Boron Supply Restores Aluminum-Blocked Auxin Transport by Modulation PIN2 Trafficking in the Arabidopsis Roots

Abstract This study tested a hypothesis that boron (B) supply alleviates aluminum (Al) toxicity by modifying auxin distribution in functionally different root zones. Auxin distribution and transport at various Al and B ratios were analyzed using the range of molecular and imaging techniques. Al stress resulted in increased auxin accumulation in root apical meristem (MZ) and transition zones (TZ) while reducing its content in elongation zone (EZ). This phenomenon was explained by reduction in basipetal auxin transport caused by Al blockage of PIN2 endocytosis, regulated at posttranscriptional level. This inhibition of PIN2 endocytosis was dependent on actin filaments and microtubules. B supply facilitated the endocytosis and exocytosis of PIN2 carriers via recycling endosomes conjugated with IAA to modify Al-induced auxin depletion in the EZ. However, disruption of auxin signaling with auxinole did not alleviate Al-induced inhibition of root growth. B supply alleviates Al-induced inhibition of root growth via restoring the endocytic recycling of PIN2 proteins involved in the basipetal (shootward) auxin transport, restoring Al-induced auxin depletion in the elongation zone. Short summary Aluminum-intensified PIN2 abundance, nontranscriptional, via repressing PIN2 endocytosis to block polar auxin transport, and this adverse effect could be alleviated by boron supply.