Correlations between axial stiffness and microstructure of a species of bamboo.

Bamboo is a ubiquitous monocotyledonous flowering plant and is a member of the true grass family Poaceae. In many parts of the world, it is widely used as a structural material especially in scaffolding and buildings. In spite of its wide use, there is no accepted methodology for standardizing a species of bamboo for a particular structural purpose. The task of developing structure-property correlations is complicated by the fact that bamboo is a hierarchical material whose structure at the nanoscopic level is not very well explored. However, we show that as far as stiffness is concerned, it is possible to obtain reliable estimates of important structural properties like the axial modulus from the knowledge of certain key elements of the microstructure. Stiffness of bamboo depends most sensitively on the size and arrangement of the fibre sheaths surrounding the vascular bundles and the arrangement of crystalline cellulose microfibrils in their secondary cell walls. For the species of bamboo studied in this work, we have quantitatively determined the radial gradation that the arrangement of fibres renders to the structure. The arrangement of the fibres gives bamboo a radially graded property variation across its cross section.

The chemical identity of intervessel pit membranes in Acer challenges hydrogel control of xylem hydraulic conductivity.

Ion-mediated enhancement of the hydraulic conductivity of xylem tissue (i.e. the ionic effect) has been reported for various angiosperm species. One explanation of the ionic effect is that it is caused by the swelling and shrinking of intervessel pit membranes due to the presence of pectins and/or other cell-wall matrix polymers such as heteroxylans or arabinogalactan-proteins (AGPs) that may contain acidic sugars. Here, we examined the ionic effect for six Acer species and their pit membrane chemistry using immunocytochemistry, including antibodies against glycoproteins. Moreover, anatomical features related to the bordered pit morphology and vessel dimensions were investigated using light and electron microscopy. The ionic effect varied from 18 % (± 9) to 32 % (± 13). Epitopes of homogalacturonan (LM18) and xylan (LM11) were not detected in intervessel pit membranes. Negative results were also obtained for glycoproteins (extensin: LM1, JIM20; AGP glycan: LM2), although AGP (JIM13)-related epitopes were detected in parenchyma cells. The mean vessel length was significantly correlated with the magnitude of the ionic effect, unlike other pit or vessel-related characteristics. Our results suggest that intervessel pit membranes of Acer are unlikely to contain pectic or other acidic polysaccharides. Therefore, alternative explanations should be tested to clarify the ionic effect.

Syncytia formed by adult female Heterodera schachtii in Arabidopsis thaliana roots have a distinct cell wall molecular architecture.

• Plant-parasitic cyst nematodes form a feeding site, termed a syncytium, through which the nematode obtains nutrients from the host plant to support nematode development. The structural features of cell walls of syncytial cells have yet to be elucidated. • Monoclonal antibodies to defined glycans and a cellulose-binding module were used to determine the cell wall architectures of syncytial and surrounding cells in the roots of Arabidopsis thaliana infected with the cyst nematode Heterodera schachtii. • Fluorescence imaging revealed that the cell walls of syncytia contain cellulose and the hemicelluloses xyloglucan and heteromannan. Heavily methyl-esterified pectic homogalacturonan and arabinan are abundant in syncytial cell walls; galactan could not be detected. This is suggestive of highly flexible syncytial cell walls. • This work provides important information on the structural architecture of the cell walls of this novel cell type and reveals factors that enable the feeding site to perform its functional requirements to support nematode development.

Analysis of crystallinity changes in cellulose II polymers using carbohydrate-binding modules.

Carbohydrate-binding modules (CBMs) are a set of tools that can be used as molecular probes for studying plant cell walls and cellulose-based substrates. CBMs from enzymes of bacterial and fungal origin present a range of recognition capabilities for crystalline and amorphous cellulose. Here cellulose-directed CBMs have been used to visualize and quantify crystallinity changes in cellulose II-based polymers following NaOH treatment. Cellulose II polymers used were in the form of lyocell fibers, which are derived from eucalyptus wood pulp. The supramolecular structure, morphology, and existence of 'skin-core' model in the fiber were examined using CBM-labeling techniques. Changes in cellulose crystallinity showed maxima at 3.33 mol dm(-3) NaOH (under treatment conditions of 49 Nm(-1) at 25 °C) and 4.48 mol dm(-3) NaOH (under treatment conditions of 147 Nm(-1) at 40 °C); CBM methods were also suitable for quantifying changes within amorphous regions. Quantification of crystallinity changes using CBM labeling techniques was achieved in combination with image analysis, which was shown to reflect the same crystallinity changes as measured using ATR-FTIR methods. It was demonstrated that CBM-labeling techniques were able to validate the proposed 'skin-core' model of lyocell fibers, comprising a semi-permeable fiber skin and a porous core.

ABA signalling modulates the detection of the LM6 arabinan cell wall epitope at the surface of Arabidopsis thaliana seedling root apices.

• The hormonal and physiological regulations underpinning the cell contexts of structural features of the heterogeneous cell wall pectic polysaccharide rhamnogalacturonan-I are far from being understood. • The effect of the modulation of abscisic acid (ABA) concentrations and sensitivity on the detection of the LM6 1,5-arabinan epitope at the surface of Arabidopsis thaliana seedling root apices was assessed by means of fluorescence imaging. • Treatment with 50 nM ABA resulted in an increase in the detection of the LM6 epitope at the root surface in the region of the meristem. An inhibitor of ABA biosynthesis and introduction of the ABA synthesis mutation aba3-2 resulted in reduced epitope detection. The same ABA application resulted in an increase in the number of epidermal root meristem cells and both this and LM6 epitope detection were specifically disrupted in the abi4 ABA-insensitive mutant. These two effects were uncoupled with the application of higher ABA concentrations, which resulted in a reduction in the number of epidermal root meristem cells but increased LM6 epitope detection. • This work demonstrates a role for ABI4-mediated ABA signalling in the modulation of pectic arabinan occurrence at the A. thaliana root meristem.

ABA promotes quiescence of the quiescent centre and suppresses stem cell differentiation in the Arabidopsis primary root meristem.

It is well known that abscisic acid (ABA) can halt meristems for long periods without loss of meristem function, and can also promote root growth at low concentrations, but the mechanisms underlying such regulation are largely unknown. Here we show that ABA promotes stem cell maintenance in Arabidopsis root meristems by both promoting the quiescence of the quiescent centre (QC) and suppressing the differentiation of stem cells and their daughters. We demonstrate that these two mechanisms of regulation by ABA involve distinct pathways, and identify components in each pathway. Our findings demonstrate a cellular mechanism for a positive role for ABA in promoting root meristem maintenance and root growth in Arabidopsis.

Enzymatic treatments reveal differential capacities for xylan recognition and degradation in primary and secondary plant cell walls.

The capacity of four xylan-directed probes (carbohydrate-binding modules CfCBM2b-1-2 and CjCBM15; monoclonal antibodies LM10 and LM11) to recognize xylan polysaccharides in primary and secondary cell walls of tobacco stem sections has been determined. Enzymatic removal of pectic homogalacturonan revealed differential recognition of xylans in restricted regions of cortical primary cell walls. Monoclonal antibody binding to these exposed xylans was more sensitive to xylanase action than carbohydrate-binding module (CBM) binding. In contrast, the recognition of xylans by CBMs in secondary cell walls of the same organ was more sensitive to xylanase action than the recognition of xylans by the monoclonal antibodies. A methodology was developed to quantify indirect immunofluorescence intensities, and to evaluate xylanase impacts. The four xylan probes were also used to detect xylan populations in chromatographic separations of solubilized cell wall materials from tobacco stems. Altogether, these observations reveal the heterogeneity of the xylans in plant cell walls. They indicate that although CBM and antibody probes can exhibit similar specificities against solubilized polymers, they can have differential capacities for xylan recognition in muro, and that the access of molecular probes and enzymes to xylan epitopes/ligands also varies between primary and secondary cell walls that are present in the same organ.

Novel cell wall architecture of isoxaben-habituated Arabidopsis suspension-cultured cells: global transcript profiling and cellular analysis.

The herbicide isoxaben is a highly specific and potent inhibitor of cellulose synthesis in plants. Nevertheless, suspension-cultured cells can be habituated to grow in high concentrations of isoxaben, and apparently compensate for the disruption of cellulose synthesis by the modulation of other cell wall components. We have habituated Arabidopsis cells to isoxaben and characterized the cellular and genetic consequences. Near whole-genome transcript profiling implicated novel genes in cell wall assembly and extended our understanding of the activity of known cell wall-related genes including glycosyltransferases involved in cellulose and pectin biosynthesis. Habituation does not appear to be mediated by stress response processes, nor by functional redundancy within the cellulose synthase (AtCesA) family. Uniquely, amongst the cellulose synthase superfamily, AtCslD5 was highly upregulated and may play a role in the biosynthesis of the novel walls of habituated cells. In silico analysis of differentially expressed genes with unknown functions identified a putative glycosyltransferase and collagen-like putative cell wall protein.