$Pyoverdin_{2112}$ of Pseudomonas fluorescens 2112 Inhibits Phytophthora capsici, a Red-Pepper Blight-Causing Fungus

Abstract

A bacterium, Pseudomonas fluorescens 2112, that is antagonistic against a red-pepper blight-causing fungus, Phytophthora capsici, was isolated from the local soil of Gyongju, Korea. This strain formed an orange-colored clear halo zone on chrome azurol S (CAS) blue agar, suggesting the production of a siderophore in addition to an antifungal antibiotic. The optimal culture conditions for siderophore production by P fluorescens 2112 were 30-h cultivation at 25C and pH 6.5 in King's B medium. The presence of 20 μg/ml of Fe 3+ ion or EDDHA promoted the production of siderophore in King's B medium. The siderophore was purified from culture broth by CM-Sephadex C-25 and Sephadex G-25 column chromatographies. The UV spectra of the purified siderophore was the same as that of pyoverdins or pseudobactins. The molecular mass was 1,958 Da determined by FAB-mass spectrometer, and the amino acid composition analysis showed that the purified siderophore consisted of glycine/threonine/serine/glutamic acid/alanine/lysine with the molar ratio of 3:2:1:1:1:1. DL-Threo-β-hydroxyaspartic acid and N δ -hydroxyornithine, two of the essential constituents of pyoverdin, were also found. The purified siderophore pyoverdin showed strong in vitro and in vivo antagonistic activities against phytophthora blight-causing P. capsici. Especially in an in viva pot test, the siderophore protected red-pepper Capsicum annum L. very well from the attack of P capsici. These results indicated that the purified siderophore of P, fluorescens 2112 played a critical role in the biocontrol of the red-pepper blight disease, equivalent to treatment by P. fluorescens 2112 cells.