GRAS51 promotes legume symbiotic nitrogen fixation and salt tolerance of nodulation by directly regulating <i>NIN</i> expression

Abiotic stresses affect the symbiotic relationship between legumes and rhizobia and nodule formation. Salt stress suppresses rhizobial infection, nodule development, and nitrogen fixation efficiency; however, the underlying genetic and molecular mechanisms remain unknown. In this study, we identified a GRAS transcription factor, designated MaGRAS51, that interacts with Nodulation Signaling Pathway 1 (MaNSP1) and jointly binds to the promoter of the Nodule Inception (MaNIN) gene to activate its expression. NSP1 and NIN genes are essential for rhizobial infection, nodule initiation, and symbiotic gene expression in legumes. Overexpression or knockdown of MaGRAS51 in Melilotus albus resulted in increased or decreased nodule number, respectively, which correspondingly led to the significant up- and downregulated expression of MaNIN. Interestingly, MaGRAS51 was highly induced by salt stress during nodulation and activated the expression of MaNIN by directly binding to its promoter independently, thereby maintaining symbiotic nodulation under salt stress. In conclusion, we identified a transcriptional activator of MaNIN and revealed the mechanism by which MaGRAS51 acts as a network node to coordinate the expression of MaNIN and symbiotic nodulation under salt stress conditions, providing insights to improve symbiotic nitrogen fixation in legumes under environmental stress conditions.