Localized proliferation of lateral roots in NO-rich patches is a striking example of the nutrient-induced plasticity of root development. In Arabidopsis, NO stimulation of lateral root elongation is apparently under the control of a NO-signaling pathway involving the ANR1 transcription factor. ANR1 is thought to transduce the NO signal internally, but the upstream NO sensing system is unknown. Here, we show that mutants of the NRT1.1 nitrate transporter display a strongly decreased root colonization of NO-rich patches, resulting from reduced lateral root elongation. This phenotype is not due to lower specific NO uptake activity in the mutants and is not suppressed when the NO-rich patch is supplemented with an alternative N source but is associated with dramatically decreased ANR1 expression. These results show that NRT1.1 promotes localized root proliferation independently of any nutritional effect and indicate a role in the ANR1-dependent NO signaling pathway, either as a NO sensor or as a facilitator of NO influx into NO-sensing cells. Consistent with this model, the NRT1.1 and ANR1 promoters both directed reporter gene expression in root primordia and root tips. The inability of NRT1.1-deficient mutants to promote increased lateral root proliferation in the NO-rich zone impairs the efficient acquisition of NO and leads to slower plant growth. We conclude that NRT1.1, which is localized at the forefront of soil exploration by the roots, is a key component of the NO-sensing system that enables the plant to detect and exploit NO-rich soil patches.