Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/13/2012
Publication Date: 6/8/2012
Citation: Kwak, Y., Bonsall, R.F., Okubara, P.A., Paulitz, T.C., Thomashow, L.S., Weller, D.M. 2012. Factors impacting the activity of 2,4-diacetylphloroglucinol-producing Pseudomonas fluorescens against take-all of wheat. Soil Biology and Biochemistry. 54:48-56. Interpretive Summary: Biological control is the application or stimulation of antagonistic microorganisms for the control of plant pathogens. Some of the most effective biocontrol agents of soilborne pathogens are strains of the bacterium Pseudomonas fluorescens that produce the antifungal metabolite 2,4-diacetylphloroglucinol (2,4-DAPG). 2,4-DAPG producers are responsible for the natural biocontrol of take-all disease known as take-all decline that occurs when wheat is grown continuously in a field. Previous studies have shown that 2,4-DAPG produced by P. fluorescens inhibits the take-all pathogen, Gaeumannomyces graminis var. tritici (Ggt) on roots. In this study, we determined whether the sensitivity of Ggt to 2,4-DAPG or the cultivar of wheat grown affects the ability of 2,4-DAPG-producing pseudomonads to control take-all. We found that both the sensitivity of Ggt and the wheat cultivar grown modulates the level of biocontrol by 2,4-DAPG producers. The results of this research are important because they demonstrated that growers can improve their level of control of take-all by simply using wheat cultivars that are more supportive of biocontrol agents.
Technical Abstract: Take-all, caused by Gaeumannomyces graminis var. tritici, is an important soilborne disease of wheat worldwide. Pseudomonas fluorescens producing the antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG) are biocontrol agents of take-all and provide natural suppression of the disease during wheat monoculture known as take-all decline. To identify factors that could contribute to the effectiveness of 2,4-DAPG producers in take-all suppression, P. fluorescens strain Q8r1-96 (genotype D) and Q2-87V1 (genotype B; reduced antibiotic production) were tested against three pathogen isolates differing in sensitivity to 2,4-DAPG (LD5, ARS-A1 and R3-111a-1) and two wheat cultivars (Tara and Buchanan). The antibiotic sensitivity of the pathogen and cultivar significantly affected the level of take-all suppression by Q8r1-96 and Q2-87V1; suppression was greatest with LD5 and Tara. Q8r1-96 suppressed ARS-A1 and R3-111a-1 on Tara but not Buchanan, and Q2-87V1 failed to suppress either pathogen isolate on either cultivar. Q8r1-96 colonized the rhizosphere of Tara and Buchanan grown in soil similarly, but 2,4-DAPG accumulation was higher on the roots of Buchanan than Tara. 2,4-DAPG at 7.5 µg ml-1 reduced the growth of roots of both cultivars, and 10 µg ml-1 caused brown necrosis and tissue collapse of seedling roots and reduced root hair development. The half-life of 2,4-DAPG in the rhizosphere was estimated to be 0.25 days. These results suggest that several interconnected factors including sensitivity of G. graminis var. tritici to 2,4-DAPG, wheat cultivar, fluctuations in populations of 2,4-DAPG producers, and antibiotics accumulation in the rhizosphere will impact the robustness of take-all suppression by P. fluorescens in the field.