Characterization of IBR5-ROP GTPase (ROP2/ROP6) Interactions in Plant Auxin Response
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The quintessential phytohormone auxin regulates many aspects of growth and development throughout the plant life cycle. Diverse auxin responses occur via multiple distinct and overlapping signaling pathways. It is well documented that auxin exerts control over gene expression by binding its nuclear co-receptors TIR1/AFB family F-box proteins and AUX/IAA transcriptional repressor proteins, thereby promoting polyubiquitination and subsequent degradation of AUX/IAAs and relieving transcriptional repression of auxin-responsive genes. More recently, auxin has also been shown to rapidly activate Rho of plant (ROP) GTPases at the plasma membrane, leading to a variety of cellular responses. The auxin signaling mutant ibr5-1 exhibits reduced auxin-responsive gene expression without acFcumulation of AUX/IAA repressor proteins, suggesting the dual-specificity protein phosphatase encoded by the gene IBR5 independently regulates the processes of AUX/IAA degradation and auxin-induced gene expression. In a previous screen for IBR5 interactors, a small GTPase was identified, prompting the question of whether IBR5 interacts with the ROP GTPases ROP2 and ROP6, which have been shown to be involved in auxin signaling pathways in the cytoplasm. In vitro interaction assays indicated IBR5 interacts with ROP2 and ROP6, and these interactions were confirmed by coimmunoprecipitation in Arabidopsis thaliana. To assess genetic interaction, an ibr5-1 rop6-2 double null mutant was generated. In root growth assays for auxin inhibition of primary root elongation or induction of lateral root formation, the double mutant exhibited auxin resistance similar to the ibr5-1 parent line. Taken together, the results suggest the dual-specificity phosphatase IBR5 interacts with the Rho-like GTPases ROP2 and ROP6, and these proteins may function in a common auxin signaling pathway.