Cloning and expression analysis of a wood-associated xylosidase gene (PtaBXL1) in poplar tension wood

In stems of woody angiosperms responding to mechanical stress, imposed for instance by tilting the stem or formation of a branch, tension wood (TW) forms above the affected part, while anatomically distinct opposite wood (OW) forms below it. In poplar TW the S3 layer of the secondary walls is substituted by a “gelatinous layer” that is almost entirely composed of cellulose and has much lower hemicellulose contents than unstressed wood. However, changes in xylan contents (the predominant hemicelluloses), their interactions with other wall components and the mechanisms involved in TW formation have been little studied. Therefore, in the study reported here we determined the structure and distribution of xylans, cloned the genes encoding the xylan remodeling enzymes β-xylosidases (PtaBXLi), and examined their expression patterns during tension wood, normal wood and opposite wood xylogenesis in poplar. We confirm that poplar wood xylans are substituted solely by 4-O-methylglucuronic acid in both TW and OW. However, although glucuronoxylans are strongly represented in both primary and secondary layers of OW, no 4-O-methylGlcA xylan was found in G-layers of TW. Four full-length BXL cDNAs encoding putative β-xylosidases were cloned. One, PtaBXL1, for which xylosidase activity was confirmed by heterologous expression in Escherichia coli, exhibited a wood-specific expression pattern in TW. In conclusion, xylan as PtaBXL1, encoding β4-xylosidase activity, are down-regulated in TW.     

Stem wood properties of mature Norway spruce after 3 years of continuous exposure to elevated [CO2] and temperature

The objective of the study was to investigate the interactive effects of elevated atmospheric carbon dioxide concentration, [CO₂], and temperature on the wood properties of mature field-grown Norway spruce ( Picea abies (L.) Karst.) trees. Material for the study was obtained from an experiment in Flakaliden, northern Sweden, where trees were grown for 3 years in whole-tree chambers at ambient (365 μmol mol⁻¹) or elevated [CO₂] (700 μmol mol⁻¹) and ambient or elevated air temperature (ambient +5.6 °C in winter and ambient +2.8 °C in summer). Elevated temperature affected both wood chemical composition and structure, but had no effect on stem radial growth. Elevated temperature decreased the concentrations of acetone-soluble extractives and soluble sugars, while mean and earlywood (EW) cell wall thickness and wood density were increased. Elevated [CO₂] had no effect on stem wood chemistry or radial growth. In wood structure, elevated [CO₂] decreased EW cell wall thickness and increased tracheid radial diameter in latewood (LW). Some significant interactions between elevated [CO₂] and temperature were found in the anatomical and physical properties of stem wood (e.g. microfibril angle, and LW cell wall thickness and density). Our results show that the wood material properties of mature Norway spruce were altered under exposure to elevated [CO₂] and temperature, although stem radial growth was not affected by the treatments.     

Isolation and Characterization of Expressed Sequence Tags (ESTs) from Cambium Tissue of Birch (<Emphasis Type="Italic">Betula platyphylla Suk</Emphasis>)

Cloning and identification of genes from the cambium of woody plants are important steps in the investigation of wood formation. In order to clone and identify the genes involved in wood formation, a cDNA library was constructed from the cambial tissue of Betula platyphylla Suk (birch). From the cDNA library, 2,878 high-quality ESTs were generated, representing 1,540 tentative unique genes (TUGs). Approximately 70% of the ESTs were matched to proteins in the NR database in GenBank and were grouped into 12 functional categories according to their functional annotation. Fifty TUGs potentially involved in wood formation, including the process of lignin biosynthesis, cell wall structure, cell wall polysaccharides synthesis or development regulation, were identified from the EST collection. The time course expression analysis of 13 wood formation related genes was further studied using RNA gel blot and semi-quantitative RT-PCR. The results showed that these genes were primarily expressed in tissue that was collected during two time periods, spring (26 April to 11 June) and later summer (30 July to 18 Sept). The bimodal expression curve of the genes indicates that both of these time periods are critical in wood formation in birch, and may be associated with cell wall expansion earlier in the year, and cell wall strengthening later in the year.     

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

Background and Aims 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. Methods 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. Key Results 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. Conclusions 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.     

Identifying candidate genes for wood formation in poplar based on microarray network analysis and graph theory

In poplar, genetic research on wood properties is very important for the improvement of wood quality. Studies of wood formation genes at each developmental stage using modern biotechnology have often been limited to several genes or gene families. Because of the complex regulatory network involved in the co-expression and interactions of thousands of genes, however, the genetic mechanisms of wood formation must be surveyed on a genome-wide scale. In this study, we identified wood formation-related genes using a differentially co-expressed (DCE) gene subset approach based on biological networks inferred from microarray data. Gene co-expression networks in leaf, root, and wood tissues were first constructed and topologically analyzed using microarray data collected from the Gene Expression Omnibus. The DCE gene modules in wood-forming tissue were then detected based on graph theory, which was followed by gene ontology (GO) enrichment analysis and GO annotation of probe sets. Finally, 72 probe sets were identified in the largest cohesive subgroup of the DCE gene network in wood tissue, with most of the probe sets associated with wood formation-related biological processes and GO cellular component categories. The approach described in this paper provides an effective strategy to identify wood formation genes in poplar and should contribute to the better understanding of the genetic and molecular mechanisms underlying wood properties in trees.     

Targeted re-sequencing and genome-wide association analysis for wood property traits in breeding population of Eucalyptus tereticornis × E. grandis

Globally, Eucalyptus plantations occupy 22 million ha area and is one of the preferred hardwood species due to their short rotation, rapid growth, adaptability and wood properties. In this study, we present results of GWAS in parents and 100 hybrids of Eucalyptus tereticornis × E. grandis using 762 genes presumably involved in wood formation. Comparative analysis between parents predicted 32,202 polymorphic SNPs with high average read depth of 269-562× per individual per nucleotide. Seventeen wood related traits were phenotyped across three diverse environments and GWAS was conducted using 13,610 SNPs. A total of 45 SNP-trait associations were predicted across two locations. Seven large effect markers were identified which explained more than 80% of phenotypic variation for fibre area. This study has provided an array of candidate genes which may govern fibre morphology in this genus and has predicted potential SNPs which can guide future breeding programs in tropical Eucalyptus.     

Novel gene expression profiles define the metabolic and physiological processes characteristic of wood and its extractive formation in a hardwood tree species,Robinia pseudoacacia

Wood is of critical importance to humans as a primary feedstock for biofuel, fiber, solid wood products, and various natural compounds including pharmaceuticals. The trunk wood of most tree species has two distinctly different regions: sapwood and heartwood. In addition to the major constituents, wood contains extraneous chemicals that can be removed by extraction with various solvents. The composition and the content of the extractives vary depending on such factors as, species, growth conditions, and time of year when the tree is cut. Despite the great commercial and keen scientific interest, little is known about the tree-specific biology of the formation of heartwood and its extractives. In order to gain insight on the molecular regulations of heartwood and its extractive formation, we carried out global examination of gene expression profiles across the trunk wood of black locust (Robinia pseudoacacia L.) trees. Of the 2,915 expressed sequenced tags (ESTs) that were generated and analyzed in the current study, 55.3% showed no match to known sequences. Cluster analysis of the ESTs identified a total of 2278 unigene sets, which were used to construct cDNA microarrays. Microarray hybridization analyses were then performed to survey the changes in gene expression profiles of trunk wood. The gene expression profiles of wood formation differ according to the region of trunk wood sampled, with highly expressed genes defining the metabolic and physiological processes characteristic of each region. For example, the gene encoding sugar transport had the highest expression in the sapwood, while the structural genes for flavonoid biosynthesis were up-regulated in the sapwood-heartwood transition zone. This analysis also established the expression patterns of 341 previously unknown genes.     

Environmental and auxin regulation of wood formation involves members of the Aux/IAA gene family in hybrid aspen

Indole acetic acid (IAA/auxin) profoundly affects wood formation but the molecular mechanism of auxin action in this process remains poorly understood. We have cloned cDNAs for eight members of the Aux/IAA gene family from hybrid aspen (Populus tremula L. x Populus tremuloides Michx.) that encode potential mediators of the auxin signal transduction pathway. These genes designated as PttIAA1-PttIAA8 are auxin inducible but differ in their requirement of de novo protein synthesis for auxin induction. The auxin induction of the PttIAA genes is also developmentally controlled as evidenced by the loss of their auxin inducibility during leaf maturation. The PttIAA genes are differentially expressed in the cell types of a developmental gradient comprising the wood-forming tissues. Interestingly, the expression of the PttIAA genes is downregulated during transition of the active cambium into dormancy, a process in which meristematic cells of the cambium lose their sensitivity to auxin. Auxin-regulated developmental reprogramming of wood formation during the induction of tension wood is accompanied by changes in the expression of PttIAA genes. The distinct tissue-specific expression patterns of the auxin inducible PttIAA genes in the cambial region together with the change in expression during dormancy transition and tension wood formation suggest a role for these genes in mediating cambial responses to auxin and xylem development.