Mycorrhizal fungi

Mycorrhizal fungi (mycobionts) form a ubiquitous mutualistic symbiotic association with the roots of higher plants (phytobionts) in coastal sand dunes worldwide. These obligate biotrophs perform vital functions in the survival, establishment and growth of plants by playing an active role in nutrient cycling. As such they serve as a crucial link between plants, fungi and soil at the soil–root interface (Rillig and Allen 1999). Mycorrhizas occur in a wide variety of habitats and ecosystems including aquatic habitats, cold or hot deserts, temperate and tropical coastal dunes, tropical rainforests, saline soils, volcanic tephra soils, prairies and coral substrates (Klironomos and Kendrick 1993). Simon et al. (1993) sequenced ribosomal DNA genes from 12 species of arbuscular mycorrhizal (AM) fungi and confirmed that mycorrhizas (fungal roots) fall into three families. He estimated that they originated about 353–462 million years ago and were instrumental in facilitating the colonization of ancient plants on land. Further evidence was provided by Remy et al. (1994) who discovered arbuscules in an early Devonian land plant, Aglaophyton major, and concluded that mycorrhizal fungi were already established on land > 400 million years ago. Thus the nutrient transfer mechanism of AM fungi was already in existence before the origin of roots. Plant roots probably evolved from rhizomes and AM fungi served as an important evolutionary step in the acquisition of water and mineral nutrients (Brundrett 2002). Over evolutionary time the divergence among these fungi has accompanied the radiation of land plants, and about 200 species of AM fungi have been recognized (Klironomos and Kendrick 1993) that exist in association with about 300 000 plant species in 90% of families (Smith and Read 1997), indicating that AM fungi are capable of colonizing many host species. Approximately 150 of the described mycorrhizal species may occur in sand dunes (Koske et al. 2004). Most host–fungus associations are beneficial to both the plant and the fungus and are thus regarded as mutualistic (++); however, the widespread use of the term mutualism (mutual benefit) for mycorrhizal interactions has been questioned because all associations are not beneficial to both the plant and fungus (Brundrett 2004).