Hydrolytic Enzymes from Arbuscular Mycorrhizae: the Current Status

Abstract

Terrestrial fungi can adopt different life strategies to exploit nutrient sources. They grow as saprotrophs on simple or complex organic substrates, or they can establish a nutritional relationship with higher plants, either as biotrophs or necrotrophs. Mycorrhizal associations are the most important mutualistic biotrophic interactions (Harley 1991). Cellular actions leading to reciprocal morphofunctional integration between symbionts during mycorrhiza establishment are complex. Plant-defence responses, which are normally weakly activated during the symbiotic state, are strongly elicited by arbuscular mycorrhizal (AM) fungi in genetically altered, resistant hosts, suggesting control over defence gene expression during establishment of a successful symbiosis. Modifications are also induced in the fungal symbionts during colonization of host tissues, with changes in wall metabolism and protein expression. They are strictly dependent on nutrition of their host plant, whereas others, like ericoid fungi, can be easily grown in pure culture (Peretto et al. 1993,1995; Varma and Bonfante 1994). Similar to other fungi in the Zygomycotina group (Lewis 1991), AM fungi possess chitin as one of their principal cell-wall components (Bonfante and Grippiolo 1984; Bonfante et al. 1990; Gianinazzi-Pearson et al. 1994). However, their hyphal wall structure and composition do not show a uniform pattern, as they differ between genera (Giovannetti and Gianinazzi-Pearson 1994; Smith and Read 1997) as well as between the different developmental stages of the fungus (Bonfante et al. 1990; Bonfante and Perotto 1995; Bonfante 1997).