Isolation of Ant Mutants ofPseudomonas fluorescensStrain Pf7-14 Altered in Antibiotic Production, Cloning ofant+DNA, and Evaluation of the Role of Antibiotic Production in the Control of Blast and Sheath Blight of Rice

Pseudomonas fluorescensstrain 7-14 (Pf7-14) isolated from the rice rhizosphere produces antibiotic(s) active againstPyriculariasp.,Rhizoctonia solani, Pythium ultimum,andGaeumannomyces graminisvar.tritici.Mutagenesis of Pf714 with ethyl methane sulfonate (EMS) or transposon (mini-Tn5-Km) yielded antibiotic-defective mutants belonging to the following classes. Class I mutants, generated by EMS or mini-Tn5-Km, are totally deficient in antifungal antibiotics, i.e., are Ant−. Class II mutants were recovered following mutagenesis with mini-Tn5-Km; they produce reduced levels of antibiotics, i.e., are Antleaky. Class III mutants overproduce antibiotics; they were generated by EMS mutagenesis. Insertions of mini-Tn5-Km into at least four different chromosomal sites caused defects in antibiotic production. To further analyzeantgenes affecting antibiotic production, we isolated Ant+plasmids from a genomic library of the strain Pf 714. Of the several plasmids that appeared to carryantloci, pAKC908 was unusual in that it restored antibiotic production in all of our Class I and Class II mutants. By mutagenesis of pAKC908 with Tn3HoHo1 we localized a DNA region required for antibiotic production. Southern hybridization data and the complementation pattern of the Ant+plasmids indicated clustering of theantgenes in Pf714. Field trials with an Ant-deficient mutant carrying a single-site mini-Tn5-Km insertion revealed that antibiotic production by Pf714 was required for the control of blast as well as sheath blight. The data also show that Pf714 was as effective as the fungicide tricyclazole in controlling these rice diseases.