Phosphate absorption by Arabidopsis thaliana: interactions between phosphorus status and inhibition by arsenate.

The effects of phosphorus status and arsenate on the absorption of phosphate by roots of intact sterile seedlings of Arabidopsis thaliana were studied by analysing the rate of depletion of phosphate from solutions initially containing 10 M KH2PO4. Depletion of phosphate from the experimental solutions was measured both chemically and by labelling with 32P. There was a substantial efflux of phosphate coincident with a rapid influx of phosphate, with efflux increasing with increasing phosphorus status. The highest rates of absorption were obtained for the plants initially grown with a high level of phosphorus but then deprived of phosphate for 5 d prior to the experiments, with the next highest rates obtained for the most phosphorus-deficient plants. Kinetic analysis suggests that changes in both the affinity and capacity of the absorption mechanism contribute to differences in the rate of phosphate influx between plants of different phosphorus status. Arsenate as 20 M KH2AsO4 inhibited phosphate influx in a manner such that all plants, regardless of their phosphorus status, had the same phosphate influx rate. This was reflected in identical values for the Michaelis constant, Km, and maximum velocity as used in Michaelis–Menten kinetics, Vmax. Arsenate had its greatest effect on phosphate movement to the shoot. The simultaneous elimination of differences in phosphate influx between plants of different phosphorus status suggest that phosphate movement to the shoot may be important in the regulation of influx by phosphorus status.