Root colonization by phenazine-1-carboxamide-producing bacterium Pseudomonas chlororaphis PCL1391 is essential for biocontrol of tomato foot and root rot

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-l-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-l-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.