Nitrogen Nutrition Special Issue doi:10.1093/jxb/erm066 Advance Access publication 22 May, 2007 SPECIAL ISSUE PAPER

Physiological measurements of nitrate (NO23) 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 NO23 show that it is more often available in the low affinity range above 1 mM concentration. Several dif-ferent regulatory mechanisms have been identified for AtNRT2.1, one of the membrane transporters encoding HATS; these include feedback regulation of expres-sion, 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 NO23 availability at the surface of the root. Another transporter protein, AtNRT1.1 also has a role in NO23 sensing that, like AtNRT2.1, is inde-pendent of their transport function. From the range of concentrations present in the soil it is proposed that the NO23-inducible part of HATS functions chiefly as a sensor for root NO23 availability. Two other key N