Mitochondrial AOX Supports Redox Balance of Photosynthetic Electron Transport, Primary Metabolite Balance, and Growth in under High Light.

When leaves receive excess light energy, excess reductants accumulate in chloroplasts. It is suggested that some of the reductants are oxidized by the mitochondrial respiratory chain. Alternative oxidase (AOX), a non-energy conserving terminal oxidase, was upregulated in the photosynthetic mutant of , , which accumulated reductants in chloroplast stroma. AOX is suggested to have an important role in dissipating reductants under high light (HL) conditions, but its physiological importance and underlying mechanisms are not yet known. Here, we compared wild-type (WT), , and a double mutant of AOX1a-knockout plant () and () grown under high- and low-light conditions, and conducted physiological analyses. The net assimilation rate () was lower in than that in the other genotypes at the early growth stage, while the leaf area ratio was higher in . We assessed detailed mechanisms in relation to . In , photosystem II parameters decreased under HL, whereas respiratory O uptake rates increased. Some intermediates in the tricarboxylic acid (TCA) cycle and Calvin cycle decreased in , whereas γ-aminobutyric acid (GABA) and N-rich amino acids increased in . Under HL, AOX may have an important role in dissipating excess reductants to prevent the reduction of photosynthetic electron transport and imbalance in primary metabolite levels.