7α- and 7β-Hydroxyorobanchyl acetate as germination stimulants for root parasitic weeds produced by cucumber

Abstract

Cucumber (Cucumis sativus) plants were found to exude at least 12 germination stimulants for root parasitic weeds including 5 known strigolactones, 7-oxoorobanchol, 7-oxoorobanchyl acetate, orobanchol, orobanchyl acetate, and 4-deoxyorobanchol. Two novel germination stimulants were purified from cucumber root exudates and their structures were determined to be 7α- and 7β-hydroxyorobanchyl acetate by 1D and 2D NMR spectroscopy, and ESI- and EI-MS spectrometry. The stereochemistry was determined by NOE measurement and by comparing the CD spectra with those of the synthetic standards of four stereoisomers of orobanchol. 7α- and 7β-Hydroxyorobanchol were detected by LC-MS/MS, and GC-MS analysis of purified sample confirmed their structures. The germination stimulation activities of 7α- and 7β-hydroxyorobanchyl acetate on Orobanche minor were comparable to those of orobanchyl acetate and 7-oxoorobanchyl acetate. By contrast, 7β-hydroxyorobanchyl acetate was a highly potent germination stimulant for Phelipanche ramosa inducing more than 50% germination at 10 pM.