HIV-1 Gag targeting to the plasma membrane reorganizes sphingomyelin-rich and cholesterol-rich lipid domains

Although the human immunodeficiency virus type 1 lipid envelope has been reported to be enriched with host cell sphingomyelin and cholesterol, the molecular mechanism of the enrichment is not well understood. Viral Gag protein plays a central role in virus budding. Here, we report the interaction between Gag and host cell lipids using different quantitative and super-resolution microscopy techniques in combination with specific probes that bind endogenous sphingomyelin and cholesterol. Our results indicate that Gag in the inner leaflet of the plasma membrane colocalizes with the outer leaflet sphingomyelin-rich domains and cholesterol-rich domains, enlarges sphingomyelin-rich domains, and strongly restricts the mobility of sphingomyelin-rich domains. Moreover, Gag multimerization induces sphingomyelin-rich and cholesterol-rich lipid domains to be in close proximity in a curvature-dependent manner. Our study suggests that Gag binds, coalesces, and reorganizes pre-existing lipid domains during assembly.

Cell density-dependent membrane distribution of ganglioside GM3 in melanoma cells

Monosialoganglioside GM3 is the simplest ganglioside involved in various cellular signaling. Cell surface distribution of GM3 is thought to be crucial for the function of GM3, but little is known about the cell surface GM3 distribution. It was shown that anti-GM3 monoclonal antibody binds to GM3 in sparse but not in confluent melanoma cells. Our model membrane study evidenced that monoclonal anti-GM3 antibodies showed stronger binding when GM3 was in less fluid membrane environment. Studies using fluorescent GM3 analogs suggested that GM3 was clustered in less fluid membrane. Moreover, fluorescent lifetime measurement showed that cell surface of high density melanoma cells is more fluid than that of low density cells. Lipidomics and fatty acid supplementation experiment suggested that monounsaturated fatty acid-containing phosphatidylcholine contributed to the cell density-dependent membrane fluidity. Our results indicate that anti-GM3 antibody senses GM3 clustering and the number and/or size of GM3 cluster differ between sparse and confluent melanoma cells.

Empagliflozin improved systolic blood pressure, endothelial dysfunction and heart remodeling in the metabolic syndrome ZSF1 rat

Background

Empagliflozin (empa), a selective sodium–glucose cotransporter (SGLT)2 inhibitor, reduced cardiovascular mortality and hospitalization for heart failure in patients with type 2 diabetes at high cardiovascular risk independent of glycemic control. The cardiovascular protective effect of empa was evaluated in an experimental model of metabolic syndrome, the obese ZSF1 rat, and its’ lean control.

Methods

Lean and obese ZSF1 rats were either non-treated or treated with empa (30 mg/kg/day) for 6 weeks. Vascular reactivity was assessed using mesenteric artery rings, systolic blood pressure by tail-cuff sphygmomanometry, heart function and structural changes by echocardiography, and protein expression levels by Western blot analysis.

Results

Empa treatment reduced blood glucose levels from 275 to 196 mg/dl in obese ZSF1 rats whereas normoglycemia (134 mg/dl) was present in control lean ZSF1 rats and was unaffected by empa. Obese ZSF1 rats showed increased systolic blood pressure, and blunted endothelium-dependent relaxations associated with the appearance of endothelium-dependent contractile responses (EDCFs) compared to control lean rats. These effects were prevented by the empa treatment. Obese ZSF1 rats showed increased weight of the heart and of the left ventricle volume without the presence of diastolic or systolic dysfunction, which were improved by the empa treatment. An increased expression level of senescence markers (p53, p21, p16), tissue factor, VCAM-1, SGLT1 and SGLT2 and a down-regulation of eNOS were observed in the aortic inner curvature compared to the outer one in the control lean rats, which were prevented by the empa treatment. In the obese ZSF1 rats, no such effects were observed. The empa treatment reduced the increased body weight and weight of lungs, spleen, liver and perirenal fat, hyperglycemia and the increased levels of total cholesterol and triglycerides in obese ZSF1 rats, and increased blood ketone levels and urinary glucose excretion in control lean and obese ZSF1 rats.

Conclusion

Empa reduced glucose levels by 28% and improved both endothelial function and cardiac remodeling in the obese ZSF1 rat. Empa also reduced the increased expression level of senescence, and atherothrombotic markers at arterial sites at risk in the control lean, but not obese, ZSF1 rat.

Geotrichum candidum gene expression and metabolite accumulation inside the cells reflect the strain oxidative stress sensitivity and ability to produce flavour compounds

Geotrichum candidum is a fungus-like yeast widely used as a starter culture for cheese ripening for its proteolytic and lipolytic activities and its contribution to the cheese flavours. The sequenced strain G. candidum CLIB 918 was isolated from cheese Pont-L’Evêque. This strain's ability to produce volatile compounds was compared to the ability of a known strong sulphur compound producer G. candidum strain (Gc203). The aminotransferase-coding genes BAT2 and ARO8 were identified to be involved in methionine catabolism. The production of volatile compounds indicated that the sequenced strain was a moderate producer compared to the strong producer strain. The major volatile compounds were produced from sulphur amino acid, branched-chain amino acid and fatty acid metabolisms. Metabolite content of the cells showed that the ability of the strain to produce volatile compounds was inversely proportional to its ability to store amino acids inside the cells. Reduced glutathione, hypotaurine and taurine intracellular concentrations and volatile fatty aldehyde production indicated the role of oxidative stress sensitivity in flavour production. The increase in expression of several genes in a Reblochon-type cheese at the end of ripening confirmed that oxygen and iron were key factors regulating cheese flavour production.

The potency of commercial blackcurrant juices to induce relaxation in porcine coronary artery rings is not correlated to their antioxidant capacity but to their anthocyanin content

OBJECTIVE

Polyphenol-rich products such as fruit juices have been found to have strong antioxidant capacities and to induce potent endothelium-dependent relaxation. We evaluated whether the commercial blackcurrant juices induced endothelium-dependent relaxation of isolated coronary arteries can be related to their antioxidant capacity and/or phenolic content.

METHODS

Six different commercial blackcurrant juices were selected. Their main phenolic compounds were measured by ultra-performance liquid chromatography and antioxidant capacity was evaluated by spectrometric methods. Vascular reactivity studies with these juices were done using isolated porcine coronary arteries.

RESULTS

The six different commercial blackcurrant juices induced relaxation ranging from 21% to 100% at the concentration of 0.5% volume per volume (v/v). The relaxation induced at 0.5% v/v was not correlated to their antioxidant capacity measured by either oxygen radical antioxidant capacity or DPPH (2,2-diphenyl-1-picrylhydrazyl) assays and also not to the ascorbic acid, total polyphenols, total flavanols, and total phenolic acid contents. In contrast, the amplitude of the relaxation was correlated to the total anthocyanins content and the individual anthocyanin concentration.

CONCLUSIONS

Correlations between relaxation amplitude and total anthocyanin or individual anthocyanin contents are of interest for the development of functional blackcurrant beverages with the potential to promote vascular protection.

Great heterogeneity of commercial fruit juices to induce endothelium-dependent relaxations in isolated porcine coronary arteries: role of the phenolic content and composition

Since polyphenol-rich products such as red wine, grape juice, and grape extracts have been shown to induce potent endothelium-dependent relaxations, we have evaluated whether commercial fruit juices such as those from berries are also able to induce endothelium-dependent relaxations of isolated coronary arteries and, if so, to determine whether this effect is related to their phenolic content. Among the 51 fruit juices tested, 2/12 grape juices, 3/7 blackcurrant juices, 4/5 cranberry juices, 1/6 apple juices, 0/5 orange juices, 2/6 red fruit and berry juices, 3/6 blends of red fruit juices, and 0/4 non-red fruit juices were able to induce relaxations achieving more than 50% at a volume of 1%. The active fruit juices had phenolic contents ranging from 0.31 to 1.86 g GAE/L, which were similar to those of most of the less active juices with the exception of one active grape juice (2.14 g GAE/L) and one active blend of red fruit juices (3.48 g GAE/L). Altogether, these findings indicate that very few commercial fruit juices have the ability to induce potent endothelium-dependent relaxations, and that this effect is not related to their quantitative phenolic content, but rather to their qualitative phenolic composition.

3-Hydroxypropionaldehyde (3-HPA) quantification by HPLC using a synthetic acrolein-free 3-hydroxypropionaldehyde system as analytical standard

HPLC-based quantification of 3-HPA using a synthetic acrolein-free 3-HPA standard obtained from commercially available 1,2,4-butanetriol through a straightforward and easy synthetic process.

Correction: 3-Hydroxypropionaldehyde (3-HPA) quantification by HPLC using a synthetic acrolein-free 3-hydroxypropionaldehyde system as analytical standard

Correction for ‘3-Hydroxypropionaldehyde (3-HPA) quantification by HPLC using a synthetic acrolein-free 3-hydroxypropionaldehyde system as analytical standard’ by G. Burgé et al., RSC Adv., 2015, 5, 92619–92627.

Elevated CO2 and/or ozone modify lignification in the wood of poplars (Populus tremula x alba)

Trees will have to cope with increasing levels of CO(2) and ozone in the atmosphere. The purpose of this work was to assess whether the lignification process could be altered in the wood of poplars under elevated CO(2) and/or ozone. Young poplars were exposed either to charcoal-filtered air (control), to elevated CO(2) (800 μl l(-1)), to ozone (200 nl l(-1)) or to a combination of elevated CO(2) and ozone in controlled chambers. Lignification was analysed at different levels: biosynthesis pathway activities (enzyme and transcript), lignin content, and capacity to incorporate new assimilates by using (13)C labelling. Elevated CO(2) and ozone had opposite effects on many parameters (growth, biomass, cambial activity, wood cell wall thickness) except on lignin content which was increased by elevated CO(2) and/or ozone. However, this increased lignification was due to different response mechanisms. Under elevated CO(2), carbon supply to the stem and effective lignin synthesis were enhanced, leading to increased lignin content, although there was a reduction in the level of some enzyme and transcript involved in the lignin pathway. Ozone treatment induced a reduction in carbon supply and effective lignin synthesis as well as transcripts from all steps of the lignin pathway and some corresponding enzyme activities. However, lignin content was increased under ozone probably due to variations in other major components of the cell wall. Both mechanisms seemed to coexist under combined treatment and resulted in a high increase in lignin content.

Biosynthesis and incorporation of side-chain-truncated lignin monomers to reduce lignin polymerization and enhance saccharification

Lignocellulosic biomass is utilized as a renewable feedstock in various agro-industrial activities. Lignin is an aromatic, hydrophobic and mildly branched polymer integrally associated with polysaccharides within the biomass, which negatively affects their extraction and hydrolysis during industrial processing. Engineering the monomer composition of lignins offers an attractive option towards new lignins with reduced recalcitrance. The presented work describes a new strategy developed in Arabidopsis for the overproduction of rare lignin monomers to reduce lignin polymerization degree (DP). Biosynthesis of these 'DP reducers' is achieved by expressing a bacterial hydroxycinnamoyl-CoA hydratase-lyase (HCHL) in lignifying tissues of Arabidopsis inflorescence stems. HCHL cleaves the propanoid side-chain of hydroxycinnamoyl-CoA lignin precursors to produce the corresponding hydroxybenzaldehydes so that plant stems expressing HCHL accumulate in their cell wall higher amounts of hydroxybenzaldehyde and hydroxybenzoate derivatives. Engineered plants with intermediate HCHL activity levels show no reduction in total lignin, sugar content or biomass yield compared with wild-type plants. However, cell wall characterization of extract-free stems by thioacidolysis and by 2D-NMR revealed an increased amount of unusual C₆C₁ lignin monomers most likely linked with lignin as end-groups. Moreover the analysis of lignin isolated from these plants using size-exclusion chromatography revealed a reduced molecular weight. Furthermore, these engineered lines show saccharification improvement of pretreated stem cell walls. Therefore, we conclude that enhancing the biosynthesis and incorporation of C₆C₁ monomers ('DP reducers') into lignin polymers represents a promising strategy to reduce lignin DP and to decrease cell wall recalcitrance to enzymatic hydrolysis.

Impact of lignin structure and cell wall reticulation on maize cell wall degradability

In this study, eight maize recombinant inbred lines were selected to assess both the impact of lignin structure and the impact of cell wall reticulation by p-hydroxycinnamic acids on cell wall degradability independently of the main "lignin content" factor. These recombinant lines and their parents were analyzed for in vitro degradability, cell wall residue content, esterified and etherified p-hydroxycinnamic acid content, and lignin content and structure. Lignin structure and esterified p-coumaric acid content showed significantly high correlation with in vitro degradability (r=-0.82 and r=-0.72, respectively). A multiple regression analysis showed that more than 80% of cell wall degradability variations within these 10 lines (eight recombinant inbred lines and their two parents) were explained by a regression model including two main explanatory factors: lignin content and estimated proportion of syringyl lignin units esterified by p-coumaric acid. This study revealed new biochemical parameters of interest to improve cell wall degradability and promote lignocellulose valorization.

Cellulose and lignin biosynthesis is altered by ozone in wood of hybrid poplar (Populus tremula × alba)

Wood formation in trees is a dynamic process that is strongly affected by environmental factors. However, the impact of ozone on wood is poorly documented. The objective of this study was to assess the effects of ozone on wood formation by focusing on the two major wood components, cellulose and lignin, and analysing any anatomical modifications. Young hybrid poplars (Populus tremula × alba) were cultivated under different ozone concentrations (50, 100, 200, and 300 l l(-1)). As upright poplars usually develop tension wood in a non-set pattern, the trees were bent in order to induce tension wood formation on the upper side of the stem and normal or opposite wood on the lower side. Biosynthesis of cellulose and lignin (enzymes and RNA levels), together with cambial growth, decreased in response to ozone exposure. The cellulose to lignin ratio was reduced, suggesting that cellulose biosynthesis was more affected than that of lignin. Tension wood was generally more altered than opposite wood, especially at the anatomical level. Tension wood may be more susceptible to reduced carbon allocation to the stems under ozone exposure. These results suggested a coordinated regulation of cellulose and lignin deposition to sustain mechanical strength under ozone. The modifications of the cellulose to lignin ratio and wood anatomy could allow the tree to maintain radial growth while minimizing carbon cost.

Disruption of LACCASE4 and 17 results in tissue-specific alterations to lignification of Arabidopsis thaliana stems

Peroxidases have been shown to be involved in the polymerization of lignin precursors, but it remains unclear whether laccases (EC 1.10.3.2) participate in constitutive lignification. We addressed this issue by studying laccase T-DNA insertion mutants in Arabidopsis thaliana. We identified two genes, LAC4 and LAC17, which are strongly expressed in stems. LAC17 was mainly expressed in the interfascicular fibers, whereas LAC4 was expressed in vascular bundles and interfascicular fibers. We produced two double mutants by crossing the LAC17 (lac17) mutant with two LAC4 mutants (lac4-1 and lac4-2). The single and double mutants grew normally in greenhouse conditions. The single mutants had moderately low lignin levels, whereas the stems of lac4-1 lac17 and lac4-2 lac17 mutants had lignin contents that were 20 and 40% lower than those of the control, respectively. These lower lignin levels resulted in higher saccharification yields. Thioacidolysis revealed that disrupting LAC17 principally affected the deposition of G lignin units in the interfascicular fibers and that complementation of lac17 with LAC17 restored a normal lignin profile. This study provides evidence that both LAC4 and LAC17 contribute to the constitutive lignification of Arabidopsis stems and that LAC17 is involved in the deposition of G lignin units in fibers.

The simultaneous repression of CCR and CAD, two enzymes of the lignin biosynthetic pathway, results in sterility and dwarfism in Arabidopsis thaliana

Cinnamoyl CoA reductase (CCR) and cinnamyl alcohol dehydrogenase (CAD) catalyze the last steps of monolignol biosynthesis. In Arabidopsis, one CCR gene (CCR1, At1g15950) and two CAD genes (CAD C At3g19450 and CAD D At4g34230) are involved in this pathway. A triple cad c cad d ccr1 mutant, named ccc, was obtained. This mutant displays a severe dwarf phenotype and male sterility. The lignin content in ccc mature stems is reduced to 50% of the wild-type level. In addition, stem lignin structure is severely affected, as shown by the dramatic enrichment in resistant inter-unit bonds and incorporation into the polymer of monolignol precursors such as coniferaldehyde, sinapaldehyde, and ferulic acid. Male sterility is due to the lack of lignification in the anther endothecium, which causes the failure of anther dehiscence and of pollen release. The ccc hypolignified stems accumulate higher amounts of flavonol glycosides, sinapoyl malate and feruloyl malate, which suggests a redirection of the phenolic pathway. Therefore, the absence of CAD and CCR, key enzymes of the monolignol pathway, has more severe consequences on the phenotype than the individual absence of each of them. Induction of another CCR (CCR2, At1g80820) and another CAD (CAD1, At4g39330) does not compensate the absence of the main CCR and CAD activities. This lack of CCR and CAD activities not only impacts lignification, but also severely affects the development of the plants. These consequences must be carefully considered when trying to reduce the lignin content of plants in order to facilitate the lignocellulose-to-bioethanol conversion process.

Acidolysis of a lignin model: investigation of heterogeneous catalysis using Montmorillonite clay

The use of heterogeneous conditions involving Montmorillonite K10 clay was investigated as a mild alternative to homogeneous acidolysis of a lignin model. Guaiacyl Dehydrogenation Oligomers (DHOs) synthesized by horseradish peroxidase were selected as starting material. Hydrolysis products were analyzed by gel permeation chromatography and by HPLC/mass spectrometry. Several experimental parameters were studied such as catalyst and substrate concentration, as well as reaction solvent composition in order to minimize high molar mass product formation generated by recombination mechanisms. In both catalytic modes, the best hydrolysis conditions were similar in terms of solvent composition with dioxane/water (90/10, v/v) and catalyst H(+) concentration of about 0.01 mol/L. Although the homogeneous catalysis generated only 28% of low molecular weight (LMW) products (monomers and dimers), clay catalysis generated 35%. In light of the qualitative analysis, both catalytic methods gave the same products, thus supporting similar hydrolysis mechanisms.

Concomitant changes in viscoelastic properties and amorphous polymers during the hydrothermal treatment of hardwood and softwood

The aim of this study was to understand how the molecular structures of amorphous polymers influence wood viscoelastic properties. Wood from oak and spruce was subjected to hydrothermal treatments at 110 or 135 degrees C. Wood rigidity, reflected by the wood storage modulus, showed different modification patterns according to the wood species or the temperature level. Because viscoelasticity is dependent on wood amorphous polymers, modifications of lignins and noncellulosic polysaccharides were examined. Hemicellulose degradation occurred only at 135 degrees C. In contrast, lignins displayed major structural alterations even at 110 degrees C. In oak lignins, the beta-O-4 bonds were extensively degraded and wood rigidity decreased dramatically during the first hours of treatment. Spruce lignins have a lower beta-O-4 content and, relative to oak, the wood rigidity decrease due to treatment was less pronounced. Wood rigidity was restored to its initial value by prolonged treatment, probably due to the formation of condensed bonds in cell wall polymers.

A BAHD acyltransferase is expressed in the tapetum of Arabidopsis anthers and is involved in the synthesis of hydroxycinnamoyl spermidines

BAHD acyltransferases catalyze the acylation of many plant secondary metabolites. We characterized the function of At2g19070, a member of the BAHD gene family of Arabidopsis thaliana. The acyltransferase gene was shown to be specifically expressed in anther tapetum cells in the early stages of flower development. The impact of gene repression was studied in RNAi plants and in a knockout (KO) mutant line. Immunoblotting with a specific antiserum raised against the recombinant protein was used to evaluate the accumulation of At2g19070 gene product in flowers of various Arabidopsis genotypes including the KO and RNAi lines, the male sterile mutant ms1 and transformants overexpressing the acyltransferase gene. Metabolic profiling of flower bud tissues from these genetic backgrounds demonstrated a positive correlation between the accumulation of acyltransferase protein and the quantities of metabolites that were putatively identified by tandem mass spectrometry as N(1),N(5),N(10)-trihydroxyferuloyl spermidine and N(1),N(5)-dihydroxyferuloyl-N(10)-sinapoyl spermidine. These products, deposited in pollen coat, can be readily extracted by pollen wash and were shown to be responsible for pollen autofluorescence. The activity of the recombinant enzyme produced in bacteria was assayed with various hydroxycinnamoyl-CoA esters and polyamines as donor and acceptor substrates, respectively. Feruloyl-CoA and spermidine proved the best substrates, and the enzyme has therefore been named spermidine hydroxycinnamoyl transferase (SHT). A methyltransferase gene (At1g67990) which co-regulated with SHT during flower development, was shown to be involved in the O-methylation of spermidine conjugates by analyzing the consequences of its repression in RNAi plants and by characterizing the methylation activity of the recombinant enzyme.

Impact of CCR1 silencing on the assembly of lignified secondary walls in Arabidopsis thaliana

A cinnamoyl-CoA reductase 1 knockout mutant in Arabidopsis thaliana was investigated for the consequences of lignin synthesis perturbation on the assembly of the cell walls. The mutant displayed a dwarf phenotype and a strong collapse of its xylem vessels corresponding to lower lignin content and a loss of lignin units of the noncondensed type. Transmission electron microscopy revealed that the transformation considerably impaired the capacity of interfascicular fibers and vascular bundles to complete the assembly of cellulose microfibrils in the S(2) layer, the S(1) layer remaining unaltered. Such disorder in cellulose was correlated with X-ray diffraction showing altered organization. Semi-quantitative immunolabeling of lignins showed that the patterns of distribution were differentially affected in interfascicular fibers and vascular bundles, pointing to the importance of noncondensed lignin structures for the assembly of a coherent secondary wall. The use of laser capture microdissection combined with the microanalysis of lignins and polysaccharides allowed these polymers to be characterized into specific cell types. Wild-type A. thaliana displayed a two-fold higher syringyl to guaiacyl ratio in interfascicular fibers compared with vascular bundles, whereas this difference was less marked in the cinnamoyl-CoA reductase 1 knockout mutant.

Stiffness gradients in vascular bundles of the palm Washingtonia robusta

Palms can grow at sites exposed to high winds experiencing large dynamic wind and gust loads. Their stems represent a system of stiff fibrous elements embedded in the soft parenchymatous tissue. The proper design of the interface of the stiffening elements and the parenchyma is crucial for the functioning of the stem. The strategy of the palm to compromise between stiff fibre caps and the soft parenchymatous tissue may serve as a model system for avoiding stress discontinuities in inhomogeneous and anisotropic fibre-reinforced composite materials. We investigated the mechanical, structural and biochemical properties of the fibre caps of the palm Washingtonia robusta at different levels of hierarchy with high spatial resolution. A gradual decrease in stiffness across the fibre cap towards the surrounding parenchymatous tissue was observed. Structural adaptations at the tissue level were found in terms of changes in cell cross sections and cell wall thickness. At the cell wall level, gradients across the fibre cap were found in the degree of orientation of the microfibrils and in the lignin level and composition. The impact of these structural variations in the local material stiffness distribution is discussed.

Biochemical composition is not the main factor influencing variability in carbon isotope composition of tree rings

From June to December, we determined the effects of variations in biochemical composition on delta(13)C of tree rings of 2-year-old oaks (Quercus petraea (Matt.) Liebl.) growing under semi-natural conditions, and the dependence of these effects of water stress during the growth season. Percent abundance, carbon concentration and delta(13)C were measured in holocellulose, lignin, extractive-free wood, starch and a water-soluble fraction. Relative concentrations of lignin and holocellulose in the extractive-free wood varied little during the season or in response to water stress, indicating that these compounds could not quantitatively explain the variations in whole-wood delta(13)C. Among all sampled tree rings, the relative concentration of each structural compound (holocellulose and lignin) accounted for less than 5% of the delta(13)C variability in whole wood. Variations in holocellulose and extractive-free wood delta(13)C between tree rings were almost identical (r > 0.95), whereas variations in lignin delta(13)C were less well correlated to these compounds. Whole-wood delta(13)C had a slightly altered isotopic signal compared with that of the structural compounds because of the presence of the extractive component. These results showed that variations in lignin delta(13)C and lignin concentration have little influence on extractive-free wood delta(13)C and whole-wood delta(13)C. Rather, holocellulose influences delta(13)C the most. Thus, we confirmed that, for climatic reconstruction from tree rings, removal of extractives by soxhlet is generally sufficient and sometimes unnecessary. Our findings also indicate that, in the case of rapid and severe water stress, the structural component did not accurately record the associated increase in delta(13)C because of dilution with previously formed organic matter and cessation of trunk growth. The effect of drought on carbon isotope ratios was more pronounced in the extractive compounds, making them good water stress indicators but only on a scale of days to months.

Redirection of the phenylpropanoid pathway to feruloyl malate in Arabidopsis mutants deficient for cinnamoyl-CoA reductase 1

Cinnamoyl-CoA reductase 1 (CCR1, gene At1g15950) is the main CCR isoform implied in the constitutive lignification of Arabidopsis thaliana. In this work, we have identified and characterized two new knockout mutants for CCR1. Both have a dwarf phenotype and a delayed senescence. At complete maturity, their inflorescence stems display a 25-35% decreased lignin level, some alterations in lignin structure with a higher frequency of resistant interunit bonds and a higher content in cell wall-bound ferulic esters. Ferulic acid-coniferyl alcohol ether dimers were found for the first time in dicot cell walls and in similar levels in wild-type and mutant plants. The expression of CCR2, a CCR gene usually involved in plant defense, was increased in the mutants and could account for the biosynthesis of lignins in the CCR1-knockout plants. Mutant plantlets have three to four-times less sinapoyl malate (SM) than controls and accumulate some feruloyl malate. The same compositional changes occurred in the rosette leaves of greenhouse-grown plants. By contrast and relative to the control, their stems accumulated unusually high levels of both SM and feruloyl malate as well as more kaempferol glycosides. These findings suggest that, in their hypolignified stems, the mutant plants would avoid the feruloyl-CoA accumulation by its redirection to cell wall-bound ferulate esters, to feruloyl malate and to SM. The formation of feruloyl malate to an extent far exceeding the levels reported so far indicates that ferulic acid is a potential substrate for the enzymes involved in SM biosynthesis and emphasizes the remarkable plasticity of Arabidopsis phenylpropanoid metabolism.

Effects of increased nitrogen supply on the lignification of poplar wood

The short-term influence of adequate and high nitrogen fertilization on poplar lignification was investigated. The high nitrogen supply decreased lignin staining in the newly formed secondary xylem, indicating that lignin deposition was affected. Acetyl bromide determinations gave a 9-10% decrease in lignin content; however, Klason lignin content was unchanged. Thioacidolysis showed that elevated N supply affected lignin structure such that there was a reduced frequency of lignin units involved in beta-O-4 bonds, a reduced syringyl/guaiacyl ratio, an increased frequency of p-hydroxyphenyl lignin units, more guaiacyl units with free phenolic groups, and more p-hydroxybenzoic acid ester-linked to poplar lignins. These features suggest that lignins from poplars grown under high N bear structural similarities to lignins formed during early stages of wood development. The findings also indicate that a gravitational stimulus inducing the formation of tension wood and high N availability lead to similar and additive effects on lignin content and structure.

Downregulation of cinnamoyl-coenzyme A reductase in poplar: multiple-level phenotyping reveals effects on cell wall polymer metabolism and structure

Cinnamoyl-CoA reductase (CCR) catalyzes the penultimate step in monolignol biosynthesis. We show that downregulation of CCR in transgenic poplar (Populus tremula x Populus alba) was associated with up to 50% reduced lignin content and an orange-brown, often patchy, coloration of the outer xylem. Thioacidolysis, nuclear magnetic resonance (NMR), immunocytochemistry of lignin epitopes, and oligolignol profiling indicated that lignin was relatively more reduced in syringyl than in guaiacyl units. The cohesion of the walls was affected, particularly at sites that are generally richer in syringyl units in wild-type poplar. Ferulic acid was incorporated into the lignin via ether bonds, as evidenced independently by thioacidolysis and by NMR. A synthetic lignin incorporating ferulic acid had a red-brown coloration, suggesting that the xylem coloration was due to the presence of ferulic acid during lignification. Elevated ferulic acid levels were also observed in the form of esters. Transcript and metabolite profiling were used as comprehensive phenotyping tools to investigate how CCR downregulation impacted metabolism and the biosynthesis of other cell wall polymers. Both methods suggested reduced biosynthesis and increased breakdown or remodeling of noncellulosic cell wall polymers, which was further supported by Fourier transform infrared spectroscopy and wet chemistry analysis. The reduced levels of lignin and hemicellulose were associated with an increased proportion of cellulose. Furthermore, the transcript and metabolite profiling data pointed toward a stress response induced by the altered cell wall structure. Finally, chemical pulping of wood derived from 5-year-old, field-grown transgenic lines revealed improved pulping characteristics, but growth was affected in all transgenic lines tested.

Both caffeoyl Coenzyme A 3-O-methyltransferase 1 and caffeic acid O-methyltransferase 1 are involved in redundant functions for lignin, flavonoids and sinapoyl malate biosynthesis in Arabidopsis

Two methylation steps are necessary for the biosynthesis of monolignols, the lignin precursors. Caffeic acid O-methyltransferase (COMT) O-methylates at the C5 position of the phenolic ring. COMT is responsible for the biosynthesis of sinapyl alcohol, the precursor of syringyl lignin units. The O-methylation at the C3 position of the phenolic ring involves the Caffeoyl CoA 3-O-methyltransferase (CCoAOMT). The CCoAOMT 1 gene (At4g34050) is believed to encode the enzyme responsible for the first O-methylation in Arabidopsis thaliana. A CCoAOMT1 promoter-GUS fusion and immunolocalization experiments revealed that this gene is strongly and exclusively expressed in the vascular tissues of stems and roots. An Arabidopsis T-DNA null mutant named ccomt 1 was identified and characterised. The mutant stems are slightly smaller than wild-type stems in short-day growth conditions and has collapsed xylem elements. The lignin content of the stem is low and the S/G ratio is high mainly due to fewer G units. These results suggest that this O-methyltransferase is involved in G-unit biosynthesis but does not act alone to perform this step in monolignol biosynthesis. To determine which O-methyltransferase assists CCoAOMT 1, a comt 1 ccomt1 double mutant was generated and studied. The development of comt 1 ccomt1 is arrested at the plantlet stage in our growth conditions. Lignins of these plantlets are mainly composed of p-hydroxyphenyl units. Moreover, the double mutant does not synthesize sinapoyl malate, a soluble phenolic. These results suggest that CCoAOMT 1 and COMT 1 act together to methylate the C3 position of the phenolic ring of monolignols in Arabidopsis. In addition, they are both involved in the formation of sinapoyl malate and isorhamnetin.

Peroxidase activity can dictate the in vitro lignin dehydrogenative polymer structure

The objective of this study was to assess the influence of the peroxidase/coniferyl alcohol (CA) ratio on the dehydrogenation polymer (DHP) synthesis. The soluble and unsoluble fractions of horseradish peroxidase (HRP)-catalyzed CA dehydrogenation mixtures were recovered in various proportions, depending on the polymerization mode (Zutropf ZT/Zulauf ZL) and HRP/CA ratio (1.6-1100purpurogallin U mmol(-1)). The ZL mode yielded 0-57%/initial CA of unsoluble condensed DHPs (thioacidolysis yields <200micromolg(-1)) with a proportion of uncondensed CA end groups increasing with the HRP/CA ratio (7.2-55.5%/total uncondensed CA). Systematically lower polymer yields (0-49%/initial CA) were obtained for the ZT mode. In that mode, a negative correlation was established between the beta-O-4 content (thioacidolysis yields: 222-660micromolg(-1)) and the HRP/CA ratio. In both modes, decreasing the HRP/CA ratio below 18Ummol(-1) favoured an end-wise polymerization process evidenced by the occurrence of tri-, tetra- and pentamers involving at least one beta-O-4 bond. At low ratio, the unsoluble ZT DHP was found to better approximate natural lignins than DHPs previously synthesized with traditional methods. Besides its possible implication in lignin biosynthesis, peroxidase activity is a crucial parameter accounting for the structural variations of in vitro DHPs.

Flavonoid accumulation in Arabidopsis repressed in lignin synthesis affects auxin transport and plant growth

In Arabidopsis thaliana, silencing of hydroxycinnamoyl-CoA shikimate/quinate hydroxycinnamoyl transferase (HCT), a lignin biosynthetic gene, results in a strong reduction of plant growth. We show that, in HCT-silenced plants, lignin synthesis repression leads to the redirection of the metabolic flux into flavonoids through chalcone synthase activity. Several flavonol glycosides and acylated anthocyanin were shown to accumulate in higher amounts in silenced plants. By contrast, sinapoylmalate levels were barely affected, suggesting that the synthesis of that phenylpropanoid compound might be HCT-independent. The growth phenotype of HCT-silenced plants was shown to be controlled by light and to depend on chalcone synthase expression. Histochemical analysis of silenced stem tissues demonstrated altered tracheary elements. The level of plant growth reduction of HCT-deficient plants was correlated with the inhibition of auxin transport. Suppression of flavonoid accumulation by chalcone synthase repression in HCT-deficient plants restored normal auxin transport and wild-type plant growth. By contrast, the lignin structure of the plants simultaneously repressed for HCT and chalcone synthase remained as severely altered as in HCT-silenced plants, with a large predominance of nonmethoxylated H units. These data demonstrate that the reduced size phenotype of HCT-silenced plants is not due to the alteration of lignin synthesis but to flavonoid accumulation.

Evidence for a role of AtCAD 1 in lignification of elongating stems of Arabidopsis thaliana

The cinnamyl alcohol dehydrogenase (AtCAD) multigene family in Arabidopsis is composed of nine genes. Our previous studies focused on the two isoforms AtCAD C and AtCAD D which show a high homology to those related to lignification in other plants. This study focuses on the seven other Arabidopsis CAD for which functions are not yet elucidated. Their expression patterns were determined in different parts of Arabidopsis. Only CAD 1 protein can be detected in elongating stems, flowers, and siliques using Western-blot analysis. Tissue specific expression of CAD 1, B1, and G genes was determined using their promoters fused to the GUS reporter gene. CAD 1 expression was observed in primary xylem in accordance with a potential role in lignification. Arabidopsis T-DNA mutants knockout for the different genes CAD genes were characterized. Their stems displayed no substantial reduction of CAD activities for coniferyl and sinapyl alcohols as well as no modifications of lignin quantity and structure in mature inflorescence stems. Only a small reduction of lignin content could be observed in elongating stems of Atcad 1 mutant. These CAD genes in combination with the CAD D promoter were used to complement a CAD double mutant severely altered in lignification (cad c cad d). The expression of AtCAD A, B1, B2, F, and G had no effect on restoring a normal lignin profile of this mutant. In contrast, CAD 1 complemented partly this mutant as revealed by the partial restoration of conventional lignin units and by the decrease in the frequency of beta-O-4 linked p-OH cinnamaldehydes.

A coumaroyl-ester-3-hydroxylase insertion mutant reveals the existence of nonredundant meta-hydroxylation pathways and essential roles for phenolic precursors in cell expansion and plant growth

Cytochromes P450 monooxygenases from the CYP98 family catalyze the meta-hydroxylation step in the phenylpropanoid biosynthetic pathway. The ref8 Arabidopsis (Arabidopsis thaliana) mutant, with a point mutation in the CYP98A3 gene, was previously described to show developmental defects, changes in lignin composition, and lack of soluble sinapoyl esters. We isolated a T-DNA insertion mutant in CYP98A3 and show that this mutation leads to a more drastic inhibition of plant development and inhibition of cell growth. Similar to the ref8 mutant, the insertion mutant has reduced lignin content, with stem lignin essentially made of p-hydroxyphenyl units and trace amounts of guaiacyl and syringyl units. However, its roots display an ectopic lignification and a substantial proportion of guaiacyl and syringyl units, suggesting the occurrence of an alternative CYP98A3-independent meta-hydroxylation mechanism active mainly in the roots. Relative to the control, mutant plantlets produce very low amounts of sinapoyl esters, but accumulate flavonol glycosides. Reduced cell growth seems correlated with alterations in the abundance of cell wall polysaccharides, in particular decrease in crystalline cellulose, and profound modifications in gene expression and homeostasis reminiscent of a stress response. CYP98A3 thus constitutes a critical bottleneck in the phenylpropanoid pathway and in the synthesis of compounds controlling plant development. CYP98A3 cosuppressed lines show a gradation of developmental defects and changes in lignin content (40% reduction) and structure (prominent frequency of p-hydroxyphenyl units), but content in foliar sinapoyl esters is similar to the control. The purple coloration of their leaves is correlated to the accumulation of sinapoylated anthocyanins.

EgMYB2, a new transcriptional activator from Eucalyptus xylem, regulates secondary cell wall formation and lignin biosynthesis

Summary EgMYB2, a member of a new subgroup of the R2R3 MYB family of transcription factors, was cloned from a library consisting of RNA from differentiating Eucalyptus xylem. EgMYB2 maps to a unique locus on the Eucalyptus grandis linkage map and co-localizes with a quantitative trait locus (QTL) for lignin content. Recombinant EgMYB2 protein was able to bind specifically the cis-regulatory regions of the promoters of two lignin biosynthetic genes, cinnamoyl-coenzyme A reductase (CCR) and cinnamyl alcohol dehydrogenase (CAD), which contain MYB consensus binding sites. EgMYB2 was also able to regulate their transcription in both transient and stable expression assays. Transgenic tobacco plants over-expressing EgMYB2 displayed phenotypic changes relative to wild-type plants, among which were a dramatic increase in secondary cell wall thickness, and an alteration of the lignin profiles. Transcript abundance of genes encoding enzymes specific to lignin biosynthesis was increased to varying extents according to the position of individual genes in the pathway, whereas core phenylpropanoid genes were not significantly affected. Together these results suggest a role for EgMYB2 in the co-ordinated control of genes belonging to the monolignol-specific pathway, and therefore in the biosynthesis of lignin and the regulation of secondary cell wall formation.

EgMYB2, a new transcriptional activator from Eucalyptus xylem, regulates secondary cell wall formation and lignin biosynthesis

Summary EgMYB2, a member of a new subgroup of the R2R3 MYB family of transcription factors, was cloned from a library consisting of RNA from differentiating Eucalyptus xylem. EgMYB2 maps to a unique locus on the Eucalyptus grandis linkage map and co-localizes with a quantitative trait locus (QTL) for lignin content. Recombinant EgMYB2 protein was able to bind specifically the cis-regulatory regions of the promoters of two lignin biosynthetic genes, cinnamoyl-coenzyme A reductase (CCR) and cinnamyl alcohol dehydrogenase (CAD), which contain MYB consensus binding sites. EgMYB2 was also able to regulate their transcription in both transient and stable expression assays. Transgenic tobacco plants over-expressing EgMYB2 displayed phenotypic changes relative to wild-type plants, among which were a dramatic increase in secondary cell wall thickness, and an alteration of the lignin profiles. Transcript abundance of genes encoding enzymes specific to lignin biosynthesis was increased to varying extents according to the position of individual genes in the pathway, whereas core phenylpropanoid genes were not significantly affected. Together these results suggest a role for EgMYB2 in the co-ordinated control of genes belonging to the monolignol-specific pathway, and therefore in the biosynthesis of lignin and the regulation of secondary cell wall formation.

In search of a maize ideotype for cell wall enzymatic degradability using histological and biochemical lignin characterization

Grass cell wall degradability is conventionally related to the lignin content and to the ferulic-mediated cross-linking of lignins to polysaccharides. To better understand the variations in degradability, 22 maize inbred lines were subjected to image analyses of Fasga- and Mäule-stained stem sections and to chemical analyses of lignins and p-hydroxycinnamic acids. For the first time, the nearness of biochemical and histological estimates of lignin levels was established. Combination of histological and biochemical traits could explain 89% of the variations for cell wall degradability and define a maize ideotype for cell wall degradability. In addition to a reduced lignin level, such an ideotype would contain lignins richer in syringyl than in guaiacyl units and preferentially localized in the cortical region rather than in the pith. Such enrichment in syringyl units would favor wall degradability in grasses, contrary to dicots, and could be related to the fact that grass syringyl units are noticeably p-coumaroylated. This might affect the interaction capabilities of lignins and polysaccharides.

CINNAMYL ALCOHOL DEHYDROGENASE-C and -D are the primary genes involved in lignin biosynthesis in the floral stem of Arabidopsis

During lignin biosynthesis in angiosperms, coniferyl and sinapyl aldehydes are believed to be converted into their corresponding alcohols by cinnamyl alcohol dehydrogenase (CAD) and by sinapyl alcohol dehydrogenase (SAD), respectively. This work clearly shows that CAD-C and CAD-D act as the primary genes involved in lignin biosynthesis in the floral stem of Arabidopsis thaliana by supplying both coniferyl and sinapyl alcohols. An Arabidopsis CAD double mutant (cad-c cad-d) resulted in a phenotype with a limp floral stem at maturity as well as modifications in the pattern of lignin staining. Lignin content of the mutant stem was reduced by 40%, with a 94% reduction, relative to the wild type, in conventional beta-O-4-linked guaiacyl and syringyl units and incorportion of coniferyl and sinapyl aldehydes. Fourier transform infrared spectroscopy demonstrated that both xylem vessels and fibers were affected. GeneChip data and real-time PCR analysis revealed that transcription of CAD homologs and other genes mainly involved in cell wall integrity were also altered in the double mutant. In addition, molecular complementation of the double mutant by tissue-specific expression of CAD derived from various species suggests different abilities of these genes/proteins to produce syringyl-lignin moieties but does not indicate a requirement for any specific SAD gene.

Water deficits affect caffeate O-methyltransferase, lignification, and related enzymes in maize leaves. A proteomic investigation

Drought is a major abiotic stress affecting all levels of plant organization and, in particular, leaf elongation. Several experiments were designed to study the effect of water deficits on maize (Zea mays) leaves at the protein level by taking into account the reduction of leaf elongation. Proteomic analyses of growing maize leaves allowed us to show that two isoforms of caffeic acid/5-hydroxyferulic 3-O-methyltransferase (COMT) accumulated mostly at 10 to 20 cm from the leaf point of insertion and that drought resulted in a shift of this region of maximal accumulation toward basal regions. We showed that this shift was due to the combined effect of reductions in growth and in total amounts of COMT. Several other enzymes involved in lignin and/or flavonoid synthesis (caffeoyl-CoA 3-O-methyltransferase, phenylalanine ammonia lyase, methylenetetrahydrofolate reductase, and several isoforms of S-adenosyl-l-methionine synthase and methionine synthase) were highly correlated with COMT, reinforcing the hypothesis that the zone of maximal accumulation corresponds to a zone of lignification. According to the accumulation profiles of the enzymes, lignification increases in leaves of control plants when their growth decreases before reaching their final size. Lignin levels analyzed by thioacidolysis confirmed that lignin is synthesized in the region where we observed the maximal accumulation of these enzymes. Consistent with the levels of these enzymes, we found that the lignin level was lower in leaves of plants subjected to water deficit than in those of well-watered plants.

Lignification in poplar plantlets fed with deuterium-labelled lignin precursors

Lignification was investigated in wild-type (WT) and in transgenic poplar plantlets with a reduced caffeic acid O-methyl-transferase (COMT) activity. Coniferin and syringin, deuterated at their methoxyl, were incorporated into the culture medium of microcuttings. The gas chromatography-mass spectrometry (GC-MS) analysis of the thioacidolysis guaiacyl (G) and syringyl (S) lignin-derived monomers revealed that COMT deficiency altered stem lignification. GC-MS analysis proved that the deuterated precursors were incorporated into root lignins and, to a lower extent, in stem lignins without major effect on growth and lignification. Deuterium from coniferin was recovered in G and S lignin units, whereas deuterium from syringin was only found in S units, which further establishes that the conversion of G to S lignin precursors may occur at the level of p-OH cinnamyl alcohols.

Silencing of hydroxycinnamoyl-coenzyme A shikimate/quinate hydroxycinnamoyltransferase affects phenylpropanoid biosynthesis

The hydroxyl group in the 3-position of the phenylpropanoid compounds is introduced at the level of coumarate shikimate/quinate esters, whose synthesis implicates an acyltransferase activity. Specific antibodies raised against the recombinant tobacco (Nicotiana tabacum) acyltransferase revealed the accumulation of the enzyme in stem vascular tissues of tobacco, in accordance with a putative role in lignification. For functional analysis, the acyltransferase gene was silenced in Arabidopsis thaliana and N. benthamiana by RNA-mediated posttranscriptional gene silencing. In Arabidopsis, gene silencing resulted in a dwarf phenotype and changes in lignin composition as indicated by histochemical staining. An in-depth study of silenced N. benthamiana plants by immunological, histochemical, and chemical methods revealed the impact of acyltransferase silencing on soluble phenylpropanoids and lignin content and composition. In particular, a decrease in syringyl units and an increase in p-hydroxyphenyl units were recorded. Enzyme immunolocalization by confocal microscopy showed a correlation between enzyme accumulation levels and lignin composition in vascular cells. These results demonstrate the function of the acyltransferase in phenylpropanoid biosynthesis.

Abiotic degradation of lignified cell walls by carbonate and copper salt

This study reports on the destructuration of Wheat straw and Spruce wood cell walls after maceration in potassium carbonate or sodium hydroxide at pH = 10 in the presence of copper acetate. The alkaline treatments had a predominant impact on the wheat straw cell wall components over copper acetate. Either K-carbonate or Na-hydroxide extracted from wheat straw a particular lignin fraction rich in condensed C-C linkages, leading to the unmasking of new ether-linked sub-structures in the cell wall. This unmasking was increased in the presence of copper salt but only in the nonextracted Wheat straw sample incubated in carbonate and not in the corresponding extractive-free sample. This difference was related to the leaching of compounds from the nonextracted cell wall, which could sustain oxidative activity of copper by hindering its precipitation into inactive hydroxide and/or carbonate species. In Spruce wood samples, copper salt was the principal factor impacting on the lignin structure over alkali alone. Its effect was, however, only detected at the level of C-C linked dimers. These results confirmed that unmasking of lignin sub-structures also occurred in Spruce wood, but probably through mechanisms different from that evidenced in Wheat straw.

Signatures of cinnamyl alcohol dehydrogenase deficiency in poplar lignins

A series of transgenic poplars down-regulated for cinnamyl alcohol dehydrogenase (CAD) was analyzed by thioacidolysis. Among the lignin-derived monomers, the indene compounds that were recently shown to originate from sinapaldehyde incorporated into lignins through 8-O-4-cross-coupling, were found to increase as a function of CAD deficiency level. While these syringyl markers were recovered in substantial amounts in the most severely depressed lines, the markers for coniferaldehyde incorporation were recovered in only low amounts. In conjunction with these additional sinapaldehyde units and relative to the control samples, lignins in CAD-deficient poplar lines had less conventional syringyl-units and beta-O-4-bonds and more free phenolic groups. We found that almost half of the polymers in the most deficient lines could be solubilized in alkali and at room temperature. This unusual behavior suggests that lignins in CAD-deficient poplars occur as small, alkali-leachable lignin domains. That mainly sinapaldehyde incorporates into the lignins of CAD-deficient poplars suggests that the recently identified sinapyl alcohol dehydrogenase (SAD), which is structurally distinct from the CAD enzyme targeted herein, does not play any substantial role in constitutive lignification in poplar.

Condensed lignins are synthesized in poplar leaves exposed to ozone

Poplar (Populus tremula x alba) trees (clone INRA 717-1-B4) were cultivated for 1 month in phytotronic chambers with two different levels of ozone (60 and 120 nL L(-1)). Foliar activities of shikimate dehydrogenase (EC 1.1.1.25), phenylalanine ammonia lyase (EC 4.3.1.5), and cinnamyl alcohol dehydrogenase (CAD, EC 1.1.1.195) were compared with control levels. In addition, we examined lignin content and structure in control and ozone-fumigated leaves. Under ozone exposure, CAD activity and CAD RNA levels were found to be rapidly and strongly increased whatever the foliar developmental stage. In contrast, shikimate dehydrogenase and phenylalanine ammonia lyase activities were increased in old and midaged leaves but not in the youngest ones. The increased activities of these enzymes involved in the late or early steps of the metabolic pathway leading to lignins were associated with a higher Klason lignin content in extract-free leaves. In addition, stress lignins synthesized in response to ozone displayed a distinct structure, relative to constitutive lignins. They were found substantially enriched in carbon-carbon interunit bonds and in p-hydroxyphenylpropane units, which is reminiscent of lignins formed at early developmental stages, in compression wood, or in response to fungal elicitor. The highest changes in lignification and in enzyme activities were obtained with the highest ozone dose (120 nL L(-1)). These results suggest that ozone-induced lignins might contribute to the poplar tolerance to ozone because of their barrier or antioxidant effect toward reactive oxygen species.

Down-regulation of the AtCCR1 gene in Arabidopsis thaliana: effects on phenotype, lignins and cell wall degradability

Cinnamoyl CoA reductase (CCR; EC 1.2.1.44) is the first enzyme specific to the biosynthetic pathway leading to monolignols. Arabidopsis thaliana (L.) Heynh. plants transformed with a vector containing a full-length AtCCR1 cDNA in an antisense orientation were obtained and characterized. The most severely down-regulated homozygous plants showed drastic alterations to their phenotypical features. These plants had a 50% decrease in lignin content accompanied by changes in lignin composition and structure, with incorporation of ferulic acid into the cell wall. Microscopic analyses coupled with immunolabelling revealed a decrease in lignin deposition in normally lignified tissues and a dramatic loosening of the secondary cell wall of interfascicular fibers and vessels. Evaluation of in vitro digestibility demonstrated an increase in the enzymatic degradability of these transgenic lines. In addition, culture conditions were shown to play a substantial role in lignin level and structure in the wild type and in the effects of AtCCR1 repression efficiency.

Down-regulation of the AtCCR1 gene in Arabidopsis thaliana: effects on phenotype, lignins and cell wall degradability

Cinnamoyl CoA reductase (CCR; EC 1.2.1.44) is the first enzyme specific to the biosynthetic pathway leading to monolignols. Arabidopsis thaliana (L.) Heynh. plants transformed with a vector containing a full-length AtCCR1 cDNA in an antisense orientation were obtained and characterized. The most severely down-regulated homozygous plants showed drastic alterations to their phenotypical features. These plants had a 50% decrease in lignin content accompanied by changes in lignin composition and structure, with incorporation of ferulic acid into the cell wall. Microscopic analyses coupled with immunolabelling revealed a decrease in lignin deposition in normally lignified tissues and a dramatic loosening of the secondary cell wall of interfascicular fibers and vessels. Evaluation of in vitro digestibility demonstrated an increase in the enzymatic degradability of these transgenic lines. In addition, culture conditions were shown to play a substantial role in lignin level and structure in the wild type and in the effects of AtCCR1 repression efficiency.

Expression pattern of two paralogs encoding cinnamyl alcohol dehydrogenases in Arabidopsis. Isolation and characterization of the corresponding mutants

Studying Arabidopsis mutants of the phenylpropanoid pathway has unraveled several biosynthetic steps of monolignol synthesis. Most of the genes leading to monolignol synthesis have been characterized recently in this herbaceous plant, except those encoding cinnamyl alcohol dehydrogenase (CAD). We have used the complete sequencing of the Arabidopsis genome to highlight a new view of the complete CAD gene family. Among nine AtCAD genes, we have identified the two distinct paralogs AtCAD-C and AtCAD-D, which share 75% identity and are likely to be involved in lignin biosynthesis in other plants. Northern, semiquantitative restriction fragment-length polymorphism-reverse transcriptase-polymerase chain reaction and western analysis revealed that AtCAD-C and AtCAD-D mRNA and protein ratios were organ dependent. Promoter activities of both genes are high in fibers and in xylem bundles. However, AtCAD-C displayed a larger range of sites of expression than AtCAD-D. Arabidopsis null mutants (Atcad-D and Atcad-C) corresponding to both genes were isolated. CAD activities were drastically reduced in both mutants, with a higher impact on sinapyl alcohol dehydrogenase activity (6% and 38% of residual sinapyl alcohol dehydrogenase activities for Atcad-D and Atcad-C, respectively). Only Atcad-D showed a slight reduction in Klason lignin content and displayed modifications of lignin structure with a significant reduced proportion of conventional S lignin units in both stems and roots, together with the incorporation of sinapaldehyde structures ether linked at Cbeta. These results argue for a substantial role of AtCAD-D in lignification, and more specifically in the biosynthesis of sinapyl alcohol, the precursor of S lignin units.