Mitochondrial serine hydroxymethyltransferase (SHMT), combined with glycine decarboxylase, catalyzes an essential sequence of the photorespiratory C 2 cycle, namely, the conversion of two molecules of glycine into one molecule each of CO 2 , NH 4 1 , and serine. The Arabidopsis (Arabidopsis thaliana) mutant shm (now designated shm1-1) is defective in mitochondrial SHMT activity and displays a lethal photorespiratory phenotype when grown at ambient CO 2 , but is virtually unaffected at elevated CO 2 . The Arabidopsis genome harbors seven putative SHM genes, two of which (SHM1 and SHM2) feature predicted mitochondrial targeting signals. We have mapped shm1-1 to the position of the SHM1 gene (At4g37930). The mutation is due to a G / A transition at the 5# splice site of intron 6 of SHM1, causing aberrant splicing and a premature termination of translation. A T-DNA insertion allele of SHM1, shm1-2, and the F 1 progeny of a genetic cross between shm1-1 and shm1-2 displayed the same conditional lethal phenotype as shm1-1. Expression of wild-type SHM1 under the control of either the cauliflower mosaic virus 35S or the SHM1 promoter in shm1-1 abrogated the photorespiratory phenotype of the shm mutant, whereas overexpression of SHM2 or expression of SHM1 under the control of the SHM2 promoter did not rescue the mutant phenotype. Promoter-b-glucuronidase analyses revealed that SHM1 is predominantly expressed in leaves, whereas SHM2 is mainly transcribed in the shoot apical meristem and roots. Our findings establish SHM1 as the defective gene in the Arabidopsis shm1-1 mutant.

SummaryThe Arabidopsis mutant dicarboxylate transport (dct ) is one of the classic mutants in the photorespiratory pathway. It requires high CO 2 levels for survival. Physiologic and biochemical characterization of dct indicated that dct is de®cient in the transport of dicarboxylates across the chloroplast envelope membrane. Hence, re-assimilation of ammonia generated by the photorespiratory cycle is blocked. However, the defective gene in dct has not been identi®ed at the molecular level. Here, we report on the molecular characterization of the defective gene in dct, on the complementation of the mutant phenotype with a wild-type cDNA, and on the functional characterization of the gene product, DiT2, in a recombinant reconstituted system. Furthermore, we provide the kinetic constants of recombinant DiT1 and DiT2, and we discuss these data with respect to their functions in ammonia assimilation. Moreover, an analysis of the transcript levels of DiT1 and DiT2 in different C 3 -and C 4 -type plant species is presented, and we demonstrate that the substrate speci®city of DiT2 from the C 4 -plant Flaveria bidentis is similar to its counterpart from C 3 plants.