Redox basis of photosynthesis inhibition at supra-optimal bicarbonate in mesophyll protoplasts of Arabidopsis thaliana

We examined the patterns of photosynthetic O2 evolution at 1 mM (optimal) and 10 mM (supra-optimal) bicarbonate in mesophyll protoplasts of Arabidopsis thaliana. The photosynthetic rate of protoplasts reached the maximum at an optimal concentration of 1 mM bicarbonate and got suppressed at supra-optimal levels of bicarbonate. We examined the basis of such photosynthesis inhibition by mesophyll protoplasts at supra-optimal bicarbonate. The wild-type protoplasts exposed to supra-optimal bicarbonate showed up signs of oxidative stress. Besides the wild-type, two mutants were used: nadp-mdh (deficient in chloroplastic NADP-MDH) and vtc1 (deficient in mitochondrial ascorbate biosynthesis). The protoplasts of the nadp-mdh mutant exhibited a higher photosynthetic rate and greater sensitivity to supra-optimal bicarbonate than the wild-type. The ascorbatedeficient vtc1 mutant had a low photosynthetic rate and no significant inhibition at high bicarbonate. The nadp-mdh mutants had elevated activities, protein, and transcript levels of key antioxidant enzymes. On the other hand, the antioxidant enzyme systems in vtc1 mutants were not much affected at supra-optimal bicarbonate. We propose that the inhibition of photosynthesis at supra-optimal bicarbonate depends on the redox state of mesophyll protoplasts. The robust antioxidant enzyme systems in protoplasts of nadp-mdh mutant might be priming the plants to sustain high photosynthesis at supra-optimal bicarbonate.

Automated summary

We studied the effects of different concentrations of bicarbonate on photosynthesis in Arabidopsis thaliana mesophyll protoplasts. The photosynthetic rate was highest at 1 mM bicarbonate and decreased at higher concentrations. We found that exposure to supra-optimal bicarbonate caused oxidative stress in wild-type protoplasts, but the nadp-mdh mutant had a higher photosynthetic rate and greater sensitivity to high bicarbonate, while the vtc1 mutant had a low photosynthetic rate and no significant inhibition. We concluded that the inhibition of photosynthesis at supra-optimal bicarbonate levels depends on the redox state of mesophyll protoplasts and that the antioxidant enzyme systems in the nadp-mdh mutant may play a role in sustaining high photosynthesis under these conditions.