Identification of Genes from Trichoderma Virens Involved in the Colonization of Maize Roots and Induced Systemic Resistance

Abstract

The filamentous fungus, Trichoderma virens, is a well-known mycoparasitic plant symbiont, valued for its biocontrol capabilities. T. virens initiates a symbiotic relationship with a plant host through the colonization of its roots. To achieve colonization, the fungus must communicate with the host and evade its innate defenses. In this study, I explored genes involved with the colonization process through transcriptomic profiling and reverse genetic characterization of genes of interest. Transcriptome profiles of the T. virens colonization of maize roots over time revealed that 24 hours post inoculation appeared to be a key time for plant-microbe communication, with many key gene categories peaking in differential expression. The transcriptomic profiles of Sm1 and Sir1 deletion mutants in the presence of plants demonstrated that Sir1, rather than Sm1, appears to be the key regulator of the fungal response to maize. Additionally, we developed a novel algorithm to select potential colonization related gene targets for characterization. About 40% of the genes identified by the algorithm would have been missed using previous methods for selecting genes. The class I hydrophobin, HFB9a, from T. virens plays a potential role in root colonization as the deletion mutants colonized less than the wild-type strain. The mutants were also unable to induce systemic resistance against Colletotrichum graminicola, and showed decreased cell wall degrading enzyme activity, which could be complemented by the purified HFB9a protein. HFB9a protein also induced phosphorylation of Arabidopsis MAP kinases, suggesting it might function as a microbe-associated molecular pattern to trigger immune responses. The mutants of a T. virens non-ribosomal peptide synthetase, Tex7, resulted in the production of a large amount of the antibiotics heptelidic acid and viridin. Heptelidic acid is not typically produced by the wild-type T. virens strain and is used as a characteristic for classification of T. virens strains. In addition to heptelidic acid production by the mutant, a small amount of the compound was detected in the wild-type strain, suggesting that this compound should not be used for strain classification. Finally, plants treated with Tex7 deletion mutants were significantly smaller than control and wild-type T. virens treated plants.