Role of gliotoxin in plant root colonization and induction of systemic resistance by Trichoderma virens

Trichoderma virens colonizes the outer root tissues of a wide range of plants. This interaction is generally beneficial, often promoting growth and systemically priming plant defenses against infection by pathogens. Q strains of T. virens produce the epipolythiodioxopiperazine (ETP) metabolite gliotoxin, while P strains produce, instead, a different ETP, gliovirin (Sherkane et al. 2017, ChemistrySelect). Mutants in the GliP locus encoding the non-ribosomal peptide synthetase of the gliotoxin biosynthetic gene cluster of the reference Q strain Gv29-8 were constructed by homologous recombination: these strains produced no gliotoxin (Vargas et al. 2014, Microbiology). While characterizing the ability of Q and P strains to colonize Arabidopsis roots, we noted that several Q strains did not promote growth in soil, and in plate assays even destroyed the seedlings. To answer the question of whether gliotoxin is a major factor in this phytotoxic effect of Trichoderma, we compared the interaction of wild type Gv29-8 and its gliP mutants (Vargas et al. 2014) with Arabidopsis and tomato seedlings. Apparently, seedlings treated with Trichoderma mutants that do not biosynthesize gliotoxin survive much better than seedlings exposed to their corresponding wild-type strain. Moreover, both mutants and wild-type strains activate the plant’s induced systemic resistance. We are following up an initial observation that only the wild-type activates systemic acquired resistance (SAR) - related genes. Secondary metabolite production is thus an important factor to take into account in development of biocontrol strains of T. virens. The ETP biosynthetic clusters are not the only differences between the genomes of P and Q strains, and we are beginning to characterize other strain-specific differences in genomic content and gene expression.