Phenylboronic acid interacts with pectic rhamnogalacturonan-II and displays anti-auxinic effects during Arabidopsis thaliana root growth and development

Boron dimerizes rhamnogalacturonan-II (RG-II) in the plant cell wall and is crucial for cell elongation; however, studying RG-II dimerization is challenging because of the severe phenotypes or lethality of RG-II mutants. Boron deprivation negates both RG-II dimerization and plant growth, but whether or how these phenotypes are functionally linked has remained unclear. Boric acid analogues can serve as experimental tools to interfere with RG-II cross-linking. In this study, we treated Arabidopsis seedlings with the boric acid analogue phenylboronic acid (PBA) to examine whether the resulting developmental phenotypes were attributable to alteration of RG-II dimerization or to other putative functions of boron in plants. We found that PBA treatment altered root development in the seedlings while RG-II dimerization and its distribution were not affected. Surprisingly, under low boron conditions, PBA treatment had no effect on root size but still prevented lateral root development, and it restored RG-II dimerization. PBA treatment also disrupted auxin levels, potentially explaining the absence of lateral roots in the seedlings treated with this analogue. We conclude that PBA interacts both with RG-II and other cellular targets such as auxin signaling components, and that the phenotypes caused by PBA arise from interference with multiple functions of boron.