Nitrate sensing and response in Plants: From calcium signaling to phytohormone regulation.

Plants need to acquire sufficient nitrogen (N) from the soil for their growth and development. Nitrate (NO) is the major source of N for plants in aerobic soils. In addition to its role as a nutrient, nitrate also acts as a signaling molecule to reprogram plant metabolism and trigger changes in plant architecture. With the development of genomics technologies and genetic tools, breakthroughs in the understanding of the nitrate signaling network have been made over the past years. In this review, we will discuss the mechanisms of nitrate sensing and its transcriptional response throughout the plant, with an emphasis on the effect of nitrate-elicited calcium signal on the primary nitrate response (PNR). Recent studies have not only identified a second nitrate sensor, NLP7, but also identified calcium-dependent kinases (CPKs) as a molecular link between membrane-localized nitrate receptor NRT1.1 (CHL1/NPF6.3) and NLP transcription factors, which bridges the nitrate gap. We also discuss the latest progress on the interaction between nitrate signal and hormonal pathways for local and systematical developmental responses in the model plant Arabidopsis thaliana roots. A holistic view of how all the identified signals crosstalk to orchestrate the thousands of N responses is the key for the sustainable development of agriculture.