Dual function of coronatine as a bacterial virulence factor against plants: Possible COI1-JAZ-independent role

Coronatine (COR, 1) is a phytotoxin and structural mimic of the plant hormone (+)-7-iso-jasmonoyl-L-isoleucine (2). COR (1) functions as a ligand of the COI1-JAZ co-receptor, which is the exclusive receptor of 2. Recently, a new role for 1 as a plant virulence factor for Pseudomonas syringae has attracted the attention of plant scientists. Bacteria invade the plant apoplast through stomatal pores. The host plant then responds to the bacterial invasion by closing the stomatal pores (stomatal defense). COR (1) functions as a bacterial chemical weapon that secures the path of infection by reopening the closed stomata. The mechanism is thought to involve inhibition of abscisic acid-signaling through the COI1-JAZ pathway. Thus, 1 plays an important role in plant-microbe interactions by abrogating the plant immune response. In this study, we synthesized seven analogues of 1 with naturally occurring α-amino acids and assessed their effect on stomata in a model plant, Arabidopsis thaliana. Structure-activity relationship studies of the analogues coupled with genetic studies and in silico docking analyses with COI1-JAZ strongly suggested that stomatal reopening induced by 1 may not rely on the COI1-JAZ signaling pathway. Our results suggest that stomatal reopening is triggered by 1 in conjunction with the conventional COI1-JAZ mode of action.