Changes in populations of rhizosphere bacteria associated with take-all disease of wheat.
Take-all, caused by Gaeumannomyces graminis var. tritici, is one of the most important fungal diseases of wheat worldwide. Knowing that microbe-based suppression of the disease occurs in monoculture wheat fields following severe outbreaks of take-all, we analyzed the changes in rhizosphere bacterial communities following infection by the take-all pathogen. Several bacterial populations were more abundant on diseased plants than on healthy plants, as indicated by higher counts on a Pseudomonas-selective medium and a higher fluorescence signal in terminal restriction fragment length polymorphism analyses of amplified 16S ribosomal DNA (rDNA). Amplified rDNA restriction analysis (ARDRA) of the most abundant cultured populations showed a shift in dominance from Pseudomonas to Chryseobacterium species in the rhizosphere of diseased plants. Fluorescence-tagged ARDRA of uncultured rhizosphere washes revealed an increase in ribotypes corresponding to several bacterial genera, including those subsequently identified by partial 16S sequencing as belonging to species of alpha-, beta-, and gamma-proteobacteria, sphingobacteria, and flavobacteria. The functional significance of some of these populations was investigated in vitro. Of those isolated, only a small subset of the most abundant Pseudomonas spp. and a phlD(+) Pseudomonas sp. showed any significant ability to inhibit G. graminis var. tritici directly. When cultured strains were mixed with the inhibitory phlD(+) Pseudomonas strain, the Chryseobacterium isolates showed the least capacity to inhibit this antagonist of the pathogen, indicating that increases in Chryseobacterium populations may facilitate the suppression of take-all by 2,4-diacetylphloroglucinol-producing phlD(+) pseudomonads.
McSpadden Gardener
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Root Disease and Biological Control Research Unit, USDA Agricultural Research Service, Pullman, Washington, USA.
Weller
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D000898:Antibiosis; D001203:Ascomycota / Q000502:physiology*; D001419:Bacteria / Q000145:classification / Q000235:genetics / Q000254:growth & development* / Q000302:isolation & purification; D015169:Colony Count, Microbial; D004275:DNA, Ribosomal / Q000032:analysis / Q000235:genetics; D008969:Molecular Sequence Data; D010696:Phloroglucinol / Q000031:analogs & derivatives / Q000378:metabolism; D010935:Plant Diseases / Q000382:microbiology; D018517:Plant Roots / Q000382:microbiology*; D011549:Pseudomonas / Q000145:classification / Q000235:genetics / Q000254:growth & development / Q000302:isolation & purification; D012336:RNA, Ribosomal, 16S / Q000235:genetics; D015183:Restriction Mapping / Q000379:methods; D017422:Sequence Analysis, DNA; D012988:Soil Microbiology; D014908:Triticum / Q000382:microbiology*
D004275:DNA, Ribosomal; D012336:RNA, Ribosomal, 16S; C059817:2,4-diacetylphloroglucinol; D010696:Phloroglucinol