Nitrate Fluxes and Nitrate Reductase Activity of Suspension-Cultured Tobacco Cells (Effects of Internal and External Nitrate Concentrations).

Cell suspensions of tobacco (Nicotiana tabacum L., cv KY14) were used to determine the responses of NO3- uptake and NO3- reductase activity (NRA) to exogenous NO3- levels in the absence of long-distance NO3- transport. Tobacco cells grown with complete Murashige and Skoog medium for 7 d were subcultured for 3 d with NH4+-free media containing 0, 5, 10, 20, 30, and 40 mM NO3-. Cell NO3-, in vitro NRA, NO3- influx, and efflux of cell NO3- were determined. The NRA increased as cell NO3- increased. Cell NO3- efflux values increased as cell NO3- level increased. Cells with low intracellular NO3- had greater NO3- influx than cells with high intracellular NO3-. Woolf-Augustinsson-Hofstee transformations of the NO3- influx kinetic data revealed patterns characteristic of a high- and low-affinity two-component NO3- uptake system. Apparent Vmax values generally decreased and Km values increased as cell NO3- concentration increased. The NRA of cells supplied with 10 and 20 mM NO3- after 3-d growth in N- free medium increased about 5-fold within 2 h and then remained constant for the next 2 h, whereas NRA of cells supplied with 5 mM NO3- increased only 2-fold during the 4-h period. Intracellular NO3- and other N metabolites associated with cell NO3- levels exerted differential effects on the NO3- influx activity and NRA of tobacco cells cultured in suspension. Expression of high NRA was correlated with both high external and intracellular NO3-, whereas maximum NO3- influx activity required a low (depleted) level of cell NO3-.