The rhizosphere is a habitat with high microbial activity due to release of nutrients from root exudates and decaying cells. Like other micro-organisms, nematophagous fungi are more frequent in the root zone, with up to 19 times more propagules than in the surrounding nonrhizosphere soil (Persmark and Jansson 1997). Numbers of nematodes are also higher in the rhizosphere than in bulk soil. Since plant-parasitic nematodes attack their plant hosts in the root, understanding of the rhizosphere biology of nematophagous fungi is important in order to use nematophagous fungi for biological control of such nematodes. Nematodes are generally attracted to their host roots by specific or unspecific compounds, e.g., CO2 (Green 1971). Besides, some nematophagous fungi, e.g., A. oligospora, grow chemotropically towards plant roots (Bordallo et al. 2002). Recently, egg-parasitic and nematode-trapping fungi have been found to invade and grow endophytically in epidermal and cortical cells of plant roots (Figure 1C) (Bordallo et al. 2002; Lopez-Llorca 2002b). Root colonization by nematophagous fungi will be further discussed in "Fungi—Root-Interactions."
The interactions between nematophagous fungi and their hosts involve several steps from recognition (attraction phenomena, contact), production of adhesives and lytic enzymes, differentiation of infection structures, e.g., appres-soria and trapping organs, to host penetration and digestion of the host cell contents (Tunlid et al. 1992). These events have been studied using a variety of techniques (microscopic, biochemical, and molecular). In the following section we will discuss some of these interactions in connection with two of the hosts of nematophagous fungi: the nematode and the plant.
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