INVOLVEMENT OF PYOCHELIN AND PYOVERDIN IN SUPPRESSION OF PYTHIUM-INDUCED DAMPING-OFF OF TOMATO BY PSEUDOMONAS-AERUGINOSA 7NSK2

The plant growth-promoting rhizobacterium Pseudomonas aeruginosa 7NSK2 produces three siderophores when iron is limited: the yellow-green fl uorescent pyoverdin, the salicylate derivative pyochelin, and salicyli c acid, This Pseudomonas strain was shown to be an efficient antagonis t of Pythium-induced damping-off. The role of pyoverdin and pyochelin in the suppression of Pythium splendens was investigated by using vari ous siderophore-deficient mutants derived from P. aeruginosa 7NSK2 in a bioassay with tomato (Lycopersicon esculentum). To provide more insi ght into the role of pyochelin in antagonism, mutant KMPCH, deficient in the production of pyoverdin and pyochelin, was complemented for pyo chelin production. The complementing clone was further characterized b y subcloning and transposon mutagenesis and used to generate a pyochel in-negative, pyoverdin-positive mutant by marker exchange, All mutants were able to reduce Pythium-induced preemergence damping-off to some extent, Production of either pyoverdin or pyochelin proved to be neces sary to achieve wild-type levels of protection against Pythium-induced postemergence damping-off. Mutant KMPCH inhibited P. splendens but wa s less active than the parental strain. This residual protection could be due to the production of salicylic acid. Since pyoverdin and pyoch elin are both siderophores, siderophore-mediated iron competition coul d explain the observed antagonism and the apparent interchangeability of the two compounds. We cannot, however, exclude the possibility that both siderophores act in an indirect way.