Biological Control of Rhizoctonia damping-off of Cucumber by a Transformed Pseudomonas putida Strain Expressing a Chitinase from a Marine Bacterium

To construct an effective biological control agent for plant root diseases, we isolated Pseudomonas putida strains from the rhizospheres of cucumber plants and used them as hosts of an expression vector for an antipathogenic gene. The antipathogenic gene encoded a chitinase, and was derived from the marine bacterium Alteromonas sp. strain 79401. We screened the genome of P. putida PaW8 to obtain a strong promoter for the expression of this gene. The chitinase expression vector pKAC9-p07, containing the chitinase gene and the promoter from PaW8, was introduced into P. putida strains that could survive well in the rhizosphere. The transformed P. putida strains showed chitinase production and suppressed the damping-off of cucumber seedlings in soils infected with the plant-pathogenic fungus Rhizoctonia solani. This study demonstrates a strategy for the construction of a biological control agent for practical use in the rhizosphere.