Metabolomic study reveals a selective accumulation of l-arginine in the d-ornithine treated tobacco cell suspension culture

The non-protein amino acid ornithine (Orn) plays essential roles in regulation of the urea cycle and polyamines biosynthesis in tobacco. Herein, we show that d-enantiomer of Orn, can actively participate in metabolites production in tobacco cells, functioning a positive role in plant cells metabolism, as opposed to the common l-enantiomer. Using a comprehensive amino acids and amines profiling method by liquid chromatography-electrospray ionization triple quadruple mass spectrometry (LC-ESI-QqQ-MS) in combination with chiral LC-ESI-MS, it was shown that d-Orn has a potential advantage in promoting selective and large accumulation of l-arginine (l-Arg) in tobacco cells. Exogenous d-Orn resulted in a selective up-regulation of l-Arg by 80-fold, while l-Orn slightly increased the levels of all amino acids. Changes of all the urea cycle related intermediates, e.g. citrulline, Arg and Orn were also shown to be critical following change of Orn's stereochemistry. GC/MS profiling of the metabolites revealed that high nicotine production was the dominant change driven by l-Orn treatments. From these observations, d-Orn was shown to be a selective regulator of l-Arg biosynthesis and the urea cycle. We propose that d-Orn has a potential function in the tobacco cells which through some previously unidentified mechanism result in l-Arg accumulation.