Xylem Secondary Cell Wall Research Articles

A semi-thin section technique-based approach to quantify the xylem secondary cell wall deposition process

Summary Identification of wood formation, cell wall deposition, lignification, and the maturation process contribute to a better understanding of biomass accumulation processes. Traditional methods for studying xylem development are limited by dyeing effects and discrimination experience, are non-quantitative for the degree of cell wall deposition and lignification, or are unsuitable for broad-leaved trees. In this study, we integrated several already existing methods to improve the discriminative accuracy of the cell development stage and to quantitatively describe the cell wall deposition and lignification degree for both softwood and hardwood tree species. To do this, we collected tree microcores every 7–14 days during a growing season for two species, one conifer (Platycladus orientalis) and one broad-leaved tree (Acer truncatum), in the mountainous areas in Beijing, China. We tracked the xylem development using semi-thin section technology combined with polarization microscopy. This integrated approach allows a quantitative description of the xylem cell wall deposition and lignification process of both hardwood and softwood tree species. This approach can be applied to demonstrate the dynamic process between the cambium layer and the production of wood cells and to describe the cell wall deposition and lignification process of wood cells from generation to maturation. This approach has certain application prospects for exploring scientific issues related to wood formation and accumulation processes in forestry and ecological studies.

Uncovering miRNA-mRNA Regulatory Modules in Developing Xylem of Pinus massoniana via Small RNA and Degradome Sequencing.

Xylem is required for the growth and development of higher plants to provide water and mineral elements. The thickening of the xylem secondary cell wall (SCW) not only improves plant survival, but also provides raw materials for industrial production. Numerous studies have found that transcription factors and non-coding RNAs regulate the process of SCW thickening. Pinus massoniana is an important woody tree species in China and is widely used to produce materials for construction, furniture, and packaging. However, the target genes of microRNAs (miRNAs) in the developing xylem of P. massoniana are not known. In this study, a total of 25 conserved miRNAs and 173 novel miRNAs were identified via small RNA sequencing, and 58 differentially expressed miRNAs were identified between the developing xylem (PM_X) and protoplasts isolated from the developing xylem (PM_XP); 26 of these miRNAs were significantly up-regulated in PM_XP compared with PM_X, and 32 were significantly down-regulated. A total of 153 target genes of 20 conserved miRNAs and 712 target genes of 113 novel miRNAs were verified by degradome sequencing. There may be conserved miRNA-mRNA modules (miRNA-MYB, miRNA-ARF, and miRNA-LAC) involved in softwood and hardwood formation. The results of qRT-PCR-based parallel validation were in relatively high agreement. This study explored the potential regulatory network of miRNAs in the developing xylem of P. massoniana and provides new insights into wood formation in coniferous species.

Tissue and cell-specific transcriptomes in cotton reveal the subtleties of gene regulation underlying the diversity of plant secondary cell walls

BackgroundKnowledge of plant secondary cell wall (SCW) regulation and deposition is mainly based on the Arabidopsis model of a ‘typical’ lignocellulosic SCW. However, SCWs in other plants can vary from this. The SCW of mature cotton seed fibres is highly cellulosic and lacks lignification whereas xylem SCWs are lignocellulosic. We used cotton as a model to study different SCWs and the expression of the genes involved in their formation via RNA deep sequencing and chemical analysis of stem and seed fibre.ResultsTranscriptome comparisons from cotton xylem and pith as well as from a developmental series of seed fibres revealed tissue-specific and developmentally regulated expression of several NAC transcription factors some of which are likely to be important as top tier regulators of SCW formation in xylem and/or seed fibre. A so far undescribed hierarchy was identified between the top tier NAC transcription factors SND1-like and NST1/2 in cotton. Key SCW MYB transcription factors, homologs of Arabidopsis MYB46/83, were practically absent in cotton stem xylem. Lack of expression of other lignin-specific MYBs in seed fibre relative to xylem could account for the lack of lignin deposition in seed fibre. Expression of a MYB103 homolog correlated with temporal expression of SCW CesAs and cellulose synthesis in seed fibres. FLAs were highly expressed and may be important structural components of seed fibre SCWs. Finally, we made the unexpected observation that cell walls in the pith of cotton stems contained lignin and had a higher S:G ratio than in xylem, despite that tissue’s lacking many of the gene transcripts normally associated with lignin biosynthesis.ConclusionsOur study in cotton confirmed some features of the currently accepted gene regulatory cascade for ‘typical’ plant SCWs, but also revealed substantial differences, especially with key downstream NACs and MYBs. The lignocellulosic SCW of cotton xylem appears to be achieved differently from that in Arabidopsis. Pith cell walls in cotton stems are compositionally very different from that reported for other plant species, including Arabidopsis. The current definition of a ‘typical’ primary or secondary cell wall might not be applicable to all cell types in all plant species.

Long cold exposure induces transcriptional and biochemical remodelling of xylem secondary cell wall in Eucalyptus.

Although eucalypts are the most planted hardwood trees worldwide, the majority of them are frost sensitive. The recent creation of frost-tolerant hybrids such as Eucalyptus gundal plants (E. gunnii × E. dalrympleana hybrids), now enables the development of industrial plantations in northern countries. Our objective was to evaluate the impact of cold on the wood structure and composition of these hybrids, and on the biosynthetic and regulatory processes controlling their secondary cell-wall (SCW) formation. We used an integrated approach combining histology, biochemical characterization and transcriptomic profiling as well as gene co-expression analyses to investigate xylem tissues from Eucalyptus hybrids exposed to cold conditions. Chilling temperatures triggered the deposition of thicker and more lignified xylem cell walls as well as regulation at the transcriptional level of SCW genes. Most genes involved in lignin biosynthesis, except those specifically dedicated to syringyl unit biosynthesis, were up-regulated. The construction of a co-expression network enabled the identification of both known and potential new SCW transcription factors, induced by cold stress. These regulators at the crossroads between cold signalling and SCW formation are promising candidates for functional studies since they may contribute to the tolerance of E. gunnii × E. dalrympleana hybrids to cold.

In silico comparative analysis of glycoside hydrolase (GH) family 10 endo-(1-4)-beta-xylanase genes from Eucalyptus grandis and Arabidopsis thaliana

Background The hemicellulose xylan constitutes the major non-cellulosic component of plant secondary cell walls. It has been shown that xylan adsorbs to cellulose fibres and also covalently binds a carbon moiety of lignin [1,2]. Eucalyptus is an important hardwood tree genus used in the pulp and paper industry and has potential as biofuel feedstock. Xylan removal is expensive and uses environmentally harsh chemical treatments [3]. Previous studies have shown that endo-(1-4)-b-xylanase enzymes belonging to glycoside hydrolase (GH) family 10 internally attacks the xylan backbone resulting in shorter xylo-saccharide chains [4]. The recently sequenced Eucalyptusgrandis genome (DOE-JGI, http://www.phytozome.net) provides a unique opportunity to analyze the native endo-(1-4)-beta-xylanase proteins involved in xylan modification in eucalypt fibre cell walls. Detailed knowledge of endogenous xylanolytic enzymes from Eucalyptus could facilitate the development of strategies to enhance the processing of woody biomass for cellulose and biofuel production. The aims of this study are to identify xylem secondary cell wall-related endo-(1-4)-bxylanase genes in the E. grandis genome and to perform a comparative analysis of the Eucalyptus xylanasepeptide sequences with those of previously studied Arabidopsis orthologs to provide a framework for assigning function to the Eucalyptus enzymes. Results Analysis of the E. grandis genome sequence on Phytozome v7.0 (http://www.phytozome.net) for putative endo-(1-4)-b-xylanase genes resulted in the identification of 18 putative GH10 family members. The expression profile of each family member was assessed (via mRNA-Seq analysis, http://eucspresso.bi.up.ac.za/) to identify members with putative roles in xylem secondary cell wall metabolism. Egrandis_v1_0.001952m (designated EgrXYN1) showed the highest xylem to phloem and xylem to leaf expression ratios of the expressed E. grandis GH10 genes [5]. BLAST analysis (<1e-10) of the A. thaliana genome for putative orthologs to EgrXYN1 and co-phylogenetic analysis of all 18 E. grandis enzymes with the putative A. thaliana xylanases revealed that AtXYN1 (At1g58370) [4] was one of the closest putative orthologs to EgrXYN1 (Figure 1). Alignment of the predicted amino acid sequences of EgrXYN1 and AtXYN1 Jalview 2.6.1 revealed 68.76% identity between the two sequences. In silico biochemical analysis predicted that EgrXYN1 has a molecular weight of 103 kDa with a pI of 6.08. This is very similar to AtXYN1 which is 102 kDa with a pI of 6.1. The protein domain view in Phytozome (http://www.phytozome.net) revealed that EgrXYN1 contains three successive N-terminal b-sandwich carbohydrate binding modules IV (at amino acid positions 53-185, 216-357 and 387-532) which were also observed in AtXYN1. A protein motif search (http://motif.genome.jp) revealed that EgrXYN1 also contained a conserved and identical C-terminal GH10 active site sequence “GLPIWFTELDV” at amino acid position 802812. Finally, de novo motif search of both AtXYN1 and * Correspondence: ritesh.mewalal@tuks.co.za Department of Genetics, Forestry and Agricultural Biotechnology Institute (FABI), University of Pretoria, Pretoria, 0002, South Africa Full list of author information is available at the end of the article Mewalal et al. BMC Proceedings 2011, 5(Suppl 7):P168 http://www.biomedcentral.com/1753-6561/5/S7/P168

Sucrose synthase affects carbon partitioning to increase cellulose production and altered cell wall ultrastructure

Overexpression of the Gossypium hirsutum sucrose synthase (SuSy) gene under the control of 2 promoters was examined in hybrid poplar (Populus alba x grandidentata). Analysis of RNA transcript abundance, enzyme activity, cell wall composition, and soluble carbohydrates revealed significant changes in the transgenic lines. All lines showed significantly increased SuSy enzyme activity in developing xylem. This activity manifested in altered secondary cell wall cellulose content per dry weight in all lines, with increases of 2% to 6% over control levels, without influencing plant growth. The elevated concentration of cellulose was associated with an increase in cell wall crystallinity but did not alter secondary wall microfibril angle. This finding suggests that the observed increase in crystallinity is a function of altered carbon partitioning to cellulose biosynthesis rather than the result of tension wood formation. Furthermore, the augmented deposition of cellulose in the transgenic lines resulted in thicker xylem secondary cell wall and consequently improved wood density. These findings clearly implicate SuSy as a key regulator of sink strength in poplar trees and demonstrate the tight association of SuSy with cellulose synthesis and secondary wall formation.

Identification and characterization of Arabidopsis thaliana genes involved in xylem secondary cell walls

The xylem of higher plants offers support to aerial portions of the plant body and serves as conduit for the translocation of water and nutrients. Terminal differentiation of xylem cells typically involves deposition of thick secondary cell walls. This is a dynamic cellular process accompanied by enhanced rates of cellulose deposition and the induction of synthesis of specific secondary-wall matrix polysaccharides and lignin. The secondary cell wall is essential for the function of conductive and supportive xylem tissues. Recently, significant progress has been made in identifying the genes responsible for xylem secondary cell wall formation. However, our present knowledge is still insufficient to account for the molecular processes by which this complex system operates. To acquire further information about xylem secondary cell walls, we initially focused our research effort on a set of genes specifically implicated in secondary cell wall formation, as well as on loss-of-function mutants. Results from two microarray screens identified several key candidate genes responsible for secondary cell wall formation. Reverse genetic analyses led to the identification of a glycine-rich protein involved in maintaining the stable structure of protoxylem, which is essential for the transport of water and nutrients. A combination of expression analyses and reverse genetics allows us to systematically identify new genes required for the development of physical properties of the xylem secondary wall.