Information on the structure and function of ABA receptors has created opportunities for agrochemical development. Current lead molecules have low and short-lived bioactivity in some relevant crop species, including wheat, the world’s most widely grown staple crop. This liability is likely a consequence of incomplete activation of different ABA receptor subclasses. We reasoned that the idiosyncratic activity of these molecules was due, in part, to a lack of interaction between the agonist and a conserved lysine in ABA receptors that forms a salt bridge to ABA’s carboxylate. We performed virtual screening to identify candidate agonists that interact with this lysine.

RESULTS

Two ABA receptor structures were used to screen against the ZINC database, a collection of commercially available ligands, using Glide docking protocols, requiring that hits interact with the conserved lysine. Candidate agonists were obtained and tested for receptor activation using in vitro assays. A family of substituted phenyl acetamido-cyclohexane carboxylic acid ABA receptor panagonists was identified. Scaffold merging was used to improve binding: We grafted an optimized headgroup of an existing sulfonamide onto our phenyl acetamido-cyclohexane carboxylic acid scaffold to yield a chimeric ligand (3CB) that displayed an improvement toward target sites of up to three orders of magnitude. Analysis of a 3CB-PYL10 crystal structure suggested that addition of appropriately situated hydrophobes to 3CB might improve activity. A 3CB derivative was synthesized, yielding an agonist that we have named opabactin (OP) for overpowered ABA receptor activation. Thermodynamic characterization of 3CB or OP receptor binding reactions indicates that the newly generated scaffold’s improvements are enthalpically driven relative to sulfonamides, consistent with the salt bridge observed in our crystal structure. Biological studies show that OP is ~10-fold more active in inhibiting seed germination (a response driven by ABA) than ABA itself. Experiments in wheat, tomato, barley, Arabidopsis, and Commelina demonstrate bioactivity in vegetative tissues across diverse species. Time course experiments in wheat and tomato using thermal imaging show that OP induces a more sustained antitranspirant response than ABA, and they document poor activity of sulfonamide agonists in those two species. To understand which receptors are necessary for OP action, we used Arabidopsis mutant strains to show that OP requires the subfamily III receptors PYR1, PYL1, and PYL2 for maximal activity. Thus, our virtual screening experiments yielded an ABA receptor agonist that functions as an antitranspirant.