Engineering Tropane Alkaloid Production Based on Metabolic Characterization of Ornithine Decarboxylase in Atropa belladonna

Ornithine decarboxylase (ODC) plays an important role in various biological processes; however, its role in plant secondary metabolism, especially in the biosynthesis of tropane alkaloids (TAs) such as pharmaceutical hyoscyamine, anisodamine, and scopolamine, remains largely unknown. In this study, we characterized the physiological and metabolic functions of the ODC gene of Atropa belladonna (AbODC) and determined its role in TA production using metabolic engineering approaches. Feeding assays with enzyme inhibitors indicated that ODC, rather than arginine decarboxylase (ADC), plays a major role in TA biosynthesis. Tissue-specific AbODC expression analysis and β-glucuronidase (GUS) staining assays showed that AbODC was highly expressed in secondary roots, especially in the cylinder tissue. Enzymatic assays indicated that AbODC was able to convert ornithine to putrescine, with the highest activity at pH 8.0 and 30 °C. Additionally, AbODC showed higher catalytic efficiency than other plant ODCs, as evident from the Km, Vmax, and Kcat values of AbODC using ornithine as the substrate. In A. belladonna root cultures, suppression of AbODC greatly reduced the production of putrescine, N-methylputrescine, and TAs, whereas overexpression of AbODC significantly increased the biosynthesis of putrescine, N-methylputrescine, hyoscyamine, and anisodamine. Moreover, transgenic A. belladonna plants overexpressing AbODC showed a significantly higher production of hyoscyamine and anisodamine compared with control plants. These findings indicate that AbODC plays a key role in TA biosynthesis and therefore is a valuable candidate for increasing TA production in A. belladonna.