Exogenous Hydrogen Sulfide Enhances Photosynthesis Under Thiocyanate Stress by Regulating Rubisco Energy Metabolism and Activation in Rice Seedlings

Thiocyanate (SCN^-), a persistent inorganic contaminant widely present in industrial wastewater, poses severe risks to plant growth and photosynthesis. Hydrogen sulfide (H₂S) is an emerging gaseous signaling molecule involved in the regulation of plant stress responses; however, its role in modulating Rubisco energy metabolism and activation under SCN^- stress remains unclear. Here, we investigated the effects of exogenous H₂S on magnesium homeostasis, ATP/NADPH metabolism, Rubisco activation, and photosynthetic performance in rice seedlings exposed to SCN^- stress via physiological, biochemical, and transcriptional approaches. We found that exogenous H₂S significantly increased Mg²⁺ accumulation, enhanced H⁺-ATPase and Mg²⁺-ATPase activities, and promoted Rubisco activase (RCA) abundance and activity. These changes were accompanied by reduced steady-state ATP and NADPH contents, indicating that increased energy consumption was driven by accelerated Calvin cycle turnover. At the transcriptional level, H₂S regulated key genes involved in ATP hydrolysis, Mg²⁺ transport, Rubisco activation, and chlorophyll biosynthesis. Consequently, the chlorophyll content, stomatal conductance, and transpiration rate improved under SCN^- stress. Collectively, our results demonstrate that exogenous H₂S enhances photosynthetic efficiency and Rubisco carboxylation capacity by coordinating Rubisco energy metabolism and activation.