Update on Symbiosis Carbon Metabolism and Transport in Arbuscular Mycorrhizas

Colonization of the land by plants some 400 million years ago was associated with the colonization of their primitive roots by soil-borne filamentous fungi (Nicolson, 1975; Simon et al., 1993; Taylor et al., 1995). Today, 90% to 95% of land plants still maintain some type of mycorrhizal association so that “mycorrhizas, not roots, are the chief organs of nutrient uptake by land plants” (Smith and Read, 1997). Of the several mycorrhizal symbioses, arbuscular mycorrhizas are much the most abundant. These are formed by a very wide variety of host plants (including angiosperms, gymnosperms, pteridophytes, and some mosses, lycopods, and psilotales) and a comparatively small group of aseptate filamentous fungi, the Glomales. The study of arbuscular mycorrhizal (AM) fungi has fundamental and practical importance. First because in most environments “root biology” is actually “mycorrhizal biology”, and second because of the practical importance of AM in fields as diverse as sustainable agriculture, horticulture, reforestation, and ecosystem management (Bethlenfalvay and Schüepp, 1994; Barea and Jeffries, 1995). There have been a number of reviews and at least one excellent text (Smith and Read, 1997) in recent years covering various aspects of the AM symbiosis. These include: development and morphology (Smith and Smith, 1997; Barker et al., 1998; Azcón-Aguilar et al., 1999; Bago, 2000), molecular and cellular biology (Gianinazzi-Pearson et al., 1995; Harrison, 1999), and physiology and nutrient transfer at symbiotic interfaces (Cooper, 1984; Smith and Smith, 1990; Koide and Schreiner, 1992; Azcón-Aguilar and Bago, 1994; George et al., 1995; Jakobsen, 1999). A current resource on mycorrhizal research worldwide is maintained at www.mycorrhiza.ag.utk.edu. Here we review the biochemistry of carbon metabolism and transport in the AM symbiosis. Progress in this area has been significant thanks to the use in recent years of diverse methods including molecular biology, cytology, in vitro AM cultures, microscopy, and nuclear magnetic resonance (Bago et al., 2000).