A specific exocytic pathway is essential for polarization of PIN proteins in the establishment and maintenance of auxin gradients

The plant hormone auxin and its directional intercellular transport play a major role in diverse aspects of plant growth and development. PIN-FORMED (PIN) proteins determine the directionality of intercellular auxin flow, and the establishment of auxin gradients in plants requires asymmetric distribution of members of the auxin efflux carrier PIN protein family to the plasma membrane. An endocytic pathway regulates the recycling of PIN proteins between the plasma membrane and endosomes, providing a mechanism for dynamic localization. N-ethylmaleimide-sensitive factor adaptor protein receptors (SNAP receptors, SNAREs) mediate fusion between vesicles and target membranes and are classed as Q- or R-SNAREs based on their sequence. To date, very little is know about the function of R-SNAREs. Here we demonstrate that an R-SNARE-dependent exocytic pathway is essential for the insertion of PINs into the plasmamembrane from the Golgi. The Arabidopsis R-SNARE VAMP714 co-localizes with Golgi vesicles and PIN1. We show that loss-of-function mutants, and dominant negative transgenics, exhibit aberrant PIN localization, reduced auxin transport, and altered root gravitropism and morphogenesis. Moreover, gene expression is upregulated by auxin. It is proposed that VAMP714 mediates the exocytic fusion of PIN-carrying vesicles to the plasma membrane, and that this forms part of a positive regulatory loop in which auxin activates a VAMP714-dependent PIN/auxin transport system.