Distribution of yieldin, a regulatory protein of the cell wall yield threshold, in etiolated cowpea seedlings

We examined the distribution and the immunohistochemical localization of yieldin in etiolated cowpea seedlings with an anti-yieldin antibody. An immunoblotting analysis revealed that the yieldin was located in the aerial organs (plumule, epicotyl and hypocotyl) but not in the roots. The intensity of the yieldin signal in the hypocotyls was highest in the apical pre-elongation region (the hook region) and decreased toward the elongated mature base indicating that the yieldin disappeared with the ceasing of cell elongation. Tissue-print immunoblotting analysis using hypocotyls in different germination stages supports this view because the apical yieldin-rich regions, just beneath the cotyledonary node (the hook and rapidly elongating regions), acropetally migrated together with hypocotyl elongation. Immunohistochemical microscopy demonstrated that yieldin was localized in the cell walls of the cortex and epidermis of the germ axes. The present results are consistent with the view that yieldin participates in the regulation of cell wall yielding during elongation growth.     

The pH-dependent yield threshold of the cell wall in a glycerinated hollow cylinder (in vitro system) of cowpea hypocotyl

In order to determine whether the pH-dependent yield threshold of the cell wall still exists in an in vitro system, an extensometer was devised to enable the perfusion of any experimental solution through the hollow cylinder of a hypocotyl segment excised from a cowpea seedling. Stress-strain experiments on glycerinated hollow cylinders revealed the existence of a definite yield threshold (y) of the cell wall in this in vitro system. The y value decreased reversibly with acidification (pH 4) to the same extent as the decrease of the yield threshold obtained in vivo (Y) with auxin-induced growth acceleration of hypocotyl segments. Heat treatment of the glycerinated hollow cylinder completely inhibited the decrease in y with acidification. The increase in the extensibility of the cell wall with acidification was inhibited significantly but not completely by heat treatment. These results support strongly the ‘acid growth’ theory and provide evidence that the acid-induced decrement of the yield threshold is mediated by an enzymatic reaction of a wall-binding protein. The combination of in vitro and in vivo studies presented here provides a basis for the establishment of a molecular theory on the nature of the growth parameters Y and Ф which control the yielding of the cell wall.     

The isolation of wall-bound proteins regulating yield threshold tension in glycerinated hollow cylinders of cowpea hypocotyl

To isolate and purify the factor regulating the yield threshold tension (y) through acidification of the cell wall, proteins were extracted from hypocotyls of Vigna unguiculata L. Their effects on the pH‐dependencies of the wall extensibility (φ) and y were examined with reconstitution experiments by incorporating them into the heat‐denatured glycerinated hollow cylinders (GHCs). The wall mechanical properties of the reconstituted GHCs were determined using stress–strain experiments. Only the proteins extracted with 1 kmol m^–3 of NaCl from the wall of elongation region restored the pH‐dependencies of φ and y once extinguished with heat‐denaturation, but proteins extracted from the other cell constituents or from the mature region of hypocotyl affect neither properties. Fractionation of the wall‐bound proteins by a hydrophobic column chromatography showed that the two different fractions affected φ or y independently. The sodium dodecyl sulphate‐polyacrylamide gel electrophoresis showed that the active fraction which restored the pH‐dependency of y still consists of two proteins of 30 and 32 kDa after purification by the sequential fractionation with cation‐exchange and gel filtration. These two proteins were named as ‘yieldin 30’ and ‘32’. Western blotting analysis using the rabbit‐antiserum against the cucumber expansin indicated that the yieldins are independent of cucumber expansin.     

Characterization of peptidoglycan hydrolase in Cag pathogenicity island of Helicobacter pylori

The Cag Type IV secretion apparatus proteins in Helicobacter pylori can mediate the injection of effector CagA protein into eukaryotic target cells. Although this apparatus forms an important pathway for bacterium–host interaction, its assembly process in vivo is poorly understood, and the proteins which contribute to break the bacterial cell walls in Cag-PAI have not yet been identified. The cagγ gene in Cag-PAI is a unique member that contains a conserved SLT catalysis domain, which makes it an attracting question whether cagy gene has the capacity to digest the bacterial cell wall. In the current study, therefore, the cagγ gene was cloned from the H. pylori NCTC 11637 and expressed in Escherichia coli, and its lytic effect on cell walls in vitro was observed. Results indicated that Cagγ protein has a lytic activity against bacterial cell walls. An allelic-exchange mutant (Δcagγ) was further constructed to investigate the relationship between Cagγ and effector CagA translocation. These results suggested that Cagγ contributed to the assembly of Cag Type IV secretion apparatus by digesting the peptidoglycan meshwork of bacterial cell walls.     

Oxidative coupling of a feruloyl-arabinoxylan trisaccharide (FAXX) in the walls of living maize cells requires endogenous hydrogen peroxide and is controlled by a low-M<Subscript>r</Subscript> apoplastic inhibitor

Feruloyl-polysaccharides can be oxidatively coupled in isolated cell walls by peroxidase plus exogenous H₂O₂ in vitro, but the extent to which similar reactions may occur in the apoplast in vivo was unclear. Numerous cellular factors potentially control feruloyl coupling in vivo, and their net controlling influence is not readily studied in vitro. Therefore, we have monitored apoplastic feruloyl coupling in cultured maize cells in vivo using a radiolabelled model substrate, 5-O-feruloyl-α-L-arabinofuranosyl-(1→3)-β-D-xylopyranosyl-(1→4)-D-xylose (FAXX). FAXX was expected to permeate the wall and to undergo reactions analogous to those normally exhibited by apoplastic feruloyl-polysaccharides in vivo. Little difference was found between the fates of [feruloyl−¹⁴C]FAXX and [pentosyl−³H]FAXX, indicating negligible apoplastic hydrolase or transferase activities. Very little radioactivity entered the protoplasm. Maize cells that had recently been washed in fresh medium were able to bind most of the FAXX (90%) in their cell walls, regardless of the age of the culture. During wall-binding, the [¹⁴C]feruloyl groups were converted to [¹⁴C]dehydrodiferulates and larger coupling products, as revealed by TLC after alkaline hydrolysis. As expected for an oxidative reaction, wall-binding was delayed by added anti-oxidants (ascorbate, ferulate, sinapate, chlorogenate or rutin). It was also completely inhibited by iodide, an H₂O₂-scavenger, indicating a role for peroxidase rather than oxidase. The observations indicate that oxidative coupling of feruloyl groups occurred within the cell wall, dependent on endogenous apoplastic H₂O₂ and wall-localised peroxidase, in vivo. Cells that had not recently been washed in fresh medium were much less able to bind FAXX, indicating the presence in the apoplast of an endogenous inhibitor of oxidative coupling. This inhibitor was of low M_r, was destroyed by heating, and remained in the aqueous phase (pH ≈3.5) when shaken with ethyl acetate. Its effectiveness was not altered by ascorbate oxidase. It is thus a small, heat-labile, hydrophilic inhibitor (not ascorbate) which we suggest plays a natural role in the control of wall cross-linking, and thus potentially in the control of cell growth.     

A (1→3,6)-β-d-galactosyl epitope containing uronic acids associated with bioactive pectins occurs in discrete cell wall domains in hypocotyl and root tissues of flax seedlings

We have used a well-characterized antibody specific for an epitope consisting of (1→3,6)-β-d-galactosyl residues with terminal glucuronic or 4-O-methylglucuronic acids of a bioactive pectin and immunocytochemistry to investigate its secretion and wall distribution in the hypocotyl and root tissues of flax seedlings. Our results show that this antigenic epitope is associated with flax pectins and is expressed by all the cells of the hypocotyl and root tissues. In the hypocotyl, it is abundant in the primary wall of epidermal cells as well as in the secondary wall of fiber cells, and is relatively less abundant in parenchyma cell walls. In contrast, the epitope is not detected in the middle lamellae and cell junction regions. In the root tip cells, immunogold electron microscopy shows that the cell walls of peripheral, columella, meristematic, cortical, and epidermal cells contain significant amounts of this epitope and that the distribution patterns are distinct. Together, these findings show that the antigenic epitope occurs in discrete domains of the wall implying a strict spatial regulation of the epitope-containing molecules. The results also show that, in root cells, the epitope is present within Golgi cisternae and is predominantly assembled in the trans and the trans-Golgi network compartments. Accepted: 21 October 1999     

Changes in cell wall composition of deformedras1 − cells ofSchizosaccharomyces pombe

Disruption of theSchizosaccharomyces pombe ras1 gene results in a morphological transformation to large spheres, in contrast to wild-type cells which grow as rods. Chemical analysis of isolated cell walls showed no significant changes in saccharide content but an increase in protein and phosphate contents inras1 ⁻ walls relative to parent walls. Polymers tightly bound to the cell wall were solubilized by SDS treatment. Several compounds with molar mass ranging from 22 to 130 kDa and more were resolved by gel filtration and SDS-PAGE. Among low-molar-mass species, a component moving as a band at 31 kDa was conspicuous inras1 ⁻ cell walls. It was solubilized by heating in Tris-HCl buffer and shown to have a β-1,3-glucanase activity against laminarin. The level of the enzyme was by 30% higher in theras1 ⁻ cell wall than in the wild-type cell wall. This enzyme may participate in the remodelling of the rigid glucan network and account (at least partially) for the aberrant cell shape. Theras1 ⁻ cell wall contained a high level of charged polymers, especially phosphoproteins, raising the appealing possibility thatras1 ⁻ is involved in a putative kinase cascade required to sense and respond to external stimuli destined for the cell wall. Although the present study shows thatras1 loss of function and altered cell wall composition are closely linked defects, it has still to be shown that theras1 protein is directly involved in alterations found in the mutant cell walls.     

The role of wall calcium in the extension of cell walls of soybean hypocotyls

Calcium crosslinks are load-bearing bonds in soybean (Glycine max (L.) Merr.) hypocotyl cell walls, but they are not the same load-bearing bonds that are broken during acid-mediated cell elongation. This conclusion is reached by studying the relationship between wall calcium, pH and the facilitated creep of frozenthawed soybean hypocotyl sections. Supporting data include the following observations: 1) 2-[(2-bis-[carboxy-methyl]amino-5-methylphenoxy)methyl]-6-methoxy-8-bis[carboxymethyl]aminoquinoline (Quin 2) and ethylene glycol-bis(2-aminoethyl ether)-N,N,N′,N′-tetraacetic acid (EGTA) caused only limited facilitated creep as compared with acid, despite removal of comparable or larger amounts of wall calcium; 2) the pH-response curves for calcium removal and acid-facilitated creep were different; 3) reversible acid-extension occurred even after removal of almost all wall calcium with Quin 2; and 4) growth of abraded sections did not involve a proportional loss of wall calcium. Removal of wall calcium, however, increased the capacity of the walls to undergo acid-facilitated creep. These data indicate that breakage of calcium crosslinks is not a major mechanism of cell-wall loosening in soybean hypocotyl tissues. This research was supported by Department of Energy grant DE-FG06-88ER13830 and NASA grant NAGW 1394. The authors are grateful to Dr. David Rayle (San Diego State University, Cal.) for stimulating discussions and comments during the course of this work.     

Modulation of interleukin-12 synthesis by DNA lacking the CpG motif and present in a mycobacterial cell wall complex

 A mycobacterial cell wall complex prepared from the non-pathogenic microorganism Mycobacterium phlei, where mycobacterial DNA is preserved and complexed to cell wall fragments, possesses anticancer and immunomodulatory activity. DNA from a number of prokaryotes has been found to modulate the immune system and to induce cytokine synthesis. We have therefore determined whether the DNA associated with this complex has the ability to induce the synthesis of interleukin-12 (IL-12), a potent anticancer cytokine. Mycobacterial DNA complexed with cell wall fragments or DNA purified from M. phlei induced IL-12 synthesis by murine and human monocytes and macrophages in vitro, and was capable of inducing IL-12 synthesis in vivo in mice following i.p. administration. Neutralization of DNA with cationic liposomes or digestion with DNase I significantly decreased the ability of the cell wall complex to induce IL-12. CpG methylation of DNA extracted from these cell walls or from M. phlei did not affect the induction of IL-12 synthesis by monocytes and macrophages. In contrast, CpG methylation of DNA from Escherichia coli abolished its ability to induce IL-12 synthesis. These results demonstrate that unmethylated CpG motifs present in M. phlei DNA are not a prerequisite for the induction of IL-12 synthesis. The size of the mycobacterial DNA, in the range of 5 bp to genomic DNA, did not influence its capacity to induce IL-12. Our results emphasize that M. phlei DNA associated with the cell wall complex makes a significant contribution to the overall immunomodulatory and anticancer activity of this mycobacterial cell wall preparation and that these activities are not correlated with the presence of CpG motifs. Received: 23 November 1999 / Accepted: 28 March 2000     

Turgor and Longitudinal Tissue 12Helianthus annuus

The quantitative relationship between turgor and the pressureexerted by the inner tissues (cortex, vascular tissue, and pith)on the peripheral cell walls (longitudinal tissue pressure)was investigated in hypocotyls of sunflower seedlings (Helianthusannuus L.) In etiolated hypocotyls cell turgor pressures, asmeasured with the pressure probe, were in the range 0·38to 0·55 MPa with an average of 0·48 MPa. In irradiatedhypocotyls turgor pressures varied from 0·40 to 0·57MPa with a, mean at 0·49 MPa. The pressure exerted bythe inner tissues on the outer walls was estimated by incubatingpeeled sections in a series of osmotic test solutions (polyethyleneglycol 8000). The length change was measured with a transducer.In both etiolated and irradiated hypocotyls an external osmoticpressure of 0·5 MPa was required to inhibit elongationof the inner tissues, i.e. the average cell turgor and the longitudinaltissue pressure are very similar quantities. The results indicatethat the turgor of the inner tissues is displaced to and borneby the thick, growth-limiting peripheral cell walls of the hypocotyl. Key words: Helianthus annuus, hypocotyl growth, tissue pressure, turgor pressure, wall stress     

Change in XET activities,cell wall extensibility and hypocotyl elongation of soybean seedlings at low water potential

In dark-grown soybean (Glycine max [L.] Merr.) seedlings, exposing the roots to water-deficient vermiculite (_w=–0.36 MPa) inhibited hypocotyl (stem) elongation. The inhibition was associated with decreased extensibility of the cell walls in the elongation zone. A detailed spatial analysis showed xyloglucan endotransglucosylase (XET; EC 2.4.1.207) activity on the basis of unit cell wall dry weight was decreased in the elongation region after seedlings were transplanted to low _w. The decrease in XET activity was at least partially due to an accumulation of cell wall mass. Since cell number was only slightly altered, wall mass had increased per cell and probably led to increased wall thickness and decreased cell wall extensibility. Alternatively, an increase in cell wall mass may represent a mechanism for regulating enzyme activity in cell walls, XET in this case, and therefore cell wall extensibility. Hypocotyl elongation was partially recovered after seedlings were grown in low-_w vermiculate for about 80 h. The partial recovery of hypocotyl elongation was associated with a partial recovery of cell wall extensibility and an enhancement of XET activity in the hypocotyl elongation zone. Our results indicate XTH proteins may play an important role in regulating cell wall extensibility and thus cell elongation in soybean hypocotyls. Our results also showed an imperfect correlation of spatial elongation and XET activity along the hypocotyls. Other potential functions of XTH and their regulation in soybean hypocotyl growth are discussed.     

Comparative investigation of primary and tertiary endodermal cell walls isolated from the roots of five monocotyledoneous species: chemical composition in relation to fine structure

The chemical composition of isolated endodermal cell walls from the roots of the five monocotyledoneous species Monstera deliciosa Liebm., Iris germanica L., Allium cepa L., Aspidistra elatior Bl. and Agapanthus africanus (L.) Hoffmgg. was determined. Endodermal cell walls isolated from aerial roots of M. deliciosa were in their primary developmental state (Casparian bands). They contained large amounts of lignin (6.5% w/w) and only traces of suberin (0.5% w/w). Endodermal cell walls isolated from the other four species were in their tertiary developmental state. Lignin was still the more abundant cell wall polymer with amounts ranging from 3.8% (w/w, A. cepa) to 4.5% (w/w, I. germanica). However, compared to endodermal cell walls in their primary state of development (Casparian bands), tertiary endodermal cell walls contained significantly higher amounts of suberin, ranging from 1.8% (w/w, I. germanica) to 3.0% (w/w, A. africanus). Thus, chemical characterization of endodermal cell walls from five different species revealed that lignin was the dominant cell wall polymer in the Casparian band of M. deliciosa, whereas tertiary endodermal cell walls contained, in addition to lignin, increasing amounts of suberin (I. germanica, A. cepa, A. elatior and A. africanus). Besides the two biopolymers lignin and suberin, cell wall carbohydrates in the range of between 40 and 60% were also quantified. The sum of all cell wall compounds investigated by gas chromatography resulted in a recovery of 50–80% of the dry weight of the isolated cell wall material. Quantitative chromatographic results in combination with microscopic studies are consistent with the existence of a distinct suberin lamella and lignified tertiary wall deposits. From these data it can be concluded that the barrier properties of the endodermis towards the apoplastic transport of ions and water will increase from primary to tertiary endodermal cell walls due to their increasing amounts of suberin. Received: 23 August 1997 / Accepted: 28 January 1998     

Fracture mechanics of the cell wall of Chara corallina

Previous mechanical studies using algae have concentrated on cell extension and growth using creep-type experiments, but there appears to be no published study of their failure properties. The mechanical strength of single large internode cell walls (up to 2 mm diameter and 100 mm in length) of the charophyte (giant alga) Chara corallina was determined by dissecting cells to give sheets of cell wall, which were then notched and fractured under tension. Tensile tests, using a range of notch sizes, were conducted on cell walls of varying age and maturity to establish their notch sensitivity and to investigate the propagation of cracks in plant cell walls. The thickness and stiffness of the walls increased with age whereas their strength was little affected. The strength of unnotched walls was estimated as 47 ± 13 MPa, comparable to that of some grasses but an order of magnitude higher than that published for model bacterial cellulose composite walls. The strength was notch-sensitive and the critical stress intensity factor K _1c was estimated to be 0.63 ± 0.19 MNm^−3/2, comparable to published values for grasses. Received: 4 April 2000 / Accepted: 21 July 2000     

Domain-specific mechanosensory transmission of osmotic and enzymatic cell wall disturbances to the actin cytoskeleton

Summary. Plant protoplasts are embedded within surrounding cell walls and the cell wall–plasma membrane–cytoskeleton (WMC) structural continuum seems to be crucial for the proper functioning of plant cells. We have utilised the protoplast preparation methodology to study the organisation and the putative components of the WMC continuum. Application of an osmotic agent evoked plasmolysis of the Zea mays root apex cells which appeared to be cell type- and growth stage-specific. Simultaneous use of wall polysaccharide-digesting enzymes selectively severed linkages between the components of the WMC continuum which changed the plasmolytic patterns in various cell types. This was followed by a reorganisation of filamentous actin aimed to reinforce protoplast boundaries and maintain the functioning of intercellular contact sites, especially at the cross walls. Particularly strong effects were evoked by pectin-degrading enzymes. Such treatments demonstrated directly the differentiated composition of various wall domains surrounding individual cells with the pectin-enriched cross walls (synapses), and the cellulose-hemicellulose network dominating the side walls. The same wall-degrading enzymes were used for in vitro digestion of isolated Lupinus albus cell walls followed by the extraction of wall proteins. Selective release of proteins suggested the importance of wall polysaccharide–protein interactions in the maintenance of the functioning and mechanical stability of root cell walls. Correspondence and reprints: Department of Molecular and Cellular Biology, Adam Mickiewicz University, Międzychodzka 5, 60-371 Poznań, Poland.     

Development of Epidermal Cell Wall Peroxidases along the Mung Bean Hypocotyl: Possible Involvement in the Cell Wall Stiffening Process

Goldberg, R., Liberman, M., Mathieu, C, Pierron, M. and Catesson,A. M. 1987. Development of epidermal cell wall peroxidases alongthe mung bean hypocotyl: possible involvement in the cell wallstiffening process.—J. exp. Bot. 38: 1378–1390. Ultrastructural investigation showed that in the epidermis ofmung bean hypocotyls, cell wall peroxidatic activities couldbe detected mainly below the maximal elongation zone. In theepidermis the peroxidatic activities were preferentially locatedin the radial cell walls. Cell wall peroxidases were then isolatedfrom epidermal strips and further characterized. The possiblepresence of a H₂O₂-generating system in the epidermis of mungbean hypocotyls was also investigated. When whole segments wereprocessed for electron microscopy, H₂O₂ could be detected cytochemicallyin the cell walls with the CeCl₃ technique. A positive reactionwas obtained in the same location when specimens were incubatedin a 3-3'-diaminobenzidine medium for peroxidases in which H₂O₂was replaced by its possible precursors (NADH or NAD + malate).However, isolated epidermal cell walls could not generate H₂O₂at the expense of NADH although they were able to oxidize thereduced nicotinamide-adenine-dinucleotide. The possible relationshipsbetween peroxidase activities, H₂O₂, and Ca²⁺ ions are discussedwith respect to their involvement in the cell wall stiffeningprocess. Key words: Epidermis, cell wall, elongation, peroxidases     

Specific Labeling of Cell Wall Polysaccharides with myo-[2-3H] Inositol during Germination and Growth of Phaseolus vulgaris L.

myo-[2-³H]Inositol was fed to bean seeds by imbibition and itsmetabolic fate was studied during germination and seedling growth.The largest amount of myo-inositol was taken up from a 500 HIMsupply (8 mg/seed) and the highest percentage was from 1 HIM(29%). myo-Inositol was incorporated to new cell wall polysaccharidesof hypocotyl and roots, mostly as uronic acid and pentose residues.In the 80% ethanolinsoluble cell walls of hypocotyls at 3, 4and 5 days after imbibition, 47 to 52% of ³H was detected asuronic acids, 20 to 24% as arabinose and 11 to 19% as xylose.Glucogenesis from myo-inositol was low: less than 6% was recoveredas hexoses. The ³H in uronic acid and arabinose residues decreasedwith increasing age (i.e. 0 to 6 cm from cotyledons) and increasedin older segments (further than 6 cm from cotyledons). In theoldest segment of 5-day-old hypocotyl (> 10 cm), ³H in thesugar residues was more than that in the youngest part (0–2cm). On the other hand, ³H in xylose residues increased steadilyin the older part, but did not exceed that in arabinose. The results show that the myo-inositol oxidation pathway functionsin growing hypocotyls and roots of bean seedlings to provideexclusively uronic acid and pentose units for cell wall synthesis.Results also show that incorporation of arabinose and uronicacids derived from myo-[2-³H]inositol to cell wall polysaccharidesis active in two regions of the hypocotyl; first, for the constructionof the primary walls in the young, growing region of the hypocotyl,and second, for thickening of the walls after completion ofelongation growth. ¹Supported by NSERC of Canada. (Received April 10, 1984; Accepted June 12, 1984)     

Implications of the galactosidase activity of yieldin in the regulatory mechanism of yield threshold that is fundamental to cell wall extension

To understand the action mechanism of yieldin (YLD) on the regulation of the yield threshold (Y ), one of the critical parameters of cell wall extension, YLD was extracted from the cell walls of cowpea (Vigna unguiculata L.) hypocotyls and the hemagglutinin activity (HA) as well as the glycosidase activity of the protein was measured. Sedimentation assays using trypsinated rabbit erythrocytes showed that YLD possessed HA at pH 7. The digestion assays using 4‐nitrophenyl (p NP) glycopyranosides as artificial substrates showed that YLD liberated galactose residues from p NP alpha‐d ‐galactopyranoside mainly at pH 4.0, i.e. the pH level where Y was decreased at most. These results show that YLD is a bifunctional protein that switches between the HA and the galactosidase activities depending on the surrounding pH. Since D‐galactose at concentration of 0.03 g l^?1 perfectly inhibited the HA, YLD was suggested to associate with galactose residues. However, the galactose application ten times concentrated was necessary to inhibit both the galactosidase activity of YLD and the acid‐induced shift of Y regulated by YLD. In addition, the specific inhibitor of alpha‐d ‐galactosidase (deoxygalactonojirimycin) inhibited both the galactosidase activity of YLD and the shift of Y at the same concentration, but not the HA. On the basis of these results, it is suggested the galactosidase activity of YLD plays a central role in the mechanism of Y ‐regulation at acidic pH.     

Apparent and true digestibility of the Weende components, cell content and cell wall of ryegrass

For 14 samples of ventilated hay and 6 of fresh grass, digestibility was determined with young wethers. The contribution to the digestibility was calculated for the Weende components (crude protein, crude fibre, nitrogen-free extractives + fat) and for cell content and cell wall (Van Soest) components. For the cell content and crude protein, a hyperbolic relationship, y' = a − b/x between the digestibility coefficient of the components (y') and the percentage component in dry matter (DM, x) can be accepted with a sufficient degree of certainty. This means that the apparent digestibility of the cell content and crude protein increase with increasing concentration. The latter formula can be transformed into a linear regression y = ax − b where y = percentage digestible component in DM, a being an estimate of the true digestibility of the component and b an estimate of the endogenous excretion of the component. From these relationships some important practical conclusions can be drawn. For other dry matter components, such as crude fibre, nitrogen free extractives + fat and cell wall, the relationship between the digestibility coefficient and the percentage of the component in DM cannot be expressed by a simple regression line. The advantages of splitting the digestibility into a contribution from cell content and cell wall components versus splitting into contributions from the Weende components are discussed.By determination of the cell wall and its digestibility in vitro, a very good prediction for the dry matter digestibility of grass can be given, which is of great importance for plant breeding and grassland research.     

Barrier function of the cell wall during uptake of nickel ions

Cell walls were isolated from roots of six plant species to study their ion-exchange capacity for nickel ions (S _Ni) at Ni²⁺ concentration of 10⁻³ M. The S _Ni values varied depending on the plant species from 50 to 150 μmol Ni²⁺ per gram dry wt; the sorption capacity increased in a row: Poaceae < Chenopodiaceae < Fabaceae. At pH 5 the sorption capacity of cell walls for nickel ions was determined by the presence of carboxyl groups of polygalacturonic acid in the polymeric cell-wall matrix. In all cases the ion-exchange capacity of cell walls was higher at pH 8 than at pH 5, indicating that Ni²⁺ binds also to a carboxyl group different from that of polygalacturonic acid. Irrespective of plant species, the presence of EDTA in the solution diminished drastically the absorption capacity of cell walls for Ni²⁺. It is concluded that the presence of 10⁻³ M EDTA weakens the defense properties of cell walls. The sequestration of Ni²⁺ in the cell wall can be considered as an effective means of plant cell defense against elevated concentrations of nickel ions in the external medium.