Pyrroline-5-carboxylate dehydrogenase is an essential enzyme for proline dehydrogenase function during dark-induced senescence in Arabidopsis thaliana

During leaf senescence, nitrogen is remobilized and carbon backbones are replenished by amino acid catabolism, with many of the key reactions occurring in mitochondria. The intermediate Delta(1)-pyrroline-5-carboxylate (P5C) is common to some catabolic pathways, thus linking the metabolism of several amino acids, including proline and arginine. Specifically, mitochondrial proline catabolism involves sequential action of proline dehydrogenase (ProDH) and P5C dehydrogenase (P5CDH) to produce P5C and then glutamate. Arginine catabolism produces urea and ornithine, the latter in the presence of alpha-ketoglutarate being converted by ornithine delta-aminotransferase (OAT) into P5C and glutamate. Metabolic changes during dark-induced leaf senescence (DIS) were studied in Arabidopsis thaliana leaves of Col-0 and in prodh1prodh2, p5cdh and oat mutants. Progression of DIS was followed by measuring chlorophyll and proline contents for 5 days. Metabolomic profiling of 116 compounds revealed similar profiles of Col-0 and oat metabolism, distinct from prodh1prodh2 and p5cdh metabolism. Metabolic dynamics were accelerated in p5cdh by 1 day. Notably, more P5C and proline accumulated in p5cdh than in prodh1prodh2. ProDH1 enzymatic activity and protein amount were significantly down-regulated in p5cdh mutant at Day 4 of DIS. Mitochondrial P5C levels appeared critical in determining the flow through interconnected amino acid remobilization pathways to sustain senescence.