Phosphate absorption and efflux of three ectomycorrhizal fungi as affected by external phosphate, cation and carbohydrate concentrations.

A prerequisite for symbiotic phosphate transfer in an ectomycorrhizal (ECM) association is hypothesized to be conditions in the interface between both symbiotic partners, that either promote the release of inorganic phosphate (P) from the Hartig net into the interfacial apoplast and/or decrease the fungal reabsorption from this location. To get more information about conditions, which might be involved in the regulation of P efflux or P reabsorption, the effect of various external conditions on 33P-orthophosphate (33P) uptake or efflux by axenic cultures of the ECM basidiomycetes Hebeloma crustliniforme, Amanita muscaria and Laccaria laccata was analysed. In short-time experiments the following external conditions were analysed: an external supply of (1) P in the preculture, (2) cations (0.1-100 mM K, 0.1-50 mM Na, Mg and Ca), and (3) carbohydrates (0.5-50 mM glucose, fructose or sucrose). The P absorption was generally reduced in cultures previously supplied with an abundant P supply and with increased P concentrations in their tissues. The P uptake was also affected by an external supply of cations, whereas carbohydrates had only a slight effect. Compared to Na, Mg and Ca, the P absorption by H. crustuliniforme and L. laccata was increased by 0.1 mM K in the labelling solution but decreased after a supply of 100 mM K and then did not differ from the other cation treatments. Compared to other cations, an addition of 50 mM Ca led to a decrease of P absorption by A. muscaria, whereas 50 mM Mg increased the P uptake by H. crustuliniforme. The P efflux from the fungi was affected by both the cation and carbohydrate concentration of the bathing solution. High concentrations of the monovalent cations K and Na (5 mM or 50 mM) in the bathing solution increased the P efflux by H. crustuliniforme (only Na) and L. laccata (K and Na), but had little effects on A. muscaria. By contrast, the same concentrations of the divalent cation Mg reduced the P efflux from all fungal basidiomycetes. The P efflux from hyphae of H. crustuliniforme and A. muscaria was also affected by the carbohydrate concentration in the bathing solution. Compared to the hexoses, glucose and fructose, the P efflux by both basidiomycetes was stimulated even by low concentrations of sucrose (0.5 mM) in the bathing solution. The effect of carbohydrates on P efflux especially from hyphae of H. crustuliniforme was clearly concentration-dependent and the maximal efflux rates were observed after a supply of 2.5 mM sucrose or fructose or 5 mM glucose in the bathing solution. A further rise of the carbohydrate concentration led to a progressive decrease of P efflux from hyphae of H. crustuliniforme. The results are discussed with respect to the transfer processes between both symbiotic partners in a mycorrhizal association and a possible regulation of these exchange processes.