Trichoderma as a Sustainable Plant Growth Promoter: Mechanisms of IAA Production, Phosphate Solubilization, and Siderophore Activity.
The growing global population and rising food demand have intensified reliance on chemical fertilizers, leading to soil degradation, environmental pollution, and heavy metal accumulation. As a sustainable alternative, Trichoderma spp. have gained prominence as bioinoculants due to their diverse plant growth-promoting abilities. However, emerging evidence highlights strain-specific variability and regulatory complexity in their beneficial traits, aspects that remain insufficiently addressed in current literature. This review explores the molecular basis of three key mechanisms: (i) indole-3-acetic acid (IAA) biosynthesis through indole-3-pyruvic acid (IPA), indole-3-acetamide (IAM), and tryptophan-independent pathways; (ii) phosphate solubilization mediated by phytase, phosphatase, and organic acid production; and (iii) iron (Fe) acquisition through siderophore production, primarily synthesized by nonribosomal peptide synthetases (NRPSs). Not all Trichoderma species exhibit the full spectrum of plant growth-promoting traits; while some are efficient in phosphate solubilization, others may primarily contribute through IAA production or siderophore production. Genomic and bioinformatic studies have revealed that T. atroviride, T. asperellum, T. virens, T. harzianum, T. citrinoviride, T. reesei, T. longibrachiatum, and T. koningiopsis harbor putative genes encoding proteins homologous to enzymes associated with IAA production, such as flavin monooxygenase and indoleamine-2,3-dioxygenase, as well as genes involved in siderophore production, including sidA, sidF, sidL, sidC, sidD, and sidG. However, the genetic and regulatory mechanisms underlying IAA production and phosphate solubilization in Trichoderma remain poorly characterized, with limited genomic information available on genes encoding auxin-related enzymes, phytases, acid phosphatases, and those involved in organic acid biosynthesis. These findings highlight the importance of targeted molecular research to support the development of strain-specific bioformulations that enhance nutrient use efficiency, reduce chemical inputs, and advance sustainable agricultural practices.