Symbiotic mycorrhizal fungi play a central role in biological interactions and biogeochemical cycles since the carbon they obtain from their photosynthetic plant hosts is allocated via the mycorrhizal mycelium to the soil ecosystem (Finlay & Rosling, 2005). The focus of much early mycorrhizal research was on the effect of the mycorrhizal mycelium on uptake of inorganic nutrients by individual plants, often studied in the absence of other soil organisms. More recent research has a broader focus on a wider range of possible effects (Newsham et al., 1995; Finlay, 2004). In addition to interactions with their own host plant roots, the mycelia interact with a range of organic and inorganic substrates, as well as with different organisms such as bacteria, fungi, soil micro- and meso-fauna and the roots of secondary host plants. Three trends in mycorrhizal research, noted by Smith and Read in 1997, are still relevant today. New information from the fossil record and, in particular, developments in molecular phylogeny have enabled progress in understanding different evolutionary aspects of the symbiosis (Hibbett et al., 2000; Corradi et al., 2004; Bidartondo, 2005). Secondly, there is an increased emphasis on the extraradical mycelium, the most dynamic component of the symbiosis, the fungal hyphae growing from the host plant roots that make direct contact with organic and inorganic substrates and interact with other soil organisms (Leake et al. 2004a).

The Glomeromycota is the newest fungal phylum. These are the arbuscular mycorrhizal (AM) fungi that form symbioses with the majority of land plant species. Fossil and molecular evidence suggest this is an ancient symbiosis, that may have been instrumental in enabling plants to colonise terrestrial habitats. The AM fungi gain carbon from their plant host, and the primary benefit to plants is thought to be the acquisition of phosphate, a highly immobile ion in soil. However, they are thought to have many other ecological benefits. Host specificity has been considered to be low in this group, as most fungi in culture will colonise most plants, but recent evidence suggests that some AM fungi show a degree of specificity. We review the evidence revealing the genetic structure of this ancient group of fungi. Although a consensus has not yet been reached, we suggest that understanding the evolution and genetic structure of this enigmatic group may be the key to understanding how they function in ecosystems.

Ectomycorrhizas (ECM) are dual organs formed between the terminal feeder roots of many plant species and certain soil fungi. The species richness and taxonomic diversity of ECM symbionts is impressive: ca. 7-10,000 fungal and ca. 8,000 plant species may be capable of forming ECM. The latter are the dominant components of forest and woodland ecosystems over much of the earths surface. The obligate nature of the symbiosis for ECM fungi has been brought into question by reports that some species produce sporocarps under field conditions in the absence of a host plant. We suggest that there is no unequivocal evidence to support this. The spread of tree roots is often underestimated and small, overlooked hosts such as dwarf Salix spp or sedges may explain the appearance of ECM sporocarps in vegetation apparently devoid of ECM hosts. Compared to plant material, the sporocarps of ECM fungi contain high concentrations of N and P. We show that it would take between 3 and 14 million mycorrhizal tips, or 1800 km of hyphae, to supply the N in one sporocarp of Boletus edulis. The mantle formed by the fungus over the root tip is the likely site of storage for the N and P required for sporocarp production, and we discuss the chemical and structural mechanisms developed on mantles by ECM fungi to defend this resource against fungivory.

The myco-heterotrophs comprise over 400 achlorophyllous plant species in 87 genera that are parasitic upon fungi, and exploit them as their principle source of carbon. In addition, there are estimated to be over 30,000 species, comprising approximately 10% of the plant kingdom, that depend upon myco-heterotrophy for establishment from dust seeds or spores. These initially myco-heterotrophic plants require fungal-carbon during their critical early establishment phases but go on to produce green shoots on emerging into light from soil or as epiphytes on other plants. This extraordinary mode of nutrition, first recognised more than a century ago, is hidden in the initially myco-heterotrophic plants and in the fully achlorophyllous species continues to be misrepresented by many botanists as a form of ‘saprophytism’. As a consequence, the parasitic dependence of these plants upon fungi is not widely appreciated. This is the only group of parasitic plants for which, in over 90% of cases, the host species (which are fungi) providing their carbon and nutrients remain unidentified. This article reviews some of the exciting recent progress made in understanding the nature and properties of fungi parasitized by plants, and in the use of distinctive carbon and nitrogen isotope ‘signatures’ of myco-heterotrophs to quantify fungal dependence in plants with green shoots that remain partially myco-heterotrophic in adulthood.

Helen Gwynne-Vaughan and Lilian Hawker were influential British mycologists active in the early and mid-20th century respectively. Here their personalities, careers, research achievements and writings are assessed and compared.