Interactions Following Contact Parasitism And Hyphal Interference

Parasitism between fungi has been widely utilized for practical purposes in the development of mitosporic fungi as biocontrol agents for use in horticulture and agriculture (Whipps, 2001), but has been infrequently studied in saprotrophic Basidiomycota. Host recognition by lectin or agglutinin-carbohydrate interaction is followed by penetration or appression to and growth along and around host hyphae (Chet et al., 1997; Jeffries, 1997; Whipps, 2001). Both enzymes and toxins are produced by the parasitic species acting on the opposing fungal cell wall, enabling penetration of the host by the parasite, or causing lysis of host hyphae (Deane et al., 1998; Howell, 1998; Vasquez-Garciduenas et al., 1998). The nutrients exposed and/or released in these processes are then assimilated by the parasitic species for further growth and development. Various enzymes have been implicated in fungal-fungal parasitism, major examples being the ^-glucanases and chitinases produced by Trichoderma species (Vasquez-Garciduenas et al., 1998; Zeiligner et al., 1999).

Potential parasitism by Basidiomycota has rarely been reported, but examples include Pseudotrametes gibbosa parasitic on Bjerkandera species, and Lenzites betulina on Trametes species (Rayner et al., 1987). In both cases the parasitism is temporary; when the parasitized fungus has been killed its territory is taken over by the parasite which then operates other antagonistic mechanisms to defend and gain further territory.

Parasitism may be an extended form of the process known as hyphal interference. Hyphal interference occurs when hyphae of two species make contact, leading to the death of the contacted compartment of the susceptible species (Boddy, 2000). Although commonly studied in wood-decaying hymenomycetes (e.g. Ikediugwu et al., 1970), similar processes occur during interactions amongst saprotrophic Ascomycota, and between hymenomycetes and Ascomycota or mi-tosporic fungi (Woods et al., 2005). In this process, affected cells show vacuolat-ion, loss of opacity, loss of hydrostatic pressure or hyphal swelling and rupture (Ikediugwu et al., 1970; Holdenrieder, 1984; Woods et al., 2005). Competing fungi may also produce thin hyphae compared with those produced in axenic cultures. These impacts on hyphal physiology result from the action of diffusible metabolites released when the interacting species are in close proximity. Interference can be extensive resulting in death; for example, Phanerochaete magnoliae overgrows and kills colonies of Datronia mollis in vitro (Ainsworth and Rayner, 1991). Small colonies of H. annosum in pine stumps and roots may be overgrown and killed by Phlebiopsis gigantea (see below).

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