The Exocyst Complex Contributes to PIN Auxin Efflux Carrier Recycling and Polar Auxin Transport in Arabidopsis

In land plants polar auxin transport is one of the substantial processes guiding whole plant polarity and morphogenesis. Directional auxin fluxes are mediated by PIN auxin efflux carriers, polarly localized at the plasma membrane. The polarization of exocytosis in yeast and animals is assisted by the exocyst: an octameric vesicle-tethering complex and an effector of Rab and Rho GTPases. Here we show that rootward polar auxin transport is compromised in roots of Arabidopsis thaliana loss-of-function mutants in the EXO70A1 exocyst subunit. The recycling of PIN1 and PIN2 proteins from brefeldin-A compartments is delayed after the brefeldin-A washout in exo70A1 and sec8 exocyst mutants. Relocalization of PIN1 and PIN2 proteins after prolonged brefeldin-A treatment is largely impaired in these mutants. At the same time, however, plasma membrane localization of GFP:EXO70A1, and the other exocyst subunits studied (GFP:SEC8 and YFP:SEC10), is resistant to brefeldin-A treatment. In root cells of the exo70A1 mutant, a portion of PIN2 is internalized and retained in specific, abnormally enlarged, endomembrane compartments that are distinct from VHA-a1-labelled early endosomes or the trans-Golgi network, but are RAB-A5d positive. We conclude that the exocyst is involved in PIN1 and PIN2 recycling, and thus in polar auxin transport regulation.