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