Biological materials: Structure and mechanical properties

Lignin is an abundant biopolymer with a high carbon content and high aromaticity. Despite its potential as a raw material for the fuel and chemical industries, lignin remains the most poorly utilised of the lignocellulosic biopolymers. Effective valorisation of lignin requires careful fine-tuning of multiple "upstream" (i.e., lignin bioengineering, lignin isolation and "early-stage catalytic conversion of lignin") and "downstream" (i.e., lignin depolymerisation and upgrading) process stages, demanding input and understanding from a broad array of scientific disciplines. This review provides a "beginning-to-end" analysis of the recent advances reported in lignin valorisation. Particular emphasis is placed on the improved understanding of lignin's biosynthesis and structure, differences in structure and chemical bonding between native and technical lignins, emerging catalytic valorisation strategies, and the relationships between lignin structure and catalyst performance.

Paving the Way for Lignin Valorisation: Recent Advances in Bioengineering, Biorefining and Catalysis.

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

The ultimate values for compressive strength, Young's modulus, and toughness of cylindrical specimens of unitary aspect ratios and uniform grain-size distributions were extrapolated for hydroxyapatite (HAP) to 70 MPa, 9.2 GPa, and 0.36 J cm(-3), and for tricalcium phosphate (TCP), to 315 MPa, 21 GPa, and 2.34 J cm(-3). For total volume porosities of 50%, the corresponding values were determined: for HAP, 9.3 MPa, 1.2 GPa, 0.042 J cm(-3), for TCP, 13 MPa, 1.6 GP, 0.077 J cm(-3). Porosities of HAP specimens ranged from 3%-50%; TCP from 10%-70%. Two pore-size distributions were employed. Exponential dependencies of the mechanical properties were found upon porosity (p < 0.0001). No differences in measured mechanical properties, as determined in compression, could be attributed to pore size. The superiority of TCP increases with density and suggests that a larger or more selective pore-size distribution could be effectively employed in TCP biological implants. This work also suggests the dominant role of secondary calcium phosphates in increasing compressive strengths.

Mechanical properties of sintered hydroxyapatite and tricalcium phosphate ceramic.

Because lignin limits the use of wood for fiber, chemical, and energy production, strategies for its downregulation are of considerable interest. We have produced transgenic aspen (Populus tremuloides Michx.) trees in which expression of a lignin biosynthetic pathway gene Pt4CL1 encoding 4-coumarate:coenzyme A ligase (4CL) has been downregulated by antisense inhibition. Trees with suppressed Pt4CL1 expression exhibited up to a 45% reduction of lignin, but this was compensated for by a 15% increase in cellulose. As a result, the total lignin-cellulose mass remained essentially unchanged. Leaf, root, and stem growth were substantially enhanced, and structural integrity was maintained both at the cellular and whole-plant levels in the transgenic lines. Our results indicate that lignin and cellulose deposition could be regulated in a compensatory fashion, which may contribute to metabolic flexibility and a growth advantage to sustain the long-term structural integrity of woody perennials.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1001&context=for_pubs

Repression of lignin biosynthesis promotes cellulose accumulation and growth in transgenic trees

Historical Background.- Taxonomy, Designation, General Characteristics, and Determination of Compression Wood.- The Structure of Compression Wood.- Chemical Properties of Compression Wood.- Distribution of Chemical Constituents.- Physical Properties of Compression Wood.- Origin and Evolution of Compression Wood.- Formation of Compression Wood.- Incidence and Occurrence of Compression Wood.- Fundamental Factors Causing Formation of Compression Wood.- Gravitropism and Compression Wood.- Physiology of Compression Wood Formation.- Inheritance of Compression Wood.- Ecology of Compression Wood Formation.- Silviculture and Compression Wood.- Mechanism of Compression Wood Action.- Compression Wood in Lumber, Plywood, and Board Manufacture.- Compression Wood in Pulp and Paper Manufacture.- Compression Wood Induced in Firs by the Balsam Woolly Aphid (Adelges piceae).- Opposite Wood.

Compression Wood in Gymnosperms

A review of the chemistry and topochemistry of compression wood with 200 references. Compression wood contains on the average 30% cellulose, 35–40% lignin, 10% galactan, 9% galactoglucomannan, 8% xylan, and 2% of a 1,3-glucan (laricinan). The cellulose is less crystalline, and the xylan has fewer arabinose side chains than in normal wood. The lignin is composed of guaiacylpropane and p-hydroxyphenylpropane units. It is more condensed, has a higher proportion of carbon-carbon bonds, and contains fewer arylglycerol-β-aryl ether structures than a normal conifer lignin. The ray cells and the primary wall of the tracheids have the same chemical composition in normal and compression woods. The galactan is largely located in the outer region of the secondary wall. Only 5–10% of the lignin in compression wood tracheids is extracellular. The middle lamella is less lignified than in normal wood, while the S1 and inner S2 layers have a lignin concentration of 30–40% which is twice as high as in normal wood. The lignin content of the S2 (L) layer is equal to or higher than that of the intercellular region along the wall. The review is concluded with a brief reference to areas where present information is incomplete or lacking.

Recent progress in the chemistry and topochemistry of compression wood

Ultrastructual investigations and chemical determinations of compression wood from Abies balsamea, Picea mariana and Pinus strobus after decay by white or brown rot fungi demonstrated that this type of wood is more resistant to decay than normal wood. Hyphae colonizing compression wood were found in cell lumina and intercellular spaces whereas normal wood cell had hyphae only in cell lumina. Compression wood did not alter the type of cell wall degradation produced by the various fungi tested, but the rate and extent of decay were limited. The white rot fungus, Trametes versicolor, caused a nonselective attack of all cell wall components as indicated by erosion of secondary wall layers and middle lamellae. A selective removal of lignin occurred throughout the cell walls of wood decayed by the other white rot fungi (Phellinus pini, Phlebia tremellosa and Scytinostroma galatinum) that were evaluated. The brown rot fungi, Fomitopsis pinicola and Oligoporus placentus caused a diffuse removal of polysaccharides from both compression wood and normal wood. Tension wood from Populus temuloides and Acer rubrum decayed by white or brown rot fungi had similar amounts of decay to those observed in normal wood. Ultrastructual observations, however, showed striking differences in the progressive stages of decay. Hyphae of Tremetes versicolor located in cell lumina did not cause erosion or severe degradation of the adjacent, underlying gelatinous layer associated with tension wood. A typical nonselective degradation of the secondary wall layers and middle lamellae, however, occurred beneath this gelatinous layer. In areas of advanced degradation, the secondary walls and middle lamellae between cell walls were completely degraded leaving only the gelatinous layer and cell corner regions of the middle lamellae. Brown rot fungi were able to degrade the gelatinous layer and other cell wall layers resulting in extensive degradation of polysaccharides.

Biodegradation of compression wood and tension wood by white and brown rot fungi

Compression wood is formed by the Ginkgoales, Coniferaies, and Taxales, but not by the Cycadales or the Gnetales. Information on the origin and evolution of compression wood has been obtained by examination of fossils and primitive, extant gymnosperms. If produced at all, compression wood could hardly have effected any righting in the Lycophyta or the Sphenophyta, but it could probably have done so in the progymnosperms and in the seed ferns. The oldest gymnosperm fossils vvith compression wood are from the Permian and Jurassic periods. Compression wood is formed by the ancient Ginkgo biloba which has existed since the Triassic. Compression wood seems to have undergone a certain evolutionary development. The difference in anatomy and chemical composition between normal wood and compression wood is less in ginkgo than in the conifers. Helical cavities are lacking in Ginkgo biloba and in the Araucariaceae. In the Taxales, the helical thickenings present in their normal wood are retained in the compression wood but not in the more recent Pseudotsuga menziesii which develops cavities and ribs. A vital tissue in the gymnosperms, compression wood has probably existed for 300 million years.

Origin and Evolution of Compression Wood

This electron microscopy study is concerned with the dormant cambial cells in compression wood of Picea abies (L.) Karst. The cambial zone comprised 4–8 rows of fusiform cells. Each radial file contained the group known as Sanio's four, namely an initial, a mother, and two daughter cells. Groups of three, resulting from a failure of a mother or a daughter cell to divide, were also present. Undifferentiated xylem tissue cells occurred singly, in pairs, or in groups of four. These cells and the last-formed tracheids were often enclosed by a common primary wall. The fusiform cells had a large, elongated nucleus, numerous small vacuoles, and a frequently infolded plasma membrane. Golgi apparatus and endoplasmic reticulum were rare, while plastids, amyloplasts, and lipid droplets were common. Cambial ray cells contained much lipid material. The tissues were similar to cambial tissues in normal wood of conifers. The present results can only be interpreted in terms of the single layer of initials and sequence of cell divisions established by Sanio. The initial can be located by several criteria in dormant and active cambial regions in conifers. It could be shown that the existence of the group of Sanio's four is better documented in the literature than has so far been recognized. The occurrence of a variable number of primary cell walls is discussed with reference to the concentration of lignin in the middle lamella of mature xylem.

T. E. Timell

Papers

Compression Wood in Gymnosperms

Recent progress in the chemistry and topochemistry of compression wood

Biodegradation of compression wood and tension wood by white and brown rot fungi

Origin and Evolution of Compression Wood

Organization and ultrastructure of the dormant cambial zone in compression wood of Picea abies.