Plant Physiol, November 2002, Vol. 130, pp. 1162-1171

Phosphorus Effects on Metabolic Processes in Monoxenic Arbuscular Mycorrhiza Cultures¹

Department of Microbial Ecology, Ecology Building, Lund University, SE-223 62 Lund, Sweden (P.A.O., I.M.v.A., H.R.); School of Biological Sciences, The University of Sydney, New South Wales 2006, Australia (W.G.A.); and School of Biological Sciences, The University of New South Wales, Sydney 2052, Australia (A.E.A.)

The influence of external phosphorus (P) on carbon (C) allocation and metabolism as well as processes related to P metabolism was studied in monoxenic arbuscular mycorrhiza cultures of carrot (Daucus carota). Fungal hyphae of Glomus intraradices proliferated from the solid minimal medium containing the colonized roots into C-free liquid minimal medium with different P treatments. The fungus formed around three times higher biomass in P-free liquid medium than in medium with 2.5 mM inorganic P (high-P). Mycelium in the second experiment was harvested at an earlier growth stage to study metabolic processes when the mycelium was actively growing. P treatment influenced the root P content and [¹³C]glucose administered to the roots 7 d before harvest gave a negative correlation between root P content and ¹³C enrichment in arbuscular mycorrhiza fungal storage lipids in the extraradical hyphae. Eighteen percent of the enriched ¹³C in extraradical hyphae was recovered in the fatty acid 16:15 from neutral lipids. Polyphosphate accumulated in hyphae even in P-free medium. No influence of P treatment on fungal acid phosphatase activity was observed, whereas the proportion of alkaline-phosphatase-active hyphae was highest in high-P medium. We demonstrated the presence of a motile tubular vacuolar system in G. intraradices. This system was rarely seen in hyphae subjected to the highest P treatment. We concluded that the direct responses of the extraradical hyphae to the P concentration in the medium are limited. The effects found in hyphae seemed instead to be related to increased availability of P to the host root.