Abstract: Due to its essential role in life process, nitrogen (N) uptake and metabolism is precisely regulated in all kinds of life organisms. The regulation of N uptake and metabolism depends on the sensing of internal and external N signals. In the past decades, much knowledge has been acquired in this aspect in bacteria and fungi. In plants, however, the progress is relatively slow and the knowledge about N signals is scare. In this review, the major component in N signal systems in bacteria, fungi and plants were compared so as to provide new insight for N signal research in pant systems.