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.

The cell wall in wood is mainly composed of three components: cellulose, hemicellulose and lignin. According to electron microscopy observations of the innermost surface of cell walls in the tracheids of Cryptomeria japonica , cellulose microfibrils are deposited during the day and a matrix containing hemicellulose is deposited at night. This indicates that the deposition of cell wall components exhibits diurnal periodicity. To gain new insights into the diurnal periodicity of lignin deposition not revealed by microscopic observations, we examined diurnal fluctuations in the expression of genes involved in monolignol biosynthesis in C. japonica saplings grown in the field and in growth chambers under a 12 h light/dark cycle. In the field experiment, two gene expression peaks were observed daily, at dusk and dawn. In the growth chamber experiment, two daily peaks were observed 0 h and 6 - 9 h after the light switched on.

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Diurnal Periodicity of the Expression of Genes Involved in Monolignol Biosynthesis in Differentiating Xylem of Cryptomeria japonica

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A Computational Approach To Predict Warp Of Sawn Lumber Due To Residual Growth Stress In A Log

Abstract Large-diameter keyaki ( Zelkova serrata Makino) logs have long been used in Japan as high-quality material especially for traditional construction and furniture. However, unlike other major wood species in Japan, keyaki has been practically considered as a wood species of high difficulty when processing. Keyaki frequently shows processing defects due to sawing, such as warping, cracking, and so forth, which often reduces the production yield of logs. Furthermore, there are drastic differences in those behaviors between individual logs, so causing unexpected processing defects. A scientific approach is required to improve this situation, but data on the material characteristics of keyaki were scarce. This study aimed to characterize the radial pattern of the residual stress as one of the processing-related characteristics. The measurement method was also examined. This study used diametral planks obtained from large-diameter keyaki logs to measure released strain of residual stress from pith to the bark sides. The results showed the distribution of released strain often showed peculiar zig-zag patterns characterized with localized residual stress, unlike the smooth bell curve pattern seen in typical logs of other species. Because of the unique characteristics, some practical points related to the measuring methodology were investigated. The extent of influence of this residual stress in the longitudinal direction was limited to within 30 cm from the site of measurement of the strain. In addition, the length of log necessary to measure the released strain without being affected by crosscutting was more than four times the log diameter—greater than the diameter ratio theorized by previous studies. 

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Unique characteristics of residual stress distribution of large-diameter keyaki (Zelkova serrata) logs and examination of their measurement method

Abstract While sawing logs along the pith to produce lumber, warps would occur in lumber due to the release of residual stress. Correction sawing should be performed to correct warps, so warps decrease the yield of lumber. Therefore, it is crucial to find a way to produce straight lumber without correction sawing. The present study focused on a series of high temperature drying with load, and it was confirmed that residual stress was eliminated, and warp was corrected after a series of high temperature drying with load. Besides, the optimal schedule conditions of high temperature drying were determined. After 6 months of storage, the treated lumber remained straight and residual stress in lumber did not recover. By applying high temperature drying with load to warped lumber, it is possible to dry lumber and correct the warps at the same time.

Effect of high temperature drying with load on reduction of residual stress and correction of warp of Japanese cedar lumber

Abstract The physical, chemical and biological properties of wood depend on the supramolecular assembly of cellulose microfibrils (CMFs), hemicelluloses (HCs) and lignin in the growing cell walls. Based on the 13C-tracer studies of ginkgo xylem formation, a hypothetical scenario for the role of monolignol glucosides (MLGs) in the assembly is proposed as follows: (1) Both moieties, aglycone monolignols and glycone d-glucose (d-Glc), play essential roles in a cooperative manner in delivery of hydrophobic and highly reactive p-hydroxycinnamyl- (H), coniferyl- (G) alcohols to the hydrophilic site of lignin deposition. (2) The d-Glc liberated at lignification site is converted into essential HCs mainly via Golgi apparatus under the influence of diurnally changing turgor pressure, and partly converted in the apoplast. (3) At cell corner middle lamella, a pressure-resistant layer of HG-lignin-HCs-CMFs is formed, and allows expansion of new cells in cambium region by elevation of turgor pressure. The deformable G-lignin-HCs-CMFs layer at secondary wall shrinks by dehydration of the swollen gel of HCs-CMFs during differentiation, and contributes posture control of standing tree. On-demand quick supply of a large amount of monolignols and HCs can be achieved by the large storage and delivery of MLGs in the growing ginkgo xylem.

Masato Yoshida

Papers

Diurnal Periodicity of the Expression of Genes Involved in Monolignol Biosynthesis in Differentiating Xylem of Cryptomeria japonica

A Computational Approach To Predict Warp Of Sawn Lumber Due To Residual Growth Stress In A Log

Unique characteristics of residual stress distribution of large-diameter keyaki (Zelkova serrata) logs and examination of their measurement method

Effect of high temperature drying with load on reduction of residual stress and correction of warp of Japanese cedar lumber

Role of monolignol glucosides in supramolecular assembly of cell wall components in ginkgo xylem formation