Enhanced Growth and Activity of a Biocontrol Bacterium Genetically Engineered To Utilize Salicylate

Plasmid NAH7 was transferred from Pseudomonas putida PpG7 to P. putida R20 [R20(NAH7)], an antagonist of Pythium ultimum. The plasmid did not affect growth or survival of R20(NAH7) and was stably maintained under nonselective conditions in broth and soil and on sugar beet seeds. Plasmid NAH7 conferred to R20(NAH7) the ability to utilize salicylate in culture, agricultural field soil, and on sugar beet seeds. The metabolic activity of R20(NAH7), but not the wild-type R20, was greatly increased in soil by amendment with salicylate (250 μg/g) as measured by induced respiration. Population densities of R20(NAH7) were also enhanced in salicylate-amended soil, increasing from approximately 1 × 10⁵ CFU/g to approximately 3 × 10⁸ CFU/g after 35 h of incubation. In contrast, population densities of R20(NAH7) in nonamended soil were approximately 3 × 10⁶ CFU/g of soil after 35 h of incubation. The concentration of salicylate in soil affected the rate and extent of population increase by R20(NAH7). At 50 to 250 μg of salicylate per g of soil, population densities of R20(NAH7) increased to approximately 10⁸ CFU/g of soil by 48 h of incubation, with the fastest increase at 100 μg/g. A lag phase of approximately 24 h occurred before the population density increased in the presence of salicylate at 500 μg/g; at 1,000 μg/g, population densities of R20(NAH7) declined over the time period of the experiment. Population densities of R20(NAH7) on sugar beet seeds in soils amended with 100 μg of salicylate per g were not increased while ample carbon was present in the spermosphere. However, after carbon from the seed had been utilized, population densities of R20(NAH7) decreased significantly less (P = 0.005) on sugar beet seeds in soil amended with salicylate than in nonamended soil.