ABSTRACT
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.
