Characterization and modeling of the borate transporter BOR3 suggest a mode of directional transport

In Arabidopsis thaliana, BOR1 and BOR2 are well-characterized borate exporters. The role of the paralog BOR3, however, has remained undetermined. We found that the bor1 bor2 bor3 triple mutant exhibits significantly poorer root elongation than the double bor1 bor2 mutant, suggesting that BOR3 contributes to the uptake of boron from the soil to the inner tissues in the absence of BOR1 and BOR2. In contrast to BOR1 and BOR2, BOR3 localization in the plasma membrane of the root cells was not noticeably polar, as would be expected given its ability to promote directional boron transport under the triple mutant background. To understand this apparent dichotomy between function and localization, we modeled the effect of BOR3 polarity on boron flow. It revealed that a synergy of diffusive processes can lead to directional transport within a plant tissue. The counterintuitive mechanism, here termed diffusive alliance transport (DAT) consists of perfectly apolar (or even outward-facing) exporters working in conjunction with polarly localized diffusion facilitators to enable the buildup of higher-than-medium concentrations as well as inward-directional flux within the tissue. For this mechanism to be operational, the existence of apoplastic compartments between cells is essential.