Structural Insights into the Abscisic Acid Stereospecificity by the ABA Receptors PYR/PYL/RCAR

The phytohormone abscisic acid ((+)-ABA) plays a key role in many processes. The biological and biochemical activities of unnatural (-)-ABA have been extensively investigated since 1960s. However, the recognition mechanism by which only a few members among PYR/PYL/RCAR (PYLs) family can bind (-)-ABA remains largely unknown. Here we systematically characterized the affinity of PYLs binding to the (-)-ABA and reported the crystal structures of apo-PYL5, PYL3-(-)-ABA and PYL9-(+)-ABA. PYL5 showed the strongest binding affinity with (-)-ABA among all the PYLs. PYL9 is a stringently exclusive (+)-ABA receptor with interchangeable disulfide bonds shared by a subclass of PYLs. PYL3 is a dual receptor to both ABA enantiomers. The binding orientation and pocket of (-)-ABA in PYLs are obviously different from those of (+)-ABA. Steric hindrance and hydrophobic interaction are the two key factors in determining the stereospecificity of PYLs binding to (-)-ABA, which is further confirmed by gain-of-function and loss-of-function mutagenesis. Our results provide novel insights of the bioactivity of ABA enantiomers onto PYLs, and shed light on designing the selective ABA receptors agonists.