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RESEARCH PRODUCT

Structural and functional genomics of symbiotic arbuscular mycorrhizal fungi.

subject

description

The absorbing organs (roots, rhizomes) of nearly all terrestrial plant families host an intimate symbiotic association, called a mycorrhiza, with specialized functional groups of soil fungi. The most common type of root symbiosis is the arbuscular mycorrhiza where soil fungi interact with a tremendous diversity of plant species, including many forest trees and agricultural, horticultural, and fruit crops (Gianinazzi et al., 2002). The fungi involved are very ancient microorganisms compared to other true fungi. Fossil data and molecular phylogenetic analyses indicate that their origin dates back to the Ordovician-Devonian era some 460 to 400 million years ago (Remy et al., 1994; Redecker et al., 2000), coinciding with land colonization by early plants. Since then, arbuscular mycorrhizal (AM) fungi have become an integral part and key components of most terrestrial ecosystems (Smith and Read, 1997). Their ability to enhance plant resistance to biotic and abiotic stresses makes them potentially powerful biotools for low-input agriculture (Gianinazzi et al., 2002), and it is believed that their biodiversity can influence plant community structure in natural ecosystems (van der Heijden et al., 1998). More than 100 species of AM fungi have been described, and many of them are held in international culture collections (IBG/BEG, 1993; INVAM, 1996). Their taxonomical status has been a matter of debate because of their asexual nature and the difficulty to affiliate them closely to existing fungal groups. Until recently, they were organized into six genera, distributed in four families, and grouped into a unique order, Glomales (Zygomycota) which comprises Gigaspora and Scutellospora (Gigasporaceae), belonging to the suborder Gigasporineae, and Glomus, Sclerocystis (Glomaceae), Acaulospora, and Entrophospora (Acaulosporaceae), clustered in the Glomineae (Morton and Benny, 1990). However, a revised classification is presently being considered which places them in a new phylum, the Glomeromycota with four new orders (Glomerales, Archeaosporales, Paraglomales, and Diversisiporales) (Schusler et al., 2001). For clarification, the generic term “AM fungi” will therefore be used above the species level throughout this chapter.