Extracellular Protease of Pseudomonas fluorescens CHA0, a Biocontrol Factor with Activity against the Root-Knot Nematode Meloidogyne incognita

In Pseudomonas fluorescens CHA0, mutation of the GacA-controlled aprA gene (encoding the major extracellular protease) or the gacA regulatory gene resulted in reduced biocontrol activity against the root-knot nematode Meloidogyne incognita during tomato and soybean infection. Culture supernatants of strain CHA0 inhibited egg hatching and induced mortality of M. incognita juveniles more strongly than did supernatants of aprA and gacA mutants, suggesting that AprA protease contributes to biocontrol. Plant diseases caused by soilborne root pathogens account for major crop losses worldwide. Yet in a small number of environments, i.e., in suppressive soils, little or no disease is observed, despite the presence of pathogens. Disease suppression depends, in part, on microorganisms that are able to antagonize pathogens (5, 10, 14, 28). The root-colonizing bacterium Pseudomonas fluorescens CHA0, which was isolated from a suppressive soil, has been studied in detail as a model strain for the biological control of several fungal plant diseases, such as black root rot of tobacco and take-all disease of wheat (5, 27). In this strain, as well as in other biocontrol pseudomonads, antifungal secondary metabolites, e.g., 2,4-diacetylphloroglucinol, hydrogen cyanide, and pyoluteorin, are important for biocontrol activity. These biocontrol factors are synthesized in response to environmental conditions and to population densities of the producer strain, whereby the GacS/ GacA two-component system exerts a crucial role as a positive control element (6, 8, 9, 11, 26). Some rhizosphere microorganisms, including P. fluorescens CHA0, can also act as antagonists of plant-pathogenic nematodes (23). For antagonistic fungi, this biological control has been shown to involve extracellular proteases (2, 21). In strain CHA0, the production of the major extracellular EDTA-sensitive protease, AprA, is controlled by the GacS/GacA signal transduction pathway (8, 17, 26, 29). The present study was undertaken to find out whether this enzyme contributes to the biocontrol properties of strain CHA0 in plant-nematode interactions.

Keywords:Pseudomonasfluorescens,aprAgene,gacAregulatorygene,pathogens,Meloidogyne incognita,biocontrol pseudomonads, antifungal secondary metabolites.

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E-mail: Stephan.Heeb @nottingham.ac.uk