Boron Supply Restores Aluminum-Blocked Auxin Transport by Modulation PIN2 Trafficking in the Root Apical Transition Zone.

Boron (B) supply alleviates toxic effects of aluminum (Al) on root growth; however, the mechanistic basis of this process remains elusive. This study filled this knowledge gap showing that boron modifies auxin distribution and transport in Al-exposed Arabidopsis roots. In B-deprived roots, Al treatment induced an increase in auxin content in the root apical meristem (MZ) and transition zone (TZ) while in the elongation zone (EZ) auxin content was decreased beyond the level required for adequate growth. These distribution patterns are explained by the fact that basipetal auxin transport from the TZ to EZ was disrupted by Al-inhibited PIN2 endocytosis. Experiments involving modulation of protein biosynthesis by cycloheximide (CHX) and transcriptional regulation by cordycepin (COR) demonstrated that Al-induced increase of PIN2 membrane proteins was depended on the inhibition of PIN2 endocytosis rather than on the transcriptional regulation of PIN2 gene. Experiments reporting profiling of Al 3+ and PIN2 proteins revealed that the inhibition of endocytosis of PIN2 proteins was due to Al-induced limitation of the fluidity of the plasma membrane. B supply mediated the turnover of PIN2 endosomes conjugated with IAA and thus restored Al-induced inhibition of IAA transport through the TZ to EZ. Overall, reported results demonstrate that boron supply mediates PIN2 endosome-based auxin transport to alleviate Al toxicity in plant roots.