Selective regulation of nicotine and polyamines biosynthesis in tobacco cells by enantiomers of ornithine.

L- and D-amino acids have diverse functions and effects on the metabolism, growth, and development of plants. Ornithine (Orn) plays a main role in the biosynthesis of many amino acids, nicotinic alkaloids, and polyamines in tobacco. This investigation describes the impact of Orn enantiomers on the production and distribution of free, conjugated, and bound polyamines, as well as nicotine in tobacco cells. It was recognized that the biosynthesis of metabolites was differently upregulated by each enantiomer. Putrescine was abundantly produced by exogenous L-ornithine (L-Orn), and both spermidine and spermine were significantly accumulated in D-ornithine (D-Orn)-supplied tobacco cells. Furthermore, nicotine production was highly upregulated by L-Orn, while the addition of D-Orn had no effect on the nicotine content of tobacco cells. It was observed that transcript expression of S-adenosylmethionine decarboxylase, as the key enzyme of spermidine/spermine biosynthesis, is coincident with their metabolic levels and is highly upregulated by D-Orn, as opposed to L-Orn. These results indicate that both enantiomers of Orn can trigger selected biosynthetic pathways in the cells, at the transcript level. Regarding these observations, it is proposed that L- and D-Orn function differently in the same biological pathways in which the latter, D-Orn specifically regulates important polyamines in the plant cells.