We investigated the capability of the plant-growth-promoting and biocontrol fungus Trichoderma harzianum
Rifai 1295-22 (T-22) to solubilize in vitro some insoluble or sparingly soluble minerals via three possible
mechanisms: acidification of the medium, production of chelating metabolites, and redox activity. T-22 was
able to solubilize MnO2, metallic zinc, and rock phosphate (mostly calcium phosphate) in a liquid sucroseyeast
extract medium, as determined by inductively coupled plasma emission spectroscopy. Acidification was
not the major mechanism of solubilization since 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 high-performance thin-layer
chromatography in the culture filtrates. Fe2O3, MnO2, Zn, and rock phosphate were also solubilized by
cell-free culture filtrates. The chelating activity of T-22 culture filtrates was determined by a method based on
measurement of the equilibrium concentration of the chrome azurol S complex in the presence of other
chelating substances. A size exclusion chromatographic separation of the components of the culture filtrates
indicated the presence of a complexed form of Fe but no chelation of Mn. In liquid culture, T. harzianum T-22
also produced diffusible metabolites capable of reducing Fe(III) and Cu(II), as determined by the formation
of Fe(II)-Na2-bathophenanthrolinedisulfonic acid and Cu(I)-Na2-2,9-dimethyl-4,7-diphenyl-1,10-phenanthrolinedisulfonic
acid complexes. This is the first report of the ability 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. Solubilization of metal oxides by Trichoderma involves both chelation and reduction.
Both of these mechanisms also play a role in biocontrol of plant pathogens, and they may be part of a
multiple-component action exerted by T-22 to achieve effective biocontrol under a variety of environmental
conditions.