Summary• A survey of 12 plants colonized by six species of arbuscular mycorrhizal fungi was conducted to explore the diversity of Arum and Paris mycorrhizal structures.• Surveyed root material was sectioned both longitudinally and transversely, doublestained and mycorrhizal structures were identified. A detailed time course experiment using four plant, and four fungal species, was used to investigate the sequential development of hyphae, arbuscules, hyphal coils, arbusculate coils and vesicles.• The survey indicated that there was a continuum of mycorrhizal structures ranging from Arum to Paris , depending upon both the host plant and the fungus. The time course showed that total colonization increased, and that the establishment of the various mycorrhizal structures did not appear to change greatly over time.• It was concluded that identification of fungal structures and their subsequent development into morphological types is not easily defined. Visual inspection of root squashes is not always adequate, especially where transverse sections are needed to determine if longitudinal hyphae are inter or intracellular; this is essential to distinguish intermediate types.

This review commemorates and examines the significance of the work of Isobel Gallaud more than 100 years ago that first established the existence of distinct structural classes (Arum-type and Paris-type) within arbuscular mycorrhizal (AM) symbioses. We add new information from recent publications to the previous data last collated 10 years ago to consider whether any patterns have emerged on the basis of different fungal morphology within plant species or families. We discuss: (1) possible control exerted by the fungus over AM morphology; (2) apparent lack of plant phylogenetic relationships between the classes; (3) functions of the interfaces in different structural classes in relation to nutrient transfer in particular; and (4) the occurrence of plants with both of the major classes, and with intermediate AM structures, in different plant habitats. We also give suggestions for future research to help remove uncertainties about the functional and ecological significance of differences in AM morphology. Lastly, we urge retention of the terms Arum- and Paris-type, which are now well recognised by those who study AM symbioses.

Summary• This study investigated effects of plant density and arbuscular mycorrhizal (AM) colonization on growth and phosphorus (P) nutrition of a cultivar of wheat (Triticum aestivum) that often shows early AM-induced growth depressions.• Two experiments were conducted. Expt 1 had three plant densities and one soil P concentration. Expt 2 had two plant densities and two P concentrations. Plants were grown in calcareous P-fixing soil, inoculated with Glomus intraradices or Gigaspora margarita, or noninoculated (nonmycorrhizal (NM)). Glomus intraradices colonized well and caused a growth depression only in Expt 1. Gigaspora margarita caused large growth depressions in both experiments even though it colonized poorly.• The results showed that growth depressions were mitigated by changes in relative competition for soil P by NM and AM plants, and probably by decreasing carbon costs of the fungi.• The different effects of the two fungi appear to be attributable to differences in the balance between P uptake by the fungal pathway and direct uptake via the roots. These differences may be important in other AM symbioses that result in growth depressions. The results show that mycorrhizal growth responses of plants grown singly may not apply at the population or community level.

This review brings together recent work on the coordination of transport processes between fungus and plant symbionts in arbuscular mycorrhizal (AM) symbioses, and focuses on new information on the diversity in structure and function of interfaces and their potential roles in transport processes. We consider the way that fungal activity is polarised to absorb mineral nutrients (especially phosphorus, P) in soil, transport them to the root and release them to the plant. Conversely, the fungal structures within the root appear to be specialised to absorb sugars, which the external mycelium cannot do. The external mycelium depends on a supply of lipid, transported from within the root. High affinity P transporters expressed in the root apices and root hairs of non-mycorrhizal roots, and most probably mycorrhizal roots, absorb P actively. This can result in the development of P depletion zones, so that a low concentration of P at the absorbing surfaces limits further uptake. The external hyphae of AM fungi extend well beyond the depletion zone, accessing supplies of P at a distance and in narrow soil pores, that is absorbed actively by a high affinity P transporter expressed in these small diameter hyphae. Translocation of P within the hyphae and transfer to the plant results in much higher rates of uptake (inflows) by mycorrhizal than non-mycorrhizal roots. The possible role of polyphosphate (polyP) in this process is discussed in the light of new data. Within the root, P is lost from the fungal structures to the interfacial apoplast by an unknown mechanism, and is absorbed by the root cortical cells. The expression of a high affinity P transporter and H + -ATPase in arbuscule-containing cells indicates that these are probably the sites of fungus/plant P transfer. The site of sugar transfer from plant to fungus has not yet been established. At the whole plant level, plant uptake systems located in the youngest regions of the root are positioned to absorb P from undepleted soil, into which the root apex has just grown. In older regions of the roots, colonised by mycorrhizal fungi, the external mycelium will take over the absorptive role and overcome the difficulties posed by the slow diffusion of P in soil.

Methods for the objective measurement of vesicular-arbuscular mycorrhizal infection in the roots of four species of host plant are described. The methods are based on vital staining of transverse sections of fresh roots, followed by evaluation of occurrence and/or numbers of living arbuscules and hyphae per section. The most useful method involved freeze-sectioning roots embedded in gelatin containing glycerol and DMSO, followed by staining the sections with nitroblue tetrazolium overnight to reveal succinate dehydrogenese activity. Two systems of computer-aided image analysis were evaluated for measurement of the number, perimeter and cross-sectional areas of intercellular hyphae and arbuscules. This approach, which is clearly objective, is an advance on previous methods which were based on ranking sections or segments of root with respect to the intensity of development of arbuscules. The data were used to calculate the area of interface between plant and fungus in infected roots. Results are presented showing that the contribution of arbuscules to the total infection and area of interface declines markedly as plants and infections age. Differences in infection in different host/fungus combinations are also described.