A MAP-SNARE interaction links cortical microtubules to the exocytic fusion machinery during auxin-dependent growth remodeling.

Auxin exerts diverse physiological effects through local maxima and tissue-scale gradients established by directional transport, which depends on the plasma membrane (PM) distribution and vesicular trafficking of auxin transporters. Cortical microtubules are positioned beneath the PM and have been implicated in trafficking events. However, how microtubules interface with terminal vesicle trafficking machinery at the PM remains unclear. Here, using auxin-dependent apical hook opening in Arabidopsis, we identify a molecular bridge linking cortical microtubules to PM-associated SNARE machinery. The microtubule-associated protein WDL4 interacts with the PM Qa-SNARE SYP121 and connects it to cortical microtubules through separate SNARE-binding and microtubule-binding domains. This coupling enhances the interaction between SYP121 and its R-SNARE partner VAMP721. Consistently, PM-localized VAMP721 signals were reduced in wdl4 mutants. Disruption of WDL4 or SYP121 impaired PM accumulation and recycling of the auxin efflux carrier PIN1 and attenuated apical hook opening. Our findings elucidate a mechanism by which a microtubule-associated factor modulates SNARE-mediated auxin transporter trafficking during auxin-dependent growth remodeling.