RLCKs phosphorylate RopGEFs to control auxin-dependent Arabidopsis development.

Auxin plays a critical role throughout plant development. We have established earlier that auxin activates the ROP (plant RHO GTPase) molecular switch, and that guanine nucleotide exchange factors ROPGEFs, which activate ROPs, are important regulators for myriad auxin-regulated processes. Here we show that auxin induces phosphorylation of RopGEF1 and that four receptor-like cytoplasmic kinases (RLCKs), named RopGEF1-activating kinases 1/2/3/4 (RAK1/2/3/4), mediate this process. We show that RAKs interact with RopGEFs and specifically phosphorylate S488 on RopGEF1, promoting its stability and recruitment to the cell membrane, and enhance ROP activation. Knockout of these RLCKs blocked auxin-stimulated RopGEF1 phosphorylation, reduced ROP activation and led to defects in PIN-FORMED (PIN)-mediated auxin distribution and multiple developmental processes. Phospho-mimic RopGEF1 (S488D) shows enhanced guanine nucleotide exchange activity in vitro, and its expression in rak quadruple mutants reverses their phenotypes. The RLCK-RopGEF linkage represents an important functional node and elucidates a critical missing link in ROP-meditated auxin signaling.