Investigation of Biosynthetic Pathways to Hydroxycoumarins During Post-Harvest Physiological Deterioration in Cassava Roots by Using Stable Isotope Labelling

摘要

Cassava (Manihot esculenta Crontz) is an important starch-rich crop, but the storage roots only have a short shelf-life due to post-harvest physiological deterioration (PPD), which includes the over-production and polymerisation of hydroxycoumarins. Key aspects of coumarin secondary-metabolite biosynthesis remain unresolved. Here we exploit the accumulation of hydroxycoumarins to test alternative pathways for their biosynthesis. Using isotopically labelled intermediates (p-coumarote-2-C-13, caffeate-2-C-13, ferulote-2-C-13, umbelliferone-2-O-18 and esculetin-2-O-18), we show that the major biosynthetic pathway to scopoletin and its glucoside, scopolin, in cassava roots during PPD is through p-coumaric, caffeic and then ferulic acids. An alternate pathway through 2',4'-dihydroxycinnomate and umbelliferone leads to esculetin and esculin. We have used (CO2)-O-18-carboxylate-lobelled cinnamic and ferulic acids, and feeding experiments under an atmosphere of O-18(2), to investigate the o-hydroxylotion and cyclisation steps. We demonstrate that the major pathway is through o-hydroxylation and not via a proposed spirolactone-dienone intermediate.