Nitrate transport and signalling

Physiological measurements of nitrate (NO3-) uptake by roots have defined two systems of high and low affinity uptake. In Arabidopsis, genes encoding both of these two uptake systems have been identified. Most is known about the high affinity transport system (HATS) and its regulation and yet measurements of soil NO3- show that it is more often available in the low affinity range above 1 mM concentration. Several different regulatory mechanisms have been identified for AtNRT2.1, one of the membrane transporters encoding HATS; these include feedback regulation of expression, a second component protein requirement for membrane targeting and phosphorylation, possibly leading to degradation of the protein. These various changes in the protein may be important for a second function in sensing NO3- availability at the surface of the root. Another transporter protein, AtNRT1.1 also has a role in NO3- sensing that, like AtNRT2.1, is independent of their transport function. From the range of concentrations present in the soil it is proposed that the NO3- -inducible part of HATS functions chiefly as a sensor for root NO3- availability. Two other key NO3- transport steps for efficient nitrogen use by crops, efflux across membranes and vacuolar storage and remobilization, are discussed. Genes encoding vacuolar transporters have been isolated and these are important for manipulating storage pools in crops, but the efflux system is yet to be identified. Consideration is given to how well our molecular and physiological knowledge can be integrated as well to some key questions and opportunities for the future.