A cytochemical and immunocytochemical analysis of the wall labyrinth apparatus in leaf transfer cells in Elodea canadensis

Background and Aims

Transfer cells are plant cells specialized in apoplast/symplast transport and characterized by a distinctive wall labyrinth apparatus. The molecular architecture and biochemistry of the labyrinth apparatus are poorly known. The leaf lamina in the aquatic angiosperm Elodea canadensis consists of only two cell layers, with the abaxial cells developing as transfer cells. The present study investigated biochemical properties of wall ingrowths and associated plasmalemma in these cells.

Methods

Leaves of Elodea were examined by light and electron microscopy and ATPase activity was localized cytochemically. Immunogold electron microscopy was employed to localize carbohydrate epitopes associated with major cell wall polysaccharides and glycoproteins.

Key Results

The plasmalemma associated with the wall labyrinth is strongly enriched in light-dependent ATPase activity. The wall ingrowths and an underlying wall layer share an LM11 epitope probably associated with glucuronoarabinoxylan and a CCRC-M7 epitope typically associated with rhamnogalacturonan I. No labelling was observed with LM10, an antibody that recognizes low-substituted and unsubstituted xylan, a polysaccharide consistently associated with secondary cell walls. The JIM5 and JIM7 epitopes, associated with homogalacturonan with different degrees of methylation, appear to be absent in the wall labyrinth but present in the rest of cell walls.

Conclusions

The wall labyrinth apparatus of leaf transfer cells in Elodea is a specialized structure with distinctive biochemical properties. The high level of light-dependent ATPase activity in the plasmalemma lining the wall labyrinth is consistent with a formerly suggested role of leaf transfer cells in enhancing inorganic carbon inflow. The wall labyrinth is a part of the primary cell wall. The discovery that the wall ingrowths in Elodea have an antibody-binding pattern divergent, in part, from that of the rest of cell wall suggests that their carbohydrate composition is modulated in relation to transfer cell functioning.     

Early local differentiation of the cell wall matrix defines the contact sites in lobed mesophyll cells of Zea mays

Background and Aims

The morphogenesis of lobed mesophyll cells (MCs) is highly controlled and coupled with intercellular space formation. Cortical microtubule rings define the number and the position of MC isthmi. This work investigated early events of MC morphogenesis, especially the mechanism defining the position of contacts between MCs. The distributions of plasmodesmata, the hemicelluloses callose and (1 → 3,1 → 4)-β-d-glucans (MLGs) and the pectin epitopes recognized by the 2F4, JIM5, JIM7 and LM6 antibodies were studied in the cell walls of Zea mays MCs.

Methods

Matrix cell wall polysaccharides were immunolocalized in hand-made sections and in sections of material embedded in LR White resin. Callose was also localized using aniline blue in hand-made sections. Plasmodesmata distribution was examined by transmission electron microscopy.

Results

Before reorganization of the dispersed cortical microtubules into microtubule rings, particular bands of the longitudinal MC walls, where the MC contacts will form, locally differentiate by selective (1) deposition of callose and the pectin epitopes recognized by the 2F4, LM6, JIM5 and JIM7 antibodies, (2) degradation of MLGs and (3) formation of secondary plasmodesmata clusterings. This cell wall matrix differentiation persists in cell contacts of mature MCs. Simultaneously, the wall bands between those of future cell contacts differentiate with (1) deposition of local cell wall thickenings including cellulose microfibrils, (2) preferential presence of MLGs, (3) absence of callose and (4) transient presence of the pectins identified by the JIM5 and JIM7 antibodies. The wall areas between cell contacts expand determinately to form the cell isthmi and the cell lobes.

Conclusions

The morphogenesis of lobed MCs is characterized by the early patterned differentiation of two distinct cell wall subdomains, defining the sites of the future MC contacts and of the future MC isthmi respectively. This patterned cell wall differentiation precedes cortical microtubule reorganization and may define microtubule ring disposition.     

An extensin-rich matrix lines the carinal canals in Equisetum ramosissimum, which may function as water-conducting channels

Background and Aims

The anatomy of Equisetum stems is characterized by the occurrence of vallecular and carinal canals. Previous studies on the carinal canals in several Equisetum species suggest that they convey water from one node to another.

Methods

Cell wall composition and ultrastructure have been studied using immunocytochemistry and electron microscopy, respectively. Serial sectioning and X-ray computed tomography were employed to examine the internode–node–internode transition of Equisetum ramosissimum.

Key Results

The distribution of the LM1 and JIM20 extensin epitopes is restricted to the lining of carinal canals. The monoclonal antibodies JIM5 and LM19 directed against homogalacturonan with a low degree of methyl esterification and the CBM3a probe recognizing crystalline cellulose also bound to this lining. The xyloglucan epitopes recognized by LM15 and CCRC-M1 were only detected in this lining after pectate lyase treatment. The carinal canals, connecting consecutive rings of nodal xylem, are formed by the disruption and dissolution of protoxylem elements during elongation of the internodes. Their inner surface appears smooth compared with that of vallecular canals.

Conclusions

The carinal canals in E. ramosissimum have a distinctive lining containing pectic homogalacturonan, cellulose, xyloglucan and extensin. These canals might function as water-conducting channels which would be especially important during the elongation of the internodes when protoxylem is disrupted and the metaxylem is not yet differentiated. How the molecularly distinct lining relates to the proposed water-conducting function of the carinal canals requires further study. Efforts to elucidate the spatial and temporal distribution of cell wall polymers in a taxonomically broad range of plants will probably provide more insight into the structural–functional relationships of individual cell wall components or of specific configurations of cell wall polymers.     

Non-lignified helical cell wall thickenings in root cortical cells of Aspleniaceae (Polypodiales): histology and taxonomical significance

Background and Aims

Extraxylary helical cell wall thickenings in vascular plants are not well documented, except for those in orchid velamen tissues which have been studied extensively. Reports on their occurrence in ferns exist, but detailed information is missing. The aim of this study is to focus on the broad patterns of structure and composition and to study the taxonomic occurrence of helical cell wall thickenings in the fern family Aspleniaceae.

Methods

Structural and compositional aspects of roots have been examined by means of light, electron, epifluorescence and laser scanning confocal microscopy. To assess the taxonomical distribution of helical cell wall thickenings a molecular phylogenetic analysis based on rbcL sequences of 64 taxa was performed.

Key Results

The helical cell wall thickenings of all examined species showed considerable uniformity of design. The pattern consists of helical, regularly bifurcating and anastomosing strands. Compositionally, the cell wall thickenings were found to be rich in homogalacturonan, cellulose, mannan and xyloglucan. Thioacidolysis confirmed our negative phloroglucinol staining tests, demonstrating the absence of lignins in the root cortex. All taxa with helical cell wall thickenings formed a monophyletic group supported by a 100 % bootstrap value and composed of mainly epiphytic species.

Conclusions

This is the first report of non-lignified pectin-rich secondary cell walls in ferns. Based on our molecular analysis, we reject the hypothesis of parallel evolution of helical cell wall thickenings in Aspleniaceae. Helical cell wall thickenings can mechanically stabilize the cortex tissue, allowing maximal uptake of water and nutrients during rainfall events. In addition, it can also act as a boundary layer increasing the diffusive pathway towards the atmosphere, preventing desiccation of the stele of epiphytic growing species.     

The distribution of cell wall polymers during antheridium development and spermatogenesis in the Charophycean green alga, Chara corallina

Background and Aims

The production of multicellular gametangia in green plants represents an early evolutionary development that is found today in all land plants and advanced clades of the Charophycean green algae. The processing of cell walls is an integral part of this morphogenesis yet very little is known about cell wall dynamics in early-divergent green plants such as the Charophycean green algae. This study represents a comprehensive analysis of antheridium development and spermatogenesis in the green alga, Chara corallina.

Methods

Microarrays of cell wall components and immunocytochemical methods were employed in order to analyse cell wall macromolecules during antheridium development.

Key Results

Cellulose and pectic homogalacturonan epitopes were detected throughout all cell types of the developing antheridium including the unique cell wall protuberances of the shield cells and the cell walls of sperm cell initials. Arabinogalactan protein epitopes were distributed only in the epidermal shield cell layers and anti-xyloglucan antibody binding was only observed in the capitulum region that initially yields the sperm filaments. During the terminal stage of sperm development, no cell wall polymers recognized by the probes employed were found on the scale-covered sperm cells.

Conclusions

Antheridium development in C. corallina is a rapid event that includes the production of cell walls that contain polymers similar to those found in land plants. While pectic and cellulosic epitopes are ubiquitous in the antheridium, the distribution of arabinogalactan protein and xyloglucan epitopes is restricted to specific zones. Spermatogenesis also includes a major switch in the production of extracellular matrix macromolecules from cell walls to scales, the latter being a primitive extracellular matrix characteristic of green plants.     

Glucuronoarabinoxylan extracted by treatment with endoxylanase from different zones of growing maize root

Glucuronoarabinoxylan is a key tethering glucan in the primary cell wall of cereals. Glucuronoarabinoxylan was extracted from different zones of maize (Zea mays L.) roots using endoxylanase that specifically cleaves β-(1,4)-glycoside bond between two consequent unsubstituted xylose residues. Changes in polysaccharide structure during elongation growth were characterized. Glucuronoarabinoxylan extractable after the endoxylanase treatment consisted of high molecular weight (30–400 kDa) and low molecular weight (<10 kDa) fractions. The presence of high molecular weight derivatives indicated that part of the natural glucuronoarabinoxylan is not digestible by the endoxylanase. This could be due to the revealed peculiar structural features, such as high level of substitution of xylose, absence of unsubstituted xylose residues existing in sequence, and significant degree of acetylation. In maize root meristem the indigestible fraction was 98% of the total extracted glucuronoarabinoxylan. This portion decreases to 47% during elongation. Also, the average molecular weight of indigestible glucuronoarabinoxylan reduced twofold. These changes in the ratio of glucuronoarabinoxylan fragments with different structure during root cell growth could reflect a transition of polysaccharide from its separating (highly substituted indigestible glucuronoarabinoxylan) form to that binding to cellulose microfibrils or other glucuronoarabinoxylan molecules and, hence, retarding growth.     

Function of xyloglucan endotransglucosylase/hydrolases in rice

Background and Aims

Although xyloglucans are ubiquitous in land plants, they are less abundant in Poales species than in eudicotyledons. Poales cell walls contain higher levels of β-1,3/1,4 mixed-linked glucans and arabinoxylans than xyloglucans. Despite the relatively low level of xyloglucans in Poales, the xyloglucan endotransglucosylase/hydrolase (XTH) gene family in rice (Oryza sativa) is comparable in size to that of the eudicotyledon Arabidopsis thaliana. This raises the question of whether xyloglucan is a substrate for rice XTH gene products, whose enzyme activity remains largely uncharacterized.

Methods

This study focused on OsXTH19 (which belongs to Group IIIA of the XTH family and is specifically expressed in growing tissues of rice shoots), and two other XTHs, OsXTH11 (Group I/II) and OsXTH20 (Group IIIA), for reference, and measurements were made of the enzymatic activities of three recombinant rice XTHs, i.e. OsXTH11, OsXTH20 and OsXTH19.

Key Results

All three OsXTH gene products have xyloglucan endohydrolase (XEH, EC 3·2·1·151) activity, and OsXTH11 has both XEH and xyloglucan endotransglycosylase (XET, EC 2·4·1207) activities. However, these proteins had neither hydrolase nor transglucosylase activity when glucuronoarabinoxylan or mixed-linkage glucan was used as the substrate. These results are consistent with histological observations demonstrating that pOsXTH19::GUS is expressed specifically in the vicinity of tissues where xyloglucan immunoreactivity is present. Transgenic rice lines over-expressing OsXTH19 (harbouring a Cauliflower Mosaic Virus 35S promoter::OsXTH19 cDNA construct) or with suppressed OsXTH19 expression (harbouring a pOsXTH19 RNAi construct) did not show dramatic phenotypic changes, suggesting functional redundancy and collaboration among XTH family members, as was observed in A. thaliana.

Conclusions

OsXTH20 and OsXTH19 act as hydrolases exclusively on xyloglucan, while OsXTH11 exhibits both hydrolase and XET activities exclusively on xyloglucans. Phenotypic analysis of transgenic lines with altered expression of OsXTH19 suggests that OsXTH19 and related XTH(s) play redundant roles in rice growth.     

Pectic-β(1,4)-galactan,extensin and arabinogalactan–protein epitopes differentiate ripening stages in wine and table grape cell walls

Background and Aims

Cell wall changes in ripening grapes (Vitis vinifera) have been shown to involve re-modelling of pectin, xyloglucan and cellulose networks. Newer experimental techniques, such as molecular probes specific for cell wall epitopes, have yet to be extensively used in grape studies. Limited general information is available on the cell wall properties that contribute to texture differences between wine and table grapes. This study evaluates whether profiling tools can detect cell wall changes in ripening grapes from commercial vineyards.

Methods

Standard sugar analysis and infra-red spectroscopy were used to examine the ripening stages (green, véraison and ripe) in grapes collected from Cabernet Sauvignon and Crimson Seedless vineyards. Comprehensive microarray polymer profiling (CoMPP) analysis was performed on cyclohexanediaminetetraacetic acid (CDTA) and NaOH extracts of alcohol-insoluble residue sourced from each stage using sets of cell wall probes (mAbs and CBMs), and the datasets were analysed using multivariate software.

Key Results

The datasets obtained confirmed previous studies on cell wall changes known to occur during grape ripening. Probes for homogalacturonan (e.g. LM19) were enriched in the CDTA fractions of Crimson Seedless relative to Cabernet Sauvignon grapes. Probes for pectic-β-(1,4)-galactan (mAb LM5), extensin (mAb LM1) and arabinogalactan proteins (AGPs, mAb LM2) were strongly correlated with ripening. From green stage to véraison, a progressive reduction in pectic-β-(1,4)-galactan epitopes, present in both pectin-rich (CDTA) and hemicellulose-rich (NaOH) polymers, was observed. Ripening changes in AGP and extensin epitope abundance also were found during and after véraison.

Conclusions

Combinations of cell wall probes are able to define distinct ripening phases in grapes. Pectic-β-(1,4)-galactan epitopes decreased in abundance from green stage to véraison berries. From véraison there was an increase in abundance of significant extensin and AGP epitopes, which correlates with cell expansion events. This study provides new ripening biomarkers and changes that can be placed in the context of grape berry development.     

The trafficking of the cellulose synthase complex in higher plants

Background

Cellulose is an important constituent of plant cell walls in a biological context, and is also a material commonly utilized by mankind in the pulp and paper, timber, textile and biofuel industries. The biosynthesis of cellulose in higher plants is a function of the cellulose synthase complex (CSC). The CSC, a large transmembrane complex containing multiple cellulose synthase proteins, is believed to be assembled in the Golgi apparatus, but is thought only to synthesize cellulose when it is localized at the plasma membrane, where CSCs synthesize and extrude cellulose directly into the plant cell wall. Therefore, the delivery and endocytosis of CSCs to and from the plasma membrane are important aspects for the regulation of cellulose biosynthesis.

Scope

Recent progress in the visualization of CSC dynamics in living plant cells has begun to reveal some of the routes and factors involved in CSC trafficking. This review highlights the most recent major findings related to CSC trafficking, provides novel perspectives on how CSC trafficking can influence the cell wall, and proposes potential avenues for future exploration.     

Chemical and enzymatic fractionation of cell walls from Fucales: insights into the structure of the extracellular matrix of brown algae

Background and Aims

Brown algae are photosynthetic multicellular marine organisms evolutionarily distant from land plants, with a distinctive cell wall. They feature carbohydrates shared with plants (cellulose), animals (fucose-containing sulfated polysaccharides, FCSPs) or bacteria (alginates). How these components are organized into a three-dimensional extracellular matrix (ECM) still remains unclear. Recent molecular analysis of the corresponding biosynthetic routes points toward a complex evolutionary history that shaped the ECM structure in brown algae.

Methods

Exhaustive sequential extractions and composition analyses of cell wall material from various brown algae of the order Fucales were performed. Dedicated enzymatic degradations were used to release and identify cell wall partners. This approach was complemented by systematic chromatographic analysis to study polymer interlinks further. An additional structural assessment of the sulfated fucan extracted from Himanthalia elongata was made.

Key Results

The data indicate that FCSPs are tightly associated with proteins and cellulose within the walls. Alginates are associated with most phenolic compounds. The sulfated fucans from H. elongata were shown to have a regular α-(1→3) backbone structure, while an alternating α-(1→3), (1→4) structure has been described in some brown algae from the order Fucales.

Conclusions

The data provide a global snapshot of the cell wall architecture in brown algae, and contribute to the understanding of the structure–function relationships of the main cell wall components. Enzymatic cross-linking of alginates by phenols may regulate the strengthening of the wall, and sulfated polysaccharides may play a key role in the adaptation to osmotic stress. The emergence and evolution of ECM components is further discussed in relation to the evolution of multicellularity in brown algae.     

Electrophoretic profiling and immunocytochemical detection of pectins and arabinogalactan proteins in olive pollen during germination and pollen tube growth

Background and Aims

Cell wall pectins and arabinogalactan proteins (AGPs) are important for pollen tube growth. The aim of this work was to study the temporal and spatial dynamics of these compounds in olive pollen during germination.

Methods

Immunoblot profiling analyses combined with confocal and transmission electron microscopy immunocytochemical detection techniques were carried out using four anti-pectin (JIM7, JIM5, LM5 and LM6) and two anti-AGP (JIM13 and JIM14) monoclonal antibodies.

Key Results

Pectin and AGP levels increased during olive pollen in vitro germination. (1 → 4)-β-d-Galactans localized in the cytoplasm of the vegetative cell, the pollen wall and the apertural intine. After the pollen tube emerged, galactans localized in the pollen tube wall, particularly at the tip, and formed a collar-like structure around the germinative aperture. (1 → 5)-α-l-Arabinans were mainly present in the pollen tube cell wall, forming characteristic ring-shaped deposits at regular intervals in the sub-apical zone. As expected, the pollen tube wall was rich in highly esterified pectic compounds at the apex, while the cell wall mainly contained de-esterified pectins in the shank. The wall of the generative cell was specifically labelled with arabinans, highly methyl-esterified homogalacturonans and JIM13 epitopes. In addition, the extracellular material that coated the outer exine layer was rich in arabinans, de-esterified pectins and JIM13 epitopes.

Conclusions

Pectins and AGPs are newly synthesized in the pollen tube during pollen germination. The synthesis and secretion of these compounds are temporally and spatially regulated. Galactans might provide mechanical stability to the pollen tube, reinforcing those regions that are particularly sensitive to tension stress (the pollen tube–pollen grain joint site) and mechanical damage (the tip). Arabinans and AGPs might be important in recognition and adhesion phenomena of the pollen tube and the stylar transmitting cells, as well as the egg and sperm cells.     

Hybrid proline-rich proteins: novel players in plant cell elongation?

Background and Aims

Hybrid proline-rich proteins (HyPRPs) represent a large family of putative cell-wall proteins characterized by the presence of a variable N-terminal domain and a conserved C-terminal domain that is related to non-specific lipid transfer proteins. The function of HyPRPs remains unclear, but their widespread occurrence and abundant expression patterns indicate that they may be involved in a basic cellular process.

Methods

To elucidate the cellular function of HyPRPs, we modulated the expression of three HyPRP genes in tobacco (Nicotiana tabacum) BY-2 cell lines and in potato (Solanum tuberosum) plants.

Key Results

In BY-2 lines, over-expression of the three HyPRP genes with different types of N-terminal domains resulted in similar phenotypic changes, namely increased cell elongation, both in suspension culture and on solid media where the over-expression resulted in enhanced calli size. The over-expressing cells showed increased plasmolysis in a hypertonic mannitol solution and accelerated rate of protoplast release, suggesting loosening of the cell walls. In contrast to BY-2 lines, no phenotypic changes were observed in potato plants over-expressing the same or analogous HyPRP genes, presumably due to more complex compensatory mechanisms in planta.

Conclusions

Based on the results from BY-2 lines, we propose that HyPRPs, more specifically their C-terminal domains, represent a novel group of proteins involved in cell expansion.     

Arabinogalactan protein-rich cell walls,paramural deposits and ergastic globules define the hyaline bodies of rhinanthoid Orobanchaceae haustoria

Background and Aims

Parasitic plants obtain nutrients from their hosts through organs called haustoria. The hyaline body is a specialized parenchymatous tissue occupying the central parts of haustoria in many Orobanchaceae species. The structure and functions of hyaline bodies are poorly understood despite their apparent necessity for the proper functioning of haustoria. Reported here is a cell wall-focused immunohistochemical study of the hyaline bodies of three species from the ecologically important clade of rhinanthoid Orobanchaceae.

Methods

Haustoria collected from laboratory-grown and field-collected plants of Rhinanthus minor, Odontites vernus and Melampyrum pratense attached to various hosts were immunolabelled for cell wall matrix glycans and glycoproteins using specific monoclonal antibodies (mAbs).

Key Results

Hyaline body cell wall architecture differed from that of the surrounding parenchyma in all species investigated. Enrichment in arabinogalactan protein (AGP) epitopes labelled with mAbs LM2, JIM8, JIM13, JIM14 and CCRC-M7 was prominent and coincided with reduced labelling of de-esterified homogalacturonan with mAbs JIM5, LM18 and LM19. Furthermore, paramural bodies, intercellular deposits and globular ergastic bodies composed of pectins, xyloglucans, extensins and AGPs were common. In Rhinanthus they were particularly abundant in pairings with legume hosts. Hyaline body cells were not in direct contact with haustorial xylem, which was surrounded by a single layer of paratracheal parenchyma with thickened cell walls abutting the xylem.

Conclusions

The distinctive anatomy and cell wall architecture indicate hyaline body specialization. Altered proportions of AGPs and pectins may affect the mechanical properties of hyaline body cell walls. This and the association with a transfer-like type of paratracheal parenchyma suggest a role in nutrient translocation. Organelle-rich protoplasts and the presence of exceptionally profuse intra- and intercellular wall materials when attached to a nitrogen-fixing host suggest subsequent processing and transient storage of nutrients. AGPs might therefore be implicated in nutrient transfer and metabolism in haustoria.     

A cell-type-specific defect in border cell formation in the Acacia mangium root cap developing an extraordinary sheath of sloughed-off cells

Background and Aims

Root caps release border cells, which play central roles in microbe interaction and root protection against soil stresses. However, the number and connectivity of border cells differ widely among plant species. Better understanding of key border-cell phenotype across species will help define the total function of border cells and associated genes.

Methods

The spatio-temporal detachment of border cells in the leguminous tree Acacia mangium was investigated by using light and fluorescent microscopy with fluorescein diacetate, and their number and structural connectivity compared with that in soybean (Glycine max).

Key Results

Border-like cells with a sheet structure peeled bilaterally from the lateral root cap of A. mangium. Hydroponic root elongation partially facilitated acropetal peeling of border-like cells, which accumulate as a sheath that covers the 0- to 4-mm tip within 1 week. Although root elongation under friction caused basipetal peeling, lateral root caps were minimally trimmed as compared with hydroponic roots. In the meantime, A. mangium columella caps simultaneously released single border cells with a number similar to those in soybean.

Conclusions

These results suggest that cell type-specific inhibitory factors induce a distinct defective phenotype in single border-cell formation in A. mangium lateral root caps.     

Spatial regulation of cell-wall structure in response to heavy metal stress: cadmium-induced alteration of the methyl-esterification pattern of homogalacturonans

Background and Aims

In flax hypocotyls, cadmium-induced reorientation of growth coincides with marked changes in homogalacturonan (HGA) epitopes that were recognized by JIM7 and JIM5 antibodies in the external tangential wall of the epidermis. In the present study, LM7 and 2F4 monoclonal antibodies were used, in addition to JIM5 and JIM7, to extend the investigation on the methyl-esterification pattern of HGA within various domains of the cortical tissues, including the cortical parenchyma where cell cohesion is crucial.

Methods

The PATAg (periodic acid thiocarbohydrazide–silver proteinate) test was applied to ultrathin sections so that the polysaccharides could be visualized and the ultrastructure studied. The monoclonal LM7, JIM5 and JIM7 antibodies that recognize differently methyl-esterified HGA were used. The monoclonal 2F4 antibody that is specific to a particular polygalacturonic acid conformation induced by a given calcium to sodium ratio was also applied. After immunogold labelling, the grids were stained with uranyl-acetate, the samples were observed using a transmission electron microscope and the gold particles were counted.

Key Results

In the presence of cadmium, the increase of LM7 labelling in external tangential wall of the epidermis, together with a decrease of JIM7 labelling, suggested a specific role for randomly partially de-esterified HGA to counteract the radial swelling stress. Enhanced JIM5 and 2F4 labelling in the junctions of the inner tissues indicated that the presence of blockwise de-esterified HGA might oppose cell separation.

Conclusions

The response of the hypocotyl to cadmium stress was to adapt the structure of the wall of cortical tissues by differently modulating the methyl-esterification pattern of HGA in various domains.     

Evolution of mixed-linkage (1 -> 3, 1 -> 4)-β-D-glucan (MLG) and xyloglucan in Equisetum (horsetails) and other monilophytes

Background and Aims

Horsetails (Equisetopsida) diverged from other extant eusporangiate monilophytes in the Upper Palaeozoic. They are the only monilophytes known to contain the hemicellulose mixed-linkage (1 → 3, 1 → 4)-β-d-glucan (MLG), whereas all land plants possess xyloglucan. It has been reported that changes in cell-wall chemistry often accompanied major evolutionary steps. We explored changes in hemicelluloses occurring during Equisetum evolution.

Methods

Hemicellulose from numerous monilophytes was treated with lichenase and xyloglucan endoglucanase. Lichenase digests MLG to di-, tri- and tetrasaccharide repeat-units, resolvable by thin-layer chromatography.

Key Results

Among monilophytes, MLG was confined to horsetails. Our analyses support a basal trichotomy of extant horsetails: MLG was more abundant in subgenus Equisetum than in subgenus Hippochaete, and uniquely the sister group E. bogotense yielded almost solely the tetrasaccharide repeat-unit (G4G4G3G). Other species also gave the disaccharide, whereas the trisaccharide was consistently very scarce. Tetrasaccharide : disaccharide ratios varied interspecifically, but with no consistent difference between subgenera. Xyloglucan was scarce in Psilotum and subgenus Equisetum, but abundant in subgenus Hippochaete and in the eusporangiate ferns Marattia and Angiopteris; leptosporangiate ferns varied widely. All monilophytes shared a core pattern of xyloglucan repeat-units, major XEG products co-chromatographing on thin-layer chromatography with non-fucosylated hepta-, octa- and nonasaccharides and fucose-containing nona- and decasaccharides.

Conclusions

G4G4G3G is the ancestral repeat-unit of horsetail MLG. Horsetail evolution was accompanied by quantitative and qualitative modification of MLG; variation within subgenus Hippochaete suggests that the structure and biosynthesis of MLG is evolutionarily plastic. Xyloglucan quantity correlates negatively with abundance of other hemicelluloses; but qualitatively, all monilophyte xyloglucans conform to a core pattern of repeat-unit sizes.     

Towards modelling the flexible timing of shoot development: simulation of maize organogenesis based on coordination within and between phytomers

Background and Aims

Experimental evidence challenges the approximation, central in crop models, that developmental events follow a fixed thermal time schedule, and indicates that leaf emergence events play a role in the timing of development. The objective of this study was to build a structural development model of maize (Zea mays) based on a set of coordination rules at organ level that regulate duration of elongation, and to show how the distribution of leaf sizes emerges from this.

Methods

A model of maize development was constructed based on three coordination rules between leaf emergence events and the dynamics of organ extension. The model was parameterized with data from maize grown at a low plant population density and tested using data from maize grown at high population density.

Key Results

The model gave a good account of the timing and duration of organ extension. By using initial conditions associated with high population density, the model reproduced well the increase in blade elongation duration and the delay in sheath extension in high-density populations compared with low-density populations. Predictions of the sizes of sheaths at high density were accurate, whereas predictions of the dynamics of blade length were accurate up to rank 9; moderate overestimation of blade length occurred at higher ranks.

Conclusions

A set of simple rules for coordinated growth of organs is sufficient to simulate the development of maize plant structure without taking into account any regulation by assimilates. In this model, whole-plant architecture is shaped through initial conditions that feed a cascade of coordination events.