VPS45 and SYP4 Qa-SNARE proteins jointly regulate auxin distribution and plant development in Arabidopsis

The plant hormone auxin is directionally transported in plant tissues by the actions of the auxin transport proteins. PIN-FORMED (PIN) family proteins asymmetrically localize at the plasma membrane, and facilitate auxin efflux, thereby promoting cell-to-cell polar auxin transport. In previous studies, we have identified the trans-Golgi network-localized ARF GEF BEN1 (BIG5) and the Sec1/Munc18 protein BEN2 (VPS45) as membrane-trafficking components involved in PIN protein localization and auxin-dependent development. Here, we address the role of VPS45-interacting SYP4 Qa-SNARE proteins and the genetic relationship between VPS45 and SYP4 proteins. We show that ben2 and syp4 mutations affect the subcellular localization of SYP4 proteins and VPS45, respectively. Genetic interference of SYP42 and SYP43 genes results in PIN-trafficking defects and developmental alterations, such as decreased root meristem size and cotyledon venation defects, which are qualitatively reminiscent of those of ben2 and/or ben1; ben2 mutants. In addition, the ben2 mutation, which is assumed to compromise VPS45 activity, strongly enhances the developmental defects of the syp4 mutants. These results suggest that VPS45 and SYP4 Qa-SNARE proteins jointly play an important role in auxin-dependent plant development.