GSK3-mediated stress signaling inhibits legume-rhizobium symbiosis by phosphorylating GmNSP1 in soybean.

Legumes establish symbiotic associations with rhizobia for biological nitrogen fixation. This process is highly regulated by various abiotic stresses, but the underlying genetic and molecular mechanisms remain largely unknown. In this study, we discovered that the glycogen synthase kinase 3 (GSK3)-like kinase, GmSK2-8, plays an important role in inhibiting symbiotic signaling and nodule formation in soybean (Glycine max) under salt stress. We found that GmSK2-8 is strongly induced in soybean under high-salt conditions, while GmSK2-8 could interact with two G. max Nodulation Signaling Pathway 1 (GmNSP1) proteins, GmNSP1a and GmNSP1b; these key transcription factors are essential for rhizobial infection, nodule initiation, and symbiotic gene expression in soybean. Furthermore, we demonstrated that GmSK2-8 phosphorylates the LHRI domain of GmNSP1a, inhibits its binding to the promoters of symbiotic genes, and thus suppresses nodule formation under salt stress. Knockdown of GmSK2-8 and its close homologs also resulted in reduced plant sensitivity to salt stress during nodule formation. Taken together, our findings indicate that GSK3-like kinases directly regulate the activities of GmNSP1s to mediate salt-inhibited legume-rhizobium symbiosis, providing novel targets for improving symbiotic nitrogen fixation under environmental stress conditions in soybean and possibly other legumes.