Nitrate is the most common N source used by higher plants and is the ‘substrate inducer’ of nitrate and nitrite reductase activities. However, environmental nitrate also has other effects on higher plants, which include the induction of a high affinity nitrate transport system. changes in root morphology, and increased root respiration. Nitrate induces nitrate and nitrite reductase activities by altering gene expression, mainly by enhancing transcription of the respective genes. We have begun to view these responses of higher plant tissues to environmental nitrate in the context of a model for signal transduction. In our model, nitrate is a signal for developmental changes in the physiology of the plant, which are expressed in a hierarchy. The primary responses include induction of genes for nitrate and nitrite reductases, nitrate uptake and translocation systems, and DNA regulatory proteins required for expression of the secondary response gene systems. The secondary responses include more complex phenomena such as proliferation of the root system, enhancement of respiration and other changes in the physiology of the plant.