The sucrose transporter GmSWEET3c drives soybean nodulation by regulating root sucrose allocation

Symbiotic nitrogen fixation in legumes, driven by the interaction between rhizobia and host plants, provides essential nitrogen for plant growth but demands substantial energy. Sucrose, the principal product of photosynthesis, is critical in supporting this process. Despite its importance, the mechanisms underlying sucrose allocation following rhizobia inoculation remain poorly understood. Here, we identified and characterized GmSWEET3c, a rhizobia-induced sucrose transporter that is critical for sucrose allocation to the root susceptible zone. Functional analysis of the Gmsweet3c mutant revealed impaired sucrose allocation and a significant reduction in nodule formation, underscoring its critical role in symbiotic nodulation. Using a GmSWEET3c-GFP fusion protein, we found that the protein is located in both the plasma membrane of root cells and the membranes of infection threads, suggesting dual roles of GmSWEET3c in facilitating sucrose transport to the root susceptible zone and directing sucrose toward infection threads. Moreover, we demonstrated that GmNSP1, a key symbiotic transcription factor, directly binds to the promoter region of GmSWEET3c, activating its expression. Collectively, our findings highlight GmSWEET3c as a key mediator of sucrose distribution in soybean roots after rhizobia inoculation, enhancing our understanding of carbohydrate allocation in legume-rhizobia symbioses.