BÜCKING, HEIKE & WOLFGANG HEYSER.
University of Bremen Plant Physiology and Anatomy Centre for environmental Research Technology Leobener Str. 28359 GERMANY.
Compatible interactions between mycorrhizal fungi and plants are based on bidirectional transfer processes across an interfacial apoplast. Development and structure of this interface vary between different mycorrhizal associations. Microautoradiographic investigations showed, that in mycorrhizal roots a main exchange zone exists, where the Hartig net is well differentiated and the main translocation of phosphate to the host plant and of carbohydrates to the fungal symbiont occurs. In this main exchange zone also the highest uptake of phosphate by the fungal sheath can be detected. Experiments with axenic cultures showed that the phosphate uptake is regulated by the phosphate concentration in the fungal mycelium. Based on these results it can be supposed (1) that in ectomycorrhizal roots a real bidirectional transfer of phosphate and carbohydrates across the same interface surface is realised and (2) that the phosphate uptake by fungal hyphae is affected by the phosphate demand of the mycorrhizal host plant in this region. X-ray microanalytical studies after cryofixation, freeze-drying and pressure infiltration revealed, that cations in the interfacial apoplast can play a possible role in the regulation of the phosphate efflux into the ectomycorrhizal interface. A model system is shown, in which the phosphate transfer to the mycorrhizal host plant and a possible regulation of the transfer processes in mycorrhizal roots are described.
Key words: ectomycorrhiza, phosphate, Pinus, Populus, X-ray microanalysis.