Structural insights into ABA receptor agonists reveal critical features to optimize and design a broad-spectrum ABA signaling activator.

Crop yield is at increasing risk due to water scarcity and climate change. Agrochemicals can activate hormone receptors to regulate transpiration and modulate transcription and address water deficits. Structure-guided optimization of multiple abscisic acid (ABA) receptor-agonist interactions is necessary to activate the entire PYRABACTIN RESISTANCE 1 (PYR1)/PYR1-LIKE (PYL)/REGULATORY COMPONENTS OF ABA RECEPTORS (RCAR) receptor family. The new agonist iCB, produced through scaffold-merging led by X-ray structure, activates subfamilies II and III at low-nM concentrations and subfamily I receptors at higher-nM concentrations. Structural analysis of opabactin and iCB ternary complexes reveals selectivity-determining residues, making the PYL1/PYL4/PYL8 subfamilies sensitive to specific agonists and highlighting the differential sensitivity of receptor subfamilies to agonists across plant species. iCB may activate most eudicots' PYL8-like receptors, in contrast to opabactin, due to limited steric constraints. This enables iCB to activate PYL8-like receptors with a bulkier Leu residue in the 3' tunnel, such as AtPYL8, SlPYL8, and VviPYL8. In contrast, opabactin activation is limited to receptors with Val at this position, for example, TaPYL8. Therefore, iCB extends its action to more ABA receptors than CB, iSB09, and opabactin, exhibits higher affinity than ABA for dimeric receptors, and can protect tomato plants against drought. In addition to regulating stomatal conductance and lowering water consumption, iCB protects photosystem II and improves photosynthesis following prolonged water deficit. Moreover, iCB induces an ABA-like transcriptional response, upregulates the osmolyte synthesis, and can be hyperpotentiated when combined with the expression of a customized receptor. Our results provide structural insights for optimizing agonist design and aiding plants in managing water deficits.