Physiological role of AOX1a in photosynthesis and maintenance of cellular redox homeostasis under high light in Arabidopsis thaliana.

As plants are sessile, they often face high light (HL) stress that causes damage of the photosynthetic machinery leading to decreased photosynthesis. The importance of alternative oxidase (AOX) in optimizing photosynthesis is well documented. In the present study, the role of AOX in sustaining photosynthesis under HL was studied using AOX1a knockout mutants (aox1a) of Arabidopsis thaliana. Under growth light (GL; 50 μmol photons m(-2) s(-1)) conditions, aox1a plants did not show any changes in photosynthetic parameters, NAD(P)/H redox ratios, or respiratory O2 uptake when compared to wild-type (WT). Upon exposure to HL (700 μmol photons m(-2) s(-1)), respiratory rates did not vary between WT and aox1a. But, photosynthetic parameters related to photosystem II (PSII) and NaHCO3 dependent O2 evolution decreased, while the P700 reduction state increased in aox1a compared to WT. Further, under HL, the redox state of cellular NAD(P)/H pools increased with concomitant rise in reactive oxygen species (ROS) and malondialdehyde (MDA) content in aox1a compared to WT. In presence of HL, the transcript levels of several genes related to antioxidant, malate-oxaloacetate (malate-OAA) shuttle, photorespiratory and respiratory enzymes was higher in aox1a compared to WT. Taken together, these results demonstrate that under HL, in spite of significant increase in transcript levels of several genes mentioned above to maintain cellular redox homeostasis and minimize ROS production, Arabidopsis plants deficient in AOX1a were unable to sustain photosynthesis as is the case in WT plants.