Development of the arbuscular mycorrhizal symbiosis: Insights from genomics

The majority of the vascular flowering plants have the ability to enter into symbiotic associations with a unique group of soil fungi, the arbuscular mycorrhizal (AM) fungi. The symbiosis develops in the roots of the plant, where the plant provides the fungus with a source of carbon and the fungus delivers mineral nutrients to the roots. In particular, the transfer of phosphorus from the AM fungus to the plant is widely documented, but there is evidence for zinc and nitrogen transport also (Hodge italic., 2001). For both symbionts, significant quantities of nutrients may be exchanged. It is estimated that the plant allocates up to 20% of its photosynthate to the roots to support the fungal symbiont, and some studies suggest that in an AM symbiosis the plant receives all of its phosphorus via the fungus (Bago italic., 2000; Smith italic., 2003). Phosphorus is a relatively immobile nutrient and is often present at concentrations in the soil that are limiting for plant growth. Consequently, improvements in phosphorus supply resulting from the AM fungus can have a significant impact on plant health and subsequently on plant biodiversity and ecosystem productivity (van der Heijden italic., 1998). The AM symbiosis is an ancient association. Both molecular data and fossil evidence suggest that the AM fungi originated 460 MYA (Redeker et al., 2000) at a time when bryophytes were the predominant plant form.