Anthranilic acid regulates subcellular localization of auxin transporters during root gravitropism

The distribution of the phytohormone auxin within plant tissues is of great importance for developmental plasticity, including root gravitropic growth. Auxin flow is directed by the subcellular polar distribution and dynamic relocalization of plasma membrane-localized auxin transporters such as the PIN-FORMED (PIN) efflux carriers, which are in turn regulated by complex endomembrane trafficking pathways. Anthranilic acid (AA) is an important early precursor of the main natural plant auxin indole-3-acetic acid (IAA). We took advantage of an AA-deficient, and consequently IAA-deficient, mutant displaying agravitropic root growth to show that AA rescues root gravitropism at concentrations that do not rescue IAA levels. Treatments with, or deficiency in, AA result in defects in PIN polarity and gravistimulus-induced PIN relocalization within root cells. Taken together, our results reveal a previously unknown role for AA in the regulation of PIN subcellular localization and dynamics involved in root gravitropism, which is independent of its better-known role in IAA biosynthesis.