LAUSR

Hydrogen sulfide (H2S) is a signaling molecule that regulates plant hormone and stress responses. The phytohormone abscisic acid (ABA) plays important role during plant adaptation to unfavorable environmental conditions and induces the persulfidation of L-CYSTEINE DESULFHYDRASE1 (DES1) and the production of H2S in guard cells. How H2S and protein persulfidation participate in relaying ABA signaling remains largely unclear, however. In this study, we discovered that ABSCISIC ACID INSENSITIVE 4 (ABI4) acts downstream of DES1 in controlling plant ABA responses. ABI4 undergoes persulfidation at Cys250, which is triggered in a time-dependent manner by ABA, and loss of DES1 function impairs this process. Furthermore, Cys250 and its persulfidation are essential for ABI4 function in the regulation of plant responses to ABA and the H2S donor NaHS during germination, seedling establishment and stomatal closure, which were abolished in the ABI4Cys250Ala mutated variant. Induction of the ABI4Cys250Ala variant into the abi4 des1 Arabidopsis line did not rescue its hyposensitivity to ABA. Cys250 is critical for the binding of ABI4 to its cognate motif within the promoter of Mitogen-Activated Protein Kinase Kinase Kinase 18 (MAPKKK18), propagates the MAPK signaling cascade induced by ABA. Furthermore, the DES1-mediated persulfidation of ABI4 enhanced the transactivation activity of ABI4 towards MAPKKK18, and ABI4 can bind to 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 H2S persulfidates ABI4 at Cys250.