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

Altomare, C.^1,2, Norvell, W. A.³, Björkman, T.¹ and Harman, G. E.^1,4

ABSTRACT   We investigated the capability of the plant-growth promoting and biocontrol fungus Trichoderma harzianum Rifai strain 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, Zn metal and rock phosphate (mostly calcium phosphate) in a liquid sucrose-yeast extract medium, as determined by inductively-coupled plasma emission spectroscopy. 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 also solubilized by cell-free culture filtrates. The chelating activity of T-22 culture filtrates was determined by a method based on the 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. 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 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 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. Solubilization of metal oxides by Trichoderma involves both chelation and reduction. Both of these mechanisms are also 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.
 

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