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 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 MnO(2), metallic zinc, 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 since the pH of cultures never fell below 5.0 and in cultures containing MnO(2) the pH rose from 6.8 to 7.4. Organic acids were not detected by high-performance thin-layer chromatography in the culture filtrates. Fe(2)O(3), MnO(2), 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)-Na(2)-bathophenanthrolinedisulfonic acid and Cu(I)-Na(2)-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.

本研究探究了植物促生兼生防真菌哈茨木霉（Trichoderma harzianum）Rifai 1295-22（简称T-22）通过三种潜在机制体外溶解部分难溶或微溶矿物的能力：培养基酸化、螯合代谢物生成以及氧化还原活性。经电感耦合等离子体发射光谱法（inductively coupled plasma emission spectroscopy）检测证实，T-22可在液体蔗糖-酵母提取物培养基中溶解二氧化锰（MnO₂）、金属锌以及磷矿粉（主要成分为磷酸钙）。由于培养体系的pH值始终未低于5.0，且含MnO₂的培养体系pH从6.8升至7.4，因此酸化并非主要的矿物溶解机制。高效薄层色谱法（high-performance thin-layer chromatography）未在培养滤液中检测到有机酸。无细胞培养滤液同样可溶解三氧化二铁（Fe₂O₃）、MnO₂、金属锌以及磷矿粉。T-22培养滤液的螯合活性通过基于铬天青S（chrome azurol S）复合物与其他螯合物质共存时的平衡浓度测定的方法得以确定。对培养滤液组分的尺寸排阻色谱分离结果显示，体系中存在结合态铁，但未检测到锰的螯合作用。在液体培养条件下，哈茨木霉T-22还可产生可扩散性代谢物以还原Fe(III)和Cu(II)，这一结果通过Fe(II)-Na₂-浴铜灵二磺酸（bathophenanthrolinedisulfonic acid）以及Cu(I)-Na₂-2,9-二甲基-4,7-二苯基-1,10-菲咯啉二磺酸（2,9-dimethyl-4,7-diphenyl-1,10-phenanthrolinedisulfonic acid）复合物的形成得以验证。本研究为首次报道木霉属菌株具备溶解难溶或微溶矿物的能力。该活性至少可部分解释T-22促进植物生长的作用机制。木霉对金属氧化物的溶解过程同时涉及螯合与还原两种作用。这两种机制在植物病原菌生物防治中同样发挥关键作用，且可能是T-22在多种环境条件下实现有效生物防控的多组分作用体系的组成部分。