Florian Brodhun

Oilseed germination is characterized by the degradation of storage lipids. It may proceed either via the direct action of a triacylglycerol lipase, or in certain plant species via a specific lipid body 13-lipoxygenase. For the involvement of a lipoxygenase previous results suggested that the hydroxy- or oxo-group that is being introduced into the fatty acid backbone by this lipoxygenase forms a barrier to continuous β-oxidation.

The moss Physcomitrella patens contains C18- as well as C20-polyunsaturated fatty acids that can be metabolized by different enzymes to form oxylipins such as the cyclopentenone cis(+)-12-oxo phytodienoic acid. Mutants defective in the biosynthesis of cyclopentenones showed reduced fertility, aberrant sporophyte morphology and interrupted sporogenesis. The initial step in this biosynthetic route is the conversion of a fatty acid hydroperoxide to an allene oxide. This reaction is catalyzed by allene oxide synthase (AOS) belonging as hydroperoxide lyase (HPL) to the cytochrome P450 family Cyp74. In this study we characterized two AOS from P. patens, PpAOS1 and PpAOS2.

Galactolipids constitute the major lipid class in plants. In recent years oxygenated derivatives of galactolipids have been detected. They are discussed as signal molecules during leaf damage, since they accumulate in wounded leaves in high levels. Using different analytical methods such as nuclear magnetic resonance, infra-red spectroscopy, and high performance liquid chromatography/mass spectrometry (HPLC/MS) earlier reports focused on the analysis of either oxidized or non-oxidized species and needed high levels of analytes. Here, we report on the analysis of the galactolipid subfraction of the Arabidopsis leaf lipidome by an improved HPLC/MS²-based method that is fast, robust, and comparatively simple in its performance. Due to a combination of phase partitioning, solid phase fractionation, liquid chromatography, and MS² experiments this method has high detection sensitivity and requires only low amounts of plant material. With this method 167 galactolipid species were detected in leaves of Arabidopsis thaliana. Out of these 79 being newly described species. From all species the head group and acyl side chains were identified via MS² experiments. Moreover, the structural identification was supported by HPLC/time-of-flight (TOF)-MS and gas chromatography (GC)/MS analysis. The quantification of different galactolipid species that accumulated 30 min after a mechanical wounding in A. thaliana leaves showed that the oxidized acyl side chains in galactolipids are divided into 65% cyclopentenones, 27% methyl-branched ketols, 3.8% hydroperoxides/straight-chain ketols, 2.0% hydroxides, and 2.6% phytoprostanes. In comparison to the free cyclopentenone derivatives, the esterified forms occur in a 149-fold excess supporting the hypothesis that galactolipids might function as storage compounds for cyclopentenones. Additional analysis of the ratio of non-oxidized to oxidized galactolipid species in leaves of wounded plants was performed resulting in a ratio of 2.0 in case of monogalactosyl diacylglycerol (MGD), 8.1 in digalactosyl diacylglycerol (DGD), and 0.6 in the acylated MGD. This indicates that galactolipid oxidation is a major and rapid metabolic process that occurs class specific.