N uptake and distribution
Multiple uptake systems contribute to N uptake in plant roots [4• and 6]. Arabidopsis, which primarily acquires N in the form of NO3−, contains both high and low affinity transport systems that have affinities in the micromolar and millimolar range. These NO3− transporters are encoded by genes from the NRT1 and NRT2 families, respectively ( Figure 1a). Some are induced by NO3− which provides a regulatory mechanism that ensures increased uptake when substrate becomes available. NO3− uptake is also regulated by the plant N status, with glutamine acting as negative feedback signal.