Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 27, 1997
Publication Date: N/A
Interpretive Summary: Antibiotic-producing fluorescent Pseudomonas spp. have been isolated from plants grown in soils from diverse geographical regions and are widely used to control a variety of soilborne plant pathogens. For example, Pseudomonas strains that produce the antibiotics phenazine-1-carboxylic acid (PCA) and 2,4-diacetylphloroglucinol (Phl) are effective against Gaeumannomyces graminis var. tritici, the causal agent of take-all of wheat. Despite the importance of antibiotics in biocontrol of plant pathogens, little is known about the frequency and ecology of naturally occurring antibiotic-producing Pseudomonas spp. This paper describes primers and DNA-probes that enable specific detection and isolation of naturally occurring Pseudomonas strains that produce PCA and Phl. On roots of wheat grown in soils that are naturally suppressive to take-all, PCA-producing pseudomonads were not detected. However, Phl-producing pseudomonads were present in all three take-all suppressive soils at densities ranging from 5 x 10**5 to 2 x 10**6 CFU per gram of root. Knowledge of the ecology and diversity of naturally occurring Phl- producing Pseudomonas spp. will help to identify new biocontrol agents and to improve the efficacy of existing biocontrol agents of take-all, a disease for which neither resistant varieties nor effective chemical treatments are available.
Technical Abstract: The antibiotics phenazine-1-carboxylic acid (PCA) and 2,4- diacetylphloroglucinol (Phl) are major determinants of biological control of soilborne plant pathogens by various strains of fluorescent Pseudomonas spp. In this study, we described primers and probes that enable specific and efficient detection of a wide variety of fluorescent Pseudomonas strains that produce phenazine antibiotics or 2,4- diacetylphloroglucinol. PCR analysis and Southern hybridization demonstrated that specific genes within the biosynthetic loci for Phl and PCA are conserved among various Pseudomonas strains of worldwide origin. The frequency of Phl- and PCA-producing fluorescent pseudomonads was determined on roots of wheat grown in three take-all suppressive and four take-all conducive soils by colony hybridization followed by PCR. Phenazine-producing strains were not detected on roots from any of the soils. However, Phl-producing fluorescent pseudomonads were isolated from all three take-all suppressive soils at densities ranging from approximately 5x10**5 to 2x10**6 CFU per gram of root. In the complementary conducive soils, Phl-producing pseudomonads were not detected or detected at densities at least 40-fold less than in the suppressive soils. We speculate that fluorescent Pseudomonas spp. that produce 2,4-diacetylphloroglucinol play an important role in the natural suppressiveness of these soils to take-all of wheat.