The mitochondrial multienzyme glycine decarboxylase (GDC) catalyzes the tetrahydrofolate-dependent catabolism of glycine to 5,10-methylene-tetrahydrofolate and the side products NADH, CO 2 , and NH 3 . This reaction forms part of the photorespiratory cycle and contributes to one-carbon metabolism. While the important role of GDC for these two metabolic pathways is well established, the existence of bypassing reactions has also been suggested. Therefore, it is not clear to what extent GDC is obligatory for these processes. Here, we report on features of individual and combined T-DNA insertion mutants for one of the GDC subunits, P protein, which is encoded by two genes in Arabidopsis (Arabidopsis thaliana). The individual knockout of either of these two genes does not significantly alter metabolism and photosynthetic performance indicating functional redundancy. In contrast, the double mutant does not develop beyond the cotyledon stage in air enriched with 0.9% CO 2 . Rosette leaves do not appear and the seedlings do not survive for longer than about 3 to 4 weeks under these nonphotorespiratory conditions. This feature distinguishes the GDC-lacking double mutant from all other known photorespiratory mutants and provides evidence for the nonreplaceable function of GDC in vital metabolic processes other than photorespiration.The mitochondrial multienzyme complex Gly decarboxylase (GDC) contributes to the two strategically important metabolic pathways of (1) photorespiration in all photosynthesizing organs and (2) one-carbon metabolism in all biosynthetically active tissues. In each of these two metabolic contexts, GDC closely cooperates with a second mitochondrial enzyme, Ser hydroxymethyltransferase (SHM), in the conversion of Gly to Ser. In the course of the tetrahydrofolate (THF)-dependent GDC reaction cycle comprising three individual reactions, CO 2 and NH 3 are released, and NAD 1 becomes reduced to NADH. The remaining methylene moiety becomes attached to THF to produce the one-carbon donor compound 5,10-methylene-THF (CH 2 -THF). SHM subsequently synthesizes Ser from CH 2 -THF and a second molecule of Gly in a fully reversible reaction (Douce et al., 2001;Hanson and Roje, 2001).The combined GDC/SHM reaction represents the mitochondrial part of the photorespiratory C 2 cycle, which occurs in all photosynthesizing tissues of C 3 plants, extends over three cellular compartments, and converts Rubisco-generated 2-phosphoglycolate into the Calvin cycle metabolite 3-phosphoglycerate (Tolbert, 1997;Douce and Neuburger, 1999). The importance of GDC and SHM for photorespiration becomes apparent from the fact that all as yet-reported mutants and antisense plants show strong metabolic disturbations in normal air, but grow well in the nonphotorespiratory conditions of approximately 1% CO 2 (Somerville and

The mannitol cycle has been verified in a unicellular red alga (Rhodellophyceae) for the first time. All four enzymes involved in the cycle (mannitol-1-phosphate dehydrogenase, Mt1PDH: EC 1.1.1.17; mannitol-1-phosphatase, Mt1Pase: EC 3.1.3.22; mannitol dehydrogenase, MtDH: 1.1.1.67; hexokinase, HK: 2.7.1.1.) were detected and characterized in crude algal extracts from Dixoniella grisea. These enzymes, with the exception of Mt1Pase, were specific to their corresponding substrates and nucleotides. The activities of enzymes in the anabolic pathway (fructose-6-P reduction by Mt1PDH and mannitol-6-P reduction by Mt1Pase) were at least 2-to 4-fold greater than those of the catabolic pathway (mannitol oxidation by MtDH and fructose oxidation by HK). There appears to be, therefore, a net carbon flow in D. grisea towards a high intracellular mannitol pool. The mannitol cycle guarantees a rapid accumulation or degradation of mannitol within algal cells in response to changing salinity in natural habitats. Moreover, the demonstration of the mannitol cycle within the Rhodellophyceae provides evidence that this metabolic pathway is of ancient origin in the red algal lineage.