Discovery of nitrate–CPK–NLP signalling in central nutrient–growth networks ($)

Nitrate acts as a potent signal as well as a source of nutritional nitrogen, but key players in the nitrate response have been missing from our understanding. Liu et al. identified a unique calcium signal stimulated by nitrate in mesophyll cells. They then found that in vitro kinase activity of Ca²⁺-sensor protein kinases (CPKs) are also nitrate stimulated, and they assayed constitutively-active Arabidopsis CPKs for stimulation of a nitrate-responsive reporter in response to nitrate. They found that several CPK genes act synergistically in the nitrate response. Efforts to generate higher-order CPK mutants were thwarted by embryo lethality, so the authors generated inducible mutants. The ipck mutant is a triple-inducible mutant of CPK10, CPK30 and CPK32. It shows significant defects in nitrate-responsive transcription and growth, and phosphorylation of NLP transcription factors. These results reveal a new Ca²⁺-CPK-NLP signaling cascade mediating nitrate responses. Nature 10.1038/nature22077