Structure and expression of the lignin O-methyltransferase gene from Zea mays L.

The isolation and characterization of cDNA and homologous genomic clones encoding the lignin O-methyltransferase (OMT) from maize is reported. The cDNA clone has been isolated by differential screening of maize root cDNA library. Southern analysis indicates that a single gene codes for this protein. The genomic sequence contains a single 916 bp intron. The deduced protein sequence from DNA shares significant homology with the recently reported lignin-bispecific caffeic acid/5-hydroxyferulic OMTs from alfalfa and aspen. It also shares homology with OMTs from bovine pineal glands and a purple non-sulfur photosynthetic bacterium. The mRNA of this gene is present at different levels in distinct organs of the plant with the highest accumulation detected in the elongation zone of roots. Bacterial extracts from clones containing the maize OMT cDNA show an activity in methylation of caffeic acid to ferulic acid comparable to that existing in the plant extracts. These results indicate that the described gene encodes the caffeic acid 3-O-methyltransferase (COMT) involved in the lignin biosynthesis of maize.     

Improvement of in-rumen digestibility of alfalfa forage by genetic manipulation of lignin O-methyltransferases

Lignin inhibits forage digestibility by ruminant animals, and lignin levels and the proportion of dimethylated syringyl (S) lignin monomers increase with progressive maturity in stems of forage crops. We generated transgenic alfalfa (Medicago sativa L.) with reduced lignin content and altered lignin composition. Down-regulation of caffeic acid 3-O-methyltransferase (COMT) reduces lignin content, accompanied by near total loss of S lignin, whereas down-regulation of caffeoyl coenzyme A 3-O-methyltransferase (CCoAOMT) reduces lignin content without reduction in S lignin. These changes are not accompanied by altered ratios of cell wall polysaccharides. Analysis of rumen digestibility of alfalfa forage in fistulated steers revealed improved digestibility of forage from COMT down-regulated plants, but a greater improvement in digestibility following down-regulation of CCoAOMT. The results indicate that both lignin content and composition affect digestibility of alfalfa forage, and reveal a new strategy for forage quality improvement by genetic manipulation of CCoAOMT expression.     

Immunolocalization of two ligninO-methyltransferases in stems of alfalfa (Medicago sativa L.)

Summary Caffeic acid 3-O-methyltransferase (COMT) and caffeoyl CoA 3-O-methyltransferase (CCOMT) catalyze parallel reactions that are believed to be involved in the biosynthesis of lignin monomers. Antisera specific for alfalfa (Medicago sativa L.) COMT or CCOMT were raised against the enzymes expressed inEscherichia coli, and were used for immunolocalization studies in lignifying alfalfa stem tissue. Both COMT and CCOMT were localized to xylem parenchyma cells, as assessed by light microscopy and immunocytochemistry. Electron microscopy revealed that both enzymes were located in the cytoplasm of xylem parenchyma cells, and to a lesser extent, in the cytoplasm of phloem cells. There was no significant difference in the localization pattern of COMT and CCOMT, suggesting that the two enzymes may be part of a metabolic grid leading to production of lignin monomers in lignifying tissue of mature alfalfa stem internodes.     

Both caffeoyl Coenzyme A 3-<Emphasis Type="Italic">O</Emphasis>-methyltransferase 1 and caffeic acid <Emphasis Type="Italic">O</Emphasis>-methyltransferase 1 are involved in redundant functions for lignin,flavonoids and sinapoyl malate biosynthesis in Arabidopsis

Two methylation steps are necessary for the biosynthesis of monolignols, the lignin precursors. Caffeic acid O-methyltransferase (COMT) O-methylates at the C5 position of the phenolic ring. COMT is responsible for the biosynthesis of sinapyl alcohol, the precursor of syringyl lignin units. The O-methylation at the C3 position of the phenolic ring involves the Caffeoyl CoA 3-O-methyltransferase (CCoAOMT). The CCoAOMT 1 gene (At4g34050) is believed to encode the enzyme responsible for the first O-methylation in Arabidopsis thaliana. A CCoAOMT1 promoter-GUS fusion and immunolocalization experiments revealed that this gene is strongly and exclusively expressed in the vascular tissues of stems and roots. An Arabidopsis T-DNA null mutant named ccomt 1 was identified and characterised. The mutant stems are slightly smaller than wild-type stems in short-day growth conditions and has collapsed xylem elements. The lignin content of the stem is low and the S/G ratio is high mainly due to fewer G units. These results suggest that this O-methyltransferase is involved in G-unit biosynthesis but does not act alone to perform this step in monolignol biosynthesis. To determine which O-methyltransferase assists CCoAOMT 1, a comt 1 ccomt1 double mutant was generated and studied. The development of comt 1 ccomt1 is arrested at the plantlet stage in our growth conditions. Lignins of these plantlets are mainly composed of p-hydroxyphenyl units. Moreover, the double mutant does not synthesize sinapoyl malate, a soluble phenolic. These results suggest that CCoAOMT 1 and COMT 1 act together to methylate the C3 position of the phenolic ring of monolignols in Arabidopsis. In addition, they are both involved in the formation of sinapoyl malate and isorhamnetin.     

Characterization of a caffeic acid 3-O-methyltransferase from wheat and its function in lignin biosynthesis

Caffeic acid 3-O-methyltransferase (COMT) catalyzes the multi-step methylation reactions of hydroxylated monomeric lignin precursors, and is believed to occupy a pivotal position in the lignin biosynthetic pathway. A cDNA (TaCM) was identified from wheat and it was found to be expressed constitutively in stem, leaf and root tissues. The deduced amino acid sequence of TaCM showed a high degree of identity with COMT from other plants, particularly in SAM binding motif and the residues responsible for catalytic and substrate specificity. The predicted TaCM three-dimensional structure is very similar with a COMT from alfalfa (MsCOMT), and TaCM protein had high immunoreactive activity with MsCOMT antibody. Kinetic analysis indicated that the recombinant TaCM protein exhibited the highest catalyzing efficiency towards caffeoyl aldehyde and 5-hydroxyconiferaldehyde as substrates, suggesting a pathway leads to S lignin via aldehyde precursors. Authority of TaCM encoding a COMT was confirmed by the expression of antisense TaCM gene in transgenic tobacco which specifically down-regulated the COMT enzyme activity. Lignin analysis showed that the reduction in COMT activity resulted in a marginal decrease in lignin content but sharp reduction in the syringl lignin. Furthermore, the TaCM protein exhibited a strong activity towards ester precursors including caffeoyl-CoA and 5-hydroxyferuloyl-CoA. Our results demonstrate that TaCM is a typical COMT involved in lignin biosynthesis. It also supports the notion, in agreement with a structural analysis, that COMT has a broad substrate preference.     

Monolignol biosynthesis in microsomal preparations from lignifying stems of alfalfa (Medicago sativa L.)

Microsomal preparations from lignifying stems of alfalfa (Medicago sativa L.) contained coniferaldehyde 5-hydroxylase activity and immunodetectable caffeic acid 3-O-methyltransferase (COMT), and catalyzed the S-adenosyl L-methionine (SAM) dependent methylation of caffeic acid, caffeyl aldehyde and caffeyl alcohol. When supplied with NADPH and SAM, the microsomes converted caffeyl aldehyde to coniferaldehyde, 5-hydroxyconiferaldehyde, and traces of sinapaldehyde. Coniferaldehyde was a better precursor of sinapaldehyde than was 5-hydroxyconiferaldehyde. The alfalfa microsomes could not metabolize 4-coumaric acid, 4-coumaraldehyde, 4-coumaroyl CoA, or ferulic acid. No metabolism of monolignol precursors was observed in microsomal preparations from transgenic alfalfa down-regulated in COMT expression. In most microsomal preparations, the level of the metabolic conversions was independent of added recombinant COMT. Taken together, the data provide only limited support for the concept of metabolic channeling in the biosynthesis of S monolignols via coniferaldehyde.     

Toward the synthesis of fine chemicals from lactose: preparation of d-xylo and l-lyxo-aldohexos-5-ulose derivatives

The transformation of (5R)-2,6-di-O-benzyl-5-C-methoxy-β-d-galactopyranosyl-(1→4)-2,3:5,6-di-O-isopropylidene-aldehydo-d-glucose dimethyl acetal (8) into partially protected derivatives of d-xylo- and l-lyxo-aldohexos-5-ulose has been reported, applying appropriate epimerisation methods to its 3′-O- and 4′-O-protected alcoholic derivatives.     

Modification of lignin biosynthesis in transgenic Nicotiana through expression of an antisense O-methyltransferase gene from Populus

An aspen lignin-specific O-methyltransferase (bi-OMT; S-adenosyl-l-methionine: caffeic acid/5-hydroxyferulic acid 3/5-O-methyltransferase, EC 2.1.1.68) antisense sequence in the form of a synthetic gene containing the cauliflower mosaic virus 35S gene sequences for enhancer elements, promoter and terminator was stably integrated into the tobacco genome and inherited in transgenic plants with a normal phenotype. Leaves and stems of the transgenes expressed the antisense RNA and the endogenous tobacco bi-OMT mRNA was suppressed in the stems. Bi-OMT activity of stems was decreased by an average of 29% in the four transgenic plants analyzed. Chemical analysis of woody tissue of stems for lignin building units indicated a reduced content of syringyl units in most of the transgenic plants, which corresponds well with the reduced activity of bi-OMT. Transgenic plants with a suppressed level of syringyl units and a level of guaiacyl units similar to control plants were presumed to have lignins of distinctly different structure than control plants. We concluded that regulation of the level of bi-OMT expression by an antisense mechanism could be a useful tool for genetically engineering plants with modified lignin without altering normal growth and development.Abbreviations OMT O-methyltransferase - bi-OMT bispecific O-methyltransferase - CAD cinnamyl alcohol dehydrogenase - Ptomt1 Populus tremuloides bi-OMT cDNA clone     

Selective disruption of wheat secondary metabolism by herbicide safeners

In wheat (Triticum aestivum L.), treatment with herbicide safeners enhances the expression of enzymes involved in pesticide detoxification and reduces crop sensitivity to herbicides. Since these same enzymes are involved in plant secondary metabolism, it was of interest to determine whether or not the safener cloquintocet mexyl perturbed phenolic metabolism in wheat seedlings. LC/ESI/MS analysis identified 14 phenolic substrates in the shoots of young wheat plants. Fragmentation imposed by collision induced dissociation identified specific C-glycosidic conjugates of 4′,5,7-trihydroxflavone (apigenin), 3′,4′,5,7-tetrahydroxyflavone (luteolin) and 3′-O-methylluteolin. Treatment of 7-day-old wheat shoots with cloquintocet mexyl resulted in an accelerated depletion of the conjugates of all three flavones, most notably with the glycosides of luteolin. In contrast, safener treatment caused the selective accumulation of 4′,5,7-trihydroxy-3′,5′-dimethoxyflavone (tricin) and the phenylpropanoid ferulic acid. Changes in phenolic content were associated with an increase in O-methyltransferase and C-glucosyltransferase activity toward flavonoid substrates as well as the classic enhancement of detoxifying glutathione transferases. Our results suggest that in addition to altering the capacity of wheat to metabolise herbicides and other xenobiotics, safeners can also cause a selective shift in the metabolism of endogenous phenolics.     

Anthocyanins in Caprifoliaceae

The qualitative and relative quantitative anthocyanin content of 19 species belonging to the genera Sambucus, Lonicera and Viburnum in the family Caprifoliaceae has been determined. Altogether 12 anthocyanins were identified; the 3-O-glucoside (2), 3-O-galactoside (5), 3-O-(6″-O-arabinosylglucoside) (7), 3-O-(6″-O-rhamnosylglucoside) (9), 3-O-(2″-O-xylosyl-6″-O-rhamnosylglucoside) (10), 3-O-(2″-O-xylosylgalactoside) (11), 3-O-(2″-O-xylosylglucoside) (12), 3-O-(2″-O-xylosylglucoside)-5-O-glucoside (14), 3-O-(2″-O-xylosyl-6″-O-Z-p-coumaroylglucoside)-5-O-glucoside (15) and 3-O-(2″-O-xylosyl-6″-O-E-p-coumaroylglucoside)-5-O-glucoside (16) of cyanidin, in addition to the 3-O-glucosides of pelargonidin and delphinidin (1 and 3). Pigment 7 is the first complete identification of the disaccharide vicianose, 6″-O-α-arabinopyranosyl-β-glucopyranose, linked to an anthocyanidin.     

A cost-effective colorimetric assay for phenolic O-methyltransferases and characterization of caffeate 3-O-methyltransferases from Populus trichocarpa

S-Adenosyl-l-methionine (AdoMet)-dependent O-methyltransferases (OMTs) catalyze the transmethylation of a variety of phenolics in bacteria, plants, and humans. To rapidly characterize phenolic OMT activities, we adapted Gibbs’ reagent, the dye originally used for detecting phenols, to develop a convenient assay method for measuring the catalytic properties of enzymatic transmethylation of phenolics. We demonstrated that Gibbs’ reagent reacted with phenolics yielding distinct absorptive characters that we used to further develop the assay to monitor the reactivities of phenolic OMTs. To validate the method, we identified two caffeate/5-hydroxyferulate 3/5-O-methyltransferases (COMTs) from the black cottonwood, Populus trichocarpa. Together with a few other plant type I OMTs, we demonstrated that our Gibbs’ reagent-mediated colorimetric assay could reliably determine the functions and kinetic parameters of phenolic OMTs. Because Gibbs’ reagent reacting with different regioselectively modified phenolics displays different colorimetric properties, the assay method can be used to monitor both substrate specificity and the regioselectivity of phenolic OMTs.     

Substrate preferences of O-methyltransferases in alfalfa suggest new pathways for 3-O-methylation of monolignols

Measurement of relative O-methyltransferase activities against all potential substrates in the monolignol pathway in developing alfalfa stem extracts revealed activities in the order: caffeoyl CoA > caffeoyl alcohol > 5-hydroxyferulic acid > caffeoyl aldehyde > 5-hydroxyconiferyl alcohol > 5-hydroxyferuloyl CoA > 5-hydroxyconiferaldehyde > caffeic acid. Maxima for all activities occurred in the seventh internode. In stem extracts from transgenic alfalfa with antisense downregulated caffeoyl CoA O-methyltransferase (CCoAOMT), activities with all substrates except for the two coenzyme A esters were unaffected. In contrast, downregulation of caffeic acid O-methyltransferase (COMT) reduced activities against the non-esterifed substrates in the order: 5-hydroxyconiferyl alcohol > 5-hydroxyferulic acid and caffeoyl alcohol > caffeoyl aldehyde > caffeic acid > 5-hydroxyconiferaldehyde. Recombinant COMT expressed in Escherichia coli exhibited the highest V(max)/K(m) values with 5-hydroxyconiferaldehyde and caffeoyl aldehyde, and the lowest with caffeic acid. These results indicate that COMT is unlikely to methylate caffeic acid during lignin biosynthesis in vivo, and provide enzymatic evidence for an alternative pathway to monolignols involving methylation of caffeoyl aldehyde and/or caffeoyl alcohol by COMT. The concept of independent pathways to guaiacyl and syringyl monolignols is discussed.     

Polyphenols isolated from antiradical extracts of Mallotus metcalfianus

Six flavonoids including two new flavones, luteolin 7-O-(4″-O-(E)-coumaroyl)-β-glucopyranoside), chrysoeriol-7-O-(4″-O-(E)-coumaroyl)-β-glucopyranoside) and a mixture of two pairs of diastereoisomeric flavonolignans, (±)-hydnocarpin 7-O-(4″-O-(E)-coumaroyl)-β-glucopyranoside)/(±)-hydnocarpin-D 7-O-(4″-O-(E)-coumaroyl)-β-glucopyranoside) with a 2:1 ratio were isolated from the whole plant of Mallotus metcalfianus Croizat, in addition to 10 known compounds. Their structures were evaluated on the basis of different spectroscopic methods, including extensive 1D and 2D NMR spectroscopy. Some extracts have moderate antimicrobial properties and interesting antiradical (DPPH) activity, as well as some compounds isolated from this species. Tannins were also identified in some active extracts.     

Dopamine-iridoid alkaloids in Carapichea affinis (= Psychotria borucana) confirm close relationship to the vomiting root Ipecac

Broad-based HPLC-UV comparison of 57 methanolic crude extracts of the Psychotria complex (Rubiaceae), representing about 20 different species collected in Costa Rica, exhibited a clear chemical segregation of Psychotria borucana. In contrast to the more widespread tryptamine-iridoid alkaloids it deviated by an accumulation of dopamine-iridoid alkaloids. Similar to the South American vomiting root Ipecac, Psychotria ipecacuanha (= Cephaelis ipecacuanha), cephaeline and emetine were found together with four related glycosides characterized by various N-acylations. Two glycosides were identified as the known N-acetates ipecoside and 6-O-methylipecoside, whereas the two others represent corresponding N-O-methylcaffeate derivatives from which borucoside was described as a new compound. Structure elucidation of all compounds was carried out by NMR- and MS-analyses supported by CSEARCH-NMR-database system. In addition to morphological characters and analyses of nucleotide sequence variation the chemical profile of P. borucana supported a recent taxonomic rearrangement where it has been grouped together with P. ipecacuanha in a separate genus Carapichea.     

Metabolomics-oriented isolation and structure elucidation of 37 compounds including two anthocyanins from Arabidopsis thaliana

In order to conduct metabolomic studies in a model plant for genome research, such as Arabidopsis thaliana (Arabidopsis), it is a prerequisite to obtain structural information for the isolated metabolites from the plant of interest. In this study, we isolated metabolites of Arabidopsis in a relatively non-targeted way, aiming at the construction of metabolite standards and chemotaxonomic comparison. Anthocyanins (5 and 7) called A8 and A10 were isolated and their structures were elucidated as cyanidin 3-O-[2-O-(β-d-xylopyranosyl)-6-O-(4-O-(β-d-glucopyranosyl)-E-p-coumaroyl)-β-d-glucopyranoside]-5-O-[6-O-(malonyl)-β-d-glucopyranoside] and cyanidin 3-O-[2-O-(2-O-(E-sinapoyl)-β-d-xylopyranosyl)-6-O-(4-O-(β-d-glucopyranosyl)-E-p-coumaroyl)-β-d-glucopyranoside]-5-O-[β-d-glucopyranoside] from analyses of 1D NMR, 2D NMR (¹H NMR, NOE, ¹³C NMR, HMBC and HMQC), HRFABMS, FT-ESI-MS and GC-TOF-MS data. In addition, 35 known compounds, including six anthocyanins, eight flavonols, one nucleoside, one indole glucosinolate, four phenylpropanoids and a derivative, together with three indoles, one carotenoid, one apocarotenoid, three galactolipids, two chlorophyll derivatives, one steroid, one hydrocarbon, and two dicarboxylic acids, were also isolated and identified from their spectroscopic data.