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Introduction performance of a control agent during in planta assays under laboratory conditions does not guarantee a successful application in greenhouse or in the field (Montesinos et al., 2002; Shoda, 2000). The performance of a biocontrol agent under changing conditions (climate, vicinity etc.) is often unpredictable and for field applications of biocontrol products varying results are reported (Thomashow, 1996). The outcome of a biological control depends on the complex interaction of host plant, pathogen, control agent, and environment and most of these relations are poorly understood. An increased knowledge on these interactions is key towards reliable application of biological control (Duffy et al., 2003; Handelsman & Stabb, 1996; Montesinos, 2003). Tab. 1. Bacterial biological control products (USA). Commercially available biocontrol products based on bacteria in 2003 (USA only). Source of data: Agrios, 2005 Product name Control agent Crop Disease/Tagert Organism Galltrol Agrobacterium radiobacter Fruit/ornamental nurserey stock, Crown gall/ Agrobacterium tumafaciens strain 84 grape, brambles Nogall Companion Agrobacterium radiobacter strain K1026 Bacillus subtilis str. GB03 and other Fruit/ornamental nurserey stock, nut Crown gall/ Agrobacterium tumafaciens Many in greenhouse and nursery Pytium , Phytophthora , Fusarium , Rhizoctonia HiStick N/T Bacillus subtilis str. MB1600 Legumes Fusarium , Rhizoctonia , Aspergillus Kodiak Bacillus subtilis GB03 Cotton and Legumes Fusarium , Rhizoctonia , Alternaria , Aspergillus Deny Burkholderia cepacia Cotton, Legumes, grain crops Pythium , Rhizoctonia , Fusarium , and nematodes Intercept Burkholderia cepacia Maize, vegetables, cotton Pythium , Rhizoctonia , Fusarium BioJect Spot-Less Pseudomonas aureofaciens turf and other Dollar spot, anthracnose, Pythium , pink snow mold Bio-save 10LP, 110 Pseudomonas syringae Pome fruit, citrus, cherries, potatoes BlightBan A506 Pseudomonas fluorescens A506 Pome and stone fruit, potatoes, tomatoes, strawberries Postharvest Botrytis , Mucor , Penicillium , Geotrichum Frost damage, Erwinia amylovora , russeting bacteria Dagger G Pseudomonas fluorescens Field crops, vegetables Rhizoctonia , Pythium Cedemon Pseudomonas chloraphis Grain cereals Barley, oat leaf spots, Fusarium 5
Introduction 2.1.2 Host, pathogen and antagonist – more than a “ménage à trois” The first step to understand complexity of biological control is the determination of crucial features of the individual relationships; the virulence determinants of the pathogen, defense mechanisms of the host plant, and mode of action of the control agent. If these basics are known, the impact of environmental conditions and competing organisms can be assessed. The best characterized part of the biological control complex is the interaction between plant and pathogen. Since the discovery of microbes as causal agents of infectious plant disease in 1878, numerous plant diseases have been studied for virulence determinants of the pathogens and defense mechanisms of host plants (Burrill, 1878; Montesinos, 2000). The following principles have been determined as common themes in plant disease: Prior to disease development, most pathogens undergo an epiphytic growth phase. The pathogen attaches to the plant surface and proliferates in this environment until it finds an entry site, for example wounds or stomata where the pathogen can overcome structural defense barriers provided by cuticula, cell wall etc. (Agrios, 2005; Montesinos et al., 2002). The next step during disease formation would be the induction of virulence determinants. This genetic switch to the pathogenic life style is tightly regulated. Virulence factors of a pathogen could be perceived by the host and lead to defense induction, thus they are only expressed under disease-favoring conditions after the pathogen has reached a certain threshold population size. Quorum sensing is considered an important regulation mechanism for many virulence factors (Lugtenberg et al., 2002; Montesinos, 2000; Staskawicz et al., 2001). Most pathogen attacks do not result in disease formation. In case the host plant recognizes the pathogen, it starts an immediate defense reaction, the so-called “hypersensitive response” (HR). HR is a localized reaction of the infected tissue. The affected areas undergo programmed cell death consisting of cellular electrolyte leakage and “oxidative burst” i.e. the rapid formation of reactive oxygen species (ROS). Apart from the direct damage to the pathogen, 6
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Page 3 and 4: Acknowledgements This project was s
Page 5 and 6: Abbreviations Ach achromobactin-lik
Page 7 and 8: Contents 4.1.2 Pseudomonas syringae
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Page 21 and 22: Introduction Fe 2+ + H 2 O 2 → Fe
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Results Nevertheless the site of tr
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Results Achr2_KpnI_rev binding site
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Results Interestingly the mutant Ps
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Results XAD4-purification yielded i
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Results After changing the mobile p
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Results P P A Fig. 21. Isoelectic f
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Results reduced in comparison to th
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Results A-E represents single inocu
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Results Tab. 10. Development of pop
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Results Tab. 11. Distribution of th
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Discussion bacteria with redundande
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Discussion 6.2 Possible regulation
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Discussion stationary phase transit
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Discussion and bacterium (Loper et
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Literature Bultreys, A., Gheysen, I
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Literature Galperin, M. Y. (2006).
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Literature Mazzola, M. (2002). Mech
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Literature Schubert, S., Rakin, A.,
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Literature Zou, J., Rodriguez-Zas,
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