The role of photorespiration in redox and energy balance of photosyntheticplant cells: A study with a barley mutant deficient in glycine decarboxylase

Protoplasts and mitochondria were isolated from leaves of homozygous barley (Hordeum vulgare L,) mutant deficient in glycine decarboxylase complex (GD C, EC 2.1.2.10) and wild-type plants. The photosynthetic rates of isolated protoplasts from the mutant and wild-type plants under saturating CO, were similar, but the respiratory rate of the mutant was two-fold higher. Respir ation in the mutant plants was much more strongly inhibited by antimycin A than in wild-type plants and a low level of the alternative oxidase protein was found in mitochondria, The activities of NADP- and NAD-dependent malat e dehydrogenases were also increased in mutant plants, suggesting an activa tion of the malate-oxaloacetate exchange for redox transfer between organel les. Mutant plants had elevated activities of NADH- and NADPH-dependent gly oxylate/hydroxypruvate reductases, which may be involved in oxidizing exces s NAD(P)H and the scavenging of glyoxylate. We estimated distribution of po ols of adenylates, NAD(H) and NADP(H) between chloroplasts, cytosol and mit ochondria. Under photorespiratory conditions, ATP/ADP and NADPH/NADP ratios in the mutant were higher in chloroplasts as compared to wild-type plants. The cytosolic NADH/NAD ratio was increased, whereas the ratio in mitochond ria decreased. It is concluded that photorespiration serves as an effective redox transfer mechanism from the chloroplast, Plants with a lowered GDC c ontent are deficient in this mechanism, which leads to over-reduction and o ver-energization of the chloroplasts.