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P. lUacinus

The teleomorphs of most nematode-trapping species are located within Orbilia, and their type of trapping apparatus arranged their taxonomic position [52]. These fungi were also classified according to their genetic data as follows: Arthrobotrys (adhesive three-dimensional networks), Dactylellina (stalked adhesive knobs and/or non-constricting rings), Drechslerella (constricting rings) and Gamsylella (adhesive branches and unstalked knobs) [53].

4.2.1.3 Ecology

Our knowledge about growth and development of trapping fungi in soil, particularly the factors which cause the switch from a saprotrophic to a parasitic phase, is not sufficient [11]. The number of nematode-trapping species present in a specific soil and their population densities can considerably be different. The highest densities are usually found in fall and in the upper 30 cm of soil [54].

A total of 54 nematophagous fungi were isolated and recognized from Scotland. The nematode-trapping fungi included 16 species while endoparasites included 15 species. Arthrobotrys gephyropaga and Drechslerella brochopaga among nematode-trapping and Harposporium anguillulae among endoparasites had the highest incidence [55], but in Irish sheep pastures 29 nematophagous fungi were isolated of which 12 were nematode-trapping and 17 were endoparasitic. In Ireland Cystopage lateralis, Stylopage hadra, Drechmeria coniospora and Meristacrum asterosperum had the highest incidence [56]. The following species were reported from Kenya: Arthrobotrys dactyloides, A. oligospora, A. superba, Acrostalagamus obovatus, Dactyllela lobata, Harposporium aungulilae, H. liltiputanum, Haptoglosa heterospora, Monacrosporium asterospernum, M. cianopagum, Myzocytium spp., Nematoctonus georgenious and N. leptosporus [57]. Recently two new species of Dactylellina were isolated and described in china. These new nematode-trapping fungi were D. sichuanensis and D. varietas which entraped nematodes by both adhesive knobs and non-constricting rings [58].

Application of chopped organic amendment [24] and glucose [59] to soil could increase the activity of nematode-trapping species which was perhaps the consequent of increase in the number of free-living and microbivorous nematodes. Probably organic amendments stimulated population densities, however, similar population densities of trapping fungi were found in plots with and without organic amendments [60, 61]. The effect of abscisic acid (ABA) and nitric oxide (NO) on the nematode-trapping fungus Drechslerella stenobrocha AS6.1 were tested and demonstrated that the trap development and nematode-trapping capability of D. stenobrocha were increased by ABA but decreased by NO [62].

It is apparent that the trapping fungi need a carbohydrate source for their proliferation but other factors, like those which cause fungistasis are also important in their abundance and trophic state in soil [11]. It is hypothesized that Orbilia species, the teleomorph of Arthrobotrys species, that are weak wood decomposers [63], support the fungi with carbon and energy sources, while nematode cadavers act as an important supply of nitrogen [21]. It is illustrated that predaceous behavior of A. oligospora can be controlled either by physiologically active compounds (amino-acids or vitamins) present in nematodes or by nitrogen sources [64, 65].

The majority of nematode-trapping fungi colonizes the bulk soil and waits until the passing nematodes contact them. Some fungi increase their trapping chance by producing secondary attractive compounds for nematodes, like A. superba which attract J2 of Meloidogyne species [17]. Others grow in rhizosphere, which give them superior predatory activity to trap plant-parasitic nematodes on their way toward the roots. For example, A. oligospora found more abundance in rhizosphere of tomato and barley plants because of its chemotropical attraction to the root tips [66]. Plant species obviously influence on rhizosphere and external root colonization. The highest incidence and diversity of nematode-trapping fungi is seen in association with pea rhizosphere [290]. Tomato roots are successfully colonized by Dactylellina ellipsospora and D. dactyloides in a pot experiment [68].

Various fungi have different efficacy in trapping and parasitizing nematodes. It is shown that A. dactyloides is more efficient in trapping Meloidogyne graminicola than Dactylella brochopaga and Monacrosporium eudermatum [17]. Some nematode-trapping fungi are good antagonists but trap few nematodes, while others are efficient in capturing nematodes but do not establish well in soil. This subject limits the potential of this group as microbial control agents [17].

4.2.1.4 Mode of Action

Attraction of the host is the first step in infection of nematode, which includes nematode host chemotaxis towards fungal hyphae or traps [69, 70]. It is not clear that what compounds are involved in chemotaxis [66, 71]. Formation of different trapping structures can be stimulated by environmental, chemical and tactile stimuli [16]. Many studies showed that the presence of nematodes or some specific organic compounds (like amino acids and peptides) could trigger the formation of trapping structures [17]. It is also demonstrated that the presence of competitor organisms and the level of nutritious substances are important in changing the trophic state from saprotrophic into parasitic [67, 72].

Nematophagous fungi adhesives commonly include proteins and/or carbohydrates [73, 74]. A nematode recognition role is suggested for a Gal-NAc-specific lectin of A. oligospora [75]. Carbohydrates that cover the surface of nematodes play an important role in both recognition phase of lectin binding and nematode chemotaxis [76, 77]. When a nematode touches A. oligospora traps, the amorphous sticky materials on the surface of the traps change to a fibrillar appearance [78]. Nematode infection triggers a signaling cascade in fungi resulting in penetration and colonization of the nematode [79]. We know a little about the signaling cascades. During trap formation, expression of the genes that are involved in construction of the trapping devices of Dactylellina haptotyla accompanied with those involved in fungal morphogenesis, was recently demonstrated [80]. The same results were reported for an entomopathogenic fungus, M. anisopliae [81].

Nematophagous Fungi
Fig. 4.1 A nematode is entrapped by two constricting rings of Arthrobotrys dactyloides (arrowheads). I Inflated ring, U non-inflated ring, H hyphae [82]

The trapping devices are usually constructed on mycelium, while they may also be formed directly on germinating conidia [67] with variation among different taxa. For example, A. dactyloides has a greater ability for conidial trap production than A. superba and A. oligospora. Fungistasis and competition for nutritious substances can cause conidia to form traps directly, and live as parasites [17].

Sudden inflation of three cells which form the constricting ring after being touched by a nematode, result in capturing the prey (Fig. 4.1). The mechanism by which the inflation of the cells starts and ring closure happens in less than 0.1 s is not clear. Mild heat, pressure and Ca2+ can also stimulate the swelling of the cells in vitro [83]. The signaling pathways that took place in ring closure were examined and a model is suggested. According to that finding the nematode entrance exerts a pressure on the ring followed by activation of G-proteins. Consequently, cytoplasmic contents of Ca2+ increases in ring cells, calmodulin activates and at last the water channels open. Quick entrance of water via those channels make the cells inflate and entrap the prey. Calmodulin could regulate a key step in the signal transduction pathways after being activated by an increase in Ca2+, because the ring closure was inhibited by calmodulin antagonists [82].

The nematode cuticle mostly consists of proteins, hence proteolytic enzymes (Table 4.2) may be important for penetration. A. oligospora produce a serine protease, named PII, which has been characterized, cloned and sequenced. This enzyme belongs to the subtilisin family and its expression is enhanced by the presence of proteins, especially those of nematode cuticle [84].

Aozl, a PII homologue, is another serine protease secreted by A. oligospora [32]. Other nematode-trapping fungi can also produce serine proteases. Arthrobotrys microscaphoides produces Mlx [33] and Arthrobortys shizishanna secretes Ds1 [34] both with a high homology to the A. oligospora serine proteases [33, 34]. A putative serine protease gene (sprl) was cloned and characterized from Monacrosporium megalosporum, whose predicted protein sequence was similar to PII and Azol from Arthrobotrys oligospora. The fungus has a single copy of this gene [35]. Similar enzymes have also been purified and characterized from the egg-parasitic fungi Paecilomyces lilacinus [42], Pochonia chlamydosporia [40], and Lecanicillium psalliotae [44].

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