Lignification and cell wall assembly processes are also frequently altered/modified by plants in response to structural and environmental stresses/conditions. For instance, many arbores­cent gymnosperms and angiosperms form heartwood beginning at the center of the stem and this eventually extensively radiates outwards through (in part) secretion of defense molecules (73), such as lignans from adjacent parenchyma cells in the sapwood (74) (Figures 7.7A-C). That is, following lignification, the various heartwood deposition processes result in forma­tion of large quantities of diverse metabolic products, including species-specific resins and lignans, which increase the density (and presumably support) of the wood, as well as the

parenchyma

Substances secreted through pit apertures

Neighboring cell lumen

Figure 7.7 Woody cross sections showing heartwood deposition and reaction wood tissues. Heartwood in (A) tamarack (Larix laricina) and (B) ebony (Diospyros ebenum), as well as (C) secretion of heartwood constituents by ray parenchyma cells into lumen of neighboring cells; this appears to occur through pit apertures (73). Light micrograph cross section of compression (D) and "normal" (F) wood in Douglas fir (Pseudotsuga menziesii) (75) and of tension (E) and "normal" (G) wood in black cottonwood (Populus balsamifera ssp. trichocarpa) (72). Bar: 20 pm (D, F) and 10 pm (E, G). Abbreviations: f, fiber;G, G — layer;Hw, heartwood;is, intercellular space;gf, gelatinous fiber;Sw, sapwood;v, vessel. [Reprinted from (A) Phytochemistry, vol. 57, Kwon, M., Bedgar, D. L., Piastuch, W., Davin, L. B. & Lewis, N. G., Induced compression wood formation in Douglas fir (Pseudotsuga menziesii) in microgravity, pp. 847­857, Copyright 2001, with permission from Elsevier. (B) Current Opinion in Plant Biology, vol. 2, Lewis, N. G., A 20th century roller coaster ride: A short account of lignification, pp. 153-162, Copyright 1999, with permission from Elsevier. (C) ACS Symposium Series, vol. 697, Gang, D. R., Fujita, M., Davin, L. B. & Lewis, N. G., The "abnormal lignins": Mapping heartwood formation through the lignan biosynthetic pathway, pp. 389-421, Copyright 1998, with permission from American Chemical Society. (E and F) The American Journal of Botany, vol. 94, Patten, A. M., Jourdes, M., Brown, E. E., Laborie, M.-P., Davin, L. B. & Lewis, N. G., Reaction tissue formation and stem tensile modulus properties in wild type and p-coumarate — 3-hydroxylase downregulated lines of alfalfa, Medicago sativa (Fabaceae), pp. 912-925, Copyright 2007, with permission from the Botanical Society of America.] (Reproduced in color as Plate 19.)
resistance of such tissues to biodegradation (lignocellulose deconstruction) (74). Such sub­stances thus often help confer additional defensive properties to heartwood, making them more resistant to pathogen challenges and helping to increase longevity. Often particu­lar types of defense metabolites (such as lignans) are found in specific tree species, further demonstrating the quite remarkable chemical diversity that has evolved through the phenyl — propanoid pathway, e. g., the lignan — and other metabolite — enrichment of western red cedar (Thuja plicata) contributes to its life span that can potentially exceed 3000 years or so. In other species, such as poplar, a lignan-enriched tissue is also formed, which is sometimes referred to as a “ripewood.”

Lignification/cell wall formation processes are also continuously modulated when woody plant species form branches, are fast-growing, and/or are challenged by having their stems bent (as when growing on a slope). The so-called “reaction wood” formed is trivially known as compression wood (Figure 7.7D) in gymnosperms and tension wood (Figure 7.7E) in angiosperms; Figures 7.7F and 7.7G show “normal” wood for comparison pur­poses. Both types of reaction wood also show variable changes in cellulose content but, interestingly, the H-lignin contents of compression wood are increased while that of overall lignin contents in tension wood are decreased in some species. Furthermore, the means for achieving both stem and branch orientation differ profoundly between gymnosperms and angiosperms; the former produce the lignin-rich reaction (compression) wood on the un­derside of stems/branches (75,76), whereas the latter form the reaction (tension) wood with variable levels of lignin deficiency on the upper-sides of both (30, 72) (discussed below). One purpose of these (re-)orientation mechanisms appears to be to enable maximization of the exposure of the photosynthetic apparatus within the leaves for efficient photosynthetic capture of energy from the sun. The underlying biochemical/molecular mechanisms and reasons for such different lignin/cell wall forming responses to similar mechanical challenges are, however, not yet known. Nevertheless, these examples illustrate some of the quite re­markable changes that plants can undergo in order to obtain either enhanced protection of the lignocellulosic matrix and/or in modifying growth/development through programmed modulation of plant cell wall assemblies.