Integrating multi-omics reveals the mechanisms of elevated [CO2] enhances salt tolerance of tobacco plants with a photosynthetic regulatory network

Abiotic stresses often occur concurrently with global changes, especially the combination of salt stress and elevated CO₂ concentration ([CO₂]) is a common phenomenon in salt-affected soils throughout the world. The synergistic regulatory mechanisms of tobacco (Nicotiana tabacum L.) plants in response to combined salt stress and elevated [CO₂] are still poorly understood. This study aimed to reveal the potential mechanisms of elevated [CO₂] modulating leaf photosynthesis of tobacco plants subjected to salt stress by integrating physiological, transcriptional, and metabolomics analyses. The results showed that elevated [CO₂] significantly enhanced the photosynthetic and carbon assimilation capacity of tobacco plants by increasing the carboxylation efficiency of Rubisco, which counteracted damage to the photosynthetic system, mitigated water imbalance, and reduced ion toxicity caused by salt stress, thus alleviating the negative physiological effects on tobacco plants. This response was mechanistically linked to metabolic reprogramming: glyoxylate-derived succinate entered the tricarboxylic acid cycle to support adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH) production for antioxidant enzymes, while intermediates served as precursors for sucrose synthesis. At the same time, elevated [CO₂] promoted photosynthetic carbon fixation, leading to a heightened triose phosphate flux toward starch and sucrose biosynthesis and accumulation. These carbohydrates functioned as osmoprotectants, conserving energy that would otherwise be expended in antioxidant synthesis. Thus, under future climate scenarios where elevated [CO₂] coincides with intensified soil salinization, salt-induced suppression of photosynthetic carbon assimilation should be rigorously accounted for to avoid overestimation of the CO₂ fertilization effect. This study provides novel insights into plant salt tolerance mechanisms, guiding precision breeding and exogenous modulation strategies for stress-resilient crops.