SUMMARYThe ultrastructural distribution of acid and alkaline phosphatase in 6-week-old onion roots infected by Glomus mosseae has been investigated cytochennically. Significant acid phosphatase activity was only observed in the little vacuolated, immature terminal arbuscule branches of the mycorrhizal fungus whilst strong alkaline a-naphthyl phosphatase and /?-glycerophosphatase activities were localized within the vacuoles of the mature arbuscular and intercellular hyphae. In the host cells neither acid nor alkaline phosphatase distribution was modified with vesiculararbuscular mycorrhiza formation. These results are discussed in relation to previously reported mycorrhiza-specific alkaline phosphatase Gianinazzi, 1976, 1978) and the metabolism of phosphorus in vesicular-arbuscular mycorrhizal systems.

SUMMARYThe ultrastructural localization of ATPase activities was identical in onion and sycamore mycorrhizas, DES-sensitive ATPase activity was associated with the fungal plasmalemma and paramural hodies whilst a DES-insensitive ATPase activity was detected within the fungal vacuole. The development of the mycorrhizal fungus within root cells modified the distribution of DES-sensitive plasmalemma-bound ATPase activity of the host. This became concentrated around the finer arbuscule branches and appeared to be a specialization of the invaginated host membrane which occurred when arbuscule structure was favourable for nutrient exchange. These observations strengthen the hypothesis that nutrient exchange in VA mycorrhizas must occur across the living host-fungus interface and that phosphate is transferred from fungus to host by an active transport mechanism in the finer branches of the arbuscule.

SUMMARYThe ultrastructural organization and some cytochemical features (protein and polysaccharide distribution) of the mycorrhiza formed by Glomus tenuis in raspberry roots have been investigated. Certain aspects of the fine mycorrhizal endophyte (smaller hyphae, thinner walls, distinct two-layered wall structure following the PATAg test for polysaccharides, complete absence of septa) distinguish it from the coarse vesicular-arbuscular mycorrhizal fungi. The modifications occurring in the host-fungus interface during Glomus tenuis mycorrhiza development are however very similar to those that have been described in several mycorrhizae formed by coarse vesicular-arbuscular endophytes.

SUMMARYAn ultrastructural comparison of early stages of mycorrhiza formation on Eucalyptus urophylla was carried out using two isolates of P. tinctorius, one cultured from a sporocarp collected under pine and the other cultured from a sporocarp collected under eucalypt. Major differences at the fungus-root interface were observed 2 and 4 d following inoculation. Eucalypt root cells in contact with the isolate from pine developed a thickened cell wall, whereas the isolate from eucalypt was associated with an accumulation of extracellular fibrils. The fibrils stained positively for carbohydrates and protein. They were absent with the isolate from pine. Four days after inoculation with the isolate from eucalypt, acid phosphatase activity was detected along the fungal plasmalemma at the interface with host cells. This enzyme activity was not detected with the isolate from pine. Vacuolar acid phosphatase activity was detected in both fungal isolates when in contact with Eucalyptus roots. It is suggested that host recognition of ectomycorrhizal symbionts can be manifested within hours of inoculation with the fungal partner.

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