The Mechanical Properties of Xylem Tissue from Tobacco Plants (Nicotiana tabacum'Samsun')

Tobacco plants (Nicotiana tabacum‘Samsun’) havebeen grown with an antisense CAD (cinnamyl alcohol dehydrogenase)gene. This modifies lignin production, resulting in lignin witha greater aldehyde content which is easier to extract chemically.This lignin probably has a reduced crosslink density. The changedproperties of the lignin affect the longitudinal tensile modulusof the xylem tissue (wood), reducing it by one third, from 2.8GPa to 1.9 GPa. Tobacco xylem tissue cell walls are more sensitiveto changes in the properties of the matrix than can be predictedusing current cell wall mechanical models.Copyright 1998 Annalsof Botany Company Tobacco,Nicotiana tabacum, xylem tissue, Young's modulus, matrix polymer connectivity, plant biomechanics.     

Alterations in the Biosynthesis of Lignin in Transgenic Plants with Chimeric Genes for 4-Coumarate: Coenzyme A Ligase

The introduction of chimeric sense and antisense gene constructsfor 4-coumarate:coenzyme A ligase into tobacco plants causedthe reduction of the 4CL activity in the transgenic plants.In the transgenic plants, the cell walls of the xylem tissuein stems were brown and the molecular structure of lignin inthe colored cell walls was dramatically different from thatin the control plants. Analysis with different types of stainrevealed that levels of cinnamyl aldehyde residues and syringylunits in lignin were depressed in the brownish cell walls. Furthermore,the lignin content in colored tissue was lower than that inthe normal tissue. Our results indicate that 4CL has importantroles in the determination of the composition and the amountof lignin in tobacco plants. (Received December 27, 1995; Accepted July 23, 1996)     

Intra- and Extracellular Localization of Lignin Peroxidase during the Degradation of Solid Wood and Wood Fragments by Phanerochaete chrysosporium by Using Transmission Electron Microscopy and Immuno-Gold Labeling

The distribution of lignin peroxidase during degradation of both wood and woody fragments by the white rot fungus Phanerochaete chrysosporium was investigated by using anti-lignin peroxidase in conjunction with postembedding transmission electron microscopy and immuno-gold labeling techniques. The enzyme was localized in the peripheral regions of the fungal cell cytoplasm in association with the cell membrane, fungal cell wall, and extracellular slime materials. In solid wood, lignin peroxidase was detected in low concentrations associated with both superficial and degradation zones within secondary cell walls undergoing fungal attack. A similar but much greater level of extracellular peroxidase activity was associated with wood fragments degraded by the fungus grown under liquid culture conditions optimal for production of the enzyme. Efforts to infiltrate degraded wood pieces with high levels of lignin peroxidase showed the enzyme to be restricted to superficial regions of wood decay and to penetrate wood cell walls only where the wall structure had been modified. In this respect the enzyme was able to penetrate characteristic zones of degradation within the secondary walls of fibers to sites of lignin attack. This suggests a possibility for a close substrate-enzyme association during wood cell wall degradation.     

Structural Characterization of Modified Lignin in Transgenic Tobacco Plants in Which the Activity of 4-Coumarate:Coenzyme A Ligase Is Depressed

Transgenic tobacco (Nicotiana tabacum L.) plants in which the activity of 4-coumarate:coenzyme A ligase is very low contain a novel lignin in their xylem. Details of changes in hydroxycinnamic acids bound to cell walls and in the structure of the novel lignin were identified by base hydrolysis, alkaline nitrobenzene oxidation, pyrolysis-gas chromatography, and 13C-nuclear magnetic resonance analysis. In the brownish tissue of the transgenic plants, the levels of three hydroxycinnamic acids, p-coumaric, ferulic, and sinapic, which were bound to cell walls, were apparently increased as a result of down-regulation of the expression of the gene for 4-coumarate:coenzyme A ligase. Some of these hydroxycinnamic acids were linked to cell walls via ester and ether linkages. The accumulation of hydroxycinnamic acids also induced an increase in the level of condensed units in the novel lignin of the brownish tissue. Our data indicate that the behavior of some of the incorporated hydroxycinnamic acids resembles lignin monomers in the brownish tissue, and their accumulation results in dramatic changes in the biosynthesis of lignin in transgenic plants.     

Lignification in relation to the biennial growth habit in brassicas

The forage brassicas are a useful model system for the study of wood formation because the thickened cell walls of their vascular tissue can vary widely in lignin content. Solid-state 13C NMR spectroscopy was used to quantify lignin, and determine features of its structure, in the vascular cell walls of forage rape (Brassica napus L.), and Thousandhead and marrowstem cultivars of kale (Brassica oleracea L. var. acephala). During the first season of vegetative growth, lignin levels in these cell walls remained low in the upper part of the stems despite the physical resemblance of this tissue to wood. The extended flowering stems produced in the following year were thinner and their vascular tissue contained much more strongly lignified cell walls. The structure of the lignin was typical of angiosperm wood. It showed only small variations in syringyl/guaiacyl ratio, but this ratio increased with lignin content and thus with the proportion of the lignin that was associated with secondary cell-wall layers.     

Amino-Acid Composition of Wood Proteins

The ammo-acid compositions of proteins present in the sapwoodand heartwood from three species of Eucalyptus, in normal, reaction,and opposite wood from Pinus radiata seedlings, and in reactionand opposite wood from Eucalyptus goniocalyx, are listed. Thequestion of the location of protein within wood cell walls isdiscussed. The amounts of tyrosine found in the woods from P.radiata seedlings are incompatible both with the belief thatit determines the intensity of staining of reaction wood cellwalls with Millon's reagent, and that it is the source of thep-hydroxybenzaldehyde obtained when milled wood lignin is oxidizedwith nitrobenzene. Loss of amino acids in the presence of milledwood lignin, -cellulose, hemicellulose, glucose, mannose, galactose,xylose, arabinose, glucuronic, and galacturonic acids is demonstrated.The presence of numerous peaks which are not indentifiable withknown amino acids in the chromatograms of wood hydrolysatesis believed to be due largely to the products of interactionof amino acids and sugars. Some retention of nitrogen by theresidues of wood hydrolysates occurs. The significance of theresults obtained with the wood samples listed above is discussedagainst this background.     

In situ analysis of lignins in transgenic tobacco reveals a differential impact of individual transformations on the spatial patterns of lignin deposition at the cellular and subcellular levels

Using tobacco transgenic lines altered in the monolignol biosynthetic pathway and which differ in their lignin profiles we have evaluated lignin deposition at the cellular and subcellular levels using several microanalytical techniques. Surprisingly, whereas a Cinnamoyl CoA reductase (CCR) down-regulated line with a strong decrease in lignin content exhibited an overall reduction in lignin deposition in the walls of the different xylem cell types, this reduction was selectively targeted to the fibers in a double transformant (down-regulated for both CCR and Cinnamyl alcohol dehydrogenase (CAD)) displaying a similar degree of global lignin content decrease. Fiber and vessel secondary walls of the transgenic tobacco line homozygous for the ccr antisense gene (CCR.H) down-regulated plants were dramatically destructured, particularly in the S2 sublayer, whereas the deposition of lignins in the S1 sublayer was not significantly modified. In contrast, cell wall organization was slightly altered in xylem cells of the double transformant. The relative distribution of non-condensed and condensed units in lignin, evaluated microscopically with specific antibodies, was differentially affected in the transgenics studied and, in a general way, a drop in non-condensed lignin units (beta- 0-4 interunit linkages) was associated with a loss of cohesion and extensive disorganization of the secondary wall. These results demonstrate that lignification is tightly and independently regulated in individual cell types and cell wall sublayers. They also show that down-regulation of specific genes may induce targeted changes in lignin structure and in spatial deposition patterns of the polymer.     

Modelling the Mechanical Properties of Xylem Tissue from Tobacco Plants (Nicotiana tabacum'Samsun') by Considering the Importance of Molecular and Micromechanisms

Tobacco plants (Nicotiana tabacum‘Samsun’) havexylem cell walls which are more sensitive to changes in theproperties of the matrix than was predicted using current cellwall mechanical models. A model is proposed which can accountfor the importance of the cell wall matrix in determining themodulus of the material. This model is based on a helical springsystem, with micromechanisms operating at the molecular scale.Xylem tissue fibre cells can behave as helical springs whenthe crosslink density of lignin is low, or they can behave likea composite sheet with fibres at an angle to the applied loadwhen the crosslink density is high. This highlights the importanceof molecular modelling when the properties of complex biologicalmaterials are being investigated.Copyright 1998 Annals of BotanyCompany Tobacco,Nicotiana tabacum, xylem tissue, Young's modulus, matrix polymer connectivity, plant biomechanics.     

Improved Paper Pulp from Plants with Suppressed Cinnamoyl-CoA Reductase or Cinnamyl Alcohol Dehydrogenase

Transgenic plants severely suppressed in the activity of cinnamoyl-CoA reductase were produced by introduction of a partial sense CCR transgene into tobacco. Five transgenic lines with CCR activities ranging from 2 to 48% of wild-type values were selected for further study. Some lines showed a range of aberrant phenotypes including reduced growth, and all had changes to lignin structure making the polymer more susceptible to alkali extraction. The most severely CCR-suppressed line also had significantly decreased lignin content and an increased proportion of free phenolic groups in non-condensed lignin. These changes are likely to make the lignin easier to extract during chemical pulping. Direct Kraft pulping trials confirmed this. More lignin could be removed from the transgenic wood than from wild-type wood at the same alkali charge. A similar improvement in pulping efficiency was recently shown for poplar trees expressing an antisense cinnamyl alcohol dehydrogenase gene. Pulping experiments performed here on CAD-antisense tobacco plants produced near-identical results – the modified lignin was more easily removed during pulping without any adverse effects on the quality of the pulp or paper produced. These results suggest that pulping experiments performed in tobacco can be predictive of the results that will be obtained in trees such as poplar, extending the utility of the tobacco model. On the basis of our results on CCR manipulation in tobacco, we predict that CCR-suppressed trees may show pulping benefits. However, it is likely that CCR-suppression will not be the optimal target for genetic manipulation of pulping character due to the potential associated growth defects.     

Cellulolytic Activities of Botryodiplodia theobromae Pat

Botryodiplodia theobromae Pat., a ‘stain’ fungusisolated from stained and decaying wood of Bombax buonopozenseP. Beauv., attacked isolated blocks of Bombax wood. In theseit perforated the cell walls and formed elongated cavities withpointed ends, usually in chains, in the secondary wall. Thefungus attacked cellulose in a similar manner to soft rot fungi.It used starch and other saccharides present in the wood ofBombax buonopozense as initial substrates before degrading thecellulose and hemicellulose components of the wood. It did notdegrade the lignin component of the wood. Cellulase, ß-glucosidase, and amylase activities weredetected in culture filtrates of Botryodiplodia theobromae.Cellulase synthesis was delayed when the wood powder in themedium contained starch and saccharides. Amylase, too, behavedlike an inducible enzyme and disappeared from the cultures onexhaustion of starch. Electron microscopy showed that cellulose extracted from thewood of Bombax buonopozense was fragmented into small particleson treatment with the culture filtrates of Botryodiplodia theobromae.Cellobiose, glucose, and another saccharide, probably cellotriose,were identified as the products of enzymatic hydrolysis of cellulose. These results are discussed in relation to the attack and degradationof the wood of Bombax buonopozense by Botryodiplodia theobromaein nature.     

Quantification of compression wood severity in tracheids of Pinus radiata D. Don using confocal fluorescence imaging and spectral deconvolution

Confocal fluorescence microscopy was used to examine the spectral characteristics of lignin autofluorescence in secondary cell walls of normal and compression wood from Pinus radiata. Using UV excitation, fluorescence spectra of normal and compression wood sections showed significant differences, especially in the outer secondary cell wall of tracheids, with a shift in maxima from violet to blue wavelengths between normal and compression wood. A comparison of normal wood, mild and severe compression wood, showed that the wavelength shift was intermediate in the mild compression wood compared to the severe compression wood, thus offering the possibility of quantifying the severity by measuring ratios of fluorescence at violet and blue wavelengths. Fluorescence induced by blue light, rather than UV, was less well differentiated amongst wood types. Spectral deconvolution indicated the presence of a minimum of five discrete lignin fluorophores in the cell walls of both normal and compression wood tracheids. Comparison with lignin model compounds suggest that the wavelength shift may correspond in part to increased levels of p-hydroxy type lignin in the compression wood samples. The combination of confocal fluorescence imaging and related spectral deconvolution therefore offers a novel technique for characterising cell wall lignin in situ.     

Reassessment of qualitative changes in lignification of transgenic tobacco plants and their impact on cell wall assembly.

In tobacco plants the effect of antisense down-regulation of various genes encoding enzymes of the monolignol biosynthetic pathway resulted in quantitative and qualitative changes in lignin distribution and in diverse alterations of the secondary wall assembly of modified tobacco plants. Total lignin content, composition in syringyl and guaiacyl units, and absolute proportions of condensed and non-condensed substructures occurring in the cell walls, were differentially modified according to the repressed gene. Immunocytochemical characterisation and visualisation of the distribution of condensed and non-condensed lignin substructure epitopes in transmission electron microscopy (TEM) revealed that some transformations entailed profound and specific alterations in the secondary wall biogenesis. Correlation between micro-morphological cell wall alterations and semi-quantitative immuno-analysis of the topochemical distribution of lignin sub-units suggests that the mode of polymerisation of monolignols into non-condensed units, favoured by the microfibril matrix of the secondary wall, plays an important part in the lignified cell wall assembly.     

Ultrastructure of the Cell Wall of Vessel Contact Cells in the Xylem of Tomato Stems

Mature contact cells in the metaxylem of tomato stems are characterizedby a distinctive wall deposit occurring mainly over the areaof the pit membrane that separates the cell from an adjoiningvessel member but also extending as a layer on the secondarywall round the interior of the cell. Cytochemical tests showthe layer contains a low concentration of pectin, no lignin,and a high concentration of polysaccharides. It can be distinguishedfrom the primary and secondary walls by quantitative differencesin the cytochemical staining reactions and by its loose-texturedappearance under the electron microscope. The layer is similarin composition and structure to the ‘protective layer’described in woody plants. The layer has also been found inpea and cotton and may be a characteristic feature of the contactcells in herbaceous plants. Cell wall ultrastructure, contact cells, Lycopersicon esculentum, Mill., protective layer, tomato, xylem     

The Hypostase in Torenia fournieri Lind.: A Histochemical Study of the Cell Walls

A histochemical investigation on the cell walls of the hypostasein Torenia fournieri Lind. (Scrophulariaceae) revealed thatthey contain large amounts of callose, cellulose and pectins.Except in the middle lamellae, tests failed to show lignin inthe walls. It is surmised that the callose in the hypostasedevelops in order to regulate the flow of metabolites to theembryo sac. Torenia fournieri Lind., hypostase, cell wall, callose     

Penetrability of White Rot-Degraded Pine Wood by the Lignin Peroxidase of Phanerochaete chrysosporium

The penetration of enzymes into wood cell walls during white rot decay is an open question. A postembedding immunoelectron microscopic technique was the method of choice to answer that question. Infiltration of pine wood specimens with a concentrated culture filtrate greatly improved the labeling density and, thereby, reproducibility. Characterization of the concentrated culture filtrate by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting (immunoblotting) revealed three closely spaced proteins of molecular weights about 42,000 showing immunoreactivity against anti-lignin peroxidase serum. It was shown by immunogold labeling that lignin peroxidase of Phanerochaete chrysosporium is located on the surface of the wood cell wall or within areas of heavy attack. It did not diffuse into undecayed parts of the cell wall. The reasons for preventing lignin peroxidase from penetrating wood cell walls during white rot decay are discussed.     

Lignin isolated from primary walls of hybrid aspen cell cultures indicates significant differences in lignin structure between primary and secondary cell wall.

Hybrid aspen (Populus tremula x tremuloides) cell cultures were grown for 7, 14 and 21 days. The cell cultures formed primary cell walls but no secondary cell wall according to carbohydrate analysis and microscopic characterization. The primary walls were lignified, increasingly with age, according to Klason lignin analysis. Presence of lignin in the primary walls, with a higher content in 21-day old cells than in 7-day old cells, was further supported by phloroglucinol/HCl reagent test and confocal microscopy after both immunolocalization and staining with acriflavin. Both laccase and peroxidase activity were found in the cultures and the activity increased during lignin formation. The lignin from the cell culture material was compared to lignin from mature aspen wood, where most of the lignin originates in the secondary cell wall, and which served as our secondary cell wall control. Lignin from the cell walls was isolated and characterized by thioacidolysis followed by gas chromatography and mass spectrometry. The lignin in the cell cultures differed from lignin of mature aspen wood in that it consisted exclusively of guaiacyl units, and had a more condensed structure. Five lignin structures were identified by mass spectrometry in the cell suspension cultures. The results indicate that the hybrid aspen cell culture used in this investigation may be a convenient experimental system for studies of primary cell wall lignin.     

Detection of Lignin Peroxidase and Xylanase by Immunocytochemical Labeling in Wood Decayed by Basidiomycetes

The white rot fungi used in this study caused two different forms of degradation. Phanerochaete chrysosporium, strain BKM-F-1767, and Phellinus pini caused a preferential removal of lignin from birch wood, whereas Trametes (Coriolus) versicolor caused a nonselective attack of all cell wall components. Use of polyclonal antisera to H8 lignin peroxidase and monoclonal antisera to H2 lignin peroxidase followed by immunogold labeling with protein A-gold or protein G-gold, respectively, showed lignin peroxidase extra-and intracellularly to fungal hyphae and within the delignified cell walls after 12 weeks of laboratory decay. Lignin peroxidase was localized at sites within the cell wall where electron-dense areas of the lignified cell wall layers remained. In wood decayed by Trametes versicolor, lignin peroxidase was located primarily along the surface of eroded cell walls. No lignin peroxidase was evident in brown-rotted wood, but slight labeling occurred within hyphal cells. Use of polyclonal antisera to xylanase followed by immunogold labeling showed intense labeling on fungal hyphae and surrounding slime layers and within the woody cell wall, where evidence of degradation was apparent. Colloidal-gold-labeled xylanase was prevalent in wood decayed by all fungi used in this study. Areas of the wood with early stages of cell wall decay had the greatest concentration of gold particles, while little labeling occurred in cells in advanced stages of decay by brown or white rot fungi.     

Identifying the characteristic secondary ions of lignin polymer using ToF-SIMS

The chemical structure of lignin, a complex, irregular polymer of phenylpropane units that occurs in plant cell walls, was investigated using time-of-flight secondary ion mass spectrometry (ToF-SIMS). The positive ToF-SIMS spectra of lignin isolated from pine and beech wood showed prominent secondary ions possessing guaiacyl (at m/z 137 and 151) or syringyl (at m/z 167 and 181) rings, which are the basic building units of lignin polymer. This shows that ToF-SIMS is a useful tool for lignin structural analysis. The peaks at m/z 137 and 167 were assigned as the C6-C1 ion, and the peaks at m/z 151 and 181 may be double-component, the C6-C1 ion and the C6-C2 ion. We confirmed the characteristic guaiacyl ions using a synthetic lignin model compound, dehydrogenation polymer (DHP), which was formed by polymerizing of unlabeled and deuterium-labeled coniferyl alcohols. The formation mechanism of the main secondary ions was deduced by labeling specific positions of coniferyl alcohols with a stable isotope to study the relationship between chemical structure and secondary ion formation in ToF-SIMS.