(Last
modification: 27. March 2009) Eine Kondensation
Hypothese: Benzylaceton Es gibt einen interessanten Kandidaten für ein anderes Enzym, welches nur eine Kondensation durchführt. Abgeleitet von der Struktur des Moleküls: Benzylaceton sieht gerade so aus wie das 'Raspberry-Keton', aber abgeleitet von Cinnamoyl-CoA (oder vielleicht von Phenylpropionyl-CoA), nicht von 4-Coumaroyl-CoA. Die Zeichnung zeigt, wie man sich das denken kann.
Benzylaceton ist in Blüten von Nicotiana attenuata (Kessler and Baldwin, 2007) die häufigste flüchtige Substanz zur Attraktion von Befruchtern. Sicherlich würde sich ein weiterer Blick auf Typ III PKS Gene in Nicotiana Arten lohnen. Wenigstens ein Kandidat wurde schon früher bemerkt: Eine Antheren-spezifisch exprimierte Typ III PKS in Nicotiana sylvestris (Atanassov et al., 1998).
Zwei mögliche Biosynthesewege zu Benzylaceton
mittels Typ III PKS Reaktionen
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A. Cinnamoyl-CoA als
Starter, eine Kondensation mit Malonyl-CoA, Decarboxylierung des Diketids,
und Reduktion der Doppelbindung in der Seitenkette: Dies entspricht genau
der Reaktionssequenz in der Biosynthese des Himbeerketons.
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B. Phenylpropionyl-CoA
als Starter, eine Kondensation mit Malonyl-CoA, dann Decarboxylierung des
Diketids. Attraktiv: Dieser Starter eliminiert die Notwendigkeit einer
Reduktase.
Zitate
Atanassov, I., Russinova, E., Antonov, L., Atanassov, A., 1998. Expression of an anther-specfic chalcone synthase-like gene is correlated with uninucleate microspore development in Nicotina sylvestris. Plant Molecular Biology 38, 1169-1178.
Two cDNA clones,
specifically expressed in Nicotiana sylvestris in anthers during
uninucleate microspore development, were isolated using a subtractive
hybridization approach. Sequence analysis showed that one of them, NSCHSLK,
displayed a high level of similarity to several anther-specific chalcone
synthase-like (CHSLK) proteins and an ORF from chromosome 1 of
Arabidopsis thaliana. A lower, but significant, similarity to chalcone
synthases and closely related enzymes (CHSRE) was also detected. The
structure of the nschslk gene was found to be typical of the chalcone (chs)/stilbene
(sts) synthase family. Expression of NSCHSLK mRNA was confined to
microspores and tapetal cells. UV- irradiation or infection with
Phytophthora parasitica var. nicotianae of transgenic Nicotiana
benthamiana plants carrying a chimeric nschslk/GUS gene indicated that
the nschslk promoter exhibits the same anther-specific, developmentally
regulated expression pattern. Comparison of CHSRE and CHSLK polypeptide
sequences revealed some important similarities and differences between the
two groups. The data presented in this study, suggest that the
anther-specific chslk genes represent a separate sub-family of plant
polyketide synthases related to chs/sts in terms of gene structure,
polypeptide sequence and the possible catalytic mechanism, but differing in
substrate/product specificity. The putative role of CHSLK enzymes in anther
development and particularly in exine synthesis is discussed.
Zurück zum Text
Kessler, D., Baldwin, I. T., 2007. Making sense of nectar scents: the effects of nectar secondary metabolites on floral visitors of Nicotiana attenuata. The Plant Journal 49, 840-854.
Flowers produce a
plethora of secondary metabolites but only nectar sugars, floral pigments
and headspace volatiles have been examined in the context of pollinator
behavior. We identify secondary metabolites in the headspace and nectar of
glasshouse- and field-grown Nicotiana attenuata plants, infer
within-flower origins by analyzing six flower parts, and compare the
attractiveness of 16 constituents in standardized choice tests with two
guilds of natural pollinators (Manducasexta moths and Archilochus
alexandri and Selasphorus rufus hummingbirds) and one nectar
thief (Solenopsis xyloni ants) to determine whether nectar
metabolites can 'filter' flower visitors: only two could. Moths responded
more strongly than did hummingbirds to headspace presentation of nicotine
and benzylacetone, the most abundant repellent and attractant compounds,
respectively. For both pollinators, nectar repellents decreased nectaring
time and nectar volume removed, but increased visitation number,
particularly for hummingbirds. Fewer ants visited if the nectar contained
repellents. To determine whether nicotine reduced nectar removal rates in
nature, we planted transformed, nicotine-silenced plants into native
populations in Utah over 2 years. Plants completely lacking nicotine in
their nectar had 68-70% more nectar removed per night by the native
community of floral visitors than did wild-type plants. We hypothesize that
nectar repellents optimize the number of flower visitors per volume of
nectar produced, allowing plants to keep their nectar volumes small.
Zurück zum Text
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File History:
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27.03.2009: Update on Figure;
Proposal: Biosynthesis of Benzylacetone
as extra page
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11.12.2008: Figures, Abstracts of publications
.
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