Involved in the regulation of gravitropic response and basipetal auxin transport in roots. Involved in salt stress tolerance. May facilitate membrane trafficking and asymmetric cell elongation via SYT1. Binds stigmasterol and dipalmitoyl phosphoethanolamine (DPPE) in vitro. Interacts with SYT1. Expressed exclusively in roots, in epidermis and cortex cells of the root elongation zone, and lateral root cap cells at the root tip. Induced by gravity stimulation. No visible phenotype under normal growth conditions, but mutant plants exhibit decreased root basipetal auxin transport, faster root gravitropic response and increase in salt stress tolerance.