ABSTRACT
Most terrestrial plants distributed worldwide establish at least one of seven types of mutualistic symbiotic mycorrhizal associations in their roots (Brundrett, 1991; Smith and Read, 1997). The most widespread of these is an endomycorrhizal symbiosis between soilborne fungi (the mycobiont) presently classified in the phylum Zygomycota, order Glomales (Morton and Benny, 1990) and approximately 80% of terrestrial plant species (the phytobiont) (Trappe, 1987). This symbiosis has been demonstrated to be mutualistic in measurable ways. The most obvious benefit to the mycobiont is a ready supply of carbon within a relatively secure niche in root cortical cells. Jakobsen and Rosendahl (1990) indicate that the mycobiont can capture as much as 20% of the fixed carbon in the phytobiont, mostly by converting glucose to trehalose (Shachar-Hill et al., 1995). The symbiosis is termed ''arbuscular'' in this chapter because the mycobiont produces specialized arbuscules in root cortical cells that are involved in bidirectional exchange of carbon, phosphorus, and other physiologically important molecules. These arbuscules are considered the key structural evolutionary innovation because they are a conserved feature in all lineages of Glomales (Morton, 1990a). The phytobiont depends to varying degrees on the mycobiont for hyphal uptake and translocation of essential cations in soil which otherwise might not be available from root uptake alone (reviewed in Smith and Read, 1997). The magnitude of dependency often is measured in terms of plant growth responses, which usually are inversely related to plant nutrient (mostly phosphorus) content.
