OsNLP4 is required for nitrate assimilation gene expressions and nitrate-dependent growth in rice

In plants, nitrate is important nutrient and signaling molecule that modulates the expression of many genes and regulates growth. In paddy grown rice, nitrogen is mostly supplied in the form of ammonium, but nitrate also shares substantial portion of available nitrogen. A number of nitrogen transporters and nitrate assimilation enzymes have been identified and functionally characterized. However, little is known about the nitrate sensor system and regulatory mechanisms of these nitrate related genes. In recent years, NIN-like proteins (NLPs) have been described as key transcription factors of nitrogen responses in Arabidopsis thaliana. But the functions of OsNLPs in rice are still elusive. Here we report the characterization of OsNLP4 to reveal its function in rice. Growths of OsNLP4 knockout mutants were reduced under the nitrate supply, but not under ammonium supply. The mRNA accumulation of genes involved in nitrate assimilation were declined significantly and nitrate uptake rate and nitrate reductase activity were also impaired in the mutants. Using rice protoplast transient expression system, OsNLP4-GFP fusion was localized to nucleus irrespective of nitrate conditions. OsNLP4 was also required for normal yield under paddy field conditions. We propose the OsNLP4 is essential for regulation of genes involved in nitrate assimilation and nitrate-dependent growth in rice. One sentence summaryThe osnlp4 mutants exhibit abnormal nitrate response and poor growth under nitrate supply and in paddy field conditions.