Journal © 2012 Blackwell Publishing Ltd