Glyoxylate Inhibition of Ribulosebisphosphate Carboxylase/Oxygenase Activation State in vivo

Photosynthetic CO2 exchange in photorespiration mutants of Arabidopsis thaliana showed a time-dependent inhibition at 350 μl/liter CO2 in 50% O2 but not in 2% O2. In a glycolate-P phos-phatase deficient mutant, inhibition of photosynthesis was due to a depletion of ribulosebisphosphate. In the remaining mutants, which have defects in photorespiratory enzymes which metabolize amino acids, reduced photosynthesis was accompanied by a decline in the activation level of ribulosebisphosphate carboxylase/oxygenase (Chastain and Ogren 1985), a decline in ribulosebisphosphate concentration, and an accumulation of glyoxylate. Addition of glyoxylate at submillimolar concentrations to intact spinach (Spinacea oleracea L.) chloroplasts inhibited light activation of ribulosebisphosphate carboxylase/oxygenase (rubisco) and CO2 fixation. Similar concentrations of glyoxylate had no effect on A. thaliana rubisco activity in vitro. These results suggest that glyoxylate accumulation indirectly inhibited rubisco activation state in vivo. The inhibition of photosynthesis in mutants which accumulate glyoxylate may be attributed to a decline in ribulosebisphosphate concentration, a reduction in rubisco activation state, or a combination of both phenomena.