Regulation of NRT2.1 nitrate transporter by auxin response factor in Arabidopsis

Root-expressed nitrate transporters work for the uptake of nitrogen when nitrate is present as the main nitrogen source in the soil. In Arabidopsis roots, high-affinity nitrate transporter, NRT2.1, is transcriptionally activated by nitrate, suggesting significance of its presence as a major facilitator for the acquisition of nitrate; however information on upstream regulatory factors that may control the expression of NRT2.1 is still limited. In this study, we focused on transcription factors that can modulate the transcript levels of NRT2.1. A transposon-tagged collection of transcription factors of Arabidopsis was screened by RT-PCR to monitor the NRT2.1 transcript levels, allowing us to identify a loss-of-function mutant of AUXIN RESPONCE FACTOR7 (ARF7). The isolated arf7 mutant showed reduced number of lateral roots and increased levels of NRT2.1 mRNA compared to the wild type. We propose a model that ARF7 is an upstream factor that negatively controls NRT2.1 having inhibitory effects on lateral root initiation under low nitrate conditions.