Glyoxylate Inhibition of Ribulosebisphosphate Carboxylase/Oxygenase Activation State in vivo

Photosynthetic CO₂ exchange in photorespiration mutants of Arabidopsisthaliana showed a time-dependent inhibition at 350 µl/literCO₂ in 50% O₂ but not in 2% O₂. In a glycolate-P phos-phatasedeficient mutant, inhibition of photosynthesis was due to adepletion of ribulosebisphosphate. In the remaining mutants,which have defects in photorespiratory enzymes which metabolizeamino acids, reduced photosynthesis was accompanied by a declinein the activation level of ribulosebisphosphate carboxylase/oxygenase(Chastain and Ogren 1985), a decline in ribulosebisphosphateconcentration, and an accumulation of glyoxylate. Addition ofglyoxylate at submillimolar concentrations to intact spinach(Spinacea oleracea L.) chloroplasts inhibited light activationof ribulosebisphosphate carboxylase/oxygenase (rubisco) andCO₂ fixation. Similar concentrations of glyoxylate had no effecton A. thaliana rubisco activity in vitro. These results suggestthat glyoxylate accumulation indirectly inhibited rubisco activationstate in vivo. The inhibition of photosynthesis in mutants whichaccumulate glyoxylate may be attributed to a decline in ribulosebisphosphateconcentration, a reduction in rubisco activation state, or acombination of both phenomena. ³Present address: CSIRO, Division of Plant Industry, GPO Box1600, Canberra, ACT 2601, Australia. (Received May 12, 1989; Accepted July 8, 1989)