Phytomelatonin receptor PMTR 1‐mediated signaling regulates stomatal closure in Arabidopsis thaliana

Melatonin has been detected in plants in 1995; however, the function and signaling pathway of this putative phytohormone are largely undetermined due to a lack of knowledge about its receptor. Here, we discovered the first phytomelatonin receptor ( CAND 2/ PMTR 1) in Arabidopsis thaliana and found that melatonin governs the receptor‐dependent stomatal closure. The application of melatonin induced stomatal closure through the heterotrimeric G protein α subunit‐regulated H 2 O 2 and Ca 2+ signals. The Arabidopsis mutant lines lacking AtCand2 that encodes a candidate G protein‐coupled receptor were insensitive to melatonin‐induced stomatal closure. Accordingly, the melatonin‐induced H 2 O 2 production and Ca 2+ influx were completely abolished in cand2 . CAND 2 is a membrane protein that interacts with GPA 1 and the expression of AtCand2 was tightly regulated by melatonin in various organs and guard cells. CAND 2 showed saturable and specific 125 I‐melatonin binding, with apparent K d (dissociation constant) of 0.73 ± 0.10 nmol/L ( r 2 = .99), demonstrating this protein is a phytomelatonin receptor ( PMTR 1). Our results suggest that the phytomelatonin regulation of stomatal closure is dependent on its receptor CAND 2/ PMTR 1‐mediated H 2 O 2 and Ca 2+ signaling transduction cascade.