The small peptide CEP1 and the NIN-like protein NLP1 regulate NRT2.1 to mediate root nodule formation across nitrate concentrations

Legumes acquire fixed nitrogen (N) from the soil and through endosymbiotic association with diazotrophic bacteria. However, establishing and maintaining N-2-fixing nodules are expensive for the host plant, relative to taking up N from the soil. Therefore, plants suppress symbiosis when N is plentiful and enhance symbiosis when N is sparse. Here, we show that the nitrate transporter MtNRT2.1 is required for optimal nodule establishment in Medicago truncatula under low-nitrate conditions and the repression of nodulation under high-nitrate conditions. The NIN-like protein (NLP) MtNLP1 is required for MtNRT2.1 expression and regulation of nitrate uptake/transport under low- and high-nitrate conditions. Under low nitrate, the gene encoding the C-terminally encoded peptide (CEP) MtCEP1 was more highly expressed, and the exogenous application of MtCEP1 systemically promoted MtNRT2.1 expression in a compact root architecture 2 (MtCRA2)-dependent manner. The enhancement of nodulation by MtCEP1 and nitrate uptake were both impaired in the Mtnrt2.1 mutant under low nitrate. Our study demonstrates that nitrate uptake by MtNRT2.1 differentially affects nodulation at low- and high-nitrate conditions through the actions of MtCEP1 and MtNLP1. MtNLP1 promotes the expression of MtNRT2.1 to stimulate or suppress nodulation under low- and high-nitrate conditions, respectively, and CEP1 independently promotes nodulation by increasing the expression of NRT2.1 under low-nitrate conditions.

Automated summary

Legumes regulate nodulation through the expression and regulation of MtNRT2.1, MtNLP1, and MtCEP1. MtNRT2.1 affects nodulation differently in low- and high-nitrate conditions, with MtNLP1 promoting its expression under low-nitrate conditions and suppressing it under high-nitrate conditions. MtCEP1 independently enhances nodulation by increasing the expression of NRT2.1 under low-nitrate conditions.