Genetics of Antibiosis in Bacterial Strains Suppressive to Take-All

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Abstract
We initiated a genetic analysis of the interaction between the fungal pathogen, Gaumannomyces graminis var. tritici and bacterial strains involved in suppression of the take-all disease. Tn5-induced mutants of three bacterial strains (Pseudomonas fluorescens NRRL B-15133, NRRL B-15135, and strain I11) that inhibit the growth of the pathogen on agar media (antibiosis) were obtained. Mutants for auxotrophy (0.67%), increased antibiosis (0.83%), loss of antibiosis (0.38%), and loss of fluorescence on King''s Medium B (NRRL B-15135 only 0.53%) were obtained. When 48 mutants affected the antibiosis were analyzed by Southern blot hybridization, 45 contained insertions in single EcoR1 fragments. From these results it was estimated that mutation at six to 10 different loci resulted in a loss of antibiosis. The data suggest that with each strain insertions of at least one half of the prototrophic, antibiosis-negative mutants may be clustered in one or two regions of the bacterial genome. Genomic cosmid clone banks were prepared from two strains (B10 and I21) that did not inhibit the growth of the pathogen on agar media, and transferred to strain NRRL B-15135. One I21 cosmid clone inhibited the expression of antibiosis in this strain, as well as two other strains. Thus, two types of bacterial genes important for the in vitro interaction with the pathogen were identified in this study.