Boron (B) is essential for plant growth because it is required for cross-liking pectin. In contrast, B is toxic when it is accumulated at high concentrations in tissues. Under low-B conditions, a boric acid channel NIP5;1 and a borate exporter BOR1 play key roles in B uptake and translocation in Arabidopsis thaliana. In root epidermal and endodermal cells, NIP5;1 and BOR1 are localized to the plasma membrane in polar manners toward the soil- and stele-side, respectively, to facilitate the directional transport of B towards the stele. When the concentration of B in the soil fluctuates, plants regulate the abundance of these transport proteins. Upon sufficient-B supply, BOR1 undergoes polyubiquitination and is transported from the plasma membrane to the vacuole for degradation, preventing an excessive accumulation of B.
In the presentation, we would like to discuss the B-sensing mechanism controlling ubiquitination and endocytosis of BOR1. Our study on BOR1, along with other research on yeast amino acid transporters, suggests that the ubiquitination of these transporters depends on their conformational transition during the transport cycle. We will present our current attempts to prove the model of transport-coupled ubiquitination for self-regulation of BOR1.
We would also like to discuss the role of BOR1 in the anther. BOR1 is localized in the plasma membrane of the tapetum cells towards the locule and its accumulation is controlled by endocytosis during development. In the tapetum cell, BOR1 plays a vital role in supplying B to the locule for pollen development.