Symbiotic nitrogen fixation is an energy-expensive process, and the light available to plants has been proposed to be a primary influencer. We demonstrate that the light-induced soybean TGACG-motif binding factor 3/4 (GmSTF3/4) and FLOWERING LOCUS T (GmFTs), which move from shoots to roots, interdependently induce nodule organogenesis. Rhizobium-activated calcium- and calmodulin-dependent protein kinase (CCaMK) phosphorylates GmSTF3, triggering GmSTF3–GmFT2a complex formation, which directly activates expression of nodule inception (NIN) and nuclear factor Y (NF-YA1 and NF-YB1). Accordingly, the CCaMK–STF–FT module integrates aboveground light signals with underground symbiotic signaling, ensuring that the host plant informs its roots that the aboveground environment is prepared to sustainably supply the carbohydrate necessary for symbiosis. These results suggest approaches that could enhance the balance of carbon and nitrogen in the biosphere.