Ornithine-delta-aminotransferase is essential for arginine catabolism but not for proline biosynthesis.
BACKGROUND
Like many other plant species, Arabidopsis uses arginine (Arg) as a storage and transport form of nitrogen, and proline (Pro) as a compatible solute in the defence against abiotic stresses causing water deprivation. Arg catabolism produces ornithine (Orn) inside mitochondria, which was discussed controversially as a precursor for Pro biosynthesis, alternative to glutamate (Glu).


RESULTS
We show here that ornithine-delta-aminotransferase (deltaOAT, At5g46180), the enzyme converting Orn to pyrroline-5-carboxylate (P5C), is localised in mitochondria and is essential for Arg catabolism. Wildtype plants could readily catabolise supplied Arg and Orn and were able to use these amino acids as the only nitrogen source. Deletion mutants of deltaOAT, however, accumulated urea cycle intermediates when fed with Arg or Orn and were not able to utilize nitrogen provided as Arg or Orn. Utilisation of urea and stress induced Pro accumulation were not affected in T-DNA insertion mutants with a complete loss of deltaOAT expression.


CONCLUSIONS
Our findings indicate that deltaOAT feeds P5C exclusively into the catabolic branch of Pro metabolism, which yields Glu as an end product. Conversion of Orn to Glu is an essential route for recovery of nitrogen stored or transported as Arg. Pro biosynthesis occurs predominantly or exclusively via the Glu pathway in Arabidopsis and does not depend on Glu produced by Arg and Orn catabolism.
Funck
Dietmar
D
Department of Plant Physiology and Biochemistry, Biology Section, University of Konstanz, Universitätsstrasse 10, 78464 Konstanz, Germany. dietmar.funck@uni-konstanz.de
Stadelhofer
Bettina
B
Koch
Wolfgang
W
D017360:Arabidopsis / Q000166:cytology / Q000187:drug effects / Q000201:enzymology*; D001120:Arginine / Q000378:metabolism*; D005816:Genetic Complementation Test; D049452:Green Fluorescent Proteins / Q000378:metabolism; D008928:Mitochondria / Q000187:drug effects / Q000201:enzymology; D008954:Models, Biological; D009154:Mutation / Q000235:genetics; D009952:Ornithine / Q000378:metabolism; D009953:Ornithine-Oxo-Acid Transaminase / Q000378:metabolism*; D010641:Phenotype; D011392:Proline / Q000096:biosynthesis*; D021381:Protein Transport / Q000187:drug effects; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D012965:Sodium Chloride / Q000494:pharmacology; D014508:Urea / Q000378:metabolism
D011993:Recombinant Fusion Proteins; D049452:Green Fluorescent Proteins; D012965:Sodium Chloride; D014508:Urea; D001120:Arginine; D011392:Proline; D009952:Ornithine; D009953:Ornithine-Oxo-Acid Transaminase