Annette Wensing

• Suppression of a plant pathogen's growth in planta by naturally occurring antagonists offers a favorable alternative to classical methods of plant protection. The soybean epiphyte, Pseudomonas syringae pv. syringae strain 22d/93 (Pss22d), exhibits a strong potential to control P. syringae pv. glycinea 1a/96 (Psg1a), the causal agent of bacterial blight of soybean. While suppression of growth of P. syringae pv. glycinea by Pss22d has been shown in greenhouse system as well as under field conditions, underlying mechanism(s) remained unknown. In frame of this work the influence of siderophore production on the biological control system was investigated. Although antagonist and pathogen produce the typical pyoverdin-type siderophore of P. syringae pathovars, they showed clearly distinct iron-chelating phenotypes. Mutational analysis of siderophore production in Pss22d led to the identification of an additional siderophore system with high sequence similarities towards achromobactin biosynthesis genes of Dickeya chrysanthemi. In vitro production of both siderophores by Pss22d was analyzed in detail. The distribution of this second siderophore among different pathovars of P. syringae was investigated. It could be demonstrated that many but not all P. syringae strains produce it. Interestingly, Psg1a belongs to the strains tested positive for the achromobactin-like siderophore. In consequence, pathogen Psg and antagonist Pss22d produced the same set of siderophores and a direct role of competition for iron in the examined biocontrol system could be excluded. Finally, the impact of the two siderophores of Pss22d on in planta performance of respective siderophore mutants was investigated. This study presents novel information on siderophore production of plant-associated bacteria as well as on the role of iron supply for epiphytic fitness and biological control in the phyllosphere.