Dispersal of wild-type and genetically-modified Pseudomonas spp from treated seeds or soil to aerial parts of radish plants

Dispersal of wild-type and genetically-modified strains of plant growth-promoting fluorescent Pseudomonas spp to aerial parts of radish plants was investigated using different inoculation strategies. P. putida strain WCS358rr colonized the surface of cotyledons of radish plants when seeds were coated with this strain or when non-coated seeds were sown in soil treated with WCS358rr. The cotyledons were not colonized by WCS358rr when suspensions of this strain were inoculated onto roots of radish seedlings that had already grown for 4 d in soil. Strain WCS358rr not only colonized the surface, but also the interior of cotyledons. For all types of inoculation strategies tested, strain WCS358rr colonized the root surface but did not penetrate the interior of the root. These results indicate that WCS358rr was not transported up, or downwards through the vascular system. When coated on seed, dispersal to cotyledons of radish was also demonstrated for P. fluorescens WCS374 and its genetically-modified derivative WCS374pMR. Plasmid pMR was maintained in WCS374pMR throughout the experiment. No differences were observed between modified strain WCS374pMR and wild-type strain WCS374 with regard to dispersal to and colonization of cotyledons and roots of radish. Strains WCS358rr, WCS374 and WCS374pMR survived on cotyledons for a period of 21–31 d. Dispersal of the applied bacterial strains to the first true leaves of radish could not be demonstrated. The results indicate that cotyledons of radish become contaminated with strains of Pseudomonas spp during germination of treated seeds or during seedling emergence from treated soil. The implications of dispersal of wild-type and genetically-modified pseudomonads to aerial parts of the plant are discussed.