Impact of siderophore production by Pseudomonas syringae pv. syringae 22d/93 on epiphytic fitness and biocontrol activity against Pseudomonas syringae pv. glycinea 1a/96

The use of naturally occurring microbial antagonists to suppress plant diseases offers a favorable alternative to classical methods of plant protection. The soybean epiphyte, Pseudomonas syringae pv. syringae strain 22d/93 (Pss22d), shows a strong potential to control P. syringae pv. glycinea (Psg), the causal agent of bacterial blight of soybean. Its antagonism against Psg being highly reproducible even in field trials, the involved suppression mechanisms are of special interest. In the current work we demonstrated that Pss22d produced a significant higher amount of siderophores than the pathogen Psg. While both Pss22d and Psg produce the same siderophores, achromobactin and pyoverdin, regulation of siderophore biosynthesis strongly differs. Epiphytic fitness of Pss22d mutants defective in siderophore biosynthesis was determined following spray inoculation of soybean leaves. The siderophore-negative mutant Pss22dDeltaSid reached a population size two orders of magnitude lower than that of the wild type 10 days after inoculation. This growth deficiency was compensated when wound inoculation was applied, indicating the availability of iron in the presence of small lesions on the leaves. Our results suggest an indirect effect of siderophore production on the biocontrol ability of Pss22d. Though siderophore-defective mutants of Pss22d still suppressed development of bacterial blight caused by Psg, siderophore production enhanced the epiphytic fitness and thereby, the competitiveness of the antagonist.