Interactions of Pseudomonas syringae pv. glycinea with Host and Nonhost Plants in Relation to Temperature and Phytotoxin Synthesis

Abstract

Pseudomonas syringae pv. glycinea PG4180 causes bacterial blight of soybean and produces the phytotoxin coro-natine (COR) in a temperature-dependent manner. COR consists of a polyketide, coronafacic acid (CFA), and an amino acid derivative, coronamic acid, and is produced optimally at 18°C whereas no detectable synthesis occurs at 28°C. We investigated the impact of temperature on PG4180 during compatible and incompatible interactions with soybean and tobacco plants, respectively. After spray inoculation, PG4180 caused typical bacterial blight symptoms on soybean plants when the bacteria were grown at 18°C prior to inoculation but not when derived from cultures grown at 28°C. The disease outcome was quantified by determination of bacterial populations in planta. The temperature effect was not observed when PG4180 was artificially infiltrated into soybean leaves, indicating that the pre-inoculation temperature and phytotoxin synthesis were important for bacterial invasion via natural plant openings. In the incompatible interaction, PG4180 elicited the hypersensitive response (HR) on tobacco plants regardless of the bacterial pre-inoculation temperature. However, the HR was significantly delayed when tobacco plants were treated with cells of the CFA-overproducing derivative, PG4180.N9, which were derived from cultures grown at 18°C, compared with parallels incubated at 28°C. CFA biosynthesis by PG4180.N9 was optimal at 18°C and negligible at 28°C. The impact of CFA synthesis on the HR was studied with different growth media, mutants, and transconjugants of PG4180, indicating that the amount of synthesized CFA but not that of COR influenced the outcome of the HR. Feeding experiments with purified coronafacoyl compounds suggested that the observed delay of the HR was mediated by CFA, shedding further light on CFA's putative role as a molecular mimic of the plant signaling molecule, jasmonic acid.