High light acclimation of Oryza sativa L. leaves involves specific photosynthetic-sourced changes of NADPH/NADP⁺ in the midvein

Previous studies have shown that exposure of Arabidopsis leaves to high light (HL) causes a systemic acquired acclimation (SAA) response in the vasculature. It has been postulated that C₄-like photosynthesis in the leaf veins triggers this response via the Mehler reaction. To investigate this proposed connection and extend SAA to other plants, we examined the redox state of NADPH, ascorbate (ASA), and glutathione (GSH) pools; levels and histochemical localization of O₂- and H₂O₂ signals; and activities of antioxidant enzymes in the midvein and leaf lamina of rice, when they were subjected to HL and low light. The results showed that (1) high NADPH/NADP(+) was generated by C₄-like photosynthesis under HL in the midvein and (2) SAA was colocally induced by HL, as indicated by the combined signaling network, including the decrease in redox status of ASA and GSH pools, accumulation of H₂O₂ and O₂- signals, and high superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities. The high correlations between these occurrences suggest that the enhanced NADPH/NADP(+) in HL-treated midveins might alter redox status of ASA and GSH pools and trigger H₂O₂ and O₂- signals during SAA via the Mehler reaction. These changes in turn upregulate SOD and APX activities in the midvein. In conclusion, SAA may be a common regulatory mechanism for the adaptation of angiosperms to HL. Manipulation of NADPH/NADP(+) levels by C₄-like photosynthesis promotes SAA under HL stress in the midvein.