A nuclear gene encoding mitochondrial triangle(1)-pyrroline-5-carboxylate dehydrogenase and its potential role in protection from proline toxicity

Delta (1)-pyrroline-5-carboxylate (P5C), an intermediate in biosynthesis an d 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 1.5.1.12) plays a crucial role in this process by degrading the toxic Pro catabolism intermediate P5C. Als o in a Delta put2 yeast mutant, lacking P5CDH, Pro led to growth inhibition and formation of reactive oxygen species (ROS). Complementation of the Del ta put2 mutant allowed identification of the Arabidopsis P5CDH gene. AtP5CD H is a single-copy gene and the encoded protein was localized to the mitoch ondria. High homology of AtP5CDH to LuFIS1, an mRNA upregulated during susc eptible pathogen attack in flax, suggested a role for P5CDH in inhibition o f hypersensitive reactions. An Arabidopsis mutant (cpr5) displaying a const itutive pathogen response was found to be hypersensitive to external Pro. I n 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 co nditions. 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 leve ls have to be tightly controlled. These results indicate that, in addition to the well-studied functions of Pro, for example in osmoregulation, the Pr o metabolism intermediate P5C also serves as a regulator of cellular stress responses.