Thomas F. C.
Ben J. J.
1Leiden University, Institute of Molecular Plant Sciences, Clusius Laboratory, Wassenaarseweg 64, 2333 AL Leiden, The Netherlands; and 2Michael Barber Centre for Mass Spectrometry, Department of Chemistry, University of Science and Technology in Manchester (UMIST), P.O. Box 88, Manchester, M60 1QD, U.K.
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Accepted 17 April 2001.
Pseudomonas chlororaphis PCL1391 controls tomato foot and root rot caused by Fusarium oxysporum f. s radicislycopersici. Its biocontrol activity is mediated by the production of phenazine-1-carboxamide (PCN). In contrast, the take-all biocontrol strains P. fluorescens 2--79 and P. aureofaciens 30--84, which produce phenazine-1-carboxylic acid (PCA), do not control this disease. To determine the role of the amide group in biocontrol, the PCN biosynthetic genes of strain PCL1391 were identified and characterized. Downstream of phzA through phzG, the novel phenazine biosynthetic gene phzH was identified and shown to be required for the presence of the 1-carboxamide group of PCN because a phzH mutant of strain PCL1391 accumulated PCA. The deduced PhzH protein shows homology with asparagine synthetases that belong to the class II glutamine amidotransferases, indicating that the conversion of PCA to PCN occurs via a transamidase reaction catalyzed by PhzH. Mutation of phzH caused loss of biocontrol activity, showing that the 1-carboxamide group of PCN is crucial for control of tomato foot and root rot. PCN production and biocontrol activity of the mutant were restored by complementing the phzH gene in trans. Moreover, transfer of phzH under control of the tac promoter to the PCA-producing biocontrol strains P. fluorescens 2--79 and P. aureofaciens 30--84 enabled these strains to produce PCN instead of PCA and suppress tomato foot and root rot. Thus, we have shown, for what we believe is the first time, that the introduction of a single gene can efficiently extend the range of the biocontrol ability of bacterial strains.
© 2001 The American Phytopathological Society