The structure of arbuscular mycorrhizas: a cell biologist's view

Arbuscular mycorrhizal fungi form a homo- geneous group of soil fungi that are found in most terrestrial ecosystems. They all belong to Glomeromycota, a basal fungal taxon which is currently considered phylogenetically related to Mucoromycotina, on the basis of genome sequence data from Rhizophagus irregularis. Glomeromycota are estimated to form symbiotic associations with about 80% of plants, from liverworts and ferns to gymnosperms and angiosperms. This ecological success is the result of the major selective advantages that arbuscular mycorrhizas (AM) interactions provide to both the plant and fungus. When lab experiments have compared symbiotic individuals with plants that were grown in the absence of glomeromycetes, striking differences have been observed: AM fungi boost plant growth, improve their capacity to absorb water and mineral nutrients (in particular, phosphate and nitrogen) and, through both this enhancement of plant health and a basal triggering of defense responses, protect plants from pathogens. Besides improving plant overall fitness, AM play a central role in nutrient cycles, soil stability and – last but not least – the survival and diffusion of AM fungi. Similarly to ectomycorrhizal fungi, in fact, glomeromycetes only accomplish their life cycle when growing in association with their plant hosts (although AM fungal reproduction is currently considered strictly asexual). Unlike ectomycorrhizal fungi, they cannot be grown for more than a few weeks in the absence of the host, a feature that characterizes AM fungi as obligate biotrophs.