INFLUENCE OF PLANT-SPECIES ON DISEASE SUPPRESSION BY PSEUDOMONAS-FLUORESCENS STRAIN CHAO WITH ENHANCED ANTIBIOTIC PRODUCTION

Introduction of the recombinant cosmid pME3090 into Pseudomonas fluorescens strain CHAO, a good biocontrol agent of various diseases caused by soilborne pathogens, increased three- to five-fold the production of the antibiotic metabolites pyoluteorin (Plt) and 2,4-diacetylphloroglucinol (Phl) in vitro. Strain CHAO/pME3090 also overproduced Plt and Phl in the rhizosphere of wheat infected or not infected with Pythium ultimum. The biocontrol activity of the wild-type and recombinant strains was compared using various plant pathogen-host combinations in a gnotobiotic system. Antibiotic overproduction affected neither the protection of wheat against P. ultimum and Gaeumannomyces graminis var. tritici nor the growth of wheat plants. In contrast, strain CHAO/pME3090 showed an increased capacity to protect cucumber against Fusarium oxysporum f. sp. cucumerinum and Phomopsis sclerotioides, compared with the wild-type strain CHAO. The antibiotic overproducing strain protected tobacco roots significantly better against Thielaviopsis basicola than the wild-type strain but drastically reduced the growth of tobacco plants and was also toxic to the growth of sweet corn. On King's B agar and on malt agar, the recombinant strain CHAO/pME3090 inhibited all pathogens more than did the parental strain CHAO. Synthetic Plt and Phl were toxic to all fungi tested. Tobacco and sweet corn were more sensitive to synthetic Plt and Phl than were cucumber and wheat. There was no correlation between the sensitivity of the pathogens to the synthetic antibiotics and the degree of disease suppression by strain CHAO/pME3090. However, there was a correlation between the sensitivity of the plants and the toxicity of the recombinant strain. We conclude that the plant species rather than the pathogen determines whether cosmid pME3090 in P. fluorescens strain CHAO leads to improved disease suppression.