Nitrate activates an MKK3-dependent MAPK module via NLP transcription factors in Arabidopsis.

Plant responses to nutrient availability are critical for plant development and yield. Nitrate, the major form of nitrogen in most soils, serves as both a nutrient and signaling molecule. Nitrate itself triggers rapid, major changes in gene expression, especially via nodule inception (NIN)-like protein (NLP) transcription factors, and stimulates protein phosphorylation. Mitogen-activated protein kinase (MAPK)-related genes are among the early nitrate-responsive genes; however, little is known about their roles in nitrate signaling pathways. Here, we show that nitrate resupply to nitrogen-depleted Arabidopsis (Arabidopsis thaliana) plants triggers, within minutes, an MAPK cascade that requires NLP-dependent transcriptional induction of mitogen-activated protein kinase kinase kinase 13 (MAP3K13) and MAP3K14 and that the MAPK cascade is composed of MKK3 and likely C-clade MAPKs (MPK1/2/7/14). Importantly, nitrate reductase-deficient mutants exhibited nitrate-induced MPK7 activities comparable to those observed in wild-type plants, indicating that nitrate itself is the signal that stimulates the cascade. We show that the modified expression of MAP3K13 and MAP3K14 affects nitrate-stimulated BT2 expression and modulates plant responses to nitrogen availability, such as nitrate uptake and senescence. Our finding that an MAPK cascade involving MAP3K13 and MAP3K14 functions in the complex regulatory network governing responses to nitrate availability will guide future strategies to optimize plant responses to nitrogen fertilization and nitrogen use efficiency.