GNOM-dependent endocytosis maintains polar localization of the borate exporter BOR1 in Arabidopsis.

BACKGROUND INFORMATION: Plants use transporters polarly localized in the plasma membrane for the directional transport of nutrients. The boric acid/borate (B) exporter BOR1 is localized polarly in the inner lateral domain of the plasma membrane in various root cells for efficient translocation of B under B limitation. With a high B supply, BOR1 is ubiquitinated and transported to vacuoles for degradation. The polar localization and vacuolar targeting of BOR1 are maintained by different endocytosis mechanisms.

RESULTS: We demonstrated that one of the most utilized inhibitors in endosomal recycling, brefeldin A (BFA), inhibits the polar localization of BOR1. BFA inhibits a subset of guanine-nucleotide exchange factors (ARF-GEFs), regulators of vesicle formation. Using a transgenic line expressing BFA-resistant engineered GNOM, we identified GNOM as the key ARF-GEF in endocytosis and maintenance of the polar localization of BOR1.

CONCLUSIONS AND SIGNIFICANCE: We found that BFA inhibits the polar localization of BOR1 by inhibiting GNOM activity. Our results suggest that GNOM-dependent endocytosis contributes to the maintenance of the polar localization of BOR1 under B limitation. We propose a model of BOR1 transcytosis initiated from GNOM-dependent endocytosis. This article is protected by copyright. All rights reserved.