3.4 PRUEBAS DE SEGURIDAD
3.5.2 REINCORPORACION DE UN NODO AL CLUSTER 142,
Mycoparasitism can be regarded as the direct attack of one fungus on another and can be generally defined as direct antagonism (Dix & Webster, 1995). This term can be divided into four sequential steps (Chet et al., 1998). The first step is called chemotrophic growth, where the secretion of a chemical stimulus by the target
fungus attracts an antagonist fungus (Chet et al., 1981; Steyaert et al., 2003). The second step is called specific recognition, where the antagonist fungus identifies the cell surface of the pathogen (Barak et al., 1985). The third step can involve two distinct processes. One is called coiling, where the Trichoderma hyphae surround its host (Chet et al., 1981; Papavizas, 1985). The second processus involves intimate hyphal interaction and contact where the Trichoderma hyphae simply grows along the host’s hyphae. This type of mycoparasitic interaction can be observed with scanning electron microscope (SEM) (Benhamou et al., 1999), or by fluorescent microscopy. The fourth and final step involves the secretion of specific lytic enzymes which degrade the host cell wall (Chet et al., 1998). The main lytic enzymes involved in the degradation of the host cell wall are β-glucanase, chitinase and proteinases. Most Trichoderma species are able to secrete such enzymes and T. hamatum can release β-1,3-glucanase and chitinase when grown in the presence of R. solani (Chet & Baker, 1981). The secretion of lytic enzymes has a major impact on the biological control potential of Trichoderma species. This was demonstrated with a mutant strain of T. harzianum where higher chitinase, β-1,3-glucanase and β-1,6-glucanase activity was expressed compared with the wild type (Rey et al., 2001). Trichoderma
species have shown in vitro mycoparasitic activities towards plant pathogens such as
P. ultimum and S. rolfsii (Papavizas, 1985). Synergistic action of lytic enzymes and antibiotics is another important factor that can enhance the ability of Trichoderma
species to inhibit plant pathogens (Kay & Stewart, 1994b; Steyaert et al., 2003).
1.2.4.2 Antibiosis
The term antibiosis refers to the production of antibiotics by fungi and is especially relevant in the case of the Trichoderma genus. This type of interaction is generally defined as indirect antagonism since no hyphal contact is required for it to take place (Dix & Webster, 1995). As previously discussed, it is well established that
Trichoderma species are able to secrete a wide range of secondary metabolites with antifungal and antibacterial properties. Antibiosis generally occurs in synergy with mycoparasitism (Schirmbock et al., 1994), where the hydrolytic enzymes allow the antibiotics to penetrate the host cells. In turn, antibiotics can inhibit cell wall synthesis and, therefore, enhance the action of the hydrolytic enzymes (Lorito et al., 1996). Antibiotics can also affect the target fungus through a number of mechanisms such
as inhibition of growth, production of primary metabolites, uptake of nutrients and sporulation (Howell, 1998; Wilcox et al., 1992). Like mycoparasitism, antibiosis is species-specific and different Trichoderma species do not have the same biological control capabilities against the same pathogen. This can even be extrapolated to the strain level, in that, within the same species of Trichoderma different strains may exhibit different antifungal activity (Ghisalberti et al., 1990; Howell et al., 1993).
1.2.4.3 Competition
This interaction between Trichoderma species and soil microorganisms can be referred to as indirect antagonism. Trichoderma species can inhibit or reduce the growth of plant pathogens through competition for space, enzyme substrates, nutrients and/or oxygen (Dix & Webster, 1995). The fast growing nature of
Trichoderma species along with their ability to colonise a wide variety of substrates means that they are efficient soil colonisers and have the ability to displace less aggressive colonisers (Papavizas, 1985). Their colonising ability is greatly influenced by environmental factors including, soil pH, temperature and water potential (Carreiro & Koske, 1992; Danielson & Davey, 1973d; Domsch et al., 1980; Eastburn & Butler, 1988b; Klein & Eveleigh, 1998; Widden & Abitbol, 1980) and therefore competition should be regarded as an effective means of biocontrol when field conditions are optimal for the establishment and proliferation of Trichoderma species.
1.2.4.4 Plant growth promotion and induced resistance
The ability of Trichoderma species to promote and stimulate plant growth has been reported with a number of crops such as cucumber, tomato, radish, pea and flowers crops (Chang et al., 1986; Inbar et al., 1994; Kleifeld & Chet, 1992; b; Ousley et al., 1994b). Plant growth promotion by Trichoderma species can be achieved directly by stimulating the uptake of nutrients by the plant (Kleifeld & Chet, 1992; Ousley et al., 1994b) or by secreting plant growth promoting metabolites such as hormones (Windham et al., 1986). The antagonistic nature of most Trichoderma species towards phytopathogenic fungi means that they can stimulate plant growth indirectly by inhibiting plant pathogens and, therefore, increasing plant metabolism (Elad et al., 1987). Yet, the significance of such results has to be regarded with caution as most
trial results reported in the literature have been conducted in the glasshouse and often do not reflect the field situation. In the field, the potential of Trichoderma
species to stimulate plant growth can be overshadowed by more efficient plant growth promoting rhizobacteria (PGPR) such as Pseudomonas species (Schroth & Hancock, 1982).
More recently, the genus Trichoderma has been shown to be avirulent plant symbionts (Harman et al., 2004). They can induce localised or systematic resistance to diseases through the release of metabolites. The release of those chemicals tends to promote plant production of ethylene or terpenoid phytoalexins, which are involved in plant resistance to pathogens (Howell et al., 2000). When applied to the root system, T. harzianum strain T39 has been shown to induce systemic resistance in dicotyledonous plant leaves to B. cinerea (De Meyer et al., 1998). The genus
Trichoderma seems to show the ability to induce resistance in a wide range of plant species such as tomato, tobacco, lettuce, cotton (De Meyer et al., 1998; Howell et al., 2000). However, the mechanisms involved in induced plant resistance are still poorly understood and to increase the efficiency and reliability of such applications in the field, more research is needed (Harman et al., 2004).
The different modes of action exhibited by Trichoderma species to control plant pathogens have mainly been studied under laboratory conditions. The next challenge is to transfer those experiments to the field and achieve constant control of plant pathogens using Trichoderma species This requires a better understanding of the ecology, establishment and propagation of Trichoderma species in situ.