TRICHODERMA: The multi useful fungi
What is Trichoderma?
Trichoderma parasites on other fungus
Trichoderma spp. are fungi that are present in nearly all soils and other diverse habitats. In soil, they frequently are the most prevalent culturable fungi.
Trichoderma species are frequently isolated from forest or agricultural soils at all latitudes.Trichoderma is a genus of fungi that is present in all soils, where they are the most prevalent culturable fungi. Many species in this genus can be characterized as opportunistic avirulent plant symbionts.
Trichoderma are among the most common saprophytic fungi. They are within the subdivision Deuteromycotina which represents the fungi lacking or having an unknown sexual state (though many trichoderma are considered asexual).
Further, it is part of the form class hyphomycetes. They are known as early invaders of roots and quickly occupy an ecological niche on the roots. Due to their ability to utilize comples substrates, they do not completely depend on the plant in their life cycle. They are also considered cellulolytic ascomycetes and among the organisms responsible for the destruction of cellulosic fabrics.
There are actually several dozen Trichoderma species, most of which are rather difficult to distinguish from one another.
( about 89 species)
There are 89 species in the Trichoderma genus. Hypocrea are teleomorphs of Trichoderma. The most important Trichoderma species in industrial, medical and biocontrol uses include:
Trichoderma are found in nearly all agricultural soils and in other environments such as decaying wood. Most species grow rapidly, produce abundant conidia,and have a wide range of enzymes including cellulases.
Trichoderma is able to grow in soils having a pH range of 2.5 - 9.5, although most prefer a slight to moderately acidic environment. The species that prefer the more acidic soils are usually regarded as having a more stress-tolerant growth habit and are less aggressive.
Trichoderma species are frequently isolated from forest or agricultural soils at all latitudes. Hypocrea species are most frequently found on bark or on decorticated wood but many species grow on bracket fungi (e.g. H. pulvinata), Exidia (H. sulphurea) or bird's nest fungi (H. latizonata) or agarics (H. avellanea).
Teleomorphs of Trichoderma are species of the ascomycete genus Hypocrea. These are characterized by the formation of fleshy, stromata in shades of light or dark brown, yellow or orange. Typically the stroma is discoidal to pulvinate and limited in extent but stromata of some species are effused, sometimes covering extensive areas. Stromata of some species (Podostroma) are clavate or turbinate. Perithecia are completely immersed. Ascospores are bicellular but disarticulate at the septum early in development into 16 part-ascospores so that the ascus appears to contain 16 ascospores. Ascospores are hyaline or green and typically spinulose. More than 200 species of Hypocrea have been described but few have been grown in pure culture and even fewer have been described in modern terms.
Taxonomy and genetics
Most Trichoderma strains have no sexual stage but instead produce only asexual spores. However, for a few strains the sexual stage is known, but not among strains that have usually been considered for biocontrol purposes. The sexual stage, when found, is within the Ascomycetes in the genus Hypocrea.
Traditional taxonomy was based upon differences in morphology, primarily of the asexual sporulation apparatus, but more molecular approaches are now being used. Consequently, the taxa recently have gone from nine to at least thirty-three species.
Most strains are highly adapted to an asexual life cycle. In the absence of meiosis, chromosome plasticity is the norm, and different strains have different numbers and sizes of chromosomes. Most cells have numerous nuclei, with some vegetative cells possessing more than 100. Various asexual genetic factors, such as parasexual recombination, mutation and other processes contribute to variation between nuclei in a single organism (thallus). Thus, the fungi are highly adaptable and evolve rapidly. There is great diversity in the genotype and phenotype of wild strains.
While wild strains are highly adaptable and may be heterokaryotic (contain nuclei of dissimilar genotype within a single organism) (and hence highly variable), strains used for biocontrol in commercial agriculture are, or should be, homokaryotic (nuclei are all genetically similar or identical). This, coupled with tight control of variation through genetic drift, allows these commercial strains to be genetically distinct and nonvariable. This is an extremely important quality control item for any company wishing to commercialize these organisms.
The organism grows and ramifies as typical fungal hyphae, 5 to 10 µm in diameter. Asexual sporulation occurs as single-celled, usually green, conidia (typically 3 to 5 µm in diameter) that are released in large numbers. Intercalary resting chlamydospores are also formed, these also are single celled, although two or more chlamydospores may be fused together.
The typical Trichoderma conidiophore, with paired branches assumes a pyramidal aspect. Typically the conidiophore terminates in one or a few phialides. In some species (e.g.T. polysporum) the main branches are terminated by long, simple or branched, hooked, straight or sinuous, septate, thin-walled, sterile or terminally fertile elongations. The main axis may be the same width as the base of the phialide or it may be much wider.
Phialides are typically enlarged in the middle but may be cylindrical or nearly subglobose. Phialides may be held in whorls, at an angle of 90° with respect to other members of the whorl, or they may be variously penicillate (gliocladium-like). Phialides may be densely clustered on wide main axis (e.g. T. polysporum, T. hamatum) or they may be solitary (e.g.T. longibrachiatum).
All species can produce colonies which have either white to yellow to green mature fruiting areas. Colonies can have either floccose and elliptical conidia, or tufted non-floccose globose conidia.
Conidia typically appear dry but in some species they may be held in drops of clear green or yellow liquid (e.g. T. virens, T. flavofuscum). Conidia of most species are ellipsoidal, 3-5 x 2-4 µm (L/W = > 1.3); globose conidia (L/W < 1.3) are rare. Conidia are typically smooth but tuberculate to finely warted conidia are known in a few species.
Synanamorphs are formed by some species that also have typical Trichoderma pustules. Synanamorphs are recognized by their solitary conidiophores that are verticillately branched and that bear conidia in a drop of clear green liquid at the tip of each phialide.
Chlamydospores may be produced by all species, but not all species produce chlamydospores on CMD at 20° C within 10 days. Chlamydospores are typically unicellular subglobose and terminate short hyphae; they may also be formed within hyphal cells. Chlamydospores of some species are multicellular (e.g. T. stromaticum).
There are many species of Trichoderma with many differing characteristics. For example, T. harzianum is tolerant to stress imposed by nutrient scarcity. Often they are antagonistic towards one another. At high temperatures T. viride and T. polysporum are displaced by T. hamatum and T. koningii, while at low temperatures the opposite is true. Reasons like these are why some species are more prosperous during cooler months while others are more persistant during warmer months.
Cultures are typically fast growing at 25-30° C, typically not growing at 35° C. Colonies at first transparent on media such as cornmeal dextrose agar (CMD) or white on richer media such as potato dextrose agar (PDA). Mycelium typically not obvious on CMD, conidia typically forming within one week in compact or loose tufts in shades of green or yellow or less frequently white. Yellow pigment may be secreted into the agar, especially on PDA. A characteristic sweet or 'coconut' odor is produced by some species.
They are favored by the presence of high levels of plant roots, which they colonize readily. Some strains are highly rhizosphere competent, i.e., able to colonize and grow on roots as they develop. The most strongly rhizosphere competent strains can be added to soil or seeds by any method. Once they come into contact with roots, they colonize the root surface or cortex, depending on the strain. Thus, if added as a seed treatment, the best strains will colonize root surfaces even when roots a meter or more below the soil surface and they can persist at useful numbers up to 18 months after application. However, most strains lack this ability.
Several new general methods for both biocontrol and for causing enhancement of plant growth have recently been demonstrated and it is now clear that there must be hundreds of separate genes and gene products involved in these processes. A recent list of mechanisms follows.
-Competition for nutrients or space
-Tolerance to stress through enhanced root and plant development
-Solubilization and sequestration of inorganic nutrients
-Inactivation of the pathogen’s enzymes
-Mycoparasitism: Grows saprophytically or as a parasite on other fungi.
-Antibiosis: Grows toward hyphae of other fungi, coils around them, and attachs to host mycelium.
-Conidiophores are erect and produce side branches bearing whorls of short phialides.
-Branches are not swollen at the apex and bear terminal conidial heads.
-Conidia are one-celled ovoid spores and are produced successively from tips of phailides which collect in small wet masses.
-Individual cells range from 25-70 um in height and 2.5-3.5 um in diameter.
-Colonies grow quickly producing white, yellow, or green cushions of sporulating filaments.
The Uses of Trichoderma
Trichoderma is a very versatile mold: a nuisance for people, a useful fungus for industry and biocontrol, and a bane to other fungi.These versatile fungi are used commercially in a variety of ways, including the following:
Trichoderma, being a saprophyte adapted to thrive in diverse situations, produces a wide array of enzymes. By selecting strains that produce a particular kind of enzyme, and culturing these in suspension, industrial quantities of enzyme can be produced.
T. reesei is used to produce cellulase and hemicellulase.
T. longibratum is used to produce xylanase.
T. harzianum is used to produce chitinase.
Foods and textiles
Trichoderma spp. are highly efficient producers of many extracellular enzymes. They are used commercially for production of cellulases and other enzymes that degrade complex polysaccharides. They are frequently used in the food and textile industries for these purposes. The enzymes are also used in poultry feed to increase the digestibility of hemicelluloses from barley or other crops.
Cyclosporine A (CsA), a calcineurin inhibitor produced by the fungi T. polysporum and Cylindrocarpon lucidum, is an immunosuppressant prescribed in organ transplants to prevent rejection.
As noted, these fungi are used, with or without legal registration, for the control of plant diseases. However, many species are still highly antagonistic to other species of fungi by many processes. These include production of soluble antibiotics (peptides), volatile and non-volatile antibiotics, or by direct parasitism. This is achieved when they coil around the hyphae of other fungi in a process called mycoparasitsm which limits the growth and activity of plant pathogenic fungi. The fungus has probably the most heavily studied cellulase system as it excretes large quantities of cellulases in growth media. Various strains have the ability to reduce plant root rot and increase root growth.
Several strains of Trichoderma have been developed as biocontrol agents against fungal diseases of plants. The various mechanisms include antibiosis, parasitism, inducing host-plant resistance, and competition. Most biocontrol agents are from the species T. harzianum, T. virideand T. hamatum. The biocontrol agent generally grows in its natural habitat on the root surface, and so affects root disease in particular, but can also be effective against foliar diseases.
Different strains of Trichoderma control every pathogenic fungus for which control has been sought. However, most Trichoderma strains are more efficient for control of some pathogens than others, and may be largely ineffective against some fungi. The recent discovery in several labs that some strains induce plants to "turn on" their native defense mechanisms offers the likelihood that these strains also will control pathogens other than fungi.
In addition to colonizing roots, Trichoderma spp. attack, parasitize and otherwise gain nutrition from other fungi. Since Trichoderma spp. grow and proliferate best when there are abundant healthy roots, they have evolved numerous mechanisms for both attack of other fungi and for enhancing plant and root growth.
Trichoderma spp. possess innate resistance to most agricultural chemicals, including fungicides, although individual strains differ in their resistance. Some lines have been selected or modified to be resistant to specific agricultural chemicals. Most manufacturers of Trichoderma strains for biological control have extensive lists of susceptibilities or resistance to a range of pesticides.
T. aggressivum (formerly T. harzianum biotype 4) is the causal agent of green mold, a disease of cultivated button mushrooms. Trichoderma viride is the causal agent of green mold rot of onion.
Plant growth promotion
For many years, the ability of these fungi to increase the rate of plant growth and development, including, especially, their ability to cause the production of more robust roots has been known. The mechanisms for these abilities are only just now becoming known.
Some of these abilities are likely to be quite profound. Recently, we have found that one strain increases the numbers of even deep roots (at as much as a meter below the soil surface). These deep roots cause crops, such as corn, and ornamental plants, such as turfgrass, to become more resistant to drought.
Perhaps even more importantly, the recent research indicates that corn whose roots are colonized by Trichoderma strain T-22 require about 40% less nitrogen fertilizer than corn whose roots lack the fungus. Since nitrogen fertilizer use is likely to be curtailed by federal mandate to minimize damage to estuaries and other oceanic environment the use of this organism may provide a method for farmers to retain high agricultural productivity while still meeting new regulations likely to be imposed.
As a source of transgenes
Biocontrol microbes, almost by definition, must contain a large number of genes that encode products that permit biocontrol to occur. Several genes have been cloned from Trichoderma spp. that offer great promise as transgenes to produce crops that are resistant to plant diseases. No such genes are yet commercially available, but a number are in development. These genes, which are contained in Trichoderma spp. and many other beneficial microbes, are the basis for much of "natural" organic crop protection and production.
Isolation and ecology
Different media for isolation purposes are used to grow Trichoderma. Some selective media are more efficient than others. Depending on the species,Trichoderma can show no growth to broadly spreading growth on Czapek's agar. Colonies are usually first white then develop yellowish tints until they become various deep shades of green. Conidiophores will arise as branches of aerial mycelia, septate, and grow up to 70 um in height. As mentioned, Trichoderma are found in many environments but are abundant in decaying wood or in soil containing decaying wood.
Trichoderma are used in the commercial production of the enzyme cellulase. This capability makes the Trichoderma very valuable in controling certain other pathogenic fungi such as Rhizoctonia, Botrytis, Pythium, Sclerotinia, and Armillaria, which themselves are pathogens on fruits and vegetables. Currently, Trichoderma is being developed and marketed as a plant growth stimulator (by stimulation and protection).
The two species used most for commercial applications are T. harzianum and T. koningii. Though they can inhibit pathogen growth, these species will kill other fungi with a toxin and then consume them using a combination of lytic enzymes. Despite many beneficial uses, they can be serious pests nonetheless and can cause problems in cultivated mushroom beds.
Edited and posted by Hồ Đình Hải
Edited and posted by Hồ Đình Hải
2-Trichoderma and biocontrol of plant
6-http://en.wikipedia.org/wiki/Trichoderma From Wikipedia, the free encyclopedia