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Browsing by Subject "Sequiterpenes"

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    Die linearen Drüsenhaare der Sonnenblume : Morphologie, Verbreitung, Metabolitprofil und Sesquiterpenbiosynthese
    (2014) Aschenbrenner, Anna-Katharina; Spring, Otmar
    The sunflower H. annuus and related species of Heliantinae possess, in addition to the very intensively studied capitate glandular trichomes, a second type of multicellular and biosynthetically active trichome which was so far underinvestigated. In the present work, these linear glandular trichomes (LGT) were studied in detail. Microscopic investigations revealed a wide distribution of LGT within sunflower and species of the subtribe Heliantinae. LGT occurred on leaves and stems of sunflower, but the highest accumulation was found at the involucral bracts of the flower head. This suggests that they may play a chemoecological role in plant-insect or plant-herbivore interaction. The fast development of LGT within only 32h was found to take place during the emergence of leaf primordia on seedlings between the second and third day of germination. The biosynthetic activity got visible with the accumulation of substances successively in the tip cell and progressed towards the trichome base. The deposition of metabolites was accompanied by a loss of cellular vitality and was indicated by the degradation of the nucleus. Through examining the metabolite profile of LGT, it was revealed that they contain oxidized flavones. These compounds were subsequently isolated and structurally characterized. In addition, a wide variety of bisabolene-type sesquiterpenes was found. Among the identified structures were several Glandulones and Heliannuoles already known from previous metabolic studies of sunflower, but also several new compounds. Since the biosynthesis of these compounds from sunflower is fairly unknown, the focus of the present study was on the discovery and characterization of key enzymes that catalyze the first step of bisabolene biosynthesis. From the transcriptome of LGT, candidate genes for the formation of bisabolenes were identified by homology-based experiments. The complete sequences of the transcribed genes from cDNA was isolated by RACE experiments and confirmed by comparison with genomic sequences from the sunflower genome project. The functional identity of two candidate genes was achieved by means of heterologous expression in a yeast system specialized for the characterization of sesquiterpenes. The product of the transformed yeast cells was isolated and spectroscopically identified as cis-γ-bisabolene. Based on these experiments, these two enzymes could be identified as isoforms of a cis-gamma-bisabolene synthase, and represents the first known enzymes of this type characterized in sunflower.