PIN2/3/4 Auxin Carriers Mediate Root Growth Inhibition under Conditions of Boron Deprivation in Arabidopsis.

The mechanistic basis by which boron (B) deprivation inhibits root growth via the mediation of the root apical auxin transport and distribution remains elusive. This study showed that B deprivation repressed root growth of wild-type Arabidopsis seedlings, which was related to a higher auxin accumulation (observed with DII-VENUS and DR5-GFP lines) in B-deprived roots. Boron deprivation elevated the auxin content in the root apex, coinciding with up-regulation of the expression levels of auxin biosynthesis-related genes (TAA1, YUC3, YUC9, and NIT1) in shoots, but not in root apices. Phenotyping experiments using auxin transport-related mutants revealed that the PIN2/3/4 carriers are involved in root growth inhibition caused by B deprivation. B deprivation not only up-regulated the transcriptional levels of PIN2/3/4 genes, but also restrained the endocytosis of PIN2/3/4 carriers (observed with PIN-Dendra2 lines), resulting in elevated protein levels of PIN2/3/4 in the plasma membrane. Overall, these results suggest that B deprivation not only enhances auxin biosynthesis in shoots by elevating the expression levels of auxin biosynthesis-related genes but also promotes the polar auxin transport from shoots to roots by up-regulating the expression levels of PIN2/3/4 genes, as well as restrains the endocytosis of PIN2/3/4 carriers, ultimately resulting in auxin accumulation in root apices and root growth inhibition.