Tension Wood Formation Research Articles

Comprehensive transcriptome analysis of developing xylem responding to artificial bending and gravitational stimuli in Betula platyphylla.

Betula platyphylla Suk (birch) is a fast-growing woody species that is important in pulp industries and the biofuels. However, as an important pulp species, few studies had been performed on its wood formation. In the present study, we investigated the molecular responses of birch xylem to artificial bending and gravitational stimuli. After trunks of birch trees were subjected to bending for 8 weeks, the cellulose content was significantly greater in tension wood (TW) than in opposite wood (OW) or normal wood (NW), whereas the lignin content in TW was significantly lower than that in OW and NW. In addition, TW grew more rapidly than OW and generated TW-specific fibers with an additional G-layer. Three transcriptome libraries were constructed from TW, OW and NW of B. platyphylla, respectively, after the plants were subjected to artificial bending. Overall, 80,909 nonredundant unigenes with a mean size of 768 nt were assembled. Expression profiles were generated, and 9,684 genes were found to be significantly differentially expressed among the TW, OW and NW libraries. These included genes involved in secondary cell wall structure, wood composition, and cellulose or lignin biosynthesis. Our study showed that during TW formation, genes involved in cellulose synthesis were induced, while the expression of lignin synthesis-related genes decreased, resulting in increased cellulose content and decreased lignin levels in TW. In addition, fasciclin-like arabinogalactan proteins play important role in TW formation. These findings may provide important insights into wood formation at the molecular level.

An Overview of Microfibril Angle in Fiber of Tension Wood

The angle between helical windings of microfibrils in the secondary cell wall of fibers and the long axis is called microfibril angle (MFA). Stiffness of wood depends on variations in the MFA. The large MFA shows low stiffness, which is found in juvenile wood and this character make threes vulnerable to high winds breaking. Timber containing a high proportion of juvenile wood is unsuitable for use as high-grade structural timber. On the other hand, the small MFA in wood shows high stiffness, which has importance in a good view of the trend in forestry. The timber with high stiffness is commonly high economic value. They are grown mainly for construction, timber and furniture. Until date, it is under pressure for increased timber production means that ways will be sought to improve the quality of timber by reducing MFA. Commonly, MFA decrease during the formation of tension wood therefore study on tension wood related to MFA formation is important for MFA reduction in normal wood. The study on tension wood formation could predict expression patterns of genes/proteins for reduction of MFA. Herein, the orientation of microfibril and MFA in cell wall layers of normal and tension wood fiber are discussed.

Auxin gradients across wood – instructive or incidental?

Various aspects of wood formation have been linked to the action of auxin, e.g. cambial activity, dormancy, secondary cell wall deposition and tension wood formation. The presence of a radial auxin concentration gradient across wood-forming tissue has been suggested to regulate cambial activity and differentiation of cambial derivatives by providing positional information to cells within the tissue. Similar patterning mechanisms that depend on the interpretation of auxin thresholds have subsequently been proposed for shoot and root apical meristems. However, direct evidence for the existence of auxin gradients has only been obtained for the cambium of various tree species. While the auxin gradient theory is based on a plethora of descriptive and pharmacological experiments, in recent years, auxin function on wood formation has been underpinned by molecular and functional data. Here, we review the latest progress in understanding the role of auxin in wood formation and discuss how auxin concentration gradients could be established and interpreted in wood-forming tissues.

Deposition and organisation of cell wall polymers during maturation of poplar tension wood by FTIR microspectroscopy

To advance our understanding of the formation of tension wood, we investigated the macromolecular arrangement in cell walls by Fourier transform infrared microspectroscopy (FTIR) during maturation of tension wood in poplar (Populus tremula x P. alba, clone INRA 717-1B4). The relation between changes in composition and the deposition of the G-layer in tension wood was analysed. Polarised FTIR measurements indicated that in tension wood, already before G-layer formation, a more ordered structure of carbohydrates at an angle more parallel to the fibre axis exists. This was clearly different from the behaviour of opposite wood. With the formation of the S₂ layer in opposite wood and the G-layer in tension wood, the orientation signals from the amorphous carbohydrates like hemicelluloses and pectins were different between opposite wood and tension wood. For tension wood, the orientation for these bands remains the same all along the cell wall maturation process, probably reflecting a continued deposition of xyloglucan or xylan, with an orientation different to that in the S₂ wall throughout the whole process. In tension wood, the lignin was more highly oriented in the S₂ layer than in opposite wood.

Gibberellin mediates the development of gelatinous fibres in the tension wood of inclined Acacia mangium seedlings.

Gibberellin stimulates negative gravitropism and the formation of tension wood in tilted Acacia mangium seedlings, while inhibitors of gibberellin synthesis strongly inhibit the return to vertical growth and suppress the formation of tension wood. To characterize the role of gibberellin in tension wood formation and gravitropism, this study investigated the role of gibberellin in the development of gelatinous fibres and in the changes in anatomical characteristics of woody elements in Acacia mangium seedlings exposed to a gravitational stimulus. Gibberellin, paclobutrazol and uniconazole-P were applied to the soil in which seedlings were growing, using distilled water as the control. Three days after the start of treatment, seedlings were inclined at 45 ° to the vertical and samples were harvested 2 months later. The effects of the treatments on wood fibres, vessel elements and ray parenchyma cells were analysed in tension wood in the upper part of inclined stems and in the opposite wood on the lower side of inclined stems. Application of paclobutrazol or uniconazole-P inhibited the increase in the thickness of gelatinous layers and prevented the elongation of gelatinous fibres in the tension wood of inclined stems. By contrast, gibberellin stimulated the elongation of these fibres. Application of gibberellin and inhibitors of gibberellin biosynthesis had only minor effects on the anatomical characteristics of vessel and ray parenchyma cells. The results suggest that gibberellin is important for the development of gelatinous fibres in the tension wood of A. mangium seedlings and therefore in gravitropism.

Transcriptome profiling the basal region of poplar stems during the early gravitropic response

Stems of angiosperm trees are affected by gravity. Young poplar stems show primary growth in the top internodes and differential cambium activity in the basal internodes after inclination with some tension wood formed after 45 min. This study was conducted in order to characterize the early changes in poplar gene expression during the early stages of the gravitropic response. Using microarray technology, the expression of 15 000 genes was examined. Approximately 3.1 % of these genes exhibited significant expression changes within the first 45 min of gravity stimulation. Approximately 73 % of the identified genes were found to have a known or predicted molecular function. Many of these genes were involved in secondary wall formation (cellulose synthesis and lignification), cytoskeleton development, signaling, and stress response. Some identified genes seem to play a role in sensing gravitational signals during tension wood formation. A large number of the identified arabinogalactan (AGP) and transcription factor genes were involved in cell wall biosynthesis suggesting that these genes are particularly active in the first phases of signal transduction during tension wood formation.

A genome‐wide screen for ethylene‐induced Ethylene Response Factors (ERFs) in hybrid aspen stem identifies ERF genes that modify stem growth and wood properties

Ethylene Response Factors (ERFs) are a large family of transcription factors that mediate responses to ethylene. Ethylene affects many aspects of wood development and is involved in tension wood formation. Thus ERFs could be key players connecting ethylene action to wood development. We identified 170 gene models encoding ERFs in the Populus trichocarpa genome. The transcriptional responses of ERF genes to ethylene treatments were determined in stem tissues of hybrid aspen (Populus tremula×tremuloides) by qPCR. Selected ethylene-responsive ERFs were overexpressed in wood-forming tissues and characterized for growth and wood chemotypes by FT-IR. Fifty ERFs in Populus showed more than five-fold increased transcript accumulation in response to ethylene treatments. Twenty-six ERFs were selected for further analyses. A majority of these were induced during tension wood formation. Overexpression of ERFs 18, 21, 30, 85 and 139 in wood-forming tissues of hybrid aspen modified the wood chemotype. Moreover, overexpression of ERF139 caused a dwarf-phenotype with altered wood development, and overexpression of ERF18, 34 and 35 slightly increased stem diameter. We identified ethylene-induced ERFs that respond to tension wood formation, and modify wood formation when overexpressed. This provides support for their role in ethylene-mediated regulation of wood development.

Influence of growth, wood anatomical properties and specific gravity on heartwood, sapwood and tension-wood in Dalbergia sissoo Roxb.

Heartwood (HW) and tension-wood (TW) significantly varied with the trees of different diameters and along the heights. Heartwood and tension-wood formation was positively affected by growth. Heartwood formation in proportion to sapwood (SW) was low during the early phase of the tree growth thereafter stabilized after DBH of 20 cm. The fiber dimensions also stabilized after the same period of growth. SW/HW (heart-wood/sap-wood) and NW/TW (normal wood/tension wood) ratio showed decline trend with growth parameters. Tension-wood formation in proportion to normal-wood (NW) also stabilized as in case of heartwood after certain period of growth. SW/HW ratio showed negative relationship with fiber length. NW/TW also showed the similar type of relationship. However, specific gravity increased after certain period of growth then declined. Significant variations due to direction in wood anatomical properties and specific gravity indicated the influence of tension wood on the wood properties.

Chemical responses to modified lignin composition in tension wood of hybrid poplar (Populus tremula x Populus alba)

The effect of altering the expression level of the F5H gene was investigated in three wood tissues (normal, opposite and tension wood) in 1-year-old hybrid poplar clone 717 (Populus tremula × Populus alba L.), containing the F5H gene under the control of the C4H promoter. Elevated expression of the F5H gene in poplar has been previously reported to increase the percent syringyl content of lignin. The wild-type and three transgenic lines were inclined 45° for 3 months to induce tension wood formation. Tension and opposite wood from inclined trees, along with normal wood from control trees, were analyzed separately for carbohydrates, lignin, cellulose crystallinity and microfibril angle (MFA). In the wild-type poplar, the lignin in tension wood contained a significantly higher percentage of syringyl than normal wood or opposite wood. However, there was no significant difference in the percent syringyl content of the three wood types within each of the transgenic lines. Increasing the F5H gene expression caused an increase in the percent syringyl content and a slight decrease in the total lignin in normal wood. In tension wood, the addition of a gelatinous layer in the fiber walls resulted in a consistently lower percentage of total lignin in the tissue. Acid-soluble lignin was observed to increase by up to 2.3-fold in the transgenic lines. Compared with normal wood and opposite wood, cell wall crystallinity in tension wood was higher and the MFA was smaller, as expected, with no evidence of an effect from modifying the syringyl monomer ratio. Tension wood in all the lines contained consistently higher total sugar and glucose percentages when compared with normal wood within the respective lines. However, both sugar and glucose percentages were lower in the tension wood of transgenic lines when compared with the tension wood of wild-type trees. Evaluating the response of trees with altered syringyl content to gravity will improve our understanding of the changes in cell wall chemistry and ultrastructural properties of normal, opposite and tension wood in plants.

Label-Free in situ Raman Analysis of Opposite and Tension Wood in Populus nigra

Label-free in situ confocal Raman microscopy has been used to investigate the differences in cell morphology and components distribution between opposite wood (OW) and tension wood (TW) in Populus nigra. In the Raman images showing the ratio of the 1657 cm-1 to the 1603 cm-1 band, the spatial heterogeneity in coniferyl alcohol and coniferyl aldehyde (Lignin-CAA) was visualized within both samples. Enrichment of Lignin-CAA was visualized in the CC and CML regions where there was the highest lignin concentration. In fiber S2, which had the highest content of cellulose, there was apparent deficiency in the Lignin-CAA concentration. Raman spectra analysis revealed that the band at 1097.0 cm-1 in OW fiber S2 shifted to lower wavenumbers in TW fiber S2 (1094.4 cm-1) and GL (1094.0 cm-1), which resulted from the stretch of the glycosidic C-O-C bond of the cellulose molecular. The negative band shift at 1097 cm-1 indicated that the microfibrils in the TW fiber S2 and GL were stretched during TW formation and the microfibrils still kept the tensional deformation even after the fibers were transversally cut. The sub-cellular localization of cellulose and lignin together with the variation in molecular deformation will contribute to understanding the morphological and chemical properties of TW as a desirable woody biomass, as well as the development of its high tensile strength.

A Review on Structures of Secondary Wall in Reaction Wood Fiber of Hardwood Species

Due to a non-vertical orientation of the stem or branch, which may begin as responses of prevailing winds, snow, slope, or asymmetric crown shape, a specialized wood tissue forms, is called “reaction wood”. In hardwood species, the reaction wood tends to form on the upper side of a leaning stem or branch, is known to as “tension wood”. It is usually related with eccentric growth and changes in structure and chemistry of wood. During the formation of tension wood many changes found in wood cells, especially in wood fibers. The changes of wood properties in tension portion result different shrinkage characteristics during drying, creates a serious problem at adjacent portion of normal wood. However, tension wood is problematic in the hardwood industries and this wood is economically less valued. However, the studies on tension wood are necessary for beneficial applications in the wood industries sectors as well as research sectors. The knowledge on tension wood anatomy is still a matter for tree growers and timber processors, especially who are often involved in the furniture making. Besides, the understanding of tension wood formation provides a unique opportunity to obtain information on the molecular and biochemical mechanisms in the expression patterns of genes/proteins. The studies on reaction wood anatomy are not enough that we concluded any model. This review describes the features of fibers in tension wood todate, including secondary wall layers structure. Until date, six types structure of secondary cell wall layers for tension wood fibers such as, S1 + S2 + S3 + G, S1 + S2 + G, S1 + G, S1 + Ǵ, S1 + S2, and S1 + S2 + S3 have been reported, which considered being involved in plant evolution.

Plan B for Stimulating Stem Cell Division

Plan B for Stimulating Stem Cell Division

Ethylene and its interaction with other hormones in tension wood formation in Leucaena leucocephala (Lam.) de Wit

The effect of exogenous ethephon and its combination with gibberellic acid (GA3), indole-3-acetic acid (IAA) and benzyl amino purine (BAP) on tension wood formation in the vertically growing shoots of Leucaena leucocephala has been studied using light and scanning electron microscopy (SEM). The application of exogenous ethephon induced tension wood formation. The percentage of tension wood severity in the shoots was directly proportional to the concentration of ethephon applied. SEM studies revealed the replacement of S2 and S3 fibre wall layers with the gelatinous layer (G-layer) in response to high concentration of ethephon. Ethephon exhibited synergistic interaction with GA3 and BAP in tension wood formation. Fibres and vessel elements in tension wood, induced by ethylene and its combination with other hormones showed increase or decrease in their dimensions irrespective of their concentration and combination. The density of rays increased in all the treatments except in the combination of ethephon and IAA. The anatomical data indicate that ethylene plays a major role in tension wood formation and it interacts synergistically with gibberellins and cytokinin. On the other hand, such synergistic interaction was not found between ethephon and IAA in tension wood formation.

Effect of tree size and competition on tension wood production over time in beech plantations and assessing relative gravitropic response with a biomechanical model

Gravitropic movements are unexpected mechanical processes that could disturb tree design allometries derived from the physics of nonliving bodies. We investigated whether the scaling law of gravitropic performance (power of -2 of stem diameter) derived from integrative biomechanical modeling is disturbed by ontogeny or environment, then discuss the silvicultural and dendroecological consequences. In a beech (Fagus sylvatica) plantation, four plots with different initial planting densities evolved without any intervention for 26 yr. Regular tree inventories and a silvicultural model were used to monitor competition over time in each plot. The radial production of tension wood was quantified using a cross-section of the stems at 1.30-m height, and an integrative biomechanical model computed the tree gravitropic performance over time. All trees developed tension wood over the whole period, with higher amounts at the youngest age, resulting in theoretical lean corrections of ca. 20-30° on the first 4 m of the stem over the whole period. The scaling law of gravitropic performance is slightly larger than the power of -2 of stem diameter. Gravitropic performance in forest ecosystems is mainly limited by size (diameter). Ontogenic acclimation of tension wood formation allows the youngest trees to be more reactive. No additional effect of spacing was found. However, silviculture influences size and, therefore, tree reactivity at a given age. Such results will be helpful for dendroecological approaches that use wood as a marker of environmental disturbances or a trait linked to plant strategies.

Gibberellin is required for the formation of tension wood and stem gravitropism in Acacia mangium seedlings.

Angiosperm trees generally form tension wood on the upper sides of leaning stems. The formation of tension wood is an important response to gravitational stimulus. Gibberellin appears to be involved in the differentiation of secondary xylem, but it remains unclear whether gibberellin plays a key role in the formation of tension wood and plant gravitropism. Therefore, a study was designed to investigate the effects of gibberellin and of inhibitors of the synthesis of gibberellin, namely paclobutrazole and uniconazole-P, on the formation of tension wood and negative stem gravitropism in Acacia mangium seedlings. Gibberellic acid (GA(3)), paclobutrazole and uniconazole-P were applied to seedlings via the soil in which they were growing. Distilled water was applied similarly as a control. Three days after such treatment, seedlings were tilted at an angle of 45° from the vertical, and samples of stems were collected for analysis 2 weeks, 2 months and 6 months after tilting. The effects of treatments on the stem recovery degree (Rº) were analysed as an index of the negative gravitropism of seedlings, together the width of the region of tension wood in the upper part of inclined stems. It was found that GA(3) stimulated the negative gravitropism of tilted seedling stems of A. mangium, while paclobutrazole and uniconazole-P inhibited recovery to vertical growth. Moreover, GA(3) stimulated the formation of tension wood in tilted A. mangium seedlings, while paclobutrazole and uniconazole-P strongly suppressed the formation of tension wood, as assessed 2 weeks after tilting. The results suggest that gibberellin plays an important role at the initial stages of formation of tension wood and in stem gravitropism in A. mangium seedlings in response to a gravitational stimulus.

Anatomy and lignin distribution in reaction phloem fibres of several Japanese hardwoods

Although tension wood formation and the structure of gelatinous fibres (G-fibres) have been widely investigated, studies of the influence of the reaction phenomenon on phloem fibres have been few and incomplete in comparison with those of xylem wood fibres. This study was undertaken to clarify the influence of stem inclination on phloem fibres using several Japanese hardwood species that produce different G-fibre types in tension wood. Eight hardwood species were inclined at 30-45° at the beginning of April. Specimens were collected in July and December. The cell-wall structure and lignin distribution of phloem fibres on both the tension and opposite sides were compared by light microscopy, ultraviolet microscopy, confocal laser scanning microscopy after staining with acriflavine, and transmission electron microscopy after staining with potassium permanganate. Three types of changes were found in tension-side phloem fibres: (1) increases in the proportion of the syringyl unit in lignin in the S(1) and S(2) layers and compound middle lamella (Cercidiphyllum japonicum), (2) formation of unlignified gelatinous layers (Melia azedarach and Acer rufinerve) and (3) increases in the number of layers (n) in the multi-layered structure of S(1) + S(2) + n (G + L) (Mallotus japonicus). Other species showed no obvious change in cell-wall structure or lignin distribution. Phloem fibres of the tree species examined in our study showed three types of changes in lignin distribution and cell-wall structure. The reaction phenomenon may vary with tree species and may not be closely related to G-fibre type in tension wood.

The response to daylight or continuous ozone of phenylpropanoid and lignin biosynthesis pathways in poplar differs between leaves and wood

Ozone induces a stimulation of the phenylpropanoid and lignin biosynthesis pathways in leaves but the response of wood, the main lignin-producing tissue, is not well documented. The purpose of this study was to compare the responses of phenylpropanoid and lignin pathways in leaves and stem wood by a simultaneous analysis of both organs. Young poplars (Populus tremula×alba) were subjected either to daylight ozone (200 nL L(-1) during light period) or continuous ozone (200 nL L(-1) during light and dark periods) in controlled chambers. The trees were tilted so as to limit the formation of tension wood to the upper side of the stem and that of opposite wood to the lower side. Continuous ozone fumigation induced more pronounced effects in leaves than daylight ozone. Tension wood and opposite wood displayed similar responses to ozone. Enzyme activities involved in phenylpropanoid and lignin biosynthesis increased in the leaves of ozone-treated poplars and decreased in the wood. All steps involved in phenylpropanoid and monolignol synthesis in leaves and stem wood, were also altered at the transcript level (except coniferyl aldehyde 5-hydroxylase in leaves) suggesting that the responses were tightly coordinated. The response occurred rapidly in the leaves and much later in the wood. Phenylpropanoid and lignin biosynthesis is probably first involved in a defensive role against ozone in the leaves, which would lead to considerable rerouting of the carbon skeletons. The later response of phenylpropanoid and lignin metabolism in wood seemed to result from readjustment to the reduced carbon supply.

Expression analysis of LIM gene family in poplar, toward an updated phylogenetic classification.

BackgroundPlant LIM domain proteins may act as transcriptional activators of lignin biosynthesis and/or as actin binding and bundling proteins. Plant LIM genes have evolved in phylogenetic subgroups differing in their expression profiles: in the whole plant or specifically in pollen. However, several poplar PtLIM genes belong to uncharacterized monophyletic subgroups and the expression patterns of the LIM gene family in a woody plant have not been studied.FindingsIn this work, the expression pattern of the twelve duplicated poplar PtLIM genes has been investigated by semi quantitative RT-PCR in different vegetative and reproductive tissues. As in other plant species, poplar PtLIM genes were widely expressed in the tree or in particular tissues. Especially, PtXLIM1a, PtXLIM1b and PtWLIM1b genes were preferentially expressed in the secondary xylem, suggesting a specific function in wood formation. Moreover, the expression of these genes and of the PtPLIM2a gene was increased in tension wood. Western-blot analysis confirmed the preferential expression of PtXLIM1a protein during xylem differentiation and tension wood formation. Genes classified within the pollen specific PLIM2 and PLIM2-like subgroups were all strongly expressed in pollen but also in cottony hairs. Interestingly, pairs of duplicated PtLIM genes exhibited different expression patterns indicating subfunctionalisations in specific tissues.ConclusionsThe strong expression of several LIM genes in cottony hairs and germinating pollen, as well as in xylem fibers suggests an involvement of plant LIM domain proteins in the control of cell expansion. Comparisons of expression profiles of poplar LIM genes with the published functions of closely related plant LIM genes suggest conserved functions in the areas of lignin biosynthesis, pollen tube growth and mechanical stress response. Based on these results, we propose a novel nomenclature of poplar LIM domain proteins.

Salt stress induces the formation of a novel type of ‘pressure wood’ in two Populus species

• Salinity causes osmotic stress and limits biomass production of plants. The goal of this study was to investigate mechanisms underlying hydraulic adaptation to salinity. • Anatomical, ecophysiological and transcriptional responses to salinity were investigated in the xylem of a salt-sensitive (Populus × canescens) and a salt-tolerant species (Populus euphratica). • Moderate salt stress, which suppressed but did not abolish photosynthesis and radial growth in P. × canescens, resulted in hydraulic adaptation by increased vessel frequencies and decreased vessel lumina. Transcript abundances of a suite of genes (FLA, COB-like, BAM, XET, etc.) previously shown to be activated during tension wood formation, were collectively suppressed in developing xylem, whereas those for stress and defense-related genes increased. A subset of cell wall-related genes was also suppressed in salt-exposed P. euphratica, although this species largely excluded sodium and showed no anatomical alterations. Salt exposure influenced cell wall composition involving increases in the lignin : carbohydrate ratio in both species. • In conclusion, hydraulic stress adaptation involves cell wall modifications reciprocal to tension wood formation that result in the formation of a novel type of reaction wood in upright stems named 'pressure wood'. Our data suggest that transcriptional co-regulation of a core set of genes determines reaction wood composition.

Toward identifying molecules responsible for the peculiar properties of the G-layer in tension wood fibres

Background Due to its peculiar properties, tension wood formation constitutes a remarkable adaptation mechanism, that makes possible for the tree to reorientate its axes (stem and branches) in response to environnemental cues. In poplar, tension wood fibres harbour an extra cell wall layer, the G-layer, responsible for the peculiar mechanical properties of tension wood. This G-layer is very thick, most likely devoid of lignins and strongly enriched in highly cristalline cellulose. In addition, cellulose microfibril orientation is almost parallel to the fibre axis. We aim to identify molecular actors responsible for the tensioning of cellulose microfibrils and we choose as candidate, molecules containing complex carbohydrates, such as pectin and the glycosylated part of arabinogalactan proteins. Indeed, a wide array of different carbohydrates has been recently evidenced in the G-layer, suggesting the occurrence of complex polysaccharides other than cellulose within this layer (1, 2).