Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/21/1999
Publication Date: N/A
Interpretive Summary: Some rhizosphere organisms have the ability to colonize plant roots, stimulate growth and protect the root from attack by pathogens. A strain of the biocontrol fungus Trichoderma harzianum Rifai 1295-22 (T-22) has shown the ability to solubilize in vitro some insoluble or sparingly soluble minerals. Using mechanisms of chemical reduction and metal complex formation, cultures of T-22 and clear filtrates of these culture solutions have shown the capacity to increase the solubility of manganese dioxide, rock phosphate, and zinc metal, but not the iron mineral hematite. These reactions may be involved in the apparent ability of this fungus to stimulate plant growth as well as to reduce susceptibility to root-attacking pathogens.
Technical Abstract: The plant-growth promoting and biocontrol fungus Trichoderma harzianum Rifai strain 1295-22 (T-22) has the ability to solubilize in vitro some insoluble or sparingly soluble minerals. Three mechanisms were investigated: acidification of the medium, production of chelating metabolites and redox activity. T-22 was able to solubilize MnO2, Zn metal and rock phosphate (mostly calcium phosphate) in a liquid sucrose-yeast extract medium. Acidification was not the major mechanism of solubilization as shown by the fact that the pH of cultures never fell below 5.0, and in cultures containing MnO2 the pH rose from 6.8 to 7.4. Organic acids were not detected by HPTLC in the culture filtrates. Fe2O3, MnO2, Zn metal and rock phosphate were solubilized also by cell-free culture filtrates. The chelating activity of T-22 culture filtrates was determined by a method based on the complex: chrome azurol S. Size exclusion chromatography of culture filtrates indicated the presence of a complexed form of Fe. In liquid culture, T. harzianum strain T-22 also produced diffusible metabolites capable of reducing Fe(III) and Cu(II), as determined by the formation of complexes: Fe(II) with bathophenanthrolinedisulfonic acid and Cu(I) with 2,9-dimethyl-4,7-diphenyl-1,10-phenanthrolinedisulfonic acid. This is the first report of the capability of a Trichoderma strain to solubilize insoluble or sparingly soluble minerals. This activity may explain, at least partially, the ability of T-22 to increase plant growth. Complexation and reduction are known to play a role in biocontrol of plant pathogens, and they may be part of a multiple component action exerted by T-22 in order to achieve effective biocontrol under a variety of environmental conditions.