Progress and new questions in understanding the mechanisms controlling plant nitrogen responses.

Plants require larger quantities of nitrogen (N) than of other soil nutrients, making N availability critical for plant growth and crop yield. However, levels of N sources available in soils vary across different regions of the world, but are often insufficient to support optimum plant growth. In addition, N availability fluctuates both temporally and spatially. Therefore, plants must sense and respond to environmental variations in N levels or to N shortages. Nitrate is both the primary source of N for plants and a critical signaling molecule that regulates N assimilation and many other physiological processes in plants. Recent studies have unraveled the mechanism of nitrate signaling and placed it at the center of the regulatory pathways governing plant responses to N availability. This review briefly summarizes the recent advances that revealed the mechanisms controlling plant responses to N availability. It focuses particularly on the mechanism of nitrate signaling, before addressing the new questions that have emerged from recent findings. Finally, it discusses how recent insights into the mechanisms regulating plant responses to N availability can be utilized in the field to improve nitrogen use efficiency in crops.