Structural Biology Study On Abscisic Acid Receptor PYLs Protein Family

Abscisic acid (ABA), one of the most essential phytohormones in plants, plays akey role in regulating various developmental processes and improving stress resistanceto drought and salt. ABA receptors are at the core of the research of ABA signalingpathway. In2009, two research groups reported a potential soluble ABA receptorPYR/PYL/RCAR protein family in Arabidopsis.Using X-ray crystallography, we solved the structures of apo-PYL2, ABA-PYL2and ABA-PYL1-ABI1at high atomic resolution. Combined with biochemical analysis,we found PYLs are ABA receptors and revealed an ABA-mediated inhibitionmechanism towards PP2Cs, a family of phosphatase. ABA’s binding leads to aremarkable conformational change on CL2loop and weakens PYL2’s dimer interface,which helps to recognize PP2Cs and inhibit its activity. The synthetic compoundpyrabactin, whose chemical structure differs a lot from ABA, could function throughPYLs. In order to understand the mechanism, we solved the crystal structure ofpyrabactin-PYL1and proposed an “ectromagnet” model for the ligand perception ofPYLs: the ligand should contain a hydrophobic module that can close CL2loop and apolar module to be anchored in the ligand binding pocket. Unlike ABA, pyrabactin onlybinds to a selective pool of PYLs. To understand what determines the selectivity, wedetermined the structure of pyrabactin-PYL2and found that pyrabactin resides indistinct approaches compared with that in PYL1, which is caused by a single amino acidalternation between Valine and Isoleucine. To delineate the complex regulation byPYLs of ABA signaling pathway, we systematically assessed the inhibitory effect of tenPYLs on four PP2Cs. The results showed that one subfamily represented by PYL10inhibits PP2Cs without ABA. Through solving apo-PYL10and PYL10-HAB1complexstructure, we found out that the constitutively active PYLs are monomeric and contain abulky hydrophobic residue guarding the ligand-binding pocket.Our studies on PYLs help us to classify PYLs to provide a framework tounderstand PYL’s complex regulation. The insights we gained form examination ofpyrabactin provide us with a general principle to design specific ABA agonists that arebeneficial in the agricultural field.