Abstract: Cellular asymmetry, which represents a fundamental characteristic of cell polarity, is prominently illustrated by the apical-basal localization of PIN-FORMED (PIN) auxin efflux carriers in Arabidopsis thaliana. Although the maintenance of PIN polarity at the plasma membrane (PM) relies on endomembrane trafficking, the pivotal factors responsible for recruiting PIN proteins to the PM remain largely unknown. In this study, we discover that EXO70G1 displays a polarized distribution at the PM in root cells. Acting as a putative subunit of the exocyst complex, which mediates the tethering of exocytic vesicles to the PM, EXO70G1 exhibits continuous recycling foci at the PM, and its dynamic behavior is akin to that of SEC6 and SEC8. Disruption of EXO70G1 and its homolog EXO70G2 in Arabidopsis reduces auxin accumulation and primary root length. Importantly, the recycling of PIN2 from the brefeldin A (BFA) compartment to the PM is compromised, and the abundance of PIN2 at the PM is reduced in the exo70G1 exo70G2 backgrounds. Interestingly, live-cell imaging reveals that the polarity of EXO70G1 is established during cytokinesis, prior to that of PIN2, and is maintained throughout the subsequent phases of cell elongation and differentiation. When the lipid raft was disturbed, the accumulation of EXO70G1 at the PM decreased. Our findings highlight the crucial role of EXO70G1 in root development by providing positional cues that facilitate the recycling efficiency of PIN2 to the PM.

Key words: endomembrane trafficking, EXO70, PIN, polar localization, root development