Arabidopsis SYP121 acts as a ROP2 effector in the regulation of root hair tip growth.

Tip growth is an extreme form of polarized cell expansion occurring in all eukaryotic kingdoms to generate highly elongated tubular cells with specialized functions, including fungal hyphae, animal neurons, plant pollen tubes, and root hairs (RHs). RHs are tubular structures protruding from the root epidermis to facilitate water and nutrient uptake, microbial interactions, and plant anchorage. RH tip growth requires polarized vesicle-targeting and active exocytosis at apical growth sites. However, how apical exocytosis is spatially and temporally controlled during tip growth remains elusive. Here, we report that the Qa-Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) SYP121 acts as an effector of Rho of Plants 2 (ROP2), mediating regulation of RH tip growth. We show that active ROP2 promotes SYP121 targeting to the apical plasma membrane. Moreover, ROP2 directly interacts with SYP121 and promotes the interaction between SYP121 and the R-SNARE VAMP722 to form a SNARE complex, likely by counteracting with the Sec1/Munc18 protein SEC11 which suppresses SYP121's function. Thus, the ROP2-SYP121 pathway facilitates exocytic trafficking during RH tip growth. This study uncovers a direct link between a ROP GTPase and vesicular trafficking and a new mechanism controlling apical exocytosis, whereby ROP GTPase signaling spatially regulates SNARE complex assembly and the polar distribution of a Q-SNARE.