Involvement of fusaric acid detoxification by Pseudomonas fluorescens strain Pf10 in the biological control of Fusarium wilt of banana caused by Fusarium oxysporum f. sp. cubense

Several strains of Pseudomonas fluorescens were isolated from the rhizosphere of banana (Musa sp). These isolates were tested for their antagonistic effect towards the banana Fusarium wilt pathogen, Fusarium oxysporum f. sp. cubense (Foc) in vitro. Among the 11 isolates tested, Pf10 was the most effective in inhibiting the mycelial growth of Foc. The strain Pf10 was capable of growing in medium containing fusaric acid (100 μg/ml), a toxin isolated from the culture filtrate of Foc. The detoxification of fusaric acid (FA) by P. fluorescens was demonstrated in tomato by the loss of wilt-inducing ability of the culture filtrate of P. fluorescens that had been grown in medium containing FA. Several proteins were secreted by P. fluorescens strain Pf10 into the culture medium when it was grown in medium containing fusaric acid. Among them, the 60-kDa protein was predominant. The N-terminal amino acid sequence of the 60-kDa protein shows substantial sequence homology (above 90 %) with amino-peptidase of Pseudomonas aeruginosa. To investigate whether the gene (s) involved in fusaric acid detoxification resides on the plasmids in P. fluorescens, plasmid deficient strain of P. fluorescens strain Pf10 was developed by plasmid curing. The plasmid deficient strain (Pf10P^-) lost its ability to detoxify fusaric acid . The P. fluorescens strain Pf10 and Pf10P^- were tested for their efficacy in controlling Fusarium wilt of banana under greenhouse conditions. Soil application of banana with P. fluorescens strain Pf10 reduced the wilt incidence by 50 % compared with the control. Whereas, the plasmid deficient strain of P. fluorescens failed to control Fusarium wilt.