Nodulation Signaling Pathway 2: A Proposed Multifunctional Signaling Hub for Coordinating Plant Architecture, Rhizosphere Chemistry, and Symbiotic Associations Under Nutrient Stress

Under nutrient stress, plants adjust shoot branching, root remodeling, and symbiotic associations to optimize survival and growth. The GRAS-type transcription factor NODULATION SIGNALING PATHWAY 2 (NSP2) participates in these responses through context-dependent interactions with partners: NSP1, MYB40, and SLR1, and is indirectly repressed by SMAX1 via NSP1 and SLR1. NSP2 functions differ among species: it directly binds DNA in rice (Oryza sativa) strigolactone (SL) biosynthesis, whereas in M. truncatula it acts as a co-regulator in nodulation and flavonoid regulation, with NSP1 or MYB40 providing DNA-binding activity; the NSP2–MYB module also appears conserved in barley (Hordeum vulgare). Through these partnerships, NSP2 is proposed to coordinate three interconnected modules influencing plant architecture, rhizosphere chemistry, and symbiotic associations. The NSP1–NSP2 heterodimer activates SL biosynthesis under phosphate deficiency, remodeling root/shoot architecture and balancing N/P uptake in rice. The NSP1–NSP2–SLR1–SMAX1 quaternary module integrates phosphate, gibberellin, and karrikin signals to orchestrate arbuscular mycorrhizal symbiosis in rice. Under nitrogen limitation and rhizobial infection, the NSP2–MYB40 module enhances flavonoid biosynthesis to promote rhizobial nodulation in Medicago truncatula. These findings suggest that the NSP2-centered network could be targeted to improve nutrient-use efficiency, although further field validation is required.