Summary Δ1‐pyrroline‐5‐carboxylate (P5C), an intermediate in biosynthesis and degradation of proline (Pro), is assumed to play a role in cell death in plants and animals. Toxicity of external Pro and P5C supply to Arabidopsis suggested that P5C dehydrogenase (P5CDH; EC 1.2.1.12) plays a crucial role in this process by degrading the toxic Pro catabolism intermediate P5C. Also in a Δput2 yeast mutant, lacking P5CDH, Pro led to growth inhibition and formation of reactive oxygen species (ROS). Complementation of the Δput2 mutant allowed identification of the Arabidopsis P5CDH gene. AtP5CDH is a single‐copy gene and the encoded protein was localized to the mitochondria. High homology of AtP5CDH to LuFIS1, an mRNA up‐regulated during susceptible pathogen attack in flax, suggested a role for P5CDH in inhibition of hypersensitive reactions. An Arabidopsis mutant (cpr5) displaying a constitutive pathogen response was found to be hypersensitive to external Pro. In agreement with a role in prevention of cell death, AtP5CDH was expressed at a basal level in all tissues analysed. The highest expression was found in flowers that are known to contain the highest Pro levels under normal conditions. External supply of Pro induced AtP5CDH expression, but much more slowly than Pro dehydrogenase (AtProDH) expression. Uncoupled induction of the AtProDH and AtP5CDH genes further supports the hypothesis that P5C levels have to be tightly controlled. These results indicate that, in addition to the well‐studied functions of Pro, for example in osmoregulation, the Pro metabolism intermediate P5C also serves as a regulator of cellular stress responses.

Branched-chain amino acid transaminases (BCATs) play a crucial role in the metabolism of leucine, isoleucine, and valine. They catalyze the last step of the synthesis and/or the initial step of the degradation of this class of amino acids. In Arabidopsis, seven putative BCAT genes are identified by their similarity to their counterparts from other organisms. We have now cloned the respective cDNA sequences of six of these genes. The deduced amino acid sequences show between 47.5% and 84.1% identity to each other and about 30% to the homologous enzymes from yeast (Saccharomyces cerevisiae) and mammals. In addition, many amino acids in crucial positions as determined by crystallographic analyses of BCATs from Escherichia coli and human (Homo sapiens) are conserved in the AtBCATs. Complementation of a yeast ⌬bat1/⌬bat2 double knockout strain revealed that five AtBCATs can function as BCATs in vivo. Transient expression of BCAT:green fluorescent protein fusion proteins in tobacco (Nicotiana tabacum) protoplasts shows that three isoenzymes are imported into chloroplasts (AtBCAT-2, -3, and -5), whereas a single enzyme is directed into mitochondria (AtBCAT-1).

We recently identified a cDNA encoding a putative isovaleryl-coenzyme A (CoA) dehydrogenase in Arabidopsis (AtIVD). In animals, this homotetrameric enzyme is located in mitochondria and catalyzes the conversion of isovaleryl-CoA to 3-methylcrotonyl-CoA as an intermediate step in the leucine (Leu) catabolic pathway. Expression of AtIVD:smGFP4 fusion proteins in tobacco (Nicotiana tabacum) protoplasts and biochemical studies now demonstrate the in vivo import of the plant isovaleryl-CoA dehydrogenase (IVD) into mitochondria and the enzyme in the matrix of these organelles. Two-dimensional separation of mitochondrial proteins by blue native and SDS-PAGE and size determination of the native and overexpressed proteins suggest homodimers to be the dominant form of the plant IVD. Northern-blot hybridization and studies in transgenic Arabidopsis plants expressing Ativd promoter:gus constructs reveal strong expression of this gene in seedlings and young plants grown in the absence of sucrose, whereas promoter activity in almost all tissues is strongly inhibited by exogeneously added sucrose. Substrate specificity tests with AtIVD expressed in Escherichia coli indicate a strong preference toward isovaleryl-CoA but surprisingly also show considerable activity with isobutyryl-CoA. This strongly indicates a commitment of the enzyme in Leu catabolism, but the activity observed with isobutyryl-CoA also suggests a parallel involvement of the enzyme in the dehydrogenation of intermediates of the valine degradation pathway. Such a dual activity has not been observed with the animal IVD and may suggest a novel connection of the Leu and valine catabolism in plants.

No abstract

No abstract