Evolutionary-based remodeling of ABA receptors reveals the structural basis of hormone perception and regulation

Drought limits crop productivity, and effective mitigation requires a mechanistic understanding of how abscisic acid (ABA) perception translates hormone levels into physiological responses. In seed plants, ABA is sensed by PYR/PYL/RCAR (PYR/PYL) receptors, which inhibit 2C protein phosphatases (PP2Cs), thereby releasing Snf1-related protein kinases and driving stomatal closure and stress-responsive transcription. Yet how receptor architecture evolved to tune ABA dependence and dynamic range remains unclear. Here, we combine structural biology, biochemistry, evolutionary analysis, and in planta assays across algal, bryophyte, and angiosperm receptors to uncover a minimal molecular code that governs ABA sensitivity and oligomeric state. We identify a five-residue signature: Three leucines in the ligand pocket stabilize the gate in a closed conformation, conferring ligand-independent PP2C inhibition (ancestral trait), while two interface residues toggle dimerization (Leu/Lys) versus monomerization (Cys/Ser), thereby setting ABA affinity. Structure-guided swaps reciprocally convert behaviors: Introducing the three leucines plus interface substitutions renders the ABA-dependent dimeric Citrus sinensis CsPYL1 into a monomer-like with ABA-independent activity, whereas the converse changes in monomeric Marchantia polymorpha MpPYL1 enforce dimerization and lower ABA affinity. In planta, reporter assays and mutant analyses reveal complementary operating ranges: Monomeric, high-affinity receptors decode low ABA under mild stress, while dimeric, reduced-affinity receptors sustain signaling at high ABA during acute drought, expanding the system's overall dynamic range and robustness. These results resolve the apparent paradox of low-affinity dimers as an evolutionary innovation rather than a loss of sensitivity, link receptor architecture to ABA-mediated response, and provide actionable design principles for engineering ABA signaling to enhance crop drought resilience.