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

Serine hydroxymethyltransferases (SHMs) are important enzymes of cellular one-carbon metabolism and are essential for the photorespiratory glycine-into-serine conversion in leaf mesophyll mitochondria. In Arabidopsis (Arabidopsis thaliana), SHM1 has been identified as the photorespiratory isozyme, but little is known about the very similar SHM2. Although the mitochondrial location of SHM2 can be predicted, some data suggest that this particular isozyme could be inactive or not targeted into mitochondria. We report that SHM2 is a functional mitochondrial SHM. In leaves, the presequence of SHM2 selectively hinders targeting of the enzyme into mesophyll mitochondria. For this reason, the enzyme is confined to the vascular tissue of wild-type Arabidopsis, likely the protoxylem and/or adjacent cells, where it occurs together with SHM1. The resulting exclusion of SHM2 from the photorespiratory environment of mesophyll mitochondria explains why this enzyme cannot substitute for SHM1 in photorespiratory metabolism. Unlike the individual shm1 and shm2 null mutants, which require CO 2 -enriched air to inhibit photorespiration (shm1) or do not show any visible impairment (shm2), double-null mutants cannot survive in CO 2 -enriched air. It seems that SHM1 and SHM2 operate in a redundant manner in one-carbon metabolism of nonphotorespiring cells with a high demand of one-carbon units; for example, during lignification of vascular cells. We hypothesize that yet unknown kinetic properties of SHM2 might render this enzyme unsuitable for the high-folate conditions of photorespiring mesophyll mitochondria.

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