Microbial interactions with plant roots are known to affect profoundly plant nutrient status and, for manganese at least, to affect plant resistance to pathogens (Huber and McCay-Buis 1993). In addition to their biocontrol characteristics, Trichoderma species also exhibit plant-growth-promoting activity (Baker 1989; Chet 1987; Harman and Bjorkman 1998; Inbar et al. 1994; Kleifeld and Chet 1992; Naseby et al. 2000). In spite of their theoretical and practical importance, the mechanisms responsible for the growth response due to Trichoderma have not been investigated extensively. Since growth enhancement has been observed in the absence of any detectable disease (Chang et al. 1986; Harman and Bjorkman 1998; Naseby et al. 2000) and in sterile soil (Windham et al. 1986), it is not thought to be a side effect of suppression of disease or minor plant pathogens. Other mechanisms, including production of hormonelike metabolites and release of nutrients from soil or organic matter, have been proposed (Kleifeld and Chet 1992; Windham et al. 1986).
The plant-growth-promoting capacity of T. harzianum 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 was recently investigated (Altamore et al. 1999). T. harzianum was able to solubilize MnO2, metallic zinc, and rock phosphate (mostly calcium phosphate). Fe2O3, MnO2, Zn, and rock phosphate were also solubilized by cell-free culture filtrates. 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 also produced diffusible metabolites capable of reducing Fe(III) and Cu(II). Solubilization of metal oxides by Trichoderma involves both chelation and reduction (Altamore et al. 1999).
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