Elemental composition and function of polyphosphates in ectomycorrhizal fungi — an X-ray microanalytical study

Abstract

The elemental composition of vacuolar granules in different ectomycorrhizal fungi, Suillus bovinus, Paxillus involutus, Pisolithus tinctorius and Laccaria laccata was analysed by energy dispersive X-ray spectroscopy (EDXS) either after chemical preparation or cryofixation, freeze-drying and pressure infiltration. Vacuolar inclusions were present in living hyphae and were not an artifact of specimen preparation, and they can be referred to as polyphosphate granules. These granules were localized in fungal vacuoles and the main counter-ions were K and Mg. The postulated association of these granules with the divalent cation Ca has to be interpreted as an artifact of the specimen preparation and was caused by the chemical preparation of former EDXS studies. The incorporation of cations, such as K, Na and Zn, depended on the external supply and emphasized the importance of these granules for the intracellular homeostasis of fungal cells. EDXS studies of polyphosphate granules in ectomycorrhizal associations of Pinus sylvestris showed that the elemental composition of granules differed between the sheath and the Hartig net. In polyphosphate granules of the Hartig net a higher content of K was detectable, whereas the incorporation of Mg was reduced. These results indicated a possible role of K in the transfer of short chained, mobile polyphosphates through the hyphae to the Hartig net. The concentration of potassium and phosphate in the fungal vacuole and the cytoplasm was closely correlated, which can be explained by a possible linkage between the phosphate and K uptake, which would maintain the charge balance and the pH of the fungal cell. The estimation of different phosphate pools in axenic cultures and mycorrhizal roots revealed that the fungal metabolism was changed by the association with an ectomycorrhizal host plant. In a mycorrhizal association higher proportions of absorbed phosphate were translocated into the metabolically inactive polyphosphate pool, which demonstrates the importance of this pool for the nutrition of the ectomycorrhizal host plant.