Coordinated cytokinin signaling and auxin biosynthesis mediates arsenate-induced root growth inhibition

Interactions between plant hormones and environmental signals are important for the maintenance of root growth plasticity under ever-changing environmental conditions. Here, we demonstrate that arsenate (As(V)), the most prevalent form of arsenic (As) in nature, restrains elongation of the primary root through transcriptional regulation of local auxin biosynthesis genes in the root tips of Arabidopsis (Arabidopsis thaliana) plants. The ANTHRANILATE SYNTHASE ALPHA SUBUNIT 1 (ASA1) and BETA SUBUNIT 1 (ASB1) genes encode enzymes that catalyze the conversion of chorismate to anthranilate (ANT) via the tryptophan-dependent auxin biosynthesis pathway. Our results showed that As(V) upregulates ASA1 and ASB1 expression in root tips, and ASA1- and ASB1-mediated auxin biosynthesis is involved in As(V)-induced root growth inhibition. Further investigation confirmed that As(V) activates cytokinin signaling by stabilizing the type-B ARABIDOPSIS RESPONSE REGULATOR1 (ARR1) protein, which directly promotes the transcription of ASA1 and ASB1 genes by binding to their promoters. Genetic analysis revealed that ASA1 and ASB1 are epistatic to ARR1 in the As(V)-induced inhibition of primary root elongation. Overall, the results of this study illustrate a molecular framework that explains As(V)-induced root growth inhibition via crosstalk between two major plant growth regulators, auxin and cytokinin.