Phosphate transfer in VA mycorrhizas. Special mechanisms or not?

Abstract

The widespread symbiosis between plants and vesicular-arbuscular (VA) mycorrhizal fungi involves uptake of phosphate by fungal hyphae, translocation into the root, and transfer to the plant across the fungus-root interface. We have suggested that this transfer may involve membrane transport mechanisms that are only present (or are activated) once the symbiosis is established. We describe here two studies designed to clarify this issue. Firstly, we have measured phosphate inflow into non-mycorrhizal and mycorrhizal roots of A Ilium porrum (leek) and have used image analysis to calculate phosphate fluxes across the fungus — root interface. These fluxes fall in the range 4–13 nmol m⁻²s⁻¹, as in other plant tissue. Secondly, we have compared effluxes of chloride and phosphate from the macro-alga Chara, cereal roots, and the ericoid (endo-)mycorrhizal fungus Hymenoscyphus ericae in culture. The aim was to ascertain if phosphate can pass through the anion channels that, when open, allow efflux of chloride. Experimental treatments involved cytoplasmic acidification and, for Chara, increased cytoplasmic calcium produced by electrical stimulations. We conclude that the control of phosphate efflux differs from that of chloride efflux. If ‘phosphate channels’ exist, the nature of the signals for opening and closing remains unknown. With all the material tested, phosphate efflux was much lower than the calculated values for fluxes at the VA mycorrhizal interface. This supports our view that phosphate transfer at the interface indeed involves ‘special mechanisms’ such as channels or carriers that are essentially inoperative in free-living material.