Lignin biosynthesis

Sugarcane DIRIGENT and O-methyltransferase promoters confer stem-regulated gene expression in diverse monocots

Transcription profiling analysis identified Saccharum hybrid DIRIGENT (SHDIR16) and Omicron-Methyltransferase (SHOMT), putative defense and fiber biosynthesis-related genes that are highly expressed in the stem of sugarcane, a major sucrose accumulator and biomass producer. Promoters (Pro) of these genes were isolated and fused to the beta-glucuronidase (GUS) reporter gene. Transient and stable transgene expression analyses showed that both Pro( DIR16 ):GUS and Pro( OMT ):GUS retain the expression characteristics of their respective endogenous genes in sugarcane and function in orthologous monocot species, including rice, maize and sorghum. Furthermore, both promoters conferred stem-regulated expression, which was further enhanced in the stem and induced in the leaf and root by salicylic acid, jasmonic acid and methyl jasmonate, key regulators of biotic and abiotic stresses. Pro( DIR16 ) and Pro( OMT ) will enable functional gene analysis in monocots, and will facilitate engineering monocots for improved carbon metabolism, enhanced stress tolerance and bioenergy production.

Phenotypic plasticity in cell walls of maize brown midrib mutants is limited by lignin composition

The hydrophobic cell wall polymer lignin is deposited in specialized cells to make them impermeable to water and prevent cell collapse as negative pressure or gravitational force is exerted. The variation in lignin subunit composition that exists among different species, and among different tissues within the same species suggests that lignin subunit composition varies depending on its precise function. In order to gain a better understanding of the relationship between lignin subunit composition and the physico-chemical properties of lignified tissues, detailed analyses were performed of near-isogenic brown midrib2 (bm2), bm4, bm2-bm4, and bm1-bm2-bm4 mutants of maize. This investigation was motivated by the fact that the bm2-bm4 double mutant is substantially shorter, displays drought symptoms even when well watered, and will often not develop reproductive organs, whereas the phenotypes of the individual bm single mutants and double mutant combinations other than bm2-bm4 are only subtly different from the wild-type control. Detailed cell wall compositional analyses revealed midrib-specific reductions in Klason lignin content in the bm2, bm4, and bm2-bm4 mutants relative to the wild-type control, with reductions in both guaiacyl (G)- and syringyl (S)-residues. The cellulose content was not different, but the reduction in lignin content was compensated by an increase in hemicellulosic polysaccharides. Linear discriminant analysis performed on the compositional data indicated that the bm2 and bm4 mutations act independently of each other on common cell wall biosynthetic steps. After quantitative analysis of scanning electron micrographs of midrib sections, the variation in chemical composition of the cell walls was shown to be correlated with the thickness of the sclerenchyma cell walls, but not with xylem vessel surface area. The bm2-bm4 double mutant represents the limit of phenotypic plasticity in cell wall composition, as the bm1-bm2-bm4 and bm2-bm3-bm4 mutants did not develop into mature plants, unlike the triple mutants bm1-bm2-bm3 and bm1-bm3-bm4.

Surface energy and wettability of spin-coated thin films of lignin isolated from wood

The surface energy of lignin films spin-coated onto oxidized silicon wafer has been determined from contact angle measurements of different test liquids with varying polar and dispersive components. Three different lignin raw materials were used, a kraft lignin from softwood, along with milled wood lignin from softwood and hardwood. Infrared and (31)P NMR spectroscopy was used to identify any major functional group differences between the lignin samples. No significant difference in the total solid-vapor surface energy for the different lignin films was observed; however, the polar component for the kraft lignin was much greater than for either of the milled wood lignin samples consistent with the presence of carboxyl groups and higher proportion of phenolic hydroxyl groups as shown by quantitative (31)P NMR on the phosphitylated samples. Furthermore, the total surface energy of lignin of 53-56 mJ m(-2) is of a similar magnitude to cellulose, also found in the wood cell wall; however, cellulose has a higher polar component leading to a lower contact angle with water and greater wettability than the milled wood lignin. Although lignin is not hydrophobic according to the strictest definition of a water contact angle greater than 90 degrees, water may only be considered a partially wetting liquid on a lignin surface. This supports the long-held belief that one of the functions of lignin in the wood cell wall is to provide water-proofing to aid in water transport. Furthermore, these results on the solid-vapor surface energy of lignin will provide invaluable insight for many natural and industrial applications including in the design and manufacture of many sustainable products such as paper, fiberboard, and polymer composite blends.

Lignin-modifying enzymes in filamentous basidiomycetes--ecological, functional and phylogenetic review

Filamentous fungi owe powerful abilities for decomposition of the extensive plant material, lignocellulose, and thereby are indispensable for the Earth's carbon cycle, generation of soil humic matter and formation of soil fine structure. The filamentous wood-decaying fungi belong to the phyla Basidiomycota and Ascomycota, and are unique organisms specified to degradation of the xylem cell wall components (cellulose, hemicelluloses, lignins and extractives). The basidiomycetous wood-decaying fungi form brackets, caps or resupinaceous (corticioid) fruiting bodies when growing on wood for dissemination of their sexual basidiospores. In particular, the ability to decompose the aromatic lignin polymers in wood is mostly restricted to the white rot basidiomycetes. The white-rot decay of wood is possible due to secretion of organic acids, secondary metabolites, and oxidoreductive metalloenzymes, heme peroxidases and laccases, encoded by divergent gene families in these fungi. The brown rot basidiomycetes obviously depend more on a non-enzymatic strategy for decomposition of wood cellulose and modification of lignin. This review gives a current ecological, genomic, and protein functional and phylogenetic perspective of the wood and lignocellulose-decaying basidiomycetous fungi.

Abiotic and biotic stresses and changes in the lignin content and composition in plants

Lignin is a polymer of phenylpropanoid compounds formed through a complex biosynthesis route, represented by a metabolic grid for which most of the genes involved have been sequenced in several plants, mainly in the model-plants Arabidopsis thaliana and Populus. Plants are exposed to different stresses, which may change lignin content and composition. In many cases, particularly for plant-microbe interactions, this has been suggested as defence responses of plants to the stress. Thus, understanding how a stressor modulates expression of the genes related with lignin biosynthesis may allow us to develop study-models to increase our knowledge on the metabolic control of lignin deposition in the cell wall. This review focuses on recent literature reporting on the main types of abiotic and biotic stresses that alter the biosynthesis of lignin in plants.

Phenolic acids act as signaling molecules in plant-microbe symbioses

Phenolic acids are the main polyphenols made by plants. These compounds have diverse functions and are immensely important in plant-microbe interactions/symbiosis. Phenolic compounds act as signaling molecules in the initiation of legumerhizobia symbioses, establishment of arbuscular mycorrhizal symbioses and can act as agents in plant defense. Flavonoids are a diverse class of polyphenolic compounds that have received considerable attention as signaling molecules involved in plant-microbe interactions compared to the more widely distributed, simple phenolic acids; hydroxybenzoic and hydroxycinnamic acids, which are both derived from the general phenylpropanoid pathway. This review describes the well-known roles attributed to phenolic compounds as nod gene inducers of legume-rhizobia symbioses, their roles in induction of the GmGin1 gene in fungus for establishment of arbuscular mycorrhizal symbiosis, their roles in inducing vir gene expression in Agrobacterium, and their roles as defense molecules operating against soil borne pathogens that could have great implications for rhizospheric microbial ecology. Amongst plant phenolics we have a lack of knowledge concerning the roles of phenolic acids as signaling molecules beyond the relatively well-defined roles of flavonoids. This may be addressed through the use of plant mutants defective in phenolic acids biosynthesis or knock down target genes in future investigations.

Consequences of antisense down-regulation of a lignification-specific peroxidase on leaf and vascular tissue in tobacco lines demonstrating enhanced enzymic saccharification

Tobacco plants expressing an antisense construct for a cationic peroxidase, which down-regulated lignin content at the presumed level of polymerisation, have been further analysed. T(1) plants were derived from a large-scale screen of T(0) mutant lines, previously published, which identified lines demonstrating consistent lignin down-regulation. Of these, line 1074 which had the most robust changes in lignin distribution through several generations was shown to have accompanying down-regulation of transcription of most lignin biosynthesis genes, except cinnamoyl-CoA reductase. The consistent 20% reduction in lignin was not accompanied by significant gross changes in vascular polysaccharide content and composition, despite a modest up-regulation of transcripts of genes involved in cellulose and hemicellulose synthesis. Morphologically, 1074 plants have under-developed xylem with both fibers and vessels having thin cell walls and limited secondary wall thickening with an abnormal S2 layer. However, they were not compromised in overall growth. Nevertheless, these and other lines showed improved potential industrial utility through a threefold increase in enzymic saccharification efficiency compared with wild-type (wt). Therefore, they were profiled for further un-intended effects of transgenesis that might compromise their value for industrial or biofuel processes. Other phenotypic changes included increased leaf thickness and bifurcation at the tip of the leaf. wt-Plants had smaller chloroplasts and higher stomatal numbers than mutants. Transgenic lines also showed a variable leaf pigment distribution with light-green areas that contained measurably less chlorophyll a, b, and carotenoids. Changes in epidermal pavement cells of mutant lines were also observed after exposure to various chemicals, while wt leaves retained their structural integrity. Despite these changes, the mutant plants grew and were viable indicating that lignification patterns can be manipulated considerably through targeting polymerisation without serious deleterious effects.

Ozone fumigation (twice ambient) reduces leaf infestation following natural and artificial inoculation by the endophytic fungus Apiognomonia errabunda of adult European beech trees

In 2006, a controlled infection study was performed in the 'Kranzberger Forst' to address the following questions: (1) Will massive artificial inoculation with Apiognomonia errabunda override the previously observed inhibitory effect of chronic ozone? (2) Can biochemical or molecular markers be detected to account for the action of ozone? To this end six adult beech trees were chosen, three ozone fumigated (2x ozone) and three control trees (ambient = 1x ozone). Spore-sprayed branches of sun and shade crown positions of each of the trees, and uninoculated control branches, were enclosed in 100-L plastic bags for one night to facilitate infection initiation. Samples were taken within a five-week period after inoculation. A. errabunda infestation levels quantified by real-time PCR increased in leaves that were not fumigated with additional ozone. Cell wall components and ACC (ethylene precursor 1-amino cyclopropane-1-carboxylic acid) increased upon ozone fumigation and may in part lead to the repression of fungal infection.

Synthesis, regulation and utilization of lignocellulosic biomass

Increasing the range of fuels and bioproducts that are derived from lignocellulosic biomass and the efficiency at which they are produced hinges on a detailed understanding of the cell wall biosynthetic process. Herein, we review the structure and biosynthesis of lignocellulosic biomass and also highlight recent breakthroughs that demonstrate a complex regulatory system of transcription factors, small interfering RNAs and phosphorylation that ultimately dictate the development of the polyalaminate cell wall. Finally, we provide an update on cases where plant biotechnology has been used to improve lignocellulosic biomass utilization as a second-generation biofuel source.

Convergent evolution of syringyl lignin biosynthesis via distinct pathways in the lycophyte Selaginella and flowering plants

Phenotypic convergence in unrelated lineages arises when different organisms adapt similarly under comparable selective pressures. In an apparent example of this process, syringyl lignin, a fundamental building block of plant cell walls, occurs in two major plant lineages, lycophytes and angiosperms, which diverged from one another more than 400 million years ago. Here, we show that this convergence resulted from independent recruitment of lignin biosynthetic cytochrome P450-dependent monooxygenases that route cell wall monomers through related but distinct pathways in the two lineages. In contrast with angiosperms, in which syringyl lignin biosynthesis requires two phenylpropanoid meta-hydroxylases C3'H and F5H, the lycophyte Selaginella employs one phenylpropanoid dual meta-hydroxylase to bypass several steps of the canonical lignin biosynthetic pathway. Transgenic expression of the Selaginella hydroxylase in Arabidopsis thaliana dramatically reroutes its endogenous lignin biosynthetic pathway, yielding a novel lignin composition not previously identified in nature. Our findings demonstrate a unique case of convergent evolution via distinct biochemical strategies and suggest a new way to genetically reconstruct lignin biosynthesis in higher plants.

Analysis of the Rac/Rop small GTPase family in rice: expression, subcellular localization and role in disease resistance

Plant-specific Rac/Rop small GTPases function as molecular switches for numerous signal transduction events, including defense responses. To understand the function of each of the seven Rac/Rop family members in rice, we studied the tissue-specific expression patterns of Rac/Rop genes by semi-quantitative reverse transcription-PCR (RT-PCR), and also Rac/Rop subcellular localization using green fluorescent protein (GFP) fusion proteins in transient expression systems. We also investigated the roles of these genes in disease resistance by testing single Rac/Rop-RNAi (RNA interference) plants against the rice blast pathogen Magnaporthe grisea. Our studies show that expression of OsRac2, 6 and 7 is very low in leaf blades, and reveal a strong correlation between the number of lysine and/or arginine (KR) residues in the polybasic region of Rac/Rop GTPases and their subcellular distribution in vivo. Infection assays showed that OsRac1 is a positive regulator of blast resistance, confirming previous observations, whereas OsRac4 and OsRac5 are negative regulators of blast resistance. OsRac6 may make minor contributions to disease resistance, while OsRac3 and OsRac7 are probably not involved in defense. Therefore, our study suggests that the rice Rac/Rop family plays multiple roles in diverse cellular activities and has both positive and negative functions in disease resistance.

Hitherto unrecognized fluorescence properties of coniferyl alcohol

We instituted a quasi-quality assurance program for demonstrating coniferyl alcohol’s fluorescence and fluorescence diminishment following enzymatic oxidation. The magnitude of diminishment was a measure of catalysis. High throughput screening was performed in pseudo-kinetic and endpoint modes by measuring the fluorescence at 416 nm following excitation at 290, 310 or 340 nm. Dose-response tracings were linear between two and three orders of magnitude with average limits of detection and quantitation of 1.8 and 6.9 μM coniferyl alcohol, respectively. Oxidation was evident with 0.025 μg/mL laccase or 0.003 μg/mL peroxidase or inside 5 min using 0.5 μg/mL laccase or 5 μM substrate. Sodium chloride inhibited (IC₅₀, 25 mM) laccase oxidation of coniferyl alcohol. Fluorescence from 10 concentrations (1 to 1000 μM) of coniferyl alcohol was stable for 24 hours over 14 excitation/emission cycles at 3 different combinations of excitation and emission wavelengths. In conclusion, coniferyl alcohol absorption and fluorescence assays should facilitate biomass lignin analyses and improve delignification.

Oxidation of isoeugenol by salen complexes with bulky substituents

The catalytic properties of bulky water-soluble salen complexes in the oxidation of isoeugenol (2-methoxy-4-(1-propenyl) phenol) have been investigated in aqueous ethanol solutions in order to obtain a mixture of polymeric compounds through dehydrogenative polymerization. The average molecular weight of dehydrogenated polymers (DHPs) was monitored by GPC and correlated to reaction conditions such as time, concentration of substrate, concentration of catalyst, type of oxidation agent, etc. The DHP synthesized by adopting the best reaction conditions was characterized by different analytical techniques (GPC, (13)C-NMR, (31)P-NMR and LC-MS) to elucidate its structure. The lignin-like polymer resulting from isoeugenol radical coupling possesses valuable biological activity and finds applications in a variety of fields, such as packaging industry and cultural heritage conservation.

The evolution of water transport in plants: an integrated approach

This review examines the evolution of the plant vascular system from its beginnings in the green algae to modern arborescent plants, highlighting the recent advances in developmental, organismal, geochemical and climatological research that have contributed to our understanding of the evolution of xylem. Hydraulic trade-offs in vascular structure-function are discussed in the context of canopy support and drought and freeze-thaw stress resistance. This qualitative and quantitative neontological approach to palaeobotany may be useful for interpreting the water-transport efficiencies and hydraulic limits in fossil plants. Large variations in atmospheric carbon dioxide levels are recorded in leaf stomatal densities, and may have had profound impacts on the water conservation strategies of ancient plants. A hypothesis that links vascular function with stomatal density is presented and examined in the context of the evolution of wood and/or vessels. A discussion of the broader impacts of plant transport on hydrology and climate concludes this review.

Comparative analysis of expressed sequence tags from the white-rot fungi (Phanerochaete chrysosporium)

Comprehensive analysis of the transcriptome of the P. chrysosporium is a useful approach to improve our understanding of its special and unique enzyme system and fungal evolution in molecular and industrial aspects. In order to unveil the functional diversity of this white-rot fungus in gene level and the expression patterns of its genes, in this study we carried out sequencing and annotation of 4,917 P. chrysosporium expressed sequence tags (ESTs). Through our bioinformatic ESTs analysis, we elucidated that 1,751 genes were derived from the present dataset of 4,917 ESTs, based on clustering and comparative genomic analyses of the ESTs. Of the 1,751 unique ESTs, 1,006 (57.5%) had homologues and orthologues in similarity searches. Our P. chrysosporium ESTs showed many genes for encoding 23 secreted proteins, many proteins for the degradation of cellulose and hemicelluloses, and heat shock proteins for stress resistance, which explain the reason why P. chrysosporium is very important and unique white-rot fungus in dealing with contaminated resources and in degrading lignin and in applying this organism to several industrial aspects.In addition, comparative analysis has shed the fresh light on the mystery about how its unique enzyme system and stress resistance have been evolved differently from its closest relatives.

Dual role of lignin in plant litter decomposition in terrestrial ecosystems

Plant litter decomposition is a critical step in the formation of soil organic matter, the mineralization of organic nutrients, and the carbon balance in terrestrial ecosystems. Biotic decomposition in mesic ecosystems is generally negatively correlated with the concentration of lignin, a group of complex aromatic polymers present in plant cell walls that is recalcitrant to enzymatic degradation and serves as a structural barrier impeding microbial access to labile carbon compounds. Although photochemical mineralization of carbon has recently been shown to be important in semiarid ecosystems, litter chemistry controls on photodegradative losses are not understood. We evaluated the importance of litter chemistry on photodegradation of grass litter and cellulose substrates with varying levels of lignin [cellulose-lignin (CL) substrates] under field conditions. Using wavelength-specific light attenuation filters, we found that light-driven mass loss was promoted by both UV and visible radiation. The spectral dependence of photodegradation correlated with the absorption spectrum of lignin but not of cellulose. Field incubations demonstrated that increasing lignin concentration reduced biotic decomposition, as expected, but linearly increased photodegradation. In addition, lignin content in CL substrates consistently decreased in photodegradative incubations. We conclude that lignin has a dual role affecting litter decomposition, depending on the dominant driver (biotic or abiotic) controlling carbon turnover. Under photodegradative conditions, lignin is preferentially degraded because it acts as an effective light-absorbing compound over a wide range of wavelengths. This mechanistic understanding of the role of lignin in plant litter decomposition will allow for more accurate predictions of carbon dynamics in terrestrial ecosystems.

Riboflavin induces resistance in rice against Rhizoctonia solani via jasmonate-mediated priming of phenylpropanoid pathway

Vitamins are plant growth regulators and activators of defense responses against pathogens. The cytomolecular mechanisms involved in the induction of resistance by chemicals especially vitamins on monocotyledonous plants are largely unknown. Here, we show that riboflavin, which acts as a defense activator in rice against economically important sheath blight caused by Rhizoctonia solani, primed the expression of lipoxygenase (LOX) as a key gene in octadecanoid pathway, and enhanced lignification. Exogenous jasmonic acid (JA) application on rice induces resistance against R. solani in a manner similar to riboflavin. Application of jasmonate-deficient rice mutant hebiba and using a LOX inhibitor revealed the main role of octadecanoid pathway in riboflavin-induced resistance (IR). In riboflavin-treated inoculated plants, upregulation of phenylalanine ammonia-lyase (PAL) expression, as a major marker of phenylpropanoid pathway, was detected downstream of LOX upregulation. Co-application of riboflavin and 5, 8, 11, 14-eicosatetraynoic acid (ETYA) on rice leaves revealed no upregulation of PAL and no priming in lignification. Furthermore, lower levels of PAL transcripts and lignin were detected in hebiba compared with control. These findings indicate the role of octadecanoid pathway in the induction of phenylpropanoid metabolism leading to lignification as a novel mechanism of riboflavin-IR in Oryza sativa-R. solani pathosystem.

Polymorphisms in O-methyltransferase genes are associated with stover cell wall digestibility in European maize (Zea mays L.)

BACKGROUND

OMT (O-methyltransferase) genes are involved in lignin biosynthesis, which relates to stover cell wall digestibility. Reduced lignin content is an important determinant of both forage quality and ethanol conversion efficiency of maize stover.

RESULTS

Variation in genomic sequences coding for COMT, CCoAOMT1, and CCoAOMT2 was analyzed in relation to stover cell wall digestibility for a panel of 40 European forage maize inbred lines, and re-analyzed for a panel of 34 lines from a published French study. Different methodologies for association analysis were performed and compared. Across association methodologies, a total number of 25, 12, 1, 6 COMT polymorphic sites were significantly associated with DNDF, OMD, NDF, and WSC, respectively. Association analysis for CCoAOMT1 and CCoAOMT2 identified substantially fewer polymorphic sites (3 and 2, respectively) associated with the investigated traits. Our re-analysis on the 34 lines from a published French dataset identified 14 polymorphic sites significantly associated with cell wall digestibility, two of them were consistent with our study. Promising polymorphisms putatively causally associated with variability of cell wall digestibility were inferred from the total number of significantly associated SNPs/Indels.

CONCLUSIONS

Several polymorphic sites for three O-methyltransferase loci were associated with stover cell wall digestibility. All three tested genes seem to be involved in controlling DNDF, in particular COMT. Thus, considerable variation among Bm3 wildtype alleles can be exploited for improving cell-wall digestibility. Target sites for functional markers were identified enabling development of efficient marker-based selection strategies.

Stone formation in peach fruit exhibits spatial coordination of the lignin and flavonoid pathways and similarity to Arabidopsis dehiscence

BACKGROUND

Lignification of the fruit endocarp layer occurs in many angiosperms and plays a critical role in seed protection and dispersal. This process has been extensively studied with relationship to pod shatter or dehiscence in Arabidopsis. Dehiscence is controlled by a set of transcription factors that define the fruit tissue layers and whether or not they lignify. In contrast, relatively little is known about similar processes in other plants such as stone fruits which contain an extremely hard lignified endocarp or stone surrounding a single seed.

RESULTS

Here we show that lignin deposition in peach initiates near the blossom end within the endocarp layer and proceeds in a distinct spatial-temporal pattern. Microarray studies using a developmental series from young fruits identified a sharp and transient induction of phenylpropanoid, lignin and flavonoid pathway genes concurrent with lignification and subsequent stone hardening. Quantitative polymerase chain reaction studies revealed that specific phenylpropanoid (phenylalanine ammonia-lyase and cinnamate 4-hydroxylase) and lignin (caffeoyl-CoA O-methyltransferase, peroxidase and laccase) pathway genes were induced in the endocarp layer over a 10 day time period, while two lignin genes (p-coumarate 3-hydroxylase and cinnamoyl CoA reductase) were co-regulated with flavonoid pathway genes (chalcone synthase, dihydroflavanol 4-reductase, leucoanthocyanidin dioxygen-ase and flavanone-3-hydrosylase) which were mesocarp and exocarp specific. Analysis of other fruit development expression studies revealed that flavonoid pathway induction is conserved in the related Rosaceae species apple while lignin pathway induction is not. The transcription factor expression of peach genes homologous to known endocarp determinant genes in Arabidopsis including SHATTERPROOF, SEEDSTCK and NAC SECONDARY WALL THICENING PROMOTING FACTOR 1 were found to be specifically expressed in the endocarp while the negative regulator FRUITFUL predominated in exocarp and mesocarp.

CONCLUSIONS

Collectively, the data suggests, first, that the process of endocarp determination and differentiation in peach and Arabidopsis share common regulators and, secondly, reveals a previously unknown coordination of competing lignin and flavonoid biosynthetic pathways during early fruit development.

Solution-state 2D NMR of ball-milled plant cell wall gels in DMSO-d(6)/pyridine-d(5)

NMR fingerprinting of the components of finely divided plant cell walls swelled in DMSO has been recently described. Cell wall gels, produced directly in the NMR tube with perdeutero-dimethylsulfoxide, allowed the acquisition of well resolved/dispersed 2D (13)C-(1)H correlated solution-state NMR spectra of the entire array of wall polymers, without the need for component fractionation. That is, without actual solubilization, and without apparent structural modification beyond that inflicted by the ball milling and ultrasonication steps, satisfactorily interpretable spectra can be acquired that reveal compositional and structural details regarding the polysaccharide and lignin components in the wall. Here, the profiling method has been improved by using a mixture of perdeuterated DMSO and pyridine (4 : 1, v/v). Adding pyridine provided not only easier sample handling because of the better mobility compared to the DMSO-d(6)-only system but also considerably elevated intensities and improved resolution of the NMR spectra due to the enhanced swelling of the cell walls. This modification therefore provides a more rapid method for comparative structural evaluation of plant cell walls than is currently available. We examined loblolly pine (Pinus taeda, a gymnosperm), aspen (Populus tremuloides, an angiosperm), kenaf (Hibiscus cannabinus, an herbaceous plant), and corn (Zea mays L., a grass, i.e., from the Poaceae family). In principle, lignin composition (notably, the syringyl : guaiacyl : p-hydroxyphenyl ratio) can be quantified without the need for lignin isolation. Correlations for p-coumarate units in the corn sample are readily seen, and a variety of the ferulate correlations are also well resolved; ferulates are important components responsible for cell wall cross-linking in grasses. Polysaccharide anomeric correlations were tentatively assigned for each plant sample based on standard samples and various literature data. With the new potential for chemometric analysis using the 2D NMR fingerprint, this gel-state method may provide the basis for an attractive approach to providing a secondary screen for selecting biomass lines and for optimizing biomass processing and conversion efficiencies.

Solution-state 2D NMR of ball-milled plant cell wall gels in DMSO-d(6)/pyridine-d(5)

NMR fingerprinting of the components of finely divided plant cell walls swelled in DMSO has been recently described. Cell wall gels, produced directly in the NMR tube with perdeutero-dimethylsulfoxide, allowed the acquisition of well resolved/dispersed 2D (13)C-(1)H correlated solution-state NMR spectra of the entire array of wall polymers, without the need for component fractionation. That is, without actual solubilization, and without apparent structural modification beyond that inflicted by the ball milling and ultrasonication steps, satisfactorily interpretable spectra can be acquired that reveal compositional and structural details regarding the polysaccharide and lignin components in the wall. Here, the profiling method has been improved by using a mixture of perdeuterated DMSO and pyridine (4 : 1, v/v). Adding pyridine provided not only easier sample handling because of the better mobility compared to the DMSO-d(6)-only system but also considerably elevated intensities and improved resolution of the NMR spectra due to the enhanced swelling of the cell walls. This modification therefore provides a more rapid method for comparative structural evaluation of plant cell walls than is currently available. We examined loblolly pine (Pinus taeda, a gymnosperm), aspen (Populus tremuloides, an angiosperm), kenaf (Hibiscus cannabinus, an herbaceous plant), and corn (Zea mays L., a grass, i.e., from the Poaceae family). In principle, lignin composition (notably, the syringyl : guaiacyl : p-hydroxyphenyl ratio) can be quantified without the need for lignin isolation. Correlations for p-coumarate units in the corn sample are readily seen, and a variety of the ferulate correlations are also well resolved; ferulates are important components responsible for cell wall cross-linking in grasses. Polysaccharide anomeric correlations were tentatively assigned for each plant sample based on standard samples and various literature data. With the new potential for chemometric analysis using the 2D NMR fingerprint, this gel-state method may provide the basis for an attractive approach to providing a secondary screen for selecting biomass lines and for optimizing biomass processing and conversion efficiencies.

Regulation of growth response to water stress in the soybean primary root. I. Proteomic analysis reveals region-specific regulation of phenylpropanoid metabolism and control of free iron in the elongation zone

In water-stressed soybean primary roots, elongation was maintained at well-watered rates in the apical 4 mm (region 1), but was progressively inhibited in the 4-8 mm region (region 2), which exhibits maximum elongation in well-watered roots. These responses are similar to previous results for the maize primary root. To understand these responses in soybean, spatial profiles of soluble protein composition were analysed. Among the changes, the results indicate that region-specific regulation of phenylpropanoid metabolism may contribute to the distinct growth responses in the different regions. Several enzymes related to isoflavonoid biosynthesis increased in abundance in region 1, correlating with a substantial increase of isoflavonoid content in this region which could contribute to growth maintenance via various potential mechanisms. In contrast, caffeoyl-CoA O-methyltransferase, which is involved in lignin synthesis, was highly up-regulated in region 2. This response was associated with enhanced accumulation of lignin, which may be related to the inhibition of growth in this region. Several proteins that increased in abundance in both regions of water-stressed roots were related to protection from oxidative damage. In particular, an increase in the abundance of ferritin proteins effectively sequestered more iron and prevented excess free iron in the elongation zone under water stress.

Lignin biosynthesis studies in plant tissue cultures

Lignin, a phenolic polymer abundant in cell walls of certain cell types, has given challenges to scientists studying its structure or biosynthesis. In plants lignified tissues are distributed between other, non-lignified tissues. Characterization of native lignin in the cell wall has been difficult due to the highly cross-linked nature of the wall components. Model systems, like plant tissue cultures with tracheary element differentiation or extracellular lignin formation, have provided useful information related to lignin structure and several aspects of lignin formation. For example, many enzyme activities in the phenylpropanoid pathway have been first identified in tissue cultures. This review focuses on studies where the use of plant tissue cultures has been advantageous in structural and biosynthesis studies of lignin, and discusses the validity of tissue cultures as models for lignin biosynthesis.

Cell wall lignin is polymerised by class III secretable plant peroxidases in Norway spruce

Class III secretable plant peroxidases occur as a large family of genes in plants with many functions and probable redundancy. In this review we are concentrating on the evidence we have on the catalysis of lignin polymerization by class III plant peroxidases present in the apoplastic space in the xylem of trees. Some evidence exists on the specificity of peroxidase isozymes in lignin polymerization through substrate specificity studies, from antisense mutants in tobacco and poplar and from tissue and cell culture lines of Norway spruce (Picea abies) and Zinnia elegans. In addition, real time (RT-)PCR results have pointed out that many peroxidases have tissue specific expression patterns in Norway spruce. Through combining information on catalytic properties of the enzymes, on the expression patterns of the corresponding genes, and on the presence of monolignols and hydrogen peroxide in the apoplastic space, we can show that specific peroxidases catalyze lignin polymerization in the apoplastic space of Norway spruce xylem.

Cinnamyl alcohol dehydrogenases-C and D, key enzymes in lignin biosynthesis, play an essential role in disease resistance in Arabidopsis

The deposition of lignin during plant-pathogen interactions is thought to play a role in plant defence. However, the function of lignification genes in plant disease resistance is poorly understood. In this article, we provide genetic evidence that the primary genes involved in lignin biosynthesis in Arabidopsis, CAD-C and CAD-D, act as essential components of defence to virulent and avirulent strains of the bacterial pathogen Pseudomonas syringae pv. tomato, possibly through the salicylic acid defence pathway. Thus, in contrast with cellulose synthesis, whose alteration leads to an increase in disease resistance, alteration of the cell wall lignin content leads directly or indirectly to defects in some defence components.

Diversification of P450 genes during land plant evolution

Plant cytochromes P450 (P450s) catalyze a wide variety of monooxygenation/hydroxylation reactions in primary and secondary metabolism. The number of P450 genes in plant genomes is estimated to be up to 1% of total gene annotations of each plant species. This implies that diversification within P450 gene superfamilies has led to the emergence of new metabolic pathways throughout land plant evolution. The conserved P450 families contribute to chemical defense mechanisms under terrestrial conditions and several are involved in hormone biosynthesis and catabolism. Species-specific P450 families are essential for the biosynthetic pathways of species-specialized metabolites. Future genome-wide analyses of P450 gene clusters and coexpression networks should help both in identifying the functions of many orphan P450s and in understanding the evolution of this versatile group of enzymes.

Mutations in the cinnamate 4-hydroxylase gene impact metabolism, growth and development in Arabidopsis

The initial reactions of the phenylpropanoid pathway convert phenylalanine to p-coumaroyl CoA, a branch point metabolite from which many phenylpropanoids are made. Although the second enzyme of this pathway, cinnamic acid 4-hydroxylase (C4H), is well characterized, a mutant for the gene encoding this enzyme has not yet, to our knowledge, been identified, presumably because knock-out mutations in this gene would have severe phenotypes. This work describes the characterization of an allelic series of Arabidopsis reduced epidermal fluorescence 3 (ref3) mutants, each of which harbor mis-sense mutations in C4H (At2g30490). Heterologous expression of the mutant proteins in Escherichia coli yields enzymes that exhibit P420 spectra, indicative of mis-folded proteins, or have limited ability to bind substrate, indicating that the mutations we have identified affect protein stability and/or enzyme function. In agreement with the early position of C4H in phenylpropanoid metabolism, ref3 mutant plants accumulate decreased levels of several different classes of phenylpropanoid end-products, and exhibit reduced lignin deposition and altered lignin monomer content. Furthermore, these plants accumulate a novel hydroxycinnamic ester, cinnamoylmalate, which is not found in the wild type. The decreased C4H activity in ref3 also causes pleiotropic phenotypes, including dwarfism, male sterility and the development of swellings at branch junctions. Together, these observations indicate that C4H function is critical to the normal biochemistry and development of Arabidopsis.

Short-rotation woody crops for bioenergy and biofuels applications

Purpose-grown trees will be part of the bioenergy solution in the United States, especially in the Southeast where plantation forestry is prevalent and economically important. Trees provide a "living biomass inventory" with existing end-use markets and associated infrastructure, unlike other biomass species such as perennial grasses. The economic feasibility of utilizing tree biomass is improved by increasing productivity through alternative silvicultural systems, improved breeding and biotechnology. Traditional breeding and selection, as well as the introduction of genes for improved growth and stress tolerance, have enabled high growth rates and improved site adaptability in trees grown for industrial applications. An example is the biotechnology-aided improvement of a highly productive tropical Eucalyptus hybrid, Eucalyptus grandis x Eucalyptus urophylla. This tree has acquired freeze tolerance by the introduction of a plant transcription factor that up-regulates the cold-response pathways and makes possible commercial plantings in the Southeastern United States. Transgenic trees with reduced lignin, modified lignin, or increased cellulose and hemicellulose will improve the efficiency of feedstock conversion into biofuels. Reduced lignin trees have been shown to improve efficiency in the pre-treatment step utilized in fermentation systems for biofuels production from lignocellulosics. For systems in which thermochemical or gasification approaches are utilized, increased density will be an important trait, while increased lignin might be a desired trait for direct firing or co-firing of wood for energy. Trees developed through biotechnology, like all transgenic plants, need to go through the regulatory process, which involves biosafety and risk assessment analyses prior to commercialization.

Genetic resources for maize cell wall biology

Grass species represent a major source of food, feed, and fiber crops and potential feedstocks for biofuel production. Most of the biomass is contributed by cell walls that are distinct in composition from all other flowering plants. Identifying cell wall-related genes and their functions underpins a fundamental understanding of growth and development in these species. Toward this goal, we are building a knowledge base of the maize (Zea mays) genes involved in cell wall biology, their expression profiles, and the phenotypic consequences of mutation. Over 750 maize genes were annotated and assembled into gene families predicted to function in cell wall biogenesis. Comparative genomics of maize, rice (Oryza sativa), and Arabidopsis (Arabidopsis thaliana) sequences reveal differences in gene family structure between grass species and a reference eudicot species. Analysis of transcript profile data for cell wall genes in developing maize ovaries revealed that expression within families differed by up to 100-fold. When transcriptional analyses of developing ovaries before pollination from Arabidopsis, rice, and maize were contrasted, distinct sets of cell wall genes were expressed in grasses. These differences in gene family structure and expression between Arabidopsis and the grasses underscore the requirement for a grass-specific genetic model for functional analyses. A UniformMu population proved to be an important resource in both forward- and reverse-genetics approaches to identify hundreds of mutants in cell wall genes. A forward screen of field-grown lines by near-infrared spectroscopic screen of mature leaves yielded several dozen lines with heritable spectroscopic phenotypes. Pyrolysis-molecular beam mass spectrometry confirmed that several nir mutants had altered carbohydrate-lignin compositions.

Ectopic expression of MYB46 identifies transcriptional regulatory genes involved in secondary wall biosynthesis in Arabidopsis

MYB46 functions as a transcriptional switch that turns on the genes necessary for secondary wall biosynthesis. Elucidating the transcriptional regulatory network immediately downstream of MYB46 is crucial to our understanding of the molecular and biochemical processes involved in the biosynthesis and deposition of secondary walls in plants. To gain insights into MYB46-mediated transcriptional regulation, we first established an inducible secondary wall thickening system in Arabidopsis by expressing MYB46 under the control of dexamethasone-inducible promoter. Then, we used an ATH1 GeneChip microarray and Illumina digital gene expression system to obtain a series of transcriptome profiles with regard to the induction of secondary wall development. These analyses allowed us to identify a group of transcription factors whose expression coincided with or preceded the induction of secondary wall biosynthetic genes. A transient transcriptional activation assay was used to confirm the hierarchical relationships among the transcription factors in the network. The in vivo assay showed that MYB46 transcriptionally activates downstream target transcription factors, three of which (AtC3H14, MYB52 and MYB63) were shown to be able to activate secondary wall biosynthesis genes. AtC3H14 activated the transcription of all of the secondary wall biosynthesis genes tested, suggesting that AtC3H14 may be another master regulator of secondary wall biosynthesis. The transcription factors identified here may include direct activators of secondary wall biosynthesis genes. The present study discovered novel hierarchical relationships among the transcription factors involved in the transcriptional regulation of secondary wall biosynthesis, and generated several testable hypotheses.

Degradation of glyphosate and other pesticides by ligninolytic enzymes

The ability of pure manganese peroxidase (MnP), laccase, lignin peroxidase (LiP) and horseradish peroxidase (HRP) to degrade the widely used herbicide glyphosate and other pesticides was studied in separate in vitro assays with addition of different mediators. Complete degradation of glyphosate was obtained with MnP, MnSO4 and Tween 80, with or without H2O2. In the presence of MnSO4, with or without H(2)O(2), MnP also transformed the herbicide, but to a lower rate. Laccase degraded glyphosate in the presence of (a) 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid) (ABTS), (b) MnSO(4) and Tween 80 and (c) ABTS, MnSO4 and Tween 80. The metabolite AMPA was detected in all cases where degradation of glyphosate occurred and was not degraded. The LiP was tested alone or with MnSO4, Tween 80, veratryl alcohol or H2O2 and in the HRP assay the enzyme was added alone or with H2O2 in the reaction mixture. However, these enzymes did not degrade glyphosate. Further experiments using MnP together with MnSO4 and Tween 80 showed that the enzyme was also able to degrade glyphosate in its commercial formulation Roundup Bio. The same enzyme mixture was tested for degradation of 22 other pesticides and degradation products present in a mixture and all the compounds were transformed, with degradation percentages ranging between 20 and 100%. Our results highlight the potential of ligninolytic enzymes to degrade pesticides. Moreover, they suggest that the formation of AMPA, the main metabolite of glyphosate degradation found in soils, can be a result of the activity of lignin-degrading enzymes.

The poplar GT8E and GT8F glycosyltransferases are functional orthologs of Arabidopsis PARVUS involved in glucuronoxylan biosynthesis

The poplar GT8E and GT8F glycosyltransferases have previously been shown to be associated with wood formation, but their roles in the biosynthesis of wood components are not known. Here, we show that PoGT8E and PoGT8F are expressed in vessels and fibers during wood formation and their encoded proteins are predominantly located in the endoplasmic reticulum. We demonstrate that expression of PoGT8E and PoGT8F in the Arabidopsis parvus mutant rescues the defects in the content and structure of glucuronoxylan conferred by the parvus mutation. These findings suggest that PoGT8E and PoGT8F are involved in glucuronoxylan biosynthesis during wood formation in poplar.

Transcriptional regulation of lignin biosynthesis

Lignin is the second most abundant plant biopolymer mainly present in the secondary walls of tracheary elements and fibers in wood. Understanding how lignin is biosynthesized has long been an interest to plant biologists and will have a significant impact on tree biotechnology. Lignin is polymerized from monolignols that are synthesized through the lignin biosynthetic pathway. To make lignin, all the genes in the lignin biosynthetic pathway need to be coordinately turned on. It has been shown that a common cis-element, namely the AC element, is present in the majority of the lignin biosynthetic genes and required for their expression in lignifying cells. Important progress has been made in the identification of transcription factors that bind to the AC elements and are potentially involved in the coordinated regulation of lignin biosynthesis. The Arabidopsis MYB58 and MYB63 as well as their poplar ortholog PtrMYB28 are transcriptional activators of the lignin biosynthetic pathway, whereas the eucalyptus EgMYB2 and pine PtMYB4 transcription factors are likely Arabidopsis MYB46 orthologs involved in the regulation of the entire secondary wall biosynthetic program. It was found that the transcriptional regulation of lignin biosynthesis is under the control of the same transcriptional network regulating the biosynthesis of other secondary wall components, including cellulose and xylan. The identification of transcription factors directly activating lignin biosynthetic genes provides unprecedented tools to potentially manipulate the amount of lignin in wood and other plant products based on our needs.

Engineering monolignol 4-O-methyltransferases to modulate lignin biosynthesis

Lignin is a complex polymer derived from the oxidative coupling of three classical monolignols. Lignin precursors are methylated exclusively at the meta-positions (i.e. 3/5-OH) of their phenyl rings by native O-methyltransferases, and are precluded from substitution of the para-hydroxyl (4-OH) position. Ostensibly, the para-hydroxyls of phenolics are critically important for oxidative coupling of phenoxy radicals to form polymers. Therefore, creating a 4-O-methyltransferase to substitute the para-hydroxyl of monolignols might well interfere with the synthesis of lignin. The phylogeny of plant phenolic O-methyltransferases points to the existence of a batch of evolutionarily "plastic" amino acid residues. Following one amino acid at a time path of directed evolution, and using the strategy of structure-based iterative site-saturation mutagenesis, we created a novel monolignol 4-O-methyltransferase from the enzyme responsible for methylating phenylpropenes. We show that two plastic residues in the active site of the parental enzyme are vital in dominating substrate discrimination. Mutations at either one of these separate the evolutionarily tightly linked properties of substrate specificity and regioselective methylation of native O-methyltransferase, thereby conferring the ability for para-methylation of the lignin monomeric precursors, primarily monolignols. Beneficial mutations at both sites have an additive effect. By further optimizing enzyme activity, we generated a triple mutant variant that may structurally constitute a novel phenolic substrate binding pocket, leading to its high binding affinity and catalytic efficiency on monolignols. The 4-O-methoxylation of monolignol efficiently impairs oxidative radical coupling in vitro, highlighting the potential for applying this novel enzyme in managing lignin polymerization in planta.

A hydroxycinnamoyltransferase responsible for synthesizing suberin aromatics in Arabidopsis

Suberin, a polyester polymer in the cell wall of terrestrial plants, controls the transport of water and nutrients and protects plant from pathogenic infections and environmental stresses. Structurally, suberin consists of aliphatic and aromatic domains; p-hydroxycinnamates, such as ferulate, p-coumarate, and/or sinapate, are the major phenolic constituents of the latter. By analyzing the "wall-bound" phenolics of mutant lines of Arabidopsis deficient in a family of acyl-CoA dependent acyltransferase (BAHD) genes, we discovered that the formation of aromatic suberin in Arabidopsis, primarily in seed and root tissues, depends on a member of the BAHD superfamily of enzymes encoded by At5g41040. This enzyme exhibits an omega-hydroxyacid hydroxycinnamoyltransferase activity with an in vitro kinetic preference for feruloyl-CoA and 16-hydroxypalmitic acid. Knocking down or knocking out the At5g41040 gene in Arabidopsis reduces specifically the quantity of ferulate in suberin, but does not affect the accumulation of p-coumarate or sinapate. The loss of the suberin phenolic differentially affects the aliphatic monomer loads and alters the permeability and sensitivity of seeds and roots to salt stress. This highlights the importance of suberin aromatics in the polymer's function.

Comparative genome analysis of lignin biosynthesis gene families across the plant kingdom

BACKGROUND

As a major component of plant cell wall, lignin plays important roles in mechanical support, water transport, and stress responses. As the main cause for the recalcitrance of plant cell wall, lignin modification has been a major task for bioenergy feedstock improvement. The study of the evolution and function of lignin biosynthesis genes thus has two-fold implications. First, the lignin biosynthesis pathway provides an excellent model to study the coordinative evolution of a biochemical pathway in plants. Second, understanding the function and evolution of lignin biosynthesis genes will guide us to develop better strategies for bioenergy feedstock improvement.

RESULTS

We analyzed lignin biosynthesis genes from fourteen plant species and one symbiotic fungal species. Comprehensive comparative genome analysis was carried out to study the distribution, relatedness, and family expansion of the lignin biosynthesis genes across the plant kingdom. In addition, we also analyzed the comparative synteny map between rice and sorghum to study the evolution of lignin biosynthesis genes within the Poaceae family and the chromosome evolution between the two species. Comprehensive lignin biosynthesis gene expression analysis was performed in rice, poplar and Arabidopsis. The representative data from rice indicates that different fates of gene duplications exist for lignin biosynthesis genes. In addition, we also carried out the biomass composition analysis of nine Arabidopsis mutants with both MBMS analysis and traditional wet chemistry methods. The results were analyzed together with the genomics analysis.

CONCLUSION

The research revealed that, among the species analyzed, the complete lignin biosynthesis pathway first appeared in moss; the pathway is absent in green algae. The expansion of lignin biosynthesis gene families correlates with substrate diversity. In addition, we found that the expansion of the gene families mostly occurred after the divergence of monocots and dicots, with the exception of the C4H gene family. Gene expression analysis revealed different fates of gene duplications, largely confirming plants are tolerant to gene dosage effects. The rapid expansion of lignin biosynthesis genes indicated that the translation of transgenic lignin modification strategies from model species to bioenergy feedstock might only be successful between the closely relevant species within the same family.

Pleiotropic changes in Arabidopsis f5h and sct mutants revealed by large-scale gene expression and metabolite analysis

Hydrocinnamic acid esters, lignin, flavonoids, glucosinolates, and salicylic acid protect plants against UV exposure, oxidative stress, diseases, and herbivores. Through the phenylpropanoid pathway, certain Brassicaceae family members, including Arabidopsis thaliana and Brassica napus, accumulate large amounts of the anti-nutritive sinapoylcholine (sinapine) in the seed. We successfully down-regulated activities of key enzymes in the pathway including F5H and SCT and achieved reduction of sinapine and lignin in B. napus seeds. Despite this success, it was unclear how multiple agronomic traits were affected in the transgenic plants. Here, we report altered large-scale gene expression of new alleles of f5h and sct mutants of A. thaliana and resultant accumulation of sinapoylglucose, disinapoylglucose, quercetin-3-O-rhamnoside, salicylic acid glucoside, and total indolyl glucosinolates in the two mutants. Expression of several flowering genes was altered in these mutants when grown under drought and NaCl treatments. Furthermore, both mutants were more susceptible to fungal infection than the wild type. Microarray experiments identified distinctive spatial and temporal expression patterns of gene clusters involved in silique/seed developmental processes and metabolite biosynthesis in these mutants. Taken together, these findings suggest that both f5h and sct mutants exhibit major differences in accumulation of diverse metabolites in the seed and profound changes in global large-scale gene expression, resulting in differential pleiotropic responses to environmental cues. Electronic supplementary material The online version of this article (doi:10.1007/s00425-009-1007-2) contains supplementary material, which is available to authorized users.

Metabolomic and physiological responses reveal multi-phasic acclimation of Arabidopsis thaliana to chronic UV radiation

Biochemical changes in vivo and pathway interactions were investigated using integrated physiological and metabolic responses of Arabidopsis thaliana L. to ultraviolet (UV) radiation (280-400 nm) at 9.96 kJ m(-2) d(-1) over the entire life cycle from seed to seed (8 weeks). Columbia-0 (Col-0) and a UV-B sensitive accession (fah-1) showed significant (P < 0.001) reductions in leaf growth after 6 weeks. Col-0 recovered growth after 8 weeks, with recovery corresponding to a switch from production of phenylpropanoids to flavonoids. fah-1 failed to recover, indicating that sinapate production is an essential component of recovery. Epidermal features show that UV radiation caused significant (P < 0.001) increases in trichome density, which may act as a structural defence response. Stomatal indices showed a significant (P < 0.0001) reduction in Col-0 and a significant (P < 0.001) increase in fah-1. Epidermal cell density was significantly increased under UV radiation on the abaxial leaf surface, suggesting that that a fully functioning phenylpropanoid pathway is a requirement for cell expansion and leaf development. Despite wild-type acclimation, the costs of adaptation lead to reduced plant fitness by decreasing flower numbers and total seed biomass. A multi-phasic acclimation to UV radiation and the induction of specific metabolites link stress-induced biochemical responses to enhanced acclimation.

Phenolic compounds in ectomycorrhizal interaction of lignin modified silver birch

BACKGROUND

The monolignol biosynthetic pathway interconnects with the biosynthesis of other secondary phenolic metabolites, such as cinnamic acid derivatives, flavonoids and condensed tannins. The objective of this study is to evaluate whether genetic modification of the monolignol pathway in silver birch (Betula pendula Roth.) would alter the metabolism of these phenolic compounds and how such alterations, if exist, would affect the ectomycorrhizal symbiosis.

RESULTS

Silver birch lines expressing quaking aspen (Populus tremuloides L.) caffeate/5-hydroxyferulate O-methyltransferase (PtCOMT) under the 35S cauliflower mosaic virus (CaMV) promoter showed a reduction in the relative expression of a putative silver birch COMT (BpCOMT) gene and, consequently, a decrease in the lignin syringyl/guaiacyl composition ratio. Alterations were also detected in concentrations of certain phenolic compounds. All PtCOMT silver birch lines produced normal ectomycorrhizas with the ectomycorrhizal fungus Paxillus involutus (Batsch: Fr.), and the formation of symbiosis enhanced the growth of the transgenic plants.

CONCLUSION

The down-regulation of BpCOMT in the 35S-PtCOMT lines caused a reduction in the syringyl/guaiacyl ratio of lignin, but no significant effect was seen in the composition or quantity of phenolic compounds that would have been caused by the expression of PtCOMT under the 35S or UbB1 promoter. Moreover, the detected alterations in the composition of lignin and secondary phenolic compounds had no effect on the interaction between silver birch and P. involutus.