KEGG(Kyoto Encyclopedia of Genes and Genomes)〔和文〕 (特集 ゲノム医学の現在と未来--基礎と臨床) -- (データベース)

Nature’s hierarchical materials

Lignins are complex natural polymers resulting from oxidative coupling of, primarily, 4-hydroxyphenylpropanoids. An understanding of their nature is evolving as a result of detailed structural studies, recently aided by the availability of lignin-biosynthetic-pathway mutants and transgenics. The currently accepted theory is that the lignin polymer is formed by combinatorial-like phenolic coupling reactions, via radicals generated by peroxidase-H2O2, under simple chemical control where monolignols react endwise with the growing polymer. As a result, the actual structure of the lignin macromolecule is not absolutely defined or determined. The ``randomness'' of linkage generation (which is not truly statistically random but governed, as is any chemical reaction, by the supply of reactants, the matrix, etc.) and the astronomical number of possible isomers of even a simple polymer structure, suggest a low probability of two lignin macromolecules being identical. A recent challenge to the currently accepted theory of chemically controlled lignification, attempting to bring lignin into line with more organized biopolymers such as proteins, is logically inconsistent with the most basic details of lignin structure. Lignins may derive in part from monomers and conjugates other than the three primary monolignols (p-coumaryl, coniferyl, and sinapyl alcohols). The plasticity of the combinatorial polymerization reactions allows monomer substitution and significant variations in final structure which, in many cases, the plant appears to tolerate. As such, lignification is seen as a marvelously evolved process allowing plants considerable flexibility in dealing with various environmental stresses, and conferring on them a striking ability to remain viable even when humans or nature alter ``required'' lignin-biosynthetic-pathway genes/enzymes. The malleability offers significant opportunities to engineer the structures of lignins beyond the limits explored to date.

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Lignins: Natural polymers from oxidative coupling of 4-hydroxyphenyl- propanoids

L-tryptophan, L-phenylalanine, and L-tyrosine are aromatic amino acids (AAAs) that are used for the synthesis of proteins and that in plants also serve as precursors of numerous natural products, such as pigments, alkaloids, hormones, and cell wall components. All three AAAs are derived from the shikimate pathway, to which ≥30% of photosynthetically fixed carbon is directed in vascular plants. Because their biosynthetic pathways have been lost in animal lineages, the AAAs are essential components of the diets of humans, and the enzymes required for their synthesis have been targeted for the development of herbicides. This review highlights recent molecular identification of enzymes of the pathway and summarizes the pathway organization and the transcriptional/posttranscriptional regulation of the AAA biosynthetic network. It also identifies the current limited knowledge of the subcellular compartmentalization and the metabolite transport involved in the plant AAA pathways and discusses metabolic engineering efforts aimed at improving production of the AAA-derived plant natural products.

The shikimate pathway and aromatic amino acid biosynthesis in plants.

SECONDARY WALL-ASSOCIATED NAC DOMAIN PROTEIN1 (SND1) is a master transcriptional switch activating the developmental program of secondary wall biosynthesis. Here, we demonstrate that a battery of SND1-regulated transcription factors is required for normal secondary wall biosynthesis in Arabidopsis thaliana. The expression of 11 SND1-regulated transcription factors, namely, SND2, SND3, MYB103, MYB85, MYB52, MYB54, MYB69, MYB42, MYB43, MYB20, and KNAT7 (a Knotted1-like homeodomain protein), was developmentally associated with cells undergoing secondary wall thickening. Of these, dominant repression of SND2, SND3, MYB103, MYB85, MYB52, MYB54, and KNAT7 significantly reduced secondary wall thickening in fiber cells. Overexpression of SND2, SND3, and MYB103 increased secondary wall thickening in fibers, and overexpression of MYB85 led to ectopic deposition of lignin in epidermal and cortical cells in stems. Furthermore, SND2, SND3, MYB103, MYB85, MYB52, and MYB54 were able to induce secondary wall biosynthetic genes. Direct target analysis using the estrogen-inducible system revealed that MYB46, SND3, MYB103, and KNAT7 were direct targets of SND1 and also of its close homologs, NST1, NST2, and vessel-specific VND6 and VND7. Together, these results demonstrate that a transcriptional network consisting of SND1 and its downstream targets is involved in regulating secondary wall biosynthesis in fibers and that NST1, NST2, VND6, and VND7 are functional homologs of SND1 that regulate the same downstream targets in different cell types.

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A battery of transcription factors involved in the regulation of secondary cell wall biosynthesis in Arabidopsis.

Abstract An occluding substance (gum) was observed in the vessels of Phellodendron amurense Rupr. and analysed by spectroscopic and chemical methods. Following safranin-alcian blue staining, the gum in sapwood (sW) turned to blue and in heartwood (hW) to red. The gum was studied in situ by UV and Raman microscopies, time-of-flight secondary ion mass spectrometry (TOF-SIMS). The gum was isolated by laser microdissection (LMD) and it was alkali hydrolysed and the degradation products were analysed by GC-MS. The staining experiments, and the UV and Raman microscopies indicated that the major component of the sW gum is constituted of polysaccharides, while in the hW gum the aromatic character is dominating. TOF-SIMS measurements were interpreted as showing the aromatic substances in the hW gum did not contain lignin. The GC-MS analysis revealed the presence of vanillic acid in the degradation products of hW gum.

The composition and chemical alteration of gums in the vessels of Phellodendron amurense

We investigated the relationship between turgor pressure and diurnal differences in secondary wall formation of differentiating tracheids. Saplings of Cryptomeria japonica were grown in a growth chamber with 12-h light:12-h dark cycles, and the tangential strain on the inner bark surface was measured as an indicator of the volumetric changes of differentiating cells. The innermost surface of developing secondary walls was then observed using field emission scanning electron microscopy at 1-h intervals after both light and dark periods. Dramatic changes in the aspects of the innermost surface of developing secondary walls occurred 3h after the light was switched on and 4h after the light was switched off. The amorphous material containing glucomannans became evident when the differentiating cells became fully turgid during the dark period. Conversely, cellulose microfibrils became clearly visible when the cell volume was low during the light period. These results suggest that the diurnal periodicity in the supply of hemicellulose-containing matrix to developing secondary walls is associated with the changes in turgor pressure of differentiating tracheids that result from the change in light conditions during the photoperiodic cycle.

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The supply of matrix containing glucomannans to the innermost surface of S2 layers is associated with changes in turgor pressure of differentiating tracheids in Cryptomeria japonica

To obtain information on the ultrastructural assembly of lignin and polysaccharides in the cell wall, the polysaccharides were selectively removed by periodate oxidation from the differentiating ginkgo xylem, to give periodate lignin, to be subsequently analyzed by high resolution FE-SEM (field-emission scanning electron microscopy). In the lignifying cell wall, lignin is formed as bead-like modules of the lignin-polysaccharide complex surrounding the cellulose microfibrils. The growing modules are fused together to form high molecular aggregates, to fill" up the space between the cellulose microfibrils. The periodate lignin composed of fused lignin modules retained the morphological features of the original cell wall structure, although some shrinkage occurred, as a result of polysaccharides removal. The observations provided useful information on the heterogeneous distribution of the condensed lignin substructures, with respect to the cell wall layers, and also on the ultrastructural assembly of lignin and polysaccharides in ginkgo tracheids.

Ultrastructure of lignified plant cell wall observed by field-emission scanninng electron microscopy. observations on periodate lignin prepared from ginkgo biloba

The secondary cell wall of compression wood tracheids has a highly lignified region (S2L) in its outermost portion. To better understand the mechanism of S2L formation, we focussed on the activity of laccase (a monolignol oxidase) and performed in situ studies of this enzyme in differentiating compression wood. Staining of differentiating compression wood demonstrated that laccase activity began in all cell wall layers before the onset of lignification. We detected no activity of peroxidase (another monolignol oxidase) in any cell wall layer. Thus, laccase likely plays the major role in monolignol oxidisation during compression wood differentiation. Laccase activity was higher in the S2L region than in other secondary wall regions, suggesting that this enzyme was responsible for the high lignin concentration in this region of the cell wall. Immunolabelling demonstrated the expression of a compression-wood-specific laccase (CoLac1) immediately following the onset of secondary wall thickening, this enzyme was localised to the S2L region, whereas much less abundant in the S1 layer or inner S2 layer. Thus, the CoLac1 protein is most likely localised to the outer part of S2 and responsible for the high lignin concentration in the S2L region.

In situ detection of laccase activity and immunolocalisation of a compression-wood-specific laccase (CoLac1) in differentiating xylem of Chamaecyparis obtusa.

To clarify the mechanism of irreversible dimensional changes due to the hygrothermal treatment of green wood, i.e., the hygrothermal recovery (HTR), the changes in vibrational properties of compression wood and normal wood were measured after hygrothermal treatment at 60 °C, 80 °C, and 100 °C. In addition, the relationship between those changes and HTR strains were discussed. The hygrothermal treatment induced an increase in mechanical loss tangent (tanδ) and decrease in specific dynamic Young’s modulus (E′/ρ). It seems that the changes in vibrational properties due to hygrothermal treatment had a time–temperature dependency: Higher temperatures and longer treatment durations induce larger increases in tanδ and larger decreases in E′/ρ. In contrast to the quenching effect, tanδ and E′/ρ did not recover to their original state even after 60 days of conditioning in water at 20 °C. For compression wood with a large microfibril angle (MFA), there was a clear relationship between the changes in vibrational properties and HTR strains. The tanδ increased and E′/ρ decreased with hygrothermal treatment, corresponding to dimensional changes in the L-direction. This suggests that structural changes in wood components are responsible for HTR. The most likely mechanism for HTR is that the hygrothermal treatment softens the lignin to release locked-in growth stress. Subsequently, irreversible structural changes in lignin induce both the changes in vibrational properties and HTR. For normal wood, because of the small MFA, the structural changes in lignin in the L-direction are possibly restricted by crystalline cellulose. As a result, the relationship between the changes in vibrational properties and HTR is uncertain.

Changes in vibrational properties of compression wood in conifer due to hygrothermal treatment and their relationship with hygrothermal recovery strain

Masato Yoshida

Papers

The composition and chemical alteration of gums in the vessels of Phellodendron amurense

The supply of matrix containing glucomannans to the innermost surface of S2 layers is associated with changes in turgor pressure of differentiating tracheids in Cryptomeria japonica

Ultrastructure of lignified plant cell wall observed by field-emission scanninng electron microscopy. observations on periodate lignin prepared from ginkgo biloba

In situ detection of laccase activity and immunolocalisation of a compression-wood-specific laccase (CoLac1) in differentiating xylem of Chamaecyparis obtusa.

Changes in vibrational properties of compression wood in conifer due to hygrothermal treatment and their relationship with hygrothermal recovery strain