Secondary metabolism in Trichoderma--a genomic perspective.

Trichoderma spp. are a rich source of secondary metabolites (SMs). The recent publication of the genome sequences of three Trichoderma spp. has revealed a vast repertoire of genes putatively involved in the biosynthesis of SMs, such as non-ribosomal peptides, polyketides, terpenoids and pyrones. Interestingly, the genomes of the mycoparasitic species Trichoderma virens and Trichoderma atroviride are enriched in secondary metabolism-related genes compared with the biomass-degrading Trichoderma reesei: 18 and 18 polyketide synthases compared with 11; 28 and 16 non-ribosomal peptide synthetases compared with 10, respectively. All three species produce a special class of non-ribosomally synthesized peptides known as peptaibols, containing non-proteinogenic amino acids (particularly α-aminoisobutyric acid). In common with other filamentous ascomycetes, Trichoderma spp. may require siderophores (also produced by non-ribosomal peptide synthetases) to grow in iron-poor conditions and to compete with their hosts for available iron. Two generalizations can be made about fungal SM genes: they are often found in clusters, and many are not expressed under standard laboratory conditions. This has made it difficult to identify the compounds. Trichoderma, in particular, interacts with other microbes in the soil and with plant roots in the rhizosphere. A detailed metabolomic-genomic study would eventually unravel the roles of many of these SMs in natural ecosystems. Novel genetic tools developed recently, combined with biological understanding of the function of SMs as toxins or signals, should lead to 'awakening' of these 'silent' clusters. Knowledge of the SM repertoire should precede application of Trichoderma strains for biocontrol: some metabolites could be toxic to plants and their consumers, and thus should be avoided. Others could be beneficial, antagonizing pathogens or inducing resistance in crop plants.