Detection and localization of novel hordatine-like compounds and glycosylated derivates of hordatines by imaging mass spectrometry of barley seeds

Mass spectrometry imaging was applied on germinated barley for the detailed localization of metabolites in longitudinal and transversal seed sections. Among others, 20 m/z signals occurred in three regular peak clusters with specific, distinct localizations in embryo tissues. High resolution FT-ICR MS, MALDI-TOF MS/MS, and UHPLC-ESI MS/MS served for the identification and structural characterization of these compounds. Only five metabolites were published in their structures, namely the antifungal compounds hordatine A and B in non-glycosylated and glycosylated forms. All other non-identified cluster compounds were of hordatine-like structure and differed by systematic O-methylations, hydroxylations, and glycosylations. These differences in molecular structures correlated to distinct localization patterns within the embryo and might serve for the regulation of antifungal properties. Based on the structural investigations by mass spectrometry, an array of different hordatines that comprises the five published hordatines, 15 novel hordatine derivates and their six precursors could be localized in the embryo of germinated barley. Implications for the biosynthetic pathway and transport processes are discussed.