Kinetics of phosphorus uptake by the germ‐tubes of the vesicular–arbuscular mycorrhizal fungus, Gigaspora margarita

Abstract

SUMMARYWe characterized kinetically the phosphorus (P) transport systems present in the germ‐tubes of the vesicular‐arbuscular (VA) mycorrhizal fungus, Gigaspora margarita Becker & Hall. Germinated spores of G. margarita were grown without P or with 1000 μM P for 10 d. The rate of P uptake by these spores and their associated germ‐tubes was measured at external P concentrations ranging from 1 to 10000 μM and in the presence or absence of an uncoupler of oxidative phosphorylation.Two transport systems for P were identified in the germ‐tubes of G. margarita grown without P. System I, an active high affinity system with a Km of 1.8–3.1 μM and a Vmax of 3.1–3.6 nmol mg protein−1h−1and System II, a passive low affinity system with a Km of 10200–11 300 μM and a Vmax of 445–895 nmol mg protein−1h−1. These two systems operated simultaneously at concentrations of P ranging from 1 to 10000 μM. Two transport systems for P were also identified in the germ‐tubes of G. margarita grown with 1000 μM P. The Km of System I in these germ‐tubes (13.4 μM) was increased relative to the Km of System I in germ‐tubes grown without P. There was also evidence to suggest that the Km of System II was increased in germ‐tubes grown with 1000 μM P.The values of Km and Vmax for the P transport systems in the germ‐tubes of G. margarita are similar to those observed in the hyphae of other fungi and in plant roots.