Biosynthesis of benzylisoquinoline alkaloids in Corydalis bracteata: compartmentation and seasonal dynamics

Numerous species of the genus Corydalis (Papaveraceae) produce a large spectrum of benzylisoquinoline alkaloids (BIA), some of which are of potential therapeutic value, but no information on sites of their biosynthesis and compartmentation is available. This study focuses on the biosynthesis, compartmentation and seasonal dynamics of BIA in Corydalis bracteata (Steph. ex Willd) Pers., a geophyte with a very short spring vegetation period, which for the rest of the year is represented by underground tubers with buds. It was found that all organs of C. bracteata contained high levels of BIA, the highest concentrations being detected in underground tuber buds in early autumn. Neither xylem nor phloem sap contained alkaloids throughout the year but BIA were present in the apoplastic wash fluid of the tuber. The absence of long-distance transport of alkaloids was confirmed by the experiment using an isotopically labeled tracer, [ring-(13)C6]-tyramine: when whole plants were fed with the tracer with via the roots, the alkaloids became labeled in the roots only and not in other organs. However, when detached roots, leaves, tubers and stems were exposed to [ring-(13)C6]-tyramine, the label was incorporated into alkaloids in all organs. We conclude that no long-distance translocation of alkaloids occurs between organs of C. bracteata, while in the tuber the cell-to-cell transport of alkaloids could occur via the apoplast. In contrast to other BIA-producing species, every organ of C. bracteata was found to be capable of de novo biosynthesis of the full complement of alkaloids.