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Root Colonization by Phenazine-1-Carboxamide-Producing Bacterium Pseudomonas chlororaphis PCL1391 Is Essential for Biocontrol of Tomato Foot and Root Rot

December 2000 , Volume 13 , Number  12
Pages  1,340 - 1,345

Thomas F. C. Chin-A-Woeng , Guido V. Bloemberg , Ine H. M. Mulders , Linda C. Dekkers , and Ben J. J. Lugtenberg

Leiden University, Institute of Molecular Plant Sciences, Clusius Laboratory, Wassenaarseweg 64, 2333 AL Leiden, The Netherlands

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Accepted 12 August 2000.

The phenazine-1-carboxamide-producing bacterium Pseudomonas chlororaphis PCL1391 controls tomato foot and root rot caused by Fusarium oxysporum f. sp. radicis-lycopersici. To test whether root colonization is required for biocontrol, mutants impaired in the known colonization traits motility, prototrophy for amino acids, or production of the site-specific recombinase, Sss/XerC were tested for their root tip colonization and biocontrol abilities. Upon tomato seedling inoculation, colonization mutants of strain PCL1391 were impaired in root tip colonization in a gnotobiotic sand system and in potting soil. In addition, all mutants were impaired in their ability to control tomato foot and root rot, despite the fact that they produce wild-type levels of phenazine-1-carboxamide, the antifungal metabolite previously shown to be required for biocontrol. These results show, for what we believe to be the first time, that root colonization plays a crucial role in biocontrol, presumably by providing a delivery system for antifungal metabolites. The ability to colonize and produce phenazine-1-carboxamide is essential for control of F. oxysporum f. sp. radicis-lycopersici. Furthermore, there is a notable overlap of traits identified as being important for colonization of the rhizosphere and animal tissues.

Additional keywords: phenazines, root rot, site-specific recombinase, sss gene.

© 2000 The American Phytopathological Society