TRICHODERMA IN THE CONTROL OF PHYTOPATHOGENIC FUNGI

Trichoderma is a genus of filamentous fungi of worldwide distribution abundant in the soil of diverse ecosystems and climatic zones. It includes species of great value for their ability to control foliar and soil phytopatho- genic fungi that attack a wide range of plants. The control mechanisms that it exercises are: competition for space and nutrients, enzymatic antibiosis, the production of secondary metabolites and mycoparasitism. This last process includes phases such as the recognition, penetration, and death of its host. During this process, Trichoderma secretes enzymes that hydrolyze the cell wall of the host fungus, which has chitin as structural components, and glucans such as: (3-1,3-glucans, (3-1,6-glucans, and (3-1,4-glucans (cellulose). These elements are embedded in a protein matrix whose intra- structural components include mannoproteins, galacto-mannoproteins, xylo-mannoproteins, glucurono-mamioproteins, and a-l,3-glucans [12], therefore, the main mechanism of antagonism against phytopathogenic fungi is the extracellular secretion of chitinase enzymes, glucanases, and proteases [13, 14]. It is also known that the presence of Trichodenna can influence the phytohormonal network of the host plant. It has been demonstrated how strains of this genus have promoted the growth of melon seedlings in the nursery, controlling the wilting by Fusarium sp., where the stimulation of growth is associated with the induction of auxins [15] and the suppression of the disease to the stimulation of the defense mechanisms of the plants [16]. Table 9.1 shows examples of Trichodenna species that have been shown biocontrol effect against phytopathogenic fungi.

TABLE 9.1 Examples of Biocontrol of Phytopathogenic Fungi by Trichodenna

Trichotlerma Species

Phytopathogenic Fungi Target

References

T. harzianum

Alteniaria alternate

[31]

Trichodenna spp.

Botiytis cinerea

[32, 33]

Trichodenna spp.

Cercospora beticola

[34]

Trichodenna spp.

Colletotrichwn spp.

[35-37]

Trichodenna spp.

Fusarium spp.

[38—41]

Trichodenna spp.

Pythium aphanidennatum

[42, 43]

Trichodenna spp.

Phytophthora spp.

[44-47]

Trichodenna spp.

Rhizoctonia solani

[48, 49]

Trichodenna spp.

Rosellinia necatrix

[50]

Trichodenna spp.

Sderotium rolfsii

[51]

Trichodenna spp.

Sclerotinia sclerotiorum

[13, 52]

Trichodenna spp.

Septoria tritici

[53]

T. harzianum

Uromyces appendiculatus

[54]

BIOACTIVE COMPOUNDS OF TRICHODERMA

The control mechanisms of phytopathogens such as antibiosis, antagonism, mycoparasitism, and the induction of plant defense responses are characteristic strategies of the Trichodenna genus. The activity of mycoparasitism allows it to use the components of the cell wall of its prey as nutrients, whose promotion of growth results in the secretion of a diverse range of both volatile and non-volatile metabolites involved in biocontrol processes.

VOLATILE COMPOUNDS

Volatile organic compounds (VOCs) are the metabolites that plants and microorganisms release into the air. They are important in intra- and interspecific communication in the rhizosphere [17], being the structure infochemicals as mono- and sesquiteipenes, alcohols, ketones, lactones, esters or C8 compounds that are part of the wealth of the emissions of volatile microbial compounds. Its positive and/or negative effects on other organisms can become useful agricultural tools. It has been shown that these promote the development of plants and their defense system. Thus, in- in vitro tests where the plants of Arabidopsis thaliana exposed to the VW of I virens showed an increase in two times of the total fresh weight in comparison with the axenically cultivated seedlings. The stimulation of the branching of lateral roots was observed, thus increasing their capacity to absorb nutrients. The production of jasmonic acid, a phytohormone that occurs when plants interact with potential pathogens or with insects, was demonstrated by activating signaling cascades that increase the plant’s immunity through changes in gene expression. In addition, the production of hydrogen peroxide, an oxygen reactive species that, like jasmonic acid, triggers defense responses were observed. Both help in the control of diseases, as in the case of the test carried out with plants infected with Botrytis cinerea that, when they were exposed to VOCs from two strains of Trichoderma (Tvl0.4 and Tv29.8), the symptoms of induced chlorosis and the percentage of dead plants from 80% to 10 and 15% respectively were decreased [18].

Among the volatile antifungal compounds produced by Trichoderma species, 6-n-pentyl-2H-pyran-2-one (6-PAP) is a polyketide which is the most characteristic with a sweet coconut aroma. The analysis of the antifungal activity of 6-PAP in vitro, showed mycelial inhibition of Fnsarium culmonnn KF 846 in 100% for 7 days with 2.0 pg concentration of 6-PAP for each agar disc of the phytopathogenic fungus strain. This compound is involved in complex interactions against plant pathogens, as well as others, such as: 2-pentylfuran, toluene, 3-octanone, a-bergamotene, linalool isobutyrate, 2-methyl- 1-propenylbenzene, (3-cimene, isoamyl alcohol, 2-butanone, pentyl acetate, 3-octanol, 2-nonanone, (3-bisabolene, ethyl octanoate, D-limonene, 2-heptanol, (3-pinene, 2-methylbutylacetate, 1-pentanol, ethyl phenyl alcohol, a-curcumene, [3-farnesene. a-cedrene, a-pinene, 1-propanol, pyridine, methyl benzoate, geranyl acetone, among others [19].

NON-VOLATILE COMPOUNDS

The genus Trichoderma produces a great variety of secondaiy metabolites involved in signaling processes, interaction with other organisms and biological activities that have a great capacity to control phytopathogenic fungi. As mentioned previously, the fungal cell wall is a structure mostly compound of carbohydrates (polysaccharides such as glucans, chitin, cellulose among others) and in a less proportion by proteins, lipids, ions, and pigments. Trichoderma species are efficient producers of hydrolytic enzymes, such as those of the chitino lytic system (chitinases andNAGase), glucanolytic ((3-1,3/1,4-glucanases, a-l,3-glucanases) and proteolytic, breaking down polysaccharides and proteins which cause the loss of integrity of the cell wall which leads to the cellular collapsing [20]. Glio- toxin and glyovirin are antibiotics produced by Trichoderma for which their activity against phytopathogenic fungi such as Rhizoctonia soJani has been documented. The harzianic and isoharzianic acids produced by Trichoderma possess antifungal activity and promote seed germination and root growth. In addition, harzianic acid is related to siderophores properties as iron-chelating agent, solubilizing it to be assimilated and used in the plant nutrient regulation mechanisms. Peptaibols, such as suzukacillins that possess antifungal activity and trichokonins that have also been shown to induce the defense system of the plant. All this, is a little part of the great diversity of soluble metabolites that represent the source of the biocontrol efficiency exerted by Trichoderma [21, 22].

 
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