trichoderma species - Weizmann Institute of Science

REVIEWSBox 2 | MycoparasitismTrichoderma spp. parasitize a range of other fungi.aThe events leading to mycoparasitism are complex, andtake place as follows: first, Trichoderma strains detectother fungi and grow tropically towards them 88 ;remotesensing is at least partially due to the sequentialexpression of cell-wall-degrading enzymes (BOX 4).Different strains can follow different patterns ofinduction, but the fungi apparently always produce lowlevels of an extracellular exochitinase. Diffusion of thisenzyme catalyses the release of cell-wall oligomers fromRtarget fungi, and this in turn induces the expression offungitoxic endochitinases 21 T,which also diffuse and beginthe attack on the target fungus before contact is actuallymade 56,89 .Once the fungi come into contact,bTrichoderma spp. attach to the host and can coil aroundit and form APPRESSORIA on the host surface. Attachment ismediated by the binding of carbohydrates in theTrichoderma cell wall to lectins on the target fungus 90 .TOnce in contact, the Trichoderma produce severalfungitoxic cell-wall-degrading enzymes 6 , and probablyalso peptaibol antibiotics 91 . The combined activities ofAthese compounds result in parasitism of the targetRfungus and dissolution of the cell walls. At the sites of theappressoria, holes can be produced in the target fungus,and direct entry of Trichoderma hyphae into the lumenof the target fungus occurs. There are at least 20–30 cknown genes, proteins and other metabolites that aredirectly involved in this interaction, which is typical ofthe complex systems that are used by these fungi in theirinteractions with other organisms.In the figure,‘R’ indicates hyphae of the plantpathogen Rhizoctonia solani,‘T’indicates hyphae ofTrichoderma spp., and ‘A’ indicates appressoria-likeRstructures. In a, the Trichoderma strain is in the processof parasitizing a hypha of R. solani. The coiling reactionis typical of mycoparasitic interactions. b shows ahigher magnification of the Trichoderma–R. solaniinteraction, showing the appressoria-like structures,and c shows an R. solani hypha from which theTrichoderma has been removed. The scale bar in brepresents 10 µm; R. solani hyphae are typically 5–6 µmin diameter, and mycoparasitic hyphae of Trichodermaspp. are ~3 µm in diameter.Photomicrograph in b reproduced with permission from REF. 92 © (1987) British Mycological Society.Photomicrograph in c reproduced with permission from REF. 93 © (1983) American Phytopathological Society.AAPPRESSORIASpecialized pressing organs fromwhich a minute infection pegcan grow and infect a cell.PHYTOALEXINSLow-molecular-weightcompounds that haveantimicrobial activity and areproduced by plants in responseto attack by pathogens.dicotyledons, and with different Trichoderma speciesand strains (TABLE 1;see examples in FIGS 1–3). The abilityof T. harzianum strain T-22 to induce systemic resistanceto pathogens in maize (FIG. 2a) is particularly noteworthyas there are, so far as we are aware, no similar reports ofresistance being induced in this crop by any other rootassociatedcommensal or symbiotic microorganism.Therefore, the capacity to induce resistance to a range ofdiseases — which are caused by various classes of plantpathogen (including fungi, bacteria and viruses) — in avariety of plants seems to be widespread in this fungalgenus (TABLE 1). However, much more remains to beunderstood about the specific systems involved.Most of the examples in TABLE 1 show systemicresistance because disease control occurs at a site thatis distant from the location of the Trichoderma.It ismore difficult to examine the role of induced resistanceto pathogens that cause seed or root diseases, as boththe biocontrol organism and the pathogen are locatedat the same site. However, strains of Trichodermavirens have been obtained that differed in their abilitiesto produce antibiotics or to be mycoparasitic 43 .Mutantand wild-type strains were tested for their abilities tocontrol the seedling disease of cotton that is caused byRhizoctonia solani,but neither antibiotic productionnor mycoparasitism was strongly associated withbiocontrol (FIG. 1a).However, the abilities of the strainsto induce terpenoid PHYTOALEXIN defence compounds inthe seedlings were strongly correlated with diseasecontrol (FIG. 1a).NATURE REVIEWS | MICROBIOLOGY VOLUME 2 | JANUARY 2004 | 45