Plays a role in plants and microbes interactions (PubMed:22206669). G-protein coupled receptor involved in root growth mediated by the bacterial quorum-sensing signals N-acyl-homoserine lactones (AHLs) (PubMed:18671868, PubMed:22206669). Binds to melatonin (PubMed:29702752). Phytomelatonin receptor required, in collaboration with GPA1, for melatonin-mediated stomatal closure involving H(2)O(2) and Ca(2+) signals (PubMed:29702752). Interacts with GPA1. Expressed at low levels in seedlings. Induced by the N-acyl-homoserine lactones (AHLs) N-3-oxo-hexanoyl-homoserine lactone (3OC6-HSL) and N-3-oxo-octanoyl-homoserine lactone (3OC8-HSL) (PubMed:22206669). Triggered by melatonin in leaves, cotyledons, hypocotyls, roots and guard cells (PubMed:29702752). Small leaf size phenotype (PubMed:29702752). Abolished Gram-negative bacteria-mediated promotion of root elongation triggered by the N-acyl-homoserine lactones (AHLs) N-3-oxo-hexanoyl-homoserine lactone (3OC6-HSL) and N-3-oxo-octanoyl-homoserine lactone (3OC8-HSL) (PubMed:22206669). Insensitivity to melatonin-induced stomatal closure associated with abolished melatonin-induced H(2)O(2) production and Ca(2+) influx (PubMed:29702752). Belongs to the UPF0359 family.