Unraveling carbon dynamics in legume–rhizobia symbioses: toward a single-cell resolution of symbiotic metabolism

Legumes acquire nitrogen via a symbiotic interaction with diazotrophic rhizobia bacteria. In return for getting fixed nitrogen, plants deliver high amount of photosynthate to the bacteria to support the nitrogen fixation process. Hence, biological nitrogen fixation in legume plants is a highly energy-demanding process that relies on the precise coordination of carbon allocation and metabolism between the host plant and its microbial symbiont. Although significant progress has been made in understanding carbon fluxes during nodulation, how these processes are spatially and functionally organized across different cell types and developmental stages within nodules remains poorly resolved. This limitation has hindered a comprehensive understanding of how carbon metabolism supports the establishment, maintenance, and termination of symbiosis. In this review, we explore the current understanding of carbon transport and metabolism throughout the nodulation process, from early allocation during rhizobial infection to the complex metabolic, transport, and regulatory networks in mature nitrogen-fixing and senescing nodules. We highlight key knowledge gaps, especially regarding cell-type specific and spatial regulation of carbon metabolism. Finally, we discuss how emerging single-cell and spatial omics techniques offer powerful tools to resolve these gaps, enabling a deeper understanding of the metabolic and regulatory complexity that underpins legume–rhizobia symbiosis.