trichoderma species - Weizmann Institute of Science

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REVIEWSAlthough many Trichoderma strains are likely tohave this ability 13,15 , the greatest long-term effectsprobably occur with rhizosphere-competent strains. Inthe greenhouse industry, T. harzianum strain T-22 iswidely used for disease control instead of chemicalfungicides because it is safer to use for growers, its disease-controleffects last longer than those of syntheticchemical pesticides (FIG. 2b) — so it is less costly thanchemical fungicides — and root growth can be as good,or better, than that achieved using pesticides 16 (FIG. 2b).Increased root development and plant growth. In bothacademic research and commercial practice, strain T-22has been shown to increase root development in maizeand numerous other plants 16,72 (FIG. 2b–d).This effectlasts for the entire life of annual plants and can beinduced by the addition of small amounts of fungus(less than 1 g ha –1 ) applied as a seed treatment. Forexample, maize plants were grown in the field fromseeds that were either untreated or treated with T-22.Several months later, when the plants were about 2 mtall, trenches were dug adjacent to the rows and thefrequencies of root intercepts on the side of the furrowwere determined. The presence of root-colonizing T-22induced about twice as many deep-root intercepts25–75 cm below the soil surface (FIG. 2d).This resultedin increased drought tolerance, and probably resistanceto compacted soils 16 .The ability of T-22 and otherstrains of Trichoderma to induce increased root formationis not restricted to maize or greenhouse crops, andthe growth of these plants can be enhanced by thepresence of other beneficial root-colonizing microorganisms.Synergy between mycorrhizal fungi andT-22 has been shown 74,75 , as well as synergy betweenTrichoderma enzymes and bacterial antibiotics 76 .Moreover, mixtures of different root-colonizing biocontrolagents can provide better results than any oneagent used on its own 77 .However, the abilities of combinationsof beneficial root-colonizing microorganismsto improve plant performance have been inadequatelyexamined in either managed or natural ecosystems.Such improvements in root development arefrequently associated with increases in yield and biomass.There have been more than 500 commercial andacademic trials on the effects of T-22 on maize, and theaverage increase in yield over those obtained usingtypical agricultural practices was ~5% (REF. 72).Increasesin plant growth were also frequently seen in other fieldcrops, as well as in greenhouse crops 16 (FIG. 2b).Trichoderma treatments therefore have the potential toimprove overall crop yields and might be particularlyimportant in suboptimal field conditions. Generally, theincreased yield of plants is more evident under stressfulconditions; when modern crop plants are grown undernear-optimal conditions there is little opportunity foryield improvement. However, even under near-optimalconditions, seed treatments of maize sometimes giveimproved yields 72 .At least in maize, there is a strong genetic componentto the yield and plant-growth enhancement that iselicited by T-22. FIGURE 2c shows the response of theinbred line Mo17 to T-22. This line is one of the mostpositive responders; other lines respond only weakly,and a few actually show a reduction in growth andyield 72 .These differential responses provide an outstandingopportunity to study at the transcriptome orproteome level the responses of plants to beneficialmicroorganisms, such as T-22, which can give rise toimproved yields and greater resistance to pests. It islikely that proteomic or genomic analyses will identifygenes and pathways that are involved in resistance tobiotic and abiotic stresses and in growth promotion.These techniques also provide important tools foridentifying strains that are particularly suited to interactingsymbiotically with plants, and fungal metabolitesthat cause beneficial effects.In most of the cases mentioned above, it is impossibleto separate direct effects on plant growth from thecontrol of pathogenic or other deleterious microorganismsthat reduce root growth. However, in theinteraction between T-22 and maize, root and shootgrowth were increased in both sterilized and nonsterilizedsoils and in the presence of soil fungicides 72 .Moreover, T. asperellum T-203, and other Trichodermastrains, increased root growth in axenic systems 13,14 .Inmost cases, however, improved root development andconcomitant increases in plant growth are probablycaused both by biocontrol and related effects on rootassociatedmicroflora, and by a direct improvement inplant growth. Deleterious root microflora can reduceplant growth in the absence of obvious plant disease 78 .Some deleterious root-associated microflora producecyanide — probably to maintain their niche in the faceof competition 79 . Trichoderma spp. are resistant tocyanide and produce two different enzymes that arecapable of degrading it in the root zone 80 .Therefore,these fungi can directly increase root growth, controldeleterious non-pathogenic root microflora, destroy toxicmetabolites produced by deleterious microflora anddirectly control root pathogens. So, the enhancement ofroot growth by these fungi, together with concomitantimprovements in plant growth and resistance to stress,is accomplished by several different routes. Each ofthese probably involves multiple mechanisms, as hasalready been described for the biological control ofplant pathogens on roots and foliage.Nutrient-uptake interactions. Trichoderma spp.increase the uptake and concentration of a variety ofnutrients (copper, phosphorus, iron, manganese andsodium) in roots in hydroponic culture, even underaxenic conditions 14 .This increased uptake indicatesan improvement in plant active-uptake mechanisms.Moreover, maize generally responds to nitrogen-containingfertilizers by increases in leaf greenness,growth and yield up to a plateau that is generally consideredto be the maximum for specific genotypesunder the prevalent field conditions. However, plantsthat are grown from seeds treated with T-22 havebeen found to give maximum yields with as much as40% less nitrogen-containing fertilizer than similarplants that were not treated with T-22 (REFS 16,81,82).52 | JANUARY 2004 | VOLUME 2 www.nature.com/reviews/micro
REVIEWSBox 4 | Mechanisms of plant-disease control by TrichodermaThe results presented and discussed in the review allow us to propose a model of the mechanisms by which Trichodermaspp. control or reduce plant disease. The figure shows a schematic overview of the means by which Trichoderma spp.control plant pathogens. Other mechanisms also exist, including the inhibition of pathogen enzymes that are necessaryfor leaf penetration and competition for seed nutrients that are required for pathogen germination in soil.Trichoderma spores or other biomass can be added to soil by a variety of methods. As shown in a, if the strain (the tancolouredstructure in the magnified section) is rhizosphere competent, it colonizes root surfaces and the outer layers ofthe cortex. This establishes a zone of interaction into which the Trichoderma strain releases bioactive molecules. Theseinclude elicitors of resistance, such as homologues of avirulence (Avr) proteins and proteins with enzymatic or otherfunctions. The fungi also produce enzymes that release cell-wall fragments, which also enhance plant resistanceresponses. The plants produce cell-wall deposits (black layer in the magnified section) and biochemical factors that limitthe growth of the Trichoderma strain and cause it to be avirulent. A control plant is shown on the right.Pathogens (shown in blue) can attack roots — as shown in b — but, in the presence of Trichoderma,infection is reducedby the same or similar molecules and cell-wall alterations that result in the avirulence of the Trichoderma strains (left panel,upper magnified section). Furthermore, several strains induce systemic resistance in plants — even though they areaZone of interactionTrichodermaCWDEsCell wall fragmentsOther proteins/enzymesAvr homologuesRoot surfacebCell wall depositsTrichodermaZone ofinteractioncTrichodermalocalized on the roots — probably through the action of a signalling compound (curved arrow going up from the roots).Then, when pathogens attack plant leaves or stems (particles in circles), the plant is potentiated to respond rapidly byproducing defence-related enzymes and antimicrobial compounds. In addition, Trichoderma strains can attack pathogensin the soil (left panel, lower magnified section) by a variety of mechanisms. The strains respond tropically to the presence ofthe pathogens, and the interaction begins before the two organisms come into contact (yellow box). The Trichodermaproduces sensing enzymes (orange rectangles) that release cell-wall fragments from the hyphae of the target pathogen,which increases the release of additional enzymes. Antibiotics can also be produced (purple rectangle). The next step of theinteraction is the actual parasitism (which frequently results in the coiling of the Trichoderma fungus around thepathogen) and the production of a number of synergistic cell-wall-degrading enzymes and other substances, followed bythe infection and death of the target fungus. In the absence of Trichoderma,root pathogens (right panel) infect roots andcause disease. Plants are also unprotected against foliar, stem and flower pathogens.As a consequence of the interactions between Trichoderma fungi and plants, a variety of pathogens of roots and theabove-ground parts of plants cause less disease in plants in which the roots are colonized by Trichoderma (c, left panel).Even in the absence of pathogens or disease, plants frequently have larger roots and higher levels of productivity in thepresence of Trichoderma.In the absence of Trichoderma,either the above-ground or below-ground portions of plantsusually have more disease, and are often less robust (right panel).NATURE REVIEWS | MICROBIOLOGY VOLUME 2 | JANUARY 2004 | 53
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trichoderma species - Weizmann Institute of Science

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