Abstract: Brassinosteroids (BRs) are potent regulators of photosynthesis and crop yield in agricultural crops; however, the mechanism by which BRs increase photosynthesis is not fully understood. Here, we show that foliar application of 24-epibrassinolide (EBR) resulted in increases in CO₂ assimilation, hydrogen peroxide (H₂O₂) accumulation, and leaf area in cucumber. H₂O₂ treatment induced increases in CO₂ assimilation whilst inhibition of the H₂O₂ accumulation by its generation inhibitor or scavenger completely abolished EBR-induced CO₂ assimilation. Increases of light harvesting due to larger leaf areas in EBR- and H₂O₂-treated plants were accompanied by increases in the photochemical efficiency of photosystem II (Φ_PSII) and photochemical quenching coefficient (q_P). EBR and H₂O₂ both activated carboxylation efficiency of ribulose-1,5-bisphosphate oxygenase/carboxylase (Rubisco) from analysis of CO₂ response curve and in vitro measurement of Rubisco activities. Moreover, EBR and H₂O₂ increased contents of total soluble sugar, sucrose, hexose, and starch, followed by enhanced activities of sugar metabolism such as sucrose phosphate synthase, sucrose synthase, and invertase. Interestingly, expression of transcripts of enzymes involved in starch and sugar utilization were inhibited by EBR and H₂O₂. However, the effects of EBR on carbohydrate metabolisms were reversed by the H₂O₂ generation inhibitor diphenyleneodonium (DPI) or scavenger dimethylthiourea (DMTU) pretreatment. All of these results indicate that H₂O₂ functions as a secondary messenger for EBR-induced CO₂ assimilation and carbohydrate metabolism in cucumber plants. Our study confirms that H₂O₂ mediates the regulation of photosynthesis by BRs and suggests that EBR and H₂O₂ regulate Calvin cycle and sugar metabolism via redox signaling and thus increase the photosynthetic potential and yield of crops.

Key words: Metabolism, Photosynthesis, Reactive oxygen species, Rubisco, Sucrose

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