Coordinated PIN7 and AUX1 responses to arsenite-restrained root growth in Arabidopsis

Arsenite (As^III), regarded as a hazard to human health and food safety, restrains root growth. This event has received little attention, and the mechanism underlying how As^III affects auxin dynamics to repress root growth remains unknown. Here, our results have suggested that As^III-inhibited root growth possibly involved an elevated auxin level in roots, as supported in multiple experiments such as (1) transgenic DII-VENUS and DR5rev::GFP; (2) IAA determination by enzyme-linked immunosorbent assay; (3) phenotype analysis of taa1 (defective in auxin biosynthesis) mutant; and (4) the external application of 1-naphthylphthalamic acid (NPA) to manipulate auxin transport. This consequence could be explained through up-regulated transcriptional levels of auxin biosynthesis-related genes in whole plants. Phenotypes of auxin transport-related carriers have displayed that loss of PIN7 and AUX1, but not PIN1/2/3 transporters, ameliorated the extent of As^III-induced root growth inhibition. Moreover, As^III specifically enhanced the abundance of PM-localized PIN7 and AUX1 involved in transcriptional but not post-transcriptional regulation. A real-time in vitro observation of PM-localized PIN2 using the transgenic pPIN2::PIN2-Dendra2 line has revealed that As^III did not influence the endocytosis of PM-localized PIN2 carriers in root apices. Overall, our results propose that As^III stress-elevated auxin level in the root apex possibly involves up-regulation transcriptional levels of auxin biosynthesis- and auxin transport-related genes but does not target PIN2 dynamics, finally, leading to auxin accumulation in root apices and root growth inhibition.