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

The effects of phosphorus status and arsenate on the absorption of phosphat e by roots of intact sterile seedlings of Arabidopsis thaliana were studied by analysing the rate of depletion of phosphate from solutions initially c ontaining 10 mu M KH2PO4. Depletion of phosphate from the experimental solu tions was measured both chemically and by labelling with P-32. There was a substantial efflux of phosphate coincident with a rapid influx of phosphate , with efflux increasing with increasing phosphorus status. The highest rat es of absorption were obtained for the plants initially grown with a high l evel of phosphorus but then deprived of phosphate for 5 d prior to the expe riments, with the next highest rates obtained for the most phosphorus- defi cient plants. Kinetic analysis suggests that changes in both the affinity a nd capacity of the absorption mechanism contribute to differences in the ra te of phosphate influx between plants of different phosphorus status. Arsen ate as 20 mu M KH2AsO4 inhibited phosphate influx in a manner such that all plants, regardless of their phosphorus status, had the same phosphate infl ux rate. This was reflected in identical values for the Michaelis constant, K-m, and maximum velocity as used in Michaelis- Menten kinetics, V-max. Ar senate had its greatest effect on phosphate movement to the shoot. The simu ltaneous elimination of differences in phosphate influx between plants of d ifferent phosphorus status suggest that phosphate movement to the shoot may be important in the regulation of influx by phosphorus status.