Fig. 1.

Simplified scheme of nitrate signalling. Changes in nitrate concentration are perceived by transceptors (e.g. low-affinity nitrate transporter NRT1), enabling changes in expression of enzymes catalysing nitrate reduction and amino acid assimilation, as well as nitrate transporters, such as NRT2 (high-affinity transporter). Initial upregulation of NRT2 expression following N re-supply to previously N-starved plants is subsequently followed by feedback downregulation resulting from the accumulation of the products of nitrate metabolism. This is probably mediated by transcription factors LBD37/38/39 (lateral organ boundary domain, negative regulator of the primary nitrate response). Gene expression for transcription factors LBD37/38/39 is very low under N-limited conditions. NRT1 acts upstream of ANR1 (a nitrate-regulated member of the MADS-box family of transcription factors), regulating increased root branching under a localized supply of nitrate. At low nitrate concentrations, NRT1 functions as an auxin transporter out of the root tip, thus inhibiting lateral root growth. Lines ending in arrows and bars indicate positive and negative effects, respectively. Dotted lines indicate uncertainty in the response.