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Functional analysis of arbuscular mycorrhiza (AM) symbiosis

The interaction between plant roots and fungi from the phylum Glomeromycota — the arbuscular mycorrhiza (AM) — is probably the most important symbiotic association in terrestrial ecosystems. More than 80% of all higher land plant species are able to benefit from the fungus’ ability to extract nutrients, mostly phosphorus, from the soil. In exchange, it has been estimated that up to 20% of all photosynthetically fixed carbon might be delivered from a plant to the fungal partner. Yet, despite its ecological importance, astonishingly little is known about the molecular mechanisms involved in this interaction, which is one of the most widespread symbioses on earth.

Fossil evidence shows AM symbiosis to be at least as old as the earliest land plants. It has been suggested that colonization of the land surface was dependent on the fungal symbiont’s ability to forage into the soil for inorganic nutrients and water. This apparently proved to be so successful that even after development of functional plant root structures, it was still retained in most plant families.

Apart from their evident ecological importance, AM fungi are fascinating study objects in their own right. They are aseptate organisms with hundreds or thousands of nuclei within one "cell" (cytoplasmatic compartment). This, together with the lack of sexuality and the obligatory biotrophic life style, brings up the obvious question how cellular organization is achieved under such circumstances.