Nutrient storage and release in uninfected cells of soybean nodules support symbiotic nitrogen fixation in infected cells.

Symbiotic nitrogen fixation (SNF) between legumes and rhizobia contributes to sustainable agriculture. In root nodules, infected cells (ICs) are the primary sites of rhizobial colonization and nitrogen fixation. However, the function of the neighboring uninfected cells (UCs) has received little attention and is poorly understood. In this study, we employed a symplastic tracing approach to elucidate the role of UCs in nutrient storage and transport within root nodules. We uncovered an extensive network of plasmodesmata connecting ICs and UCs, while direct IC-IC connections were absent. By artificially inducing callose deposition at plasmodesmata, we demonstrate that plasmodesmata permeability between ICs and UCs regulates nutrient import into ICs, thereby influencing nutrient homeostasis and the SNF ability of nodules. Furthermore, high nitrogen levels triggered callose deposition at plasmodesmata, restricting nutrient transport, which may represent one mechanism by which excessive nitrogen inhibits SNF. These findings provide insights into the regulatory mechanisms of SNF and underscore the crucial role of UCs in optimizing nitrogen fixation efficiency. Supplementary Information: The online version contains supplementary material available at 10.1007/s42994-025-00247-y.