Phosphate efflux from intraradical hyphae of Gigaspora margarita in vitro and its implication for phosphorus translocation

The enzymatic separation of physiologically active intraradical hyphae of arbuscular mycorrhiza (AAA) fungi from host tissue is an effective way to analyse the physiological characteristics of intraradical hyphae. Applying this technique, first, both biomass and phosphorus (P) distribution were quantified in the intraradical and extraradical part of Gigaspora margarita colonizing onion. Second, the separated intraradical hyphae were incubated in vitro; efflux of phosphate from the hyphae and the hyphal polyphosphate (poly P) content were determined. A substantial proportion of P in mycorrhizal roots was fungal P, while the proportion of fungal biomass to root biomass was < 2 %. Phosphate efflux and the decrease in poly-P content in the hyphae were both enhanced by the addition of glucose and 2-deoxyglucose, an analogue of glucose. The degrees of enhancement for phosphate efflux and poly-P decrease were comparable, suggesting that P efflux from intraradical hyphae was coupled with poly-P hydrolysis. Based on these findings, the translocation of P from fungus to host was estimated in relation to the distribution of hyphal P in extraradical and intraradical parts of the AM fungus.