Tethering of cellulose synthase complex to the plasma membrane relies on the isoform of EXO70A1 in Arabidopsis

In yeast and mammals, the EXO70 subunit of the exocyst complex plays a key role in mediating the tethering of exocytic vesicles to the plasma membrane (PM). In plants, however, the role of EXO70 in regulating vesicle tethering during exocytosis remains unclear. In land plants, EXO70 has undergone significant evolutionary expansion, resulting in multiple EXO70 paralogues that may allow the exocyst to form various isoforms with specific functions. Previous research in Arabidopsis has shown that generally disrupting exocyst function leads to various defects in cellulose synthase (CESA) complex (CSC) trafficking. In this study, we utilized real-time imaging combined with genetic approaches to explore the role of EXO70A1, a member of the EXO70 family in Arabidopsis, in CSC trafficking. The exo70a1 mutant exhibited a decrease in crystalline cellulose content and a reduced density of functional CSCs in the PM. Moreover, the delivery of tdTomato-CESA6 from the cortex to the PM was compromised in the mutant, leading to the accumulation of CSC vesicles at the cell cortex. However, the velocity of tdTomato-CESA6 in the PM was unaffected in exo70a1. These findings suggest that EXO70A1 has a specific role in tethering CSCs to the PM.