2′-Epi-orobanchol and solanacol, two unique strigolactones, germination stimulants for root parasitic weeds, produced by tobacco

Germination stimulants for root holoparasitic weeds broomrapes (Orobanche and Phelipanche spp.) produced by tobacco (Nicotiana tabacum L.) were purified and characterized. The root exudates of tobacco contained at least five different stimulants, and LC-MS/MS analyses revealed that four of them were strigolactones; a tetraclehydrostrigol isomer, a didehydrostrigol isomer, and two strigol isomers. The two isomers of strigol were identified as (+)-orobanchol and its 2'-epimer by comparison of NMR and GC- and LC-MS data with those of synthetic standards. The structure of the tetradehydrostrigol isomer, the major stimulant of the bright yellow tobacco cultivars, was determined as 4-(x-hydroxy-5,8-dimethyl-GR24 [(E)-4-(x-hydroxy-5,8-dimethyl-3-(4-methyl-5-oxo-2,5-dihydrofuran2-yloxy)methylene)-3a,4-dihydro-3H-indeno[1,2-b]furan-2(8bH)-one] and named solanacol. 2'-Epiorobanchol and solanacol are the first natural strigolactones having a 2'-epi stereochemistry and a benzene ring, respectively.