Abstract
Abscisic acid (ABA) plays a vital role in many developmental processes and the response to adaptive stress in plants. Under drought stress, plants enhance levels of ABA and activate ABA receptors, but under harsh environmental stress, plants usually cannot efficiently synthesize and release sufficient quantities of ABA. The response of plants to harsh environmental stress may be improved through ABA-independent activation of ABA receptors. The molecular basis of ABA-independent inhibition of group A protein phosphatases type 2C (PP2Cs) by pyrabactin resistance/Pyr1-like (PYR1/PYLs) is not yet clear. Here, we used our previously reported structures of PYL3 to first obtain the monomeric PYL3 mutant and then to introduce bulky hydrophobic residue substitutions to promote the closure of the Gate/L6/CL2 loop, thereby mimicking the conformation of ABA occupancy. Through structure-guided mutagenesis and biochemical characterization, we investigated the mechanism of ABA-independent activation of PYL3. Two types of PYL3 mutants were obtained: (a) PYL3 V108K V107L V192F can bind to ABA and effectively inhibit HAB1 without ABA; (b) PYL3 V108K V107F V192F, PYL3 V108K V107L V192F L111F and PYL3 V108K V107F V192F L111F cannot recognize ABA but can greatly inhibit HAB1 without ABA. Intriguingly, the ability of PYL3 mutants to bind to ABA was severely compromised if any two of three variable residues (V107, V192 and L111) were mutated into a bulky hydrophobic residue. The introduction of PYL3 mutants into transgenic plants will help elucidate the functionality of PYL3 in vivo and may facilitate the future production of transgenic crops with high yield and tolerance of abiotic stresses.