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Trichoderma virens, an avirulent plant symbiotic fungus ubiquitous in soils \nworldwide, is remarkable as it induces systemic resistance in plants, enhances plant \ngrowth, and acts as a mycoparasite of plant-pathogenic fungi. For all microorganisms, \nthe ability to acquire environmental iron is essential for fitness and survival. The ability \nof T. virens to acquire iron from the soil determines its success as a beneficial \nmicroorganism for plants and saprophyte of organic material. The most successful \nstrategy developed by microbes for iron acquisition has been the production of \nsiderophores, secondary metabolites that bind iron tightly and are produced in both \nintracellular and extracellular forms. Extensive genomic analysis of the genome \nsequence of T. virens revealed the presence of three genes, Tex10, Tex20, and Tex21 that \nencode siderophore-producing enzymes known as non-ribosomal peptide synthetases \n(NRPSs), but their regulation and function are largely unknown. To study the function of \nthese secondary metabolites in the life strategy of T. virens, two genes encoding the \nNRPSs for the biosynthesis of siderophores (Tex10 and Tex20) were disrupted by fungal \ntransformation. While the Tex20 mutants are phenotypically very similar to wild type, \nthe Tex10 mutant shows some striking differences. Additionally, the Tex10 mutant \nshows less ability to respond to oxidative stress. Gene expression of these NRPSencoding \ngenes was also analyzed in wild type and mutant strains. Siderophore \nproduction reaches a maximum at three days in iron depleted medium, while cultures \ngrown in ferrated medium show a dramatic reduction in siderophore production, as well \nas NRPS gene expression. Correspondingly, several NRPS-encoding gene deletion \nstrains reveal both lower mRNA and siderophore levels. Siderophore production at the \npeak expression time was corroborated by High Performance Liquid Chromatography. \nThis information is essential for the enhancement of the beneficial capabilities of T. \nvirens for global agricultural improvement.