Progress has been made in the mechanism of nitrate inhibiting root nodule formation

Recently, Xie Fang, a researcher at the State Key Laboratory of plant molecular genetics of the center for excellence and innovation in molecular plant science of the Chinese Academy of Sciences, cooperated with Florian frugier, a professor at the University of Sacre in Paris / Sacre Institute of Plant Sciences in Paris, France, and published a research paper on new Phytologist under the title of nlp1 directly controls expression of the peptide encoding cep1 gene in response to nitrate, The molecular mechanism of nitrate inhibiting the expression of mtnlp1 transcription factor in Medicago truncatula was revealed by binding to the half NRE (hnre) component on the promoter of small peptide coding gene mtcep1.

Nitrogen (n) is one of the essential nutrients for plants. Nitrate nitrogen can not only be directly absorbed and utilized by plants, but also be used as an important signal molecule to regulate the expression of genes related to plant response, absorption and metabolism to nitrogen, so as to affect plant growth and development. Legumes can obtain nitrogen for growth by symbiotic interaction with rhizobia, but symbiotic nitrogen fixation is a process that consumes plant energy. When there is a high concentration of nitrogen in soil, nitrogen as a signal molecule will affect the function of symbiotic nitrogen fixation genes, so as to inhibit the process of symbiotic nitrogen fixation. Previous studies have shown that the transcription factor mtnlp1 is the core component of nitrate nitrogen inhibiting nodulation, which inhibits the formation of root nodules by regulating local and systemic pathways.

CEPS (C-terminally encoded peptides) encode a class of small peptides, which are synthesized in plant roots and transported to aboveground parts. Through binding with its receptors, CEPS systematically regulate the absorption of nitrogen. The expression level of CEPS gene depends on the nitrogen concentration in soil. It is induced under the condition of low nitrogen and inhibited under the condition of high nitrogen. However, its expression regulation mechanism is still unclear. According to the research report in Arabidopsis, atnlp7 not only has the ability to start, but also has the function of inhibition. Therefore, the researchers speculated that Tribulus terrestris alfalfa mtnlp1 may be involved in nitrate inhibition of CEPS gene expression.

In this study, the synthesized mtcep1 small peptide was added to wild-type and nlp1 mutants respectively to analyze whether the promoting effect of mtcep1 small peptide on nodulation would be affected. The results showed that mtcep1 small peptide promoted nodulation independent of mtnlp1, suggesting that mtcep1 may act downstream of mtnlp1. Further transcriptional level analysis confirmed that nitrate inhibition of cep1 gene expression was dependent on mtnlp1. Using dexamethasone (DEX) induced expression system, the researchers found that mtnlp1 could inhibit the expression of mtcep1; The results of double luciferase reporter gene detection system also showed that mtnlp1 could inhibit the activity of mtcep1 promoter in response to nitrate. Through truncation experiment and sequence alignment, it was found that the NRE (nitrate response element) component of CEP gene promoter was shorter than that of mtnlp1 promoter gene, so it was named hnre (half NRE). Further studies found that the hnre on the cep1 gene promoter is necessary for nlp1 binding, and the hnre is also conserved on the CEP gene promoter in Arabidopsis, suggesting that this regulation does not exist only in legumes.

This study revealed that mtnlp1 can not only act as a transcription promoter to start the expression of mtcle35 gene, thus mediating the inhibition of nodulation by nitrate, but also act as an inhibitor to inhibit its expression by binding with the promoter of cep1 gene, so it has bifunctional transcriptional activity. At the same time, this study also revealed the mechanism of nitrogen inhibiting CEP gene expression.