On the Inside Nitrite Signaling

It is well known that nitrate is an important nutrient that supports plant growth and development. The application of nitrate also causes extensive changes in the expression of genes coding for proteins involved in nitrogen (N) metabolism. Indeed, genomic analyses have provided a comprehensive dataset of more than a thousand nitrate-responsive genes in Arabidopsis (Arabidopsis thaliana). Much less, however, is known about the signaling role of nitrite, the direct product of nitrate reduction. In this issue, Wang et al. (1735–1745) report that nitrite increases mRNA levels as quickly as nitrate in N-starved Arabidopsis roots. Both nitrite and nitrate inductions occur at concentrations as low as 100 nM. The response at low nitrite concentrations was not due to contaminating nitrate—a problem in several earlier studies. The speed of the response suggests that it is unlikely that reprovision of N is the cause of the nitrite response. Another possible mechanism is that nitrite is converted to nitric oxide, which elicits the response, but treatment with 250 mM nitrite revealed no increase in the fluorescence of roots stained with a nitric oxide-reactive dye. Transcriptome analysis using nitrate or nitrite showed that more than half of the nitrate-induced genes, which included genes involved in nitrate and ammonium assimilation, energy production, and carbon and N metabolism, responded equivalently to nitrite; however, the nitrite response was more robust and there were many genes that responded specifically to nitrite. Thus, it appears that nitrite can serve as a signal as well as if not better than nitrate.