Role of Siderophores in Suppression ofPythiumSpecies and Production of Increased-Growth Response of Wheat by Fluorescent Pseudomonads

Abstract

About 7% of nearly 5,000 strains of baceria isolated from roots of wheat produced a zone of inhibition (.gtoreq. 10 mm) against Pythium ultimum var. Sporangiiferum (pathogenic to wheat) on either King''s medium B (KMB), potato-dextrose agar (PDA), or both media. One-third of these strains, in turn, resulted in 10-30% taller seedlings (equal to the response to metalaxyl) when applied at 107-108 colony-forming units per wheat seed sown in a Thatuna silt loam (pH 5.3 in 0.01 M CaCl2) naturally infested with Pythnium. None of the isolates were positive for growth promotion on wheat in the same soil made Pythium-free with methyl bromide. Of 10 growth-promoting fluorescent pseudomonads, six were inhibitory to one or more of seven Pythium isolates (representing five species or varieties, all pathogenic on wheat) on KMB and four on both KMB and PDA. All seven Pythium isolates were inhibited on KMB amended with ethylene-diamine-di-o-hydroxyphenylacetic acid (EDDA), but none were inhibited on KMB amended with EDDA+FeCl3 at 50 .mu.g/ml, nor by any of the six fluorescent pseudomonads on KMB amended with FeCl3 at 10 .mu.g/ml. Strain B324, with characteristics intermediate between Pseudomonas fluorescens and P. putida, was inhibitory to all seven Pythium isolates on KMB but not on PDA and produced a strong growth-promoting effect on wheat in the Thatuna silt loam that was nullified by FeCl3. None of five siderophore-negative mutants, produced by N-methyl-N''-nitro-N-nitrosoguanidine mutagenesis of B324, were inhibitory of Pythium on KMB, and none produced a significant growth response on wheat in natural soil. The results indicate that the plant growth-promoting activity of some strains of fluorescent pseudomonads on wheat results from ability of the strains to suppress Pythium by production of siderophores, including in relatively low-pH soil.