Solubilization of phosphates and micronutrients by the plant-growth-promoting and biocontrol fungus Trichoderma harzianum Rifai 1295-22

We investigated the capability of the plant-growth-promoting and biocontrol fungus Trichoderma harzianum Rifai 1295-22 (T-22) to solubilize in vitro s ome 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 phosph ate (mostly calcium phosphate) in a liquid sucrose yeast extract medium, as determined by inductively coupled plasma emission spectroscopy. Acidificat ion 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 t o 7.4. Organic acids were not detected by high-performance thin-layer chrom atography in the culture filtrates. Fe2O3, MnO2, Zn, and rock phosphate wer e also solubilized by cell-free culture filtrates. The chelating activity o f T-22 culture filtrates was determined by a method based on measurement of the equilibrium concentration of the chrome azurol S complex in the presen ce of other chelating substances. A size exclusion chromatographic separati on of the components of the culture filtrates indicated the presence of a c omplexed 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)-Na-2-bathophenanthrolinedi sulfonic acid and CuO)-Na-2-2,9-dimethyl-4,7-diphenyl-1,10-phenanthrolinedi sulfonic acid complexes. This is the first report of the ability of a Trich oderma strain to solubilize insoluble or sparingly soluble minerals. This a ctivity may explain, at least partially, the ability of T-22 to increase pl ant growth. Solubilization of metal oxides by Trichoderma involves both che lation and reduction. Both of these mechanisms also play a role in biocontr ol 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 environm ental conditions.