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Introduction the following necrosis of the adjacent tissue cuts the pathogen off from its nutrients. It thus is unable to spread and multiply and will finally die. (Hutcheson, 2001; Klement, 1963). The localized response is followed by a systemic defense reaction, which is mediated by the plant messenger molecules salicylic acid (Baker et al., 1997). This reaction is referred to as systemic acquired resistance (SAR; (Ross, 1962; van Loon et al., 1998a). A pathogen that is recognized by the plant is called “incompatible” to the respective host plant. The plant is non-susceptible. In case of a so-called “compatible” pathogen host interaction, the pathogen suppresses or modifies plant defense in such a way, that it is not attacked (Staskawicz et al., 2001). It proliferates causing visible disease symptoms as it nourishes on plant material. New inoculum is spread from the infested tissue and the disease cycle continues (Fig.2). Fig. 2. Life cycle of plant pathogens. The simplified life cycle of plant pathogenic microorganisms divides into two parts. During the epiphytic phase (left half of the cycle) the pathogen is distributed to new locations were it colonizes plant surfaces. Under favourable environmental conditions and in the presence of suitable entry sites, the pathogen invades the plant tissue. In an incompatible system the pathogen is recognized by the host and plant defence is activated. During hypersensitive response (HR) the pathogen is killed and the respective plant tissue becomes necrotic. The surviving plant enters a state of enhanced disease resistance (SAR). In a compatible system the pathogen remains undetected or is able to manipulate plant defence reactions. After achieving a threshold density it enters into the pathogenic phase (right half of the cycle). By expression of virulence factors it attacks surrounding tissue and further proliferates on the nutrients gained from the plant. The pathogen is distributed from the infected tissue and again enters into the epiphytic phase. 7
Introduction The relationship between plants and epiphytic microorganism is understood to a much lesser extent. One well-characterized feature is the correlation between root exudates and the high number of microorganisms associated to the rhizosphere. By this mechanism plants are able to recruit biocontrol agents, according to the type of pathogen that threatens the plant (Weller et al., 2002). An example for this is the frequently observed spontaneous decline of take-all disease. Take-all disease of wheat is caused by the fungus Gaeumannomyces graminis var. tritici which results (as the name implies) in disastrous losses. The take-all decline is defined as the spontaneous decrease in the incidence and severity of take-all that occurs with monoculture of wheat or other susceptible host crops after one or more severe outbreaks of the disease (Mazzola, 2002). It was observed many times in different locations all over the world and has been correlated to 2,4-diacetylphloroglucinol (DAPG)- producing bacteria (Keel et al., 1996). The relative abundance of DAPG producers in the wheat rhizosphere increases significantly during take-all disease, while it does not increase if the pathogen is absent. Furthermore, a temporal break in monocultures of wheat by a non-susceptible crop leads to a decrease in DAPG producer populations and a loss of take-all suppression by the respective soil. This stresses the active part of the host plant in supporting the biocontrol agent (Mazzola, 2002; Weller et al., 2002). Rhizobacteria can actively support the growth of their host plants. The respective group of bacteria is called plant growth-promoting rhizobacteria (PGPR). They produce plant hormones that influence root morphogenesis and result in the overproduction of root hairs and lateral roots. The subsequent increase of ion uptake accompanied by enhanced solubilisation of this ions due to microbial activity results in enhanced plant nourishment and subsequently in better plant health and growth (Persello-Cartieaux et al., 2003; Schippers et al., 1987). Screening of PGNRs for biological control abilities revealed an interesting new mechanism of induced plant resistance. Some PGNRs induce a plant defense reaction similar to SAR. This so-called induced systemic 8
Page 1 and 2: Siderophore production of Pseudomon
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
Page 9 and 10: Contents 5.3 Influence of sideropho
Page 11 and 12: Introduction 2 Introduction Like hu
Page 13 and 14: Introduction In contrast to the rhi
Page 15: Introduction 2.1.2 Host, pathogen a
Page 19 and 20: Introduction developement of resist
Page 21 and 22: Introduction Fe 2+ + H 2 O 2 → Fe
Page 23 and 24: Introduction 1997a; May et al., 199
Page 25 and 26: Introduction Pss22d demonstrated a
Page 27 and 28: Introduction 2.4 Aim of this study
Page 29 and 30: Material and Methods Tab. 4. Antibi
Page 31 and 32: Material and Methods Iron-free 5b m
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Page 35 and 36: Material and Methods 3.6 Plasmids T
Page 37 and 38: Material and Methods PCR-Screening
Page 39 and 40: Material and Methods on ice for 10
Page 41 and 42: Material and Methods stained in eth
Page 43 and 44: Material and Methods 4.2.5.3 Conver
Page 45 and 46: Material and Methods 4.2.5.9 Ligati
Page 47 and 48: Material and Methods strains were s
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Page 51 and 52: Material and Methods gentle shaking
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Page 61 and 62: Results Yersiniabactin is a siderop
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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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