ABSTRACT
The most widely distributed mycorrhizal symbiosis in tropical agroecosystems is the arbuscular mycorrhizal symbiosis. This sym biosis is established between plant root systems and fungi (Figure 6.1) which belong to the phylum Glomeromycota (Schiibler, Schwarzott, and Walker, 2001). The name of this symbiosis is based on the pres ence of specific fungal structures, called arbuscules, which colonize the cortical cells of roots. Based on current root analysis, it is consid ered that arbuscular mycorrhizal symbiosis is present in more than 80 percent of all known terrestrial plants. The importance of arbuscular mycorrhizal (AM) fungi is related not only to their beneficial effect on plant growth and nutrition but to the fact that these fungi appear to have played a role in plant evolution and adaptation (Malloch, Pyrosinsky, and Raven, 1980; Taylor et al., 1995). According to fossil evi-
The authors sincerely thank Chantal Hamel for the kind invitation to write this contribution, and are indebted to C. Plenchette, Associate Editor, and an anonymous referee for their very valuable comments and corrections of previous manuscripts. They also wish to thank Alicia Franco-Ramfrez and Mauricio Ivan Andrade Luna, MSc students at the Colegio de Postgraduados, Mexico, for kindly providing the photographs for Figure 6.1 A, C, and D. Financial support from the Mexican govern ment through Project SEMARNAT-CONACyT 2004-01-45 "Los hongos silvestres comestibles del Parque Nacional Izta-Popo, Zoquiapan y Anexos ” is also acknowl edged.
