Inhibition of photosynthesis in Arabidopsis mutants lacking leaf glutamate synthase activity

Abstract

The CO₂ released in photorespiration is believed to result from a complex mitochondrial reaction in which glycine is converted to equimolar amounts of CO₂, NH₃ and the C-1 group of N⁵,N¹⁰-methylene-tetrahydrofolate ^1–3. Because photorespiratory CO₂ production may amount to 80 μmol per h per g fresh weight^4,5 the stoichiometry of the decarboxylation reaction indicates that NH₃ release from glycine exceeds primary NO₃⁻ reduction^6,7. Leaf cells must therefore be capable of rapid NH₃ reassimilation in photorespiratory conditions. It has been suggested⁸ that NH₃ could be refixed directly into glutamate by mitochondrial glutamate dehydrogenase (GDH), utilizing the NADH generated during glycine decarboxylation^1,9,10. This seems unlikely^11,12 because of the high K_m for NH₃ exhibited by GDH in vitro. An alternative suggestion^11,12, that photorespiratory NH₃ could be re-assimilated into glutamate by the sequential action of cytoplasmic glutamine synthetase (GS) and the chloroplast enzyme glutamate synthase (GOGAT)^13,14, is supported by circumstantial evidence from in vitro studies^12,15. Here we provide direct evidence for such a pathway, based on the results of experiments with mutants of Arabidopsis thaliana (L.) Heynh which are deficient in leaf GOGAT activity.