Comparative compositional analysis of walls with two different morphologies: archetypical versus transfer-cell-like

Summary Abnormally thick cell walls of a clonal maize cell line with the labyrinth wall morphology found in transfer cells were analyzed and compared to the relatively thin and even archetypical walls of a sister cell line. Despite a drastic difference in wall morphology between the transfer and archetypical cell walls, the chemical composition of the walls was essentially the same. There were no major differences in the glycosyl residue composition, in the amount of total lipid, and in the amount of total protein. The amounts of wall material released by chemical extraction of cellulosic, hemicellulosic, and pectic fractions were the same for the two types of walls. There were some differences in the protein profile and in the inorganic ion content between the transfer and archetypical walls. These results indicate that profound changes in wall morphology can be brought about in the absence of gross changes in wall composition and suggest that major changes in time- or place-dependent deposition and/or subtle changes in arrangement of rare wall constituents may be responsible.     

Distribution of pectins in cell walls of maize coleoptiles and evidence against their involvement in auxin-induced extension growth

Summary Aiming to elucidate the possible involvement of pectins in auxin-mediated elongation growth the distribution of pectins in cell walls of maize coleoptiles was investigated. Antibodies against defined epitopes of pectin were used: JIM 5 recognizing pectin with a low degree of esterification, JIM 7 recognizing highly esterified pectin and 2F4 recognizing a pectin epitope induced by Ca²⁺. JIM 5 weakly labeled the outer third of the outer epidermal wall and the center of filled cell corners in the parenchyma. A similar labeling pattern was obtained with 2F4. In contrast, JIM 7 densely labeled the whole outer epidermal wall except the innermost layer, the middle lamellae, and the inner edges of open cell corners in the parenchyma. Enzymatic de-esterification with pectin methylesterase increased the labeling by JIM 5 and 2F4 substantially. A further increase of the labeling density by JIM 5 and 2F4 and an extension of the labeling over the whole outer epidermal wall could be observed after chemical de-esterification with alkali. This indicates that both methyl- and other esters exist in maize outer epidermal walls. Thus, in the growth-controlling outer epidermal wall a clear zonation of pectin fractions was observed: the outermost layer (about one third to one half of wall thickness) contains unesterified pectin epitopes, presumably cross-linked by Ca²⁺ extract. Tracer experiments with³H-myo-inositol showed rapid accumulation of tracer in all extractable pectin fractions and in a fraction tightly bound to the cell wall. A stimulatory effect of IAA on tracer incorporation could not be detected in any fraction. Summarizing the data a model of the pectin distribution in the cell walls of maize coleoptiles was developed and its implications for the mechanism of auxin-induced wall loosening are discussed.Abbreviations CDTA trans-1,2-diaminocyclohexane-N,N,N,N-tetraacetic acid - CWP cell-wall pellet - IAA indole-3-acetic acid - LSE low-salt extract - TCA trichloroacetic acid; Tris tris-(hydroxy-methyl)aminoethane     

Electron-translucent angular areas in developing tapetum cell walls and pollen grains ofTilia platyphyllos

Summary InTilia platyphyllos, the anther tapetal cell walls undergo significant modifications from the tetrad stage onwards. During the tetrad stage the inner tangential and radial parts of the tapetal walls begin to dissolve, while the distal parts swell. After the tetrad stage, the distal and outer radial tapetal cell walls become covered by a thick, irregular, highly electron-dense, polysaccharide layer. Striking features of the maturing tapetal walls (microspore stage and later) are electron-translucent, structureless, unstainable angular areas of variable dimensions. Similar electron-translucent areas occur in the exine arcades and apertures, but also isolated in the locular fluid ofT. platyphyllos. Electron-translucent areas, that are also found in the exine arcades and tapetal cells of other angiosperms, can be interpreted as the products of poorly understood metabolic processes.     

Isolation of diferulic bridges ester-linked to arabinan in sugar beet cell walls

After degradation of sugar beet cell walls with Driselase and fractionation of the solubilised products by hydrophobic interaction chromatography, a dehydrodiferuloylated oligoarabinan was isolated. Its structure was assigned to two dimers of (1-->5)-linked arabinose units esterified by a central 8-O-4' ferulic dimer. These results provide the first direct evidence that pectic arabinans in sugar beet cell walls may be covalently cross-linked through dehydrodiferulates.     

Regulators of cell division in plant tissues

The cytokinins in certain fractions prepared from extracts of immature sweet-corn (Zea mays L.) kernels using polystyrene ion-exchange resins have been further investigated. Cytokinins active in the radish cotyledon bioassay were purified from these fractions and identified as 9--D-glucopyranosylzeatin, 9--D-glucopyranosyldihydrozeatin, O--D-glucopyranosylzeatin. and O--D-glucopyranosyl-9--D-ribofuranosylzeatin. In addition, compounds which resemble zeatin and its glycosides in chromatographic behaviour and in ultraviolet absorption characteristics were purified from extracts of the same material by high-performance liquid chromatography. In addition to zeatin and zeatin riboside, the following compounds were identified unambiguously: O--D-glucopyranosyl-9--D-ribofuranosyldihydrozeatin, O--D-glucopyranosyldihydrozeatin, and hihydrozeatin riboside. A further compound was tentatively identified as O--D-glucopyranosylzeatin, and at least two unidentified compounds appeared to be new derivatives of zeatin. In identifying the above compounds, chemical-ionization mass spectrometry proved to be an invaluable complementary technique, yielding spectra showing intense protonated-molecular-ion peaks and also prominent structure-related fragmentation that was either not evident or very minor in the electron-impact spectra. An assessment of the relative importance of the various possible mechanisms for cytokinin modification and inactivation in mature sweet-corn kernels was made by supplying [³H]zeatin and [³H]zeatin riboside to such kernels after excision. The principal metabolites of zeatin were adenine nucleotides, adenosine and adenine, while little of the metabolite radioactivity was attributable to known O-glucosides. Adenine nucleotides and adenine were the principal metabolites of zeatin riboside, while lesser metabolites were identified as adenosine, dihydrozeatin, and the O-glucosides of dihydrozeatin and dihydrozeatin riboside. Side-chain cleavage, rather than side-chain modification, appears to be the dominant form of cytokinin metabolism in mature sweet-corn kernels.Abbreviations CI-MS chemical-ionization mass spectrum - EIMS electron-impact mass spectrum - GC-MS combined gas chromatography-mass spectrometry - HPLC high performance liquid chromatography - M⁺ molecular ion - MH⁺ protonated molecular ion - TLC thin-layer chromatography - TMS trimethylsilyl - UV ultraviolet XXVII=Letham et al. (1979)     

Pectin esterification is spatially regulated both within cell walls and between developing tissues of root apices

Monoclonal antibodies recognizing un-esterified (JIM5) and methyl-esterified (JIM7) epitopes of pectin have been used to locate these epitopes by indirect immunofluorescence and immunogold electron microscopy in the root apex of carrot (Daucus carota L.). Both antibodies labelled the walls of cells in all tissues of the developing root apex. Immunogold labelling observed at the level of the electron microscope indicated differential location of the pectin epitopes within the cell walls. The un-esterified epitope was located to the inner surface of the primary cell walls adjacent to the plasma membrane, in the middle lamella and abundantly to the outer surface at intercellular spaces. In contrast, the epitope containing methyl-esterified pectin was located evenly throughout the cell wall. In root apices of certain other species the JIM5 and JIM7 epitopes were found to be restricted to distinct tissues of the developing roots. In the root apex of oat (Avena sativa L.), JIM5 was most abundantly reactive with cell walls at the region of intercellular spaces of the cortical cells. JIM7 was reactive with cells of the cortex and the stele. Neither epitope occurred in walls of the epidermal or root-cap cells. These pattern of expression were observed to derive from the very earliest stages of the development of these tissues in the oat root meristem and were maintained in the mature root. In the coleoptile and leaf tissues of oat seedlings, JIM5 labelled all cells abundantly whereas JIM7 was unreactive. Other members of the Gramineae and also the Chenopodiaceae are shown to express similar restricted spatial patterns of distribution of these pectin epitopes in root apices.Abbreviations CDTA 1,2-diaminocyclohexane tetraacetic acid - RG rhamnogalacturonan J.P.K. was supported by the Agricultural and Food Research Council Cell Signalling and Recognition Programme. We thank J. Cooke and N. Stacey for technical assistance, H.A. Schols, Drs. P. Albersheim and A. Darvill for pectic polysaccharides, and Dr. R.R. Selvendran and M. McCann for useful discussions.     

Xylogalacturonan exists in cell walls from various tissues of Arabidopsis thaliana

Evidence is presented for the presence of xylogalacturonan (XGA) in Arabidopsis thaliana. This evidence was obtained by extraction of pectin from the seeds, root, stem, young leaves and mature leaves of A. thaliana, followed by treatment of these pectin extracts with xylogalacturonan hydrolase (XGH). Upon enzymatic treatment, XGA oligosaccharides were primarily produced from pectin extracts obtained from the young and mature leaves and to a lesser extent from those originating from the stem of A. thaliana. The oligosaccharide GalA(3)Xyl was predominantly formed from these pectin extracts. No XGA oligosaccharides were detected in digests of pectin extracts from the seeds and roots. A low number of XGA oligosaccharides was obtained from pectins of A. thaliana. This indicates a uniform distribution of xylose in XGA from A. thaliana. The predominant production of GalA(3)Xyl, as well as the release of linear GalA oligosaccharides pointed to a lower degree of xylose substitution in XGA from A. thaliana than in XGA from apple and potato. The estimated amount of XGA accounted for approximately 2.5%, 7% and 6% (w/w) of the total carbohydrate in the pectin fraction of the stem, young leaves and mature leaves, respectively.     

Investigation of ferulate deposition in endosperm cell walls of mature and developing wheat grains by using a polyclonal antibody

A polyclonal antibody has been raised against ferulic acid ester linked to arabinoxylans (AX). 5-O-feruloyl-α-l-arabinofuranosyl(1→4)-β-d-xylopyranosyl was obtained by chemical synthesis, and was coupled to bovine serum albumin for the immunization of rabbit. The polyclonal antibody designated 5-O-Fer-Ara was highly specific for 5-O-(trans-feruloyl)-l-arabinose (5-O-Fer-Ara) structure that is a structural feature of cell wall AX of plants belonging to the family of Gramineae. The antibody has been used to study the location and deposition of feruloylated AX in walls of aleurone and starchy endosperm of wheat grain. 5-O-Fer-Ara began to accumulate early in aleurone cell wall development (beginning of grain filling, 13 days after anthesis, DAA) and continued to accumulate until the aleurone cells were firmly fixed between the starchy endosperm and the nucellus epidermis (19 DAA). From 26 DAA to maturity, the aleurone cell walls changed little in appearance. The concentration of 5-O-Fer-Ara is high in both peri- and anticlinal aleurone cell walls with the highest accumulation of 5-O-Fer-Ara at the cell junctions at the seed coat interface. The situation is quite different in the starchy endosperm: whatever the stage of development, a low amount of 5-O-Fer-Ara epitope was detected. Contrary to what was observed for aleurone cell walls, no peak of accumulation of feruloylated AX was noticed between 13 and 19 DAA. Visualization of labelled Golgi vesicles suggested that the feruloylation of AX is intracellular. The distribution of (5-O-Fer-Ara) epitope is further discussed in relation to the role of ferulic acid and its dehydrodimers in cell wall structure and tissue organization of wheat grain.     

Influence of Maize Rotations on the Yield of Soybean Grown in Meloidogyne incognita Infested Soil

A replicated field study was conducted from 1972 to 1980 involving soybeans grown in 2-, 3-, and 4-year rotations with maize in soil infested with Meloidogyne incognita. Monocultured soybeans were maintained as controls. Cropping regimes involved root-knot nematode susceptible and resistant soybean cultivars and soybeans treated and not treated with nematicides. Yields of susceptible cultivars declined with reduced length of rotation. Nematicide treatment significantly increased yields of susceptible cultivars when monocultured, but bad little influence on yield when susceptible cultivars were grown in rotation. Yields of monocultured resistant cultivars were significantly lower than yields of resistant cultivars grown in rotation. However, yields of resistant cultivars grown in rotation were not influenced by the length of the rotation. Nematicide treatment significantly increased yields of monocultured resistant cultivars over the latter years of the study.     

Mannan transglycosylase: a novel enzyme activity in cell walls of higher plants

Mannan transglycosylase is a novel cell wall enzyme activity acting on mannan-based plant polysaccharides in primary cell walls of monocotyledons and dicotyledons. The enzyme activity was detected by its ability to transfer galactoglucomannan (GGM) polysaccharides to tritium-labelled GGM-derived oligosaccharides generating tritium-labelled GGM polysaccharides. Mannan transglycosylase was found in a range of plant species and tissues. High levels of the enzyme activity were present in flowers of some kiwifruit (Actinidia) species and in ripe tomato (Solanum lycopersicum L.) fruit. Low levels were detected in mature green tomato fruit and activity increased during tomato fruit ripening up to the red ripe stage. Essentially all activity was found in the tomato skin and outermost 2 mm of tissue. Mannan transglycosylase activity in tomato skin and outer pericarp is specific for mannan-based plant polysaccharides, including GGM, galactomannan, glucomannan and mannan. The exact structural requirements for valid acceptors remain to be defined. Nevertheless, a mannose residue at the second position of the sugar chain and the absence of a galactose substituent on the fourth residue (counting from the non-reducing end) appear to be minimal requirements. Mannan-based polysaccharides in the plant cell wall may have a role analogous to that of xyloglucans, introducing flexibility and forming growth-restraining networks with cellulose. Thus mannan transglycosylase and xyloglucan endotransglycosylase, the only other known transglycosylase activity in plant cell walls, may both be involved in remodelling and refining the cellulose framework in developmental processes throughout the life of a plant.Abbreviations EBM Endo--mannanase - GGM galactoglucomannan - GGMO Galactoglucomannan-derived oligosaccharide - G₂M₅ Di-galactosyl mannopentaitol - M₂–M₅ Mannobiitol to mannopentaitol oligosaccharides - SK+OP Skin plus outer pericarp - XET Xyloglucan endotransglucosylase - XG Xyloglucan     

Effect of cadmium ion excess over cell structure and functioning of Zea mays and Hordeum vulgare

Zea mays (maize) and Hordeum vulgare (barley) plants were analyzed in order to study the variation in response to Cadmium (Cd) toxicity based on development of leaf symptoms, effect in dry matter production, Cd uptake, lipid peroxidation and effect on cell ultrastructure in leaves and roots. Cd accumulation in roots of Z. mays and H. vulgare was 18–50 times higher than in the aerial parts. Malondialdehyde (MDA) content was more affected in the roots of both Z. mays and H. vulgare than in shoots (60 and 56–51 and 40%, respectively). At ultrastructural level, in Cd treated seedlings, a decline in the vacuolar content of barley roots cells and maize leaf cells was observed. Results corroborate that these gramineous crops can uptake and accumulate substantial amounts of Cd especially in roots. Therefore, H. vulgare and Z. mays could have a phytostabilization potential and thereafter could be tested in phytoremediation technologies.     

Plant and algal cell walls: diversity and functionality

Background

Although plants and many algae (e.g. the Phaeophyceae, brown, and Rhodophyceae, red) are only very distantly related they are united in their possession of carbohydrate-rich cell walls, which are of integral importance being involved in many physiological processes. Furthermore, wall components have applications within food, fuel, pharmaceuticals, fibres (e.g. for textiles and paper) and building materials and have long been an active topic of research. As shown in the 27 papers in this Special Issue, as the major deposit of photosynthetically fixed carbon, and therefore energy investment, cell walls are of undisputed importance to the organisms that possess them, the photosynthetic eukaryotes (plants and algae). The complexities of cell wall components along with their interactions with the biotic and abiotic environment are becoming increasingly revealed.

Scope

The importance of plant and algal cell walls and their individual components to the function and survival of the organism, and for a number of industrial applications, are illustrated by the breadth of topics covered in this issue, which includes papers concentrating on various plants and algae, developmental stages, organs, cell wall components, and techniques. Although we acknowledge that there are many alternative ways in which the papers could be categorized (and many would fit within several topics), we have organized them as follows: (1) cell wall biosynthesis and remodelling, (2) cell wall diversity, and (3) application of new technologies to cell walls. Finally, we will consider future directions within plant cell wall research. Expansion of the industrial uses of cell walls and potentially novel uses of cell wall components are both avenues likely to direct future research activities. Fundamentally, it is the continued progression from characterization (structure, metabolism, properties and localization) of individual cell wall components through to defining their roles in almost every aspect of plant and algal physiology that will present many of the major challenges in future cell wall research.     

Robustness of central carbohydrate metabolism in developing maize kernels

The central carbohydrate metabolism provides the precursors for the syntheses of various storage products in seeds. While the underlying biochemical map is well established, little is known about the organization and flexibility of carbohydrate metabolic fluxes in the face of changing biosynthetic demands or other perturbations. This question was addressed in developing kernels of maize (Zea mays L.), a model system for the study of starch and sugar metabolism. ¹³C-labeling experiments were carried out with inbred lines, heterotic hybrids, and starch-deficient mutants that were selected to cover a wide range of performances and kernel phenotypes. In total, 46 labeling experiments were carried out using either [U-¹³C₆]glucose or [U-¹³C₁₂]sucrose and up to three stages of kernel development. Carbohydrate flux distributions were estimated based on glucose isotopologue abundances, which were determined in hydrolysates of starch by using quantitative ¹³C-NMR and GC-MS. Similar labeling patterns in all samples indicated robustness of carbohydrate fluxes in maize endosperm, and fluxes were rather stable in response to glucose or sucrose feeding and during development. A lack of ADP-glucose pyrophosphorylase in the bt2 and sh2 mutants triggered significantly increased hexose cycling. In contrast, other mutations with similar kernel phenotypes had no effect. Thus, the distribution of carbohydrate fluxes is stable and not determined by sink strength in maize kernels.     

Proteome analysis of maize roots reveals that oxidative stress is a main contributing factor to plant arsenic toxicity

To gain insight into plant responses to arsenic, the effect of arsenic exposure on maize (Zea mays L.) root proteome has been examined. Maize seedlings were fed hydroponically with 300 microM sodium arsenate or 250 microM sodium arsenite for 24 h, and changes in differentially displayed proteins were studied by two-dimensional electrophoresis and digital image analysis. About 10% of total detected maize root proteins (67 out of 700) were up- or down-regulated by arsenic, among which 20 were selected as being quite reproducibly affected by the metalloid. These were analyzed by matrix-assisted laser desorption/ionization-time of flight mass spectrometry and 11 of them could be identified by comparing their peptide mass fingerprints against protein- and expressed sequence tag-databases. The set of identified maize root proteins highly responsive to arsenic exposure included a major and functionally homogeneous group of seven enzymes involved in cellular homeostasis for redox perturbation (e.g., three superoxide dismutases, two glutathione peroxidases, one peroxiredoxin, and one p-benzoquinone reductase) besides four additional, functionally heterogeneous, proteins (e.g., ATP synthase, succinyl-CoA synthetase, cytochrome P450 and guanine nucleotide-binding protein beta subunit). These findings strongly suggest that the induction of oxidative stress is a main process underlying arsenic toxicity in plants.     

Isolation and structural identification of diarabinosyl 8-O-4-dehydrodiferulate from maize bran insoluble fibre

The first saccharide ester of a dehydrodiferulic acid (DFA) other than 5-5-DFA has been isolated from maize bran insoluble fibre after acidic hydrolysis and fractionation by gel chromatography and semi-preparative RP-HPLC. HPLC-MS along with 1D, 2D and 3D NMR spectra provided the requisite structural evidence that it is the di-5-O-l-arabinosyl ester of 8-O-4-DFA. Although a range of DFAs have been well authenticated as components released from the cell walls of grasses, the only structural evidence for a DFA attached to polysaccharides had been from 5-5-DFA. The isolation of the 8-O-4-ester demonstrates that polysaccharides in maize cell walls, and presumably in all grasses, are cross-linked through dehydrodiferulates other than 5-5-dehydrodiferulate.     

The polysaccharide structure of potato cell walls: Chemical fractionation

Cell walls of potato tubers were fractionated by successive extraction with various reagents. A slightly degraded pectic fraction with 77% galacturonic acid was extracted in hot, oxalate-citrate buffer at pH 4. A further, major pectic fraction with 38% galacturonic acid was extracted in cold 0.1 M Na₂CO₃ with little apparent degradation. These two pectic fractions together made up 52% of the cell wall. Most of the oxalate-citrate fraction could alternatively be extracted with cold acetate-N,N,N-tetracetic acid (CDTA) buffer, a non-degradative extractant which nevertheless removed essentially all the calcium ions. This fraction was therefore probably held only by calcium binding, and the remainder of the pectins by covalent bonds. Electrophoresis showed that both pectic fractions contained a range of molecular types differing in composition, with a high arabinose: galactose ratio as well as much galacturonic acid in the most extractable fractions. From methylation data, the main side-chains were 1,4-linked galactans and 1,5-linked arabinans, with smaller quantities of covalently attached xyloglucan. Extraction with NaOH-borate removed a small hemicellulose fraction and some cellulose. The main hemicelluloses were apparently a galactoxyloglucan, a mannan or glucomannan and an arabinogalactan.Abbreviations GLC gas-liquid chromatography - MS mass spectrometry - V₀ void volume - MW weight-average molecular weight - DMSO dimethylsulphoxide - EDTA ethylenediamine tetraacetic acid - TFA trifluoroacetic acid - CDTA N,N,N-tetraacetic acid     

Osmotic Stress Increases Alcohol Dehydrogenase Activity in Maize Seedlings

Maize (Zea mays L.) seedlings were exposed to osmotic stress, and alcohol dehydrogenase (ADH) activity and abscisic acid (ABA) concentration were determined. The osmotic stress increased ADH activities in both roots and shoots, whereas the increase was 2-fold greater in roots than the shoots. The stress also increased ABA concentration in both roots and shoots and the increase was greater in the roots than in the shoots.