Unser 10. Leibniz Plant Biochemistry Symposium am 7. und 8. Mai war ein großer Erfolg. Thematisch ging es in diesem Jahr um neue Methoden und Forschungsansätze der Naturstoffchemie. Die exzellenten Vorträge über Wirkstoffe…
Omanische Heilpflanze im Fokus der Phytochemie IPB-Wissenschaftler und Partner aus Dhofar haben jüngst die omanische Heilpflanze Terminalia dhofarica unter die phytochemische Lupe genommen. Die Pflanze ist reich an…
Geschmack ist vorhersagbar: Mit FlavorMiner. FlavorMiner heißt das Tool, das IPB-Chemiker und Partner aus Kolumbien jüngst entwickelt haben. Das Programm kann, basierend auf maschinellem Lernen (KI), anhand der…
Tahara, K.; Nishiguchi, M.; Frolov, A.; Mittasch, J.; Milkowski, C.;Identification of UDP glucosyltransferases from the aluminum-resistant tree Eucalyptus camaldulensis forming β-glucogallin, the precursor of hydrolyzable tanninsPhytochemistry152154-161(2018)DOI: 10.1016/j.phytochem.2018.05.005
In the highly aluminum-resistant tree Eucalyptus camaldulensis, hydrolyzable tannins are proposed to play a role in internal detoxification of aluminum, which is a major factor inhibiting plant growth on acid soils. To understand and modulate the molecular mechanisms of aluminum detoxification by hydrolyzable tannins, the biosynthetic genes need to be identified. In this study, we identified and characterized genes encoding UDP-glucose:gallate glucosyltransferase, which catalyzes the formation of 1-O-galloyl-β-d-glucose (β-glucogallin), the precursor of hydrolyzable tannins. By homology-based cloning, seven full-length candidate cDNAs were isolated from E. camaldulensis and expressed in Escherichia coli as recombinant N-terminal His-tagged proteins. Phylogenetic analysis classified four of these as UDP glycosyltransferase (UGT) 84A subfamily proteins (UGT84A25a, -b, UGT84A26a, -b) and the other three as UGT84J subfamily proteins (UGT84J3, -4, -5). In vitro enzyme assays showed that the UGT84A proteins catalyzed esterification of UDP–glucose and gallic acid to form 1-O-galloyl-β-d-glucose, whereas the UGT84J proteins were inactive. Further analyses with UGT84A25a and −26a indicated that they also formed 1-O-glucose esters of other structurally related hydroxybenzoic and hydroxycinnamic acids with a preference for hydroxybenzoic acids. The UGT84A genes were expressed in leaves, stems, and roots of E. camaldulensis, regardless of aluminum stress. Taken together, our results suggest that the UGT84A subfamily enzymes of E. camaldulensis are responsible for constitutive production of 1-O-galloyl-β-d-glucose, which is the first step of hydrolyzable tannin biosynthesis.
Publikation
Hettwer, K.; Böttcher, C.; Frolov, A.; Mittasch, J.; Albert, A.; von Roepenack-Lahaye, E.; Strack, D.; Milkowski, C.;Dynamic metabolic changes in seeds and seedlings of Brassica napus (oilseed rape) suppressing UGT84A9 reveal plasticity and molecular regulation of the phenylpropanoid pathwayPhytochemistry12446-57(2016)DOI: 10.1016/j.phytochem.2016.01.014
In Brassica napus, suppression of the key biosynthetic enzyme UDP-glucose:sinapic acid glucosyltransferase (UGT84A9) inhibits the biosynthesis of sinapine (sinapoylcholine), the major phenolic component of seeds. Based on the accumulation kinetics of a total of 158 compounds (110 secondary and 48 primary metabolites), we investigated how suppression of the major sink pathway of sinapic acid impacts the metabolome of developing seeds and seedlings. In UGT84A9-suppressing (UGT84A9i) lines massive alterations became evident in late stages of seed development affecting the accumulation levels of 58 secondary and 7 primary metabolites. UGT84A9i seeds were characterized by decreased amounts of various hydroxycinnamic acid (HCA) esters, and increased formation of sinapic and syringic acid glycosides. This indicates glycosylation and β-oxidation as metabolic detoxification strategies to bypass intracellular accumulation of sinapic acid. In addition, a net loss of sinapic acid upon UGT84A9 suppression may point to a feedback regulation of HCA biosynthesis. Surprisingly, suppression of UGT84A9 under control of the seed-specific NAPINC promoter was maintained in cotyledons during the first two weeks of seedling development and associated with a reduced and delayed transformation of sinapine into sinapoylmalate. The lack of sinapoylmalate did not interfere with plant fitness under UV-B stress. Increased UV-B radiation triggered the accumulation of quercetin conjugates whereas the sinapoylmalate level was not affected.
A dsRNAi approach silencing a key enzyme of sinapate ester biosynthesis (UDP-glucose:sinapate glucosyltransferase, encoded by the UGT84A9 gene) in oilseed rape (Brassica napus) seeds was performed to reduce the anti-nutritive properties of the seeds by lowering the content of the major seed component sinapine (sinapoylcholine) and various minor sinapate esters. The transgenic seeds have been produced so far to the T6 generation and revealed a steady suppression of sinapate ester accumulation. HPLC analysis of the wild-type and transgenic seeds revealed, as in the previous generations, marked alterations of the sinapate ester pattern of the transformed seeds. Besides strong reduction of the amount of the known sinapate esters, HPLC analysis revealed unexpectedly the appearance of several minor hitherto unknown rapeseed constituents. These compounds were isolated and identified by mass spectrometric and NMR spectroscopic analyses. Structures of 11 components were elucidated to be 4-O-glucosides of syringate, caffeyl alcohol and its 7,8-dihydro derivative as well as of sinapate and sinapine, along with sinapoylated kaempferol glycosides, a hexoside of a cyclic spermidine alkaloid and a sinapine derivative with an ether-bridge to a C6–C3-unit. These results indicate a strong impact of the transgenic approach on the metabolic network of phenylpropanoids in B. napus seeds. Silencing of UGT84A9 gene expression disrupt the metabolic flow through sinapoylglucose and alters the amounts and nature of the phenylpropanoid endproducts.
Publikation
Stehle, F.; Brandt, W.; Stubbs, M. T.; Milkowski, C.; Strack, D.;Sinapoyltransferases in the light of molecular evolutionPhytochemistry701652-1662(2009)DOI: 10.1016/j.phytochem.2009.07.023
Acylation is a prevalent chemical modification that to a significant extent accounts for the tremendous diversity of plant metabolites. To catalyze acyl transfer reactions, higher plants have evolved acyltransferases that accept β-acetal esters, typically 1-O-glucose esters, as an alternative to the ubiquitously occurring CoA-thioester-dependent enzymes. Shared homology indicates that the β-acetal ester-dependent acyltransferases are derived from a common hydrolytic ancestor of the Serine CarboxyPeptidase (SCP) type, giving rise to the name Serine CarboxyPeptidase-Like (SCPL) acyltransferases. We have analyzed structure–function relationships, reaction mechanism and sequence evolution of Arabidopsis 1-O-sinapoyl-β-glucose:l-malate sinapoyltransferase (AtSMT) and related enzymes to investigate molecular changes required to impart acyltransferase activity to hydrolytic enzymes. AtSMT has maintained the catalytic triad of the hydrolytic ancestor as well as part of the H-bond network for substrate recognition to bind the acyl acceptor l-malate. A Glu/Asp substitution at the amino acid position preceding the catalytic Ser supports binding of the acyl donor 1-O-sinapoyl-β-glucose and was found highly conserved among SCPL acyltransferases. The AtSMT-catalyzed acyl transfer reaction follows a random sequential bi-bi mechanism that requires both substrates 1-O-sinapoyl-β-glucose and l-malate bound in an enzyme donor–acceptor complex to initiate acyl transfer. Together with the strong fixation of the acyl acceptor l-malate, the acquisition of this reaction mechanism favours transacylation over hydrolysis in AtSMT catalysis. The model structure and enzymatic side activities reveal that the AtSMT-mediated acyl transfer proceeds via a short-lived acyl enzyme complex. With regard to evolution, the SCPL acyltransferase clade most likely represents a recent development. The encoding genes are organized in a tandem-arranged cluster with partly overlapping functions. With other enzymes encoded by the respective gene cluster on Arabidopsis chromosome 2, AtSMT shares the enzymatic side activity to disproportionate 1-O-sinapoyl-β-glucoses to produce 1,2-di-O-sinapoyl-β-glucose. In the absence of the acyl acceptor l-malate, a residual esterase activity became obvious as a remnant of the hydrolytic ancestor. With regard to the evolution of Arabidopsis SCPL acyltransferases, our results suggest early neofunctionalization of the hydrolytic ancestor toward acyltransferase activity and acyl donor specificity for 1-O-sinapoyl-β-glucose followed by subfunctionalization to recognize different acyl acceptors.