Coronatine contributes Pseudomonas cannabina pv. alisalensis virulence by overcoming both stomatal and apoplastic defenses in dicot and monocot plants

Abstract

P. cannabina pv. alisalensis (Pcal) is a causative agent of bacterial blight of crucifer including cabbage, radish, and broccoli. Importantly, Pcal can infect not only a wide range of Brassicaceae, but also green manure crops such as oat. However, Pcal virulence mechanisms have not been investigated and are not fully understood. We focused on coronatine (COR) function, which is one of the well-known P. syringae pv. tomato DC3000 virulence factors, in Pcal infection processes on both dicot and monocot plants. Cabbage and oat plants dip-inoculated with a Pcal KB211 COR mutant ({Delta}cmaA) exhibited reduced virulence compared to Pcal WT. Moreover, {Delta}cmaA failed to reopen stomata on both cabbage and oat, suggesting that COR facilitates Pcal entry through stomata into both plants. Furthermore, cabbage and oat plants syringe-infiltrated with {Delta}cmaA also showed reduced virulence, suggesting that COR is involved in overcoming not only stomatal-based defense, but also apoplastic defense. Indeed, defense related genes, including PR1 and PR2, were highly expressed in plants inoculated with {Delta}cmaA compared to Pcal WT, indicating that COR suppresses defense-related genes of both cabbage and oat. Additionally, SA accumulation increases after {Delta}cmaA inoculation compared to Pcal WT. Taken together, COR contributes to cause disease by suppressing stomatal-based defense and apoplastic defense in both dicot and monocot plants. This is the first study to investigate COR functions in the interaction of Pcal and different host plants (dicot and monocot plants) using genetically and biochemically defined COR deletion mutants. Author summaryDisease outbreaks caused by new Pseudomonas syringae isolates are problems worldwide. P. cannabina pv. alisalensis (Pcal) causes bacterial blight on a wide range of cruciferous plants and bacterial brown spot on oat plants. Although P. syringae deploys a variety of virulence factors, Pcal virulence factors have not been investigated. We focused on coronatine (COR) function, which is one of the well-known P. syringae virulence factors. COR is a non-host-specific phytotoxin and contributes to P. syringae growth and lesion formation or expansion in several host plants. COR function has been mainly studied in the model pathogen P. syringae pv. tomato DC3000 and model the plant Arabidopsis thaliana. Thus, COR roles in Pcal infection especially on monocot plants have not been well studied. Therefore, we investigated COR role in Pcal interaction with both dicot and monocot plants. Here, we revealed that COR functions as a multifunctional suppressor to manage Pcal virulence on both plants.