Mycorrhizal Fungi: What We Know and What Should We Know?

Mycorrhizal fungi are symbionts in roots of the majority of higher plants. These associations vary widely in structure and functions, but the most common interaction is the arbuscular mycorrhizal (AM) association. It is estimated that more than 80% of all terrestrial plants form this type of association. These include of many agriculturally and horticulturally important crop species (Smith and Read 1997). The AM symbiosis represents an ancient symbiosis (Pirozynski and Malloch 1975; Pirozynski and Dalpe 1989). Hyphae and arbuscules have been reported in fossils of Aglaophyton isolated from the Rhynie chert, and this evidence has established the existence of AM symbiosis in the early Devonian (Remy et al. 1994; Taylor et al. 1995). Furthermore, molecular works based on the nucleotide sequence divergence of 18s rDNA suggests that the Glomales arose 350–460 million years ago and that the symbiosis was instrumental in the successful colonization of land by plants (Simon et al. 1993a). During evolution, the AM symbiosis has been lost from about 10% of plants, including whole angiosperm families (Tester et al. 1987). AM fungi are probably the most ubiquitous fungi in agricultural soils, accounting for 5–36% of the total biomass in soil and 9–55% of the biomass of soil microorganisms (Olson et al. 1999). These fungi are a critical component in agricultural systems because these organisms can increase plant growth (Smith and Read 1997), plant reproductive capacity (Lu and Koide 1994), plant water stress tolerance (Gupta and Kumar 2000), and plant health through antagonistic and competitive effects on pests and pathogens (Gange and West 1994). This colonization may also enhance the plant’s resistance to biotic and abiotic stresses (Newsham et al. 1994; Subramanian et al. 1995; von Reichenbach and Schonbeck 1995; Ricken and Hofner 1996). AM fungi develop an extensive hyphal (extraradical hyphae) network with the plant root system which makes a significant contribution to the improvement of soil texture and water relations (Bethlenfalvay and Shuepp 1994). The main benefit to the host plant in the mycorrhizae symbiosis is the enhanced uptake of immobile soil nutrients, in particular phosphorus (Jakobsen 1999). Arbuscular mycorrhizal associations increase nitrogen accumulation in plant tissues as a result of the hyphae competing for mineralized organic soil nitrogen (Ibijbijen et al. 1996).