Pseudomonas aureofaciens 30-84, a biological control bacterium for the soil-borne disease take-all of wheat, is a model system for biological control of root-infecting fungal pathogens. Strain 30-84 inhibits the causal agent of take-all, Gaeumannomyces graminis var. tritici, primarily through the production of phenazine antibiotics, which are important for survival of the bacterium in the rhizosphere. Prior to this work, phenazine production was shown to be regulated by an N-acyl-homoserine lactone (AHL) response system encoded by phzI and phzR. This work identified a second regulatory system involved in the phenazine regulatory cascade. The two-component regulatory system involving the GacS/GacA proteins regulates the production of phenazines, extracellular protease, hydrogen cyanide and fluorescent siderophores. GacS/GacA regulates the production of phenazines at multiple levels. They control the production of the AHL signal required for expression of the phenazine biosynthetic operon by tightly regulating transcription of phzI. This was the first report of a linkage between a two-component regulatory system and an AHL response system. GacS/GacA also control phenazine production through a second mechanism. Preliminary evidence suggests translational regulation of one or more genes involved in the phenazine regulatory cascade through transcriptional control of a regulatory RNA (rsmB RNA) required to neutralize the negative effects of the translational repressor RsmA. Another aspect of this work was the analysis of the formation and rhizosphere competence of spontaneous gacS and gacA mutants of strain 30-84. These are commonly isolated from laboratory cultures of all biocontrol bacteria and could pose a threat to the efficacy of biological control if they arise in the rhizosphere and displace the phenazine-producing wild type strain 30-84. This work indicated that the mutants did arise on wheat roots and did displace strain 30-84 on roots in sterile soil. However, the mutants did not displace strain 30-84 on roots in natural soil. In fact, the wild type strain 30-84 appeared to compete more favorably with indigenous microorganisms in the presence of a subpopulation of GacS/GacA mutants. Therefore, the results presented here indicate that a subpopulation of gacS and gacA mutants is a normal and beneficial part of the P. aureofaciens community in the rhizosphere.
| Identifer | oai:union.ndltd.org:arizona.edu/oai:arizona.openrepository.com:10150/279779 |
| Date | January 2001 |
| Creators | Chancey, Scott Thomas |
| Contributors | Pierson, Leland S., III |
| Publisher | The University of Arizona. |
| Source Sets | University of Arizona |
| Language | en_US |
| Detected Language | English |
| Type | text, Dissertation-Reproduction (electronic) |
| Rights | Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. |
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