The unconventional prefoldin RPB5 interactor mediates the gravitropic response by modulating cytoskeleton organization and auxin transport in Arabidopsis

Gravity-induced root curvature involves the asymmetric distribution of the phytohormone auxin. This response depends on the concerted activities of the auxin transporters such as PIN-FORMED (PIN) proteins for auxin efflux and AUXIN RESISTANT 1 (AUX1) for auxin influx. However, how the auxin gradient is established remains elusive. Here we identified a new mutant with a short root, strong auxin distribution in the lateral root cap and an impaired gravitropic response. The causal gene encoded an Arabidopsis homolog of the human unconventional prefoldin RPB5 interactor (URI). AtURI interacted with prefoldin 2 (PFD2) and PFD6, two beta-type PFD members that modulate actin and tubulin patterning in roots. The auxin reporter DR5(rev):GFP showed that asymmetric auxin redistribution after gravistimulation is disordered in aturi-1 root tips. Treatment with the endomembrane protein trafficking inhibitor brefeldin A indicated that recycling of the auxin transporter PIN2 is disrupted in aturi-1 roots as well as in pfd mutants. We propose that AtURI cooperates with PFDs to recycle PIN2 and modulate auxin distribution.

Automated summary

Gravity-induced root curvature relies on the asymmetric distribution of auxin, which is regulated by the concerted activities of PIN and AUX1 proteins. However, the mechanism underlying the establishment of auxin gradient remains unclear. In this study, a mutant with abnormal root growth and impaired gravitropic response was identified. The gene responsible for this phenotype encodes AtURI, a protein involved in endomembrane trafficking and auxin distribution regulation. AtURI interacts with PFDs to modulate PIN2 recycling and auxin distribution in roots.