Unravelling the invivo regulation and metabolic role of the alternative oxidase pathway in C3 species under photoinhibitory conditions

The mitochondrial alternative oxidase pathway (AOP) has been suggested to act as a sink for excess reducing power generated in the chloroplast under high-light (HL) stress and thus may reduce photoinhibition. The aim of this study was to compare different species to investigate the invivo regulation and role of AOP under HL stress. The invivo activities of AOP ((alt)) and the cytochrome oxidase pathway, chlorophyll fluorescence, metabolite profiles, alternative oxidase (AOX) capacity and protein amount were determined in leaves of five C-3 species under growth light and after HL treatment. Differences in respiration and metabolite levels were observed among species under growth light conditions. The HL response of (alt) was highly species dependent, correlated with the AOP capacity and independent of AOX protein content. Nevertheless, significant correlations were observed between (alt), levels of key metabolites and photosynthetic parameters. The results show that the species-specific response of (alt) is caused by the differential post-translational regulation of AOX. Significant correlations between respiration, metabolites and photosynthetic performance across species suggest that AOP may permit stress-related amino acid synthesis, whilst maintaining photosynthetic activity under HL stress.