Plant Pathology (1997) 46, 80-90

P. Schmidli-Sacherer, C. Keel and G. Défago (1997)
The global regulator GacA of Pseudomonas fluorescens CHA0 is required for suppression of root diseases in dicotyledons but not in Gramineae
Plant Pathology 46 (1), 80-90
Abstract: Pseudomonas fluorescens strain CHA0 suppresses various plant diseases caused by soil-borne fungi. The pseudomonad produces the antimicrobial metabolites 2,4-diacetylphloroglucinol (Phl), pyoluteorin (Plt) and hydrogen cyanide, which are important for disease suppression, as well as the siderophores pyoverdine (Pvd), salicylic acid (Sal) and pyochelin (Pch). In the current work, a derivative of CHA0 with a mutation in the global regulator gene gacA (GacA^-), which is unable to produce Phl, Plt and HCN, failed to protect the dicotyledonous plants cress and cucumber against damping-off caused by Pythium ultimum. In contrast, the GacA^- mutant could still protect the Gramineae wheat and maize against damping-off mediated by the same strain of P. ultimum, and wheat against take-all caused by Gaeumannomyces graminis. However, the GacA^- mutant overproduced Pch and Pvd. To gain more insight into disease protection afforded by the GacA^- mutant, a GacA^- Pvd^- double mutant (strain CHA496) was constructed by gene replacement. Strain CHA496 overproduced Pch and Sal compared with CHA0 and protected wheat against P. ultimum and G. graminis, whereas cress and cucumber were not protected. Addition of FeCl₃ repressed Pch and Sal production by strain CHA496 in vitro and impaired the protection of wheat in soil microcosms. In conclusion, a functional gacA gene was necessary for the protection of dicotyledons against root diseases, but not for that of Gramineae. Results indicated also that Pch and/or Sal were involved in the ability of the GacA^- Pvd^- mutant of CHA0 to suppress root diseases in Gramineae.
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Research topic(s) for pests/diseases/weeds:
biocontrol - natural enemies
Research topic(s) for beneficials or antagonists:
molecular biology - genes

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Pest and/or beneficial records:

Beneficial	Pest/Disease/Weed	Crop/Product	Country	Quarant.
	Gaeumannomyces tritici	Wheat (Triticum)
	Globisporangium ultimum	Maize/corn (Zea mays)
	Globisporangium ultimum	Wheat (Triticum)
	Globisporangium ultimum	Cucumber (Cucumis sativus)
Pseudomonas fluorescens (antagonist)	Gaeumannomyces tritici	Wheat (Triticum)
Pseudomonas fluorescens (antagonist)	Globisporangium ultimum	Maize/corn (Zea mays)
Pseudomonas fluorescens (antagonist)	Globisporangium ultimum	Wheat (Triticum)