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.