Gliovirin

The bicyclic tetrahydro-1,2-oxazine subunit of gliovirin is synthesized through a diastereoselective copper-catalyzed cyclization of an N-hydroxyamino ester. Oxidative elaboration to the fully functionalized bicycle was achieved through a series of mild transformations. Central to this approach was the development of the first catalytic, enantioselective propargylation of an oxime to furnish a key N-hydroyxamino ester intermediate.

Trichoderma virens produces viridin/viridiol, heptelidic (koningic) acid, several volatile sesquiterpenes and gliotoxin (Q strains) or gliovirin (P strains). We earlier reported that deletion of the terpene cyclase vir4 and a glyceraldehyde-3-phosphate dehydrogenase (GAPDH, designated as vGPD) associated with the "vir" cluster abrogated the biosynthesis of several volatile sesquiterpene metabolites. Here we show that, the deletion of this GAPDH also impairs the biosynthesis of heptelidic acid (a non-volatile sesquiterpene), viridin (steroid) and gliovirin (non-ribosomal peptide), indicating regulation of non-volatile metabolite biosynthesis by this GAPDH that is associated with a secondary metabolism gene cluster. To gain further insights into the details of this novel form of regulation, we identified the terpene cyclase gene responsible for heptelidic acid biosynthesis (hereafter designated as has1) and prove that the expression of this gene is regulated by vGPD. Interestingly, deletion of has1 impaired biosynthesis of heptelidic acid (HA), viridin and gliovirin, but not of volatile sesquiterpenes. Deletion of the vir cluster associated terpene cyclase gene (vir4), located next to the vGPD gene, did not impair biosynthesis of HA, viridin or gliovirin. We thus unveil a novel circuitry of regulation of secondary metabolism where an HA-tolerant GAPDH isoform (vGPD) regulates HA biosynthesis through the transcriptional regulation of the HA-synthase gene (which is not part of the "vir" cluster). Interestingly, impairment of HA biosynthesis leads to the down-regulation of biosynthesis of other non-volatile secondary metabolites, but not of volatile secondary metabolites. We thus provide evidence that the "vir" cluster associated, HA-tolerant GAPDH in T. virens participates in the biosynthesis of volatile sesquiterpenes as a biosynthetic enzyme, and regulates the production of non-volatile metabolites via regulation of HA biosynthesis. The orthologue of the "vir" cluster in Aspergillus oryzae was earlier reported to synthesize HA by another group. Our study thus proves that the same gene cluster can code for unrelated metabolites in different species.

Trichoderma spp. are universal saprotrophic fungi in terrestrial ecosystems, and as rhizosphere inhabitants, they mediate interactions with other soil microorganisms, plants, and arthropods at multiple trophic levels. In the rhizosphere, Trichoderma can reduce the abundance of phytopathogenic microorganisms, which involves the action of potent inhibitory molecules, such as gliovirin and siderophores, whereas endophytic associations between Trichoderma and the seeds and roots of host plants can result in enhanced plant growth and crop productivity, as well as the alleviation of abiotic stress. Such beneficial effects are mediated via the activation of endogenous mechanisms controlled by phytohormones such as auxins and abscisic acid, as well as by alterations in host plant metabolism. During either root colonization or in the absence of physical contact, Trichoderma can trigger early defense responses mediated by Ca2+ and reactive oxygen species, and subsequently stimulate plant immunity by enhancing resistance mechanisms regulated by the phytohormones salicylic acid, jasmonic acid, and ethylene. In addition, Trichoderma release volatile organic compounds and nitrogen or oxygen heterocyclic compounds that serve as signaling molecules, which have effects on plant growth, phytopathogen levels, herbivorous insects, and at the third trophic level, play roles in attracting the natural enemies (predators and parasitoids) of herbivores. In this paper, we review some of the most recent advances in our understanding of the environmental influences of Trichoderma spp., with particular emphasis on their multiple interactions at different trophic levels.