Species Rich Genus Of Micro Fungi Biology Essay

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Trichoderma, with an estimated 130 described species, is a species-rich genus of micro fungi belonging to the Ascomycota phylum. These fungi are predominant over wide geographic regions in all climatic zones and can be isolated from nearly every soil, decaying wood, compost or other organic matter (Hoyos-Carvajal et al., 2009; tr5; Harman et al., 2004; Berg et al., 2005; De Respinis et al., 2006) (TR1). They are remarkable for their rapid growth, capability of utilizing diverse substrates, and resistance to noxious chemicals (Kubicek et al., 2003). Some of the species are of economic importance because of their production of enzymes and antibiotics, or use as biocontrol agents (Gams and Bissett, 1998; Hjeljord and Tronsmo, 1998; Sivasithamparam and Ghisalberti, 1998). Kubicek et al., 2003) (TR6). According to Hoyos-Carvajal et al. (2009) these species can form intimate associations with plant roots, providing an endemic level of biological control or stimulating plant growth by producing soluble forms of mineral nutrients and growth-promoting metabolites (tr5).

Three important characteristics that some strains of Trichoderma have been shown to exhibit are the ability to (1) protect seeds and seedlings from organisms that cause damping off, (2) be rhizosphere competent and protect the subterranean portions of growing plants from attack by pathogens, and (3) enhance plant growth and development (Harman & Taylor, 1990). Most Trichoderma isolates not only rapidly colonize the rhizosphere of seedlings but also persisted at considerable population levels and remained active against plant pathogens (Abassi et al., 2002; Yang et al., 2003).

2.2.2 Trichoderma spp. and biological control of plant diseases

A review of the biology and systematic of the genus Trichoderma by Samuels (2001) showed disease controlled by Trichoderma spp. Some of these diseases include Rhizoctonia damping-off in radish (Lifshitz & Baker, 1985), com and soybean (Kommedahl et al.., 1981); cucumber fruit rot caused by Rhizoctonia solani (Lewis & Papavizas, 1980); greymould on tomato (Migheli et al., 1994), grapes and strawberry (Elad ef al., 1995; Harman et aI., 1995); take-all disease in wheat (Ghisalberti & Sivasithamparam, 1991), and sclerotinia sclerotiorum in pea (Knudsen & Eschen, 1991) and a wilt-complex, predominantly caused by Sclerotium rolfsii Sacc., Rhizoctonia solani and Fusarium oxysporum Schldl. in lentil and chickpeas. According to Howell (2003), six mechanisms are employed by Trichoderma spp to provide biological control against diseases. These include: (i) mycoparatism and production of antifungal metabolites, (ii) competition and rhizosphere competence, (iii) enzymes secretion, (iv) induction of defense responses in plants, (v) metabolism of germination stimulants and (vi) adjunct mechanisms (increased plant growth, resistance to stress, etc.)

In tomato production, the most salient biological control activity of Trichoderma spp. has been the suppression of damping-off caused by Pythium. This soilborne pathogen poses serious threats in greenhouse and field production with considerable damage to plant, particularly in the early stages of seedling growth (Blancard et al., 1994; Moulin et al., 19994; Rachniyom & Jaenaksorn, 2008). Generally, pythium-challenged seedlings are removed from the field as no chemical control is available. Verticillium wilt caused by V. dahliae is also another fungal disease, which can cause with considerable yield loss in tomato. This fungus can survive in soils for many years and infect their hosts by entering the vascular system and are transported within the conductive xylem (Green, 1981) whereby it interacts with nutrients and water movement upward and downward. Recent study by Jabnoun-Khiareddine et al. (2009) showed that Trichoderma spp. have the potential to provide disease control against this soilborne pathogen. These authors tested three different strains (T.harzianum, T. virens and T. viride) with verticillium wilt causal agents in tomato grown in growth chamber and under greenhouse conditions. Trichoderma spp. reduced the radial growth of all verticillium wilt agents. In growth chamber, the leaf damage index was reduced by 60% though all verticillium-challenged plants showed disease symptoms. Inoculating plants with T.virens increased the fresh and rootmasses by ca. 40%, whereas T.harzianum and T.viride had no effect thereof. Conversely in the greenhouse, all the Trichoderma strains increased the fresh root and shoot mass by more than 50% as compared to the untreated plants. These authors postulated that mycelial reduction growth was mainly due to the important competitive potential of the antagonists used and the reduction of resting structures abundance of verticillium spp. isolates compared to the untreated.

2.2.3 Trichoderma spp. and plant growth promotion

Trichoderma is no longer considered as biological control agent only but also as plant growth enhancer. This is supported by the reported growth promotion of several species of plants treated with Trichoderma spp. (Windham et al., 1989; Bailey and Lumsden, 1998; Björkman et al., 1998; Yedidia et al., 1999; Naseby et al., 2000; Brimner and Boland, 2003; Trico3). Enhanced tomato seedling growth with T. harzianum was investigated under greenhouse conditions (Ozbay and Brown, 2004). Four weeks after sowing, root colonization of tomato seedlings by T.harzianum strains was more 90%. In addition, T. harzianum strains T22 and T95 increased the shoot height, stem caliper, shoot fresh weight and shoot dry weight by % respectively. These isolated had no significant effect on the root fresh and dry weights. The mechanisms involved in growth promotion by Trichoderma spp. were not clearly elucidated. Gravel et al. (2007) studied the effect of T.atroviride and seven other biological control agents on growth of tomato grown under hydroponics conditions. The production or degradation of indole acetic acid (IAA) by T.atroviride was investigated as possible mechanisms for plant growth stimulation. T. atroviride synthesized IAA from different feature precursors in vitro. In fact, the addition of L-tryptophan, tryptamine and tryptophol in the culture medium stimulated the production of IAA by 417%, 718% and 3108% respectively. This supports that theory that microbial IAA could have been involved in the growth stimulation. In greenhouse conditions, the growth of seedlings inoculated withT. atroviride increased as the concentration of L-tryptophan increased in the pouches. This suggests that the synthesis of IAA through tryptophan-dependent pathways by T. atroviride, affected the growth of the tomato seedlings. These authors concluded that growth stimulation was the synergic result of numerous modes of action exhibited by T. atroviride including a regulation in the concentration of IAA in the rhizosphere and a regulation of the concentration of ethylene within the roots.

Increased mineral uptake by Trichoderma inoculated plants has also been suggested as possible mechanism for plant growth promotion. The potential of Trichoderma harzianum strain T-203 to induce a growth response in cucumber plants was studied in soil and under greenhouse conditions (Yedidia et al., 2001). Twenty eight days after treatment initiation, T. harzianum inoculated plants increased the cumulative root length, shoot length, leaf area and dry weight by 75%, 45%, 80% and 80% respectively. Similarly, an increase of 90 and 30% in P and Fe concentration respectively, was observed. To better characterize the effect of T. harzianum during the early stages of root colonization, experiments were carried under axenic hydroponic growth. T. harzianum-inoculated plants increased root length shoot length and dry weight of roots and dry shoot weight by ca. 45%, 60%, 24% and 40%, respectively as compared to controls five days after T.harzianum inoculation. Shoot Zn, P and Mn concentrations increased by 25, 30 and 70%, respectively. The findings of this study suggested that the improvement of plant nutritional level may be directly related to a general beneficial growth effect of the root system following T. harzianum inoculation