Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/12/2000
Publication Date: 12/1/2000
Citation: Timms-Wilson, T.M., Ellis, R.J., Renwick, A., Rhodes, D.J., Mavrodi, D.V., Weller, D.M., Thomashow, L.S., Bailey, M.J. 2000. Chromosomal insertion of phenazine-1-carboxylic acid biosynthetic pathway enhances efficacy of damping-off disease control by pseudomonas fluorescens. American Phytopathological Society. vol. 13, No. 12, 2000, pp.1293-1300. Interpretive Summary: Root diseases caused by soilborne pathogens are major yield-limiting factors in the production of food, fiber and ornamental crops. As agricultural practices become more sustainable, there is an increasing need for ecologically sound methods of disease control. Biological control, which exploits the natural antagonistic activity of certain root-colonizing bacteria against fungal pathogens, is one such approach. Biological control agents often perform inadequately under field conditions, however, and this has impeded acceptance of the technology as an alternative to chemical pesticides. The goal of this study was to determine whether the ability of the biological control agent Pseudomonas fluorescens to control damping-off disease of pea seedlings could be improved by engineering it to produce the antifungal compound phenazine-1-carboxylic acid (PCA). The engineered strain retained its ability to colonize the roots of pea, controlled the disease significantly better than the natural isolate, and effectively prevented the disease when introduced into the soil prior to sowing the pea seeds. These results suggest that the antifungal activity of the natural isolate is limiting to its performance as a biological control agent, and that enhancing antifungal activity is a viable approach to improving its performance against damping-off disease of pea.
Technical Abstract: A disarmed Tn5 vector (pUT::Ptac-phzABCDEFG) was used to introduce a single copy of the genes responsible for phenazine-1-carboxylic acid (PCA) biosynthesis into the chromosome of a plant-growth-promoting rhizobacterium Pseudomonas fluorescens. The PCA gene cluster was modified for expression under a constitutive Ptac promoter and lacked the phzIR regulators. PCA-producing variants significantly improved the ability of hte wild-type P. fluorescens to reduce damping-off disease of peas seedlings caused by Pythium ultimum, even under conditions of heavy soil infestation. Under conditions of oxygen limitation that are typical of hte rhizosphere, PCA production per cell in vitro was greater than that recorded in fast-growing, nutrient-rich cultures. Similarly, when the invitro nutrient supply was limited, P. fluorescens::phz variants taht produced the most PCA effectively competed against P. ultimum by suppressing mycelial development. Soil-based bioassays confirmed that the level of PCA bio-synthesis correlated directly with the efficacy of biological control and the persistence of inocula in soil microcosms. They also showed that soil pretreatment with bacteria provides a suitable method for plant protection by reducing infection, effectively decontaminating the soil. These data demonstrate that the insertion of a single chromosomal copy of the genes for a novel antifungal compound, PCA, enhances the ecological fitness of a natural isolate already adapted to the rhizosphere and capable of suppressing fungal disease.