RAC GTPases in tobacco and Arabidopsis mediate auxin-induced formation of proteolytically active nuclear protein bodies that contain AUX/IAA proteins

Auxin signaling relies on ubiquitin ligase SCFTIR1- mediated 26S proteasome- dependent proteolysis of a large family of short- lived transcription regulators, auxin/ indole acetic acid ( Aux/ IAA), resulting in the derepression of auxin- responsive genes. We have shown previously that a subset of Rac GTPases is activated by auxin, and they in turn stimulate auxin-responsive gene expression. We show here that increasing Rac signaling activity promotes Aux/ IAA degradation, whereas downregulating that activity results in the reduction of auxin- accelerated Aux/ IAA proteolysis. Observations reported here reveal a novel function for these Rac GTPases as regulators for ubiquitin/ 26S proteasome- mediated proteolysis and further consolidate their role in auxin signaling. Moreover, our study reveals a cellular process whereby auxin induces and Rac GTPases mediate the recruitment of nucleoplasmic Aux/ IAAs into proteolytically active nuclear protein bodies, into which components of the SCFTIR1, COP9 signalosome, and 26S proteasome are also recruited.