Abstract

Hydrogen sulfide (H 2 S) is a signaling molecule that regulates plant hormone and stress responses. The phytohormone abscisic acid (ABA) plays an important role in plant adaptation to unfavorable environmental conditions and induces the persulfidation of L-CYSTEINE DESULFHYDRASE1 (DES1) and the production of H 2 S in guard cells. However, it remains largely unclear how H 2 S and protein persulfidation participate in the relay of ABA signals. In this study, we discovered that ABSCISIC ACID INSENSITIVE 4 (ABI4) acts downstream of DES1 in the control of ABA responses in Arabidopsis . ABI4 undergoes persulfidation at Cys250 that is triggered in a time-dependent manner by ABA, and loss of DES1 function impairs this process. Cys250 and its persulfidation are essential for ABI4 function in the regulation of plant responses to ABA and the H 2 S donor NaHS during germination, seedling establishment, and stomatal closure, which are abolished in the ABI4 Cys250Ala mutated variant. Introduction of the ABI4 Cys250Ala variant into the abi4 des1 mutant did not rescue its hyposensitivity to ABA. Cys250 is critical for the binding of ABI4 to its cognate motif in the promoter of Mitogen-Activated Protein Kinase Kinase Kinase 18 ( MAPKKK18 ), which propagates the MAPK signaling cascade induced by ABA. Furthermore, the DES1-mediated persulfidation of ABI4 enhances the transactivation activity of ABI4 toward  MAPKKK18 , and ABI4 can bind the DES1 promoter, forming a regulatory loop. Taken together, these findings advance our understanding of a post-translational regulatory mechanism and suggest that ABI4 functions as an integrator of ABA and MAPK signals through a process in which DES1-produced H 2 S persulfidates ABI4 at Cys250.