Constancy of nitrogen turnover kinetics in the plant cell: insights into the integration of subcellular N fluxes.

Compartmental analysis with 13N was used to determine cytosolic nitrate (NO3-) pools, and their turnover rates, in roots of intact barley (Hordeum vulgare L. cv Klondike) seedlings. Influx, efflux, flux to the vacuole and assimilation, and flux to the xylem, varied as much as 300-fold over a wide range of external NO3- conditions. By contrast, the kinetic constant kc describing cytosolic NO3- turnover varied by less than 4% from a mean value of 0.0407 min(-1). Accordingly, cytosolic NO3- pools varied linearly with influx. A literature survey showed that kc constancy is observed with both NO3- and ammonium (NH4+) fluxes in many plant species, including H. vulgare, Arabidopsis thaliana, Picea glauca, and Oryza sativa. The regulatory system implied by this phenomenon is fundamentally different from that of potassium (K+) fluxes, in which cytosolic pool size is held constant while kc varies with external K+ concentrations. We further present data showing that barley plants, grown on one steady-state concentration of NH4+, restore kc within minutes of exposure to new, non-steady-state, NH4+ concentrations. We propose the existence of a high-fidelity mechanism governing the timing of cytosolic N turnover, and discuss its implications for attempts to improve plants biotechnologically.