title: Investigation of biosynthetic pathways to hydroxycoumarins during post-harvest physiological deterioration in Cassava roots by using stable isotope labelling.
abstract: Cassava (Manihot esculenta Crantz) 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-coumarate-2-(13)C, caffeate-2-(13)C, ferulate-2-(13)C, umbelliferone-2-(18)O and esculetin-2-(18)O), 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'-dihydroxycinnamate and umbelliferone leads to esculetin and esculin. We have used C(18)O(2)-carboxylate-labelled cinnamic and ferulic acids, and feeding experiments under an atmosphere of (18)O(2), to investigate the o-hydroxylation and cyclisation steps. We demonstrate that the major pathway is through o-hydroxylation and not via a proposed spirolactone-dienone intermediate.
authors.affiliation: Department of Pharmacy and Pharmacology, University of Bath, Bath, UK. prsisb@bath.ac.uk
authors.forename: Soad A L
authors.initials: SA
authors.lastname: Bayoumi
authors.forename: Michael G
authors.initials: MG
authors.lastname: Rowan
authors.forename: John R
authors.initials: JR
authors.lastname: Beeching
authors.forename: Ian S
authors.initials: IS
authors.lastname: Blagbrough
mesh_terms: D053898:Biosynthetic Pathways; D003374:Coumarins / Q000737:chemistry / Q000378:metabolism*; D005960:Glucosides / Q000737:chemistry / Q000378:metabolism; D007553:Isotope Labeling; D002365:Manihot / Q000378:metabolism*; D013058:Mass Spectrometry; D018517:Plant Roots / Q000737:chemistry* / Q000378:metabolism; D012603:Scopoletin / Q000737:chemistry / Q000378:metabolism; D014468:Umbelliferones / Q000737:chemistry
chemical_list: D003374:Coumarins; D005960:Glucosides; D014468:Umbelliferones; C417572:scopolin; D012603:Scopoletin; C007628:esculetin