Mechanism of antagonism by Streptomyces griseocarneus (strain Di944) against fungal pathogens of greenhouse-grown tomato transplants

Abstract

Antagonism by Streptomyces griseocarneus Benedict (strain Di944), a filamentous bacterium isolated from the rhizosphere of a field tomato plant, was investigated in culture and in the rhizospheres of tomato to understand the mechanisms of suppression of damping-off and root rot of tomato plug transplants. The bacterium inhibited Rhizoctonia solani Kühn, the oomycete pathogens Pythium and Phytophthora spp., and fungal pathogens including Alternaria alternata, Botrytis cinerea, Fusarium oxysporum f. sp. lycopersici, F. oxysporum f. sp. radicis-lycopersici, F. solani, Thielaviopsis basicola and Verticillium dahlia, but not bacterial pathogens of tomato as determined by agar diffusion bioassay. Standard physicochemical analyses of the metabolites extracted from cell-free culture filtrates of S. griseocarneus Di944 identified the presence of an antifungal compound, a pentaene macrolide complex, designated as rhizostreptin. This antifungal compound was also detected by ultra-violet spectrophotometry and high performance liquid chromatography in the extract from the rhizospheres of tomato transplants grown from seeds treated with S. griseocarneus Di944 and, thus, found to be suppressive to damping-off caused by R. solani. Germination of spores and growth of mycelia of pathogenic fungi of tomato were inhibited by rhizostreptin at concentrations between 0.5 and 2 µg mL−1. Rhizostreptin was found to be more fungitoxic than other known polyene macrolides, amphotericin-B, nystatin, filipin and candicidin-type heptaene, as determined by agar diffusion bioassay. A substantial growth of S. griseocarneus Di944 in mineral medium supplemented with cell-wall components of R. solani indicated the ability of the bacterium to utilize fungal cell-wall components as carbon and nitrogen sources. This was further confirmed by the production of hydrolytic enzymes chitinase, glucanase, phospholipase and proteinase, by S. griseocarneus Di944 grown in mineral medium supplemented with glucose and (NH4)2SO4 or cell-wall components of R. solani as carbon and nitrogen sources. Secretion of extracellular antifungal pentaene macrolide and fungal cell-wall degrading enzymes is believed to be the major mechanism by which the necrotrophic mycoparasite S. griseocarneus Di944 inhibits fungi and oomycetes responsible for damping-off and root rot of tomato transplants. However, studies involving mutant strains of S. griseocarneus Di944 that are incapable of producing antifungal pentaene and fungal cell-wall degrading enzymes are needed to further confirm the mechanisms of antagonism by S. griseocarneus Di944.