Nitrate Foraging By Arabidopsis Root Is Mediated By the Transcription Factor TCP20 through the Systemic Signaling Pathway (from DNA cis-Elements to Nitrate Regulatory Network)

Nitrate is the primary nitrogen (N) source for most plants. Besides acting as a vital nutrient, nitrate is a key signaling molecule that holistically regulates plant metabolism and development. However, despite the fact that the nitrate response had been extensively studied for decades, little was known about nitrate regulatory network compared to our much more detailed understanding of the nitrate assimilation process. The lack of rapid and systematic tools had limited our ability to find key regulators. Therefore, we developed a rapid transient expression system in Nicotiana benthamiana for analyzing nitrate responses and were able to identify nitrate enhancer DNA and cis-elements. By using the nitrate enhancer DNA to screen a library of Arabidopsis transcription factors (TF) in the yeast one-hybrid system, 18 potential TFs were identified. One of them, TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL FACTOR1-20 (TCP20), was identified as a novel regulator in the nitrate regulatory network and found to play a key role in systemic signaling that directs nitrate foraging by Arabidopsisroots. We anticipate that more TFs will be identified by using this systematic approach, which will eventually lead to the elaboration of a nitrate regulatory network. An understanding of nitrate regulatory mechanisms will assist in efforts to improve nitrogen use efficiency (NUE) in agriculture and thereby reduce greenhouse gas emissions and pollution of ecosystems.