An ANTHRANILATE SYNTHASE interacts with ABA and is required for ABA-dependent stomatal closing

Abscisic acid (ABA), a phytohormone that affects key biological processes, is best known for causing stomatal closure to protect plants against environmental stresses. The prevailing mechanism for ABA perception is through the PYL/PYR/RCAR family of proteins, but reports of other ABA-interacting proteins such as the guard cell outward-rectifying K+ channel (GORK) have encouraged the search for more ABA-sensitive proteins. Here, we identified an ABA-interacting site similar to GORK, in an Arabidopsis thaliana ANTHRANILATE SYNTHASE (ASA2). We found that asa2 mutant plants have an obvious aberration in ABA-dependent stomatal closing. Leaf transcriptomics revealed significantly fewer ABA-induced differentially expressed genes in asa2-1 as compared with Col-0. ABA- and other hormone-related terms were also under-represented, indicating an overall reduced genomic sensitivity to ABA. Computational analysis hinted at a plausible ABA interaction at the predicted site, and both indirect and direct in vitro interaction studies showed that ASA2 could interact with ABA in a specific and ligand-dependent manner. Importantly, single amino acid substitutions at the ABA site resulted in various degrees of reduced ABA affinity. Further examination of how ABA interaction affects the enzymatic activity of ASA2 and the flow of information in the chloroplast could reveal molecular targets for agrochemical design that will improve plant resilience.