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 shortlived 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.