Monolignol acyltransferase for lignin p-hydroxybenzoylation in Populus

Lysine 2-hydroxyisobutyrylation proteomics analyses reveal the regulatory mechanism of CaMYB61-CaAFR1 module in regulating stem development in Capsicum annuum L

Plant stems constitute the most abundant renewable resource on earth. The function of lysine (K)-2-hydroxyisobutyrylation (Khib), a novel post-translational modification (PTM), has not yet been elucidated in plant stem development. Here, by assessing typical pepper genotypes with straight stem (SS) and prostrate stem (PS), we report the first large-scale proteomics analysis for protein Khib to date. Khib-modifications influenced central metabolic processes involved in stem development, such as glycolysis/gluconeogenesis and protein translation. The high Khib level regulated gene expression and protein accumulation associated with cell wall formation in the pepper stem. Specially, we found that CaMYB61 knockdown lines that exhibited prostrate stem phenotypes had high Khib levels. Most histone deacetylases (HDACs, e.g., switch-independent 3 associated polypeptide function related 1, AFR1) potentially function as the "erasing enzymes" involved in reversing Khib level. CaMYB61 positively regulated CaAFR1 expression to erase Khib and promote cellulose and hemicellulose accumulation in the stem. Therefore, we propose a bidirectional regulation hypothesis of "Khib modifications" and "Khib erasing" in stem development, and reveal a novel epigenetic regulatory network in which the CaMYB61-CaAFR1 molecular module participating in the regulation of Khib levels and biosynthesis of cellulose and hemicellulose for the first time.

Mutating alfalfa COUMARATE 3-HYDROXYLASE using multiplex CRISPR/Cas9 leads to reduced lignin deposition and improved forage quality

Alfalfa (Medicago sativa L.) forage quality is adversely affected by lignin deposition in cell walls at advanced maturity stages. Reducing lignin content through RNA interference or antisense approaches has been shown to improve alfalfa forage quality and digestibility. We employed a multiplex CRISPR/Cas9-mediated gene-editing system to reduce lignin content and alter lignin composition in alfalfa by targeting the COUMARATE 3-HYDROXYLASE (MsC3H) gene, which encodes a key enzyme in lignin biosynthesis. Four guide RNAs (gRNAs) targeting the first exon of MsC3H were designed and clustered into a tRNA-gRNA polycistronic system and introduced into tetraploid alfalfa via Agrobacterium-mediated transformation. Out of 130 transgenic lines, at least 73 lines were confirmed to contain gene-editing events in one or more alleles of MsC3H. Fifty-five lines were selected for lignin content/composition analysis. Amongst these lines, three independent tetra-allelic homozygous lines (Msc3h-013, Msc3h-121, and Msc3h-158) with different mutation events in MsC3H were characterized in detail. Homozygous mutation of MsC3H in these three lines significantly reduced the lignin content and altered lignin composition in stems. Moreover, these lines had significantly lower levels of acid detergent fiber and neutral detergent fiber as well as higher levels of total digestible nutrients, relative feed values, and in vitro true dry matter digestibility. Taken together, these results showed that CRISPR/Cas9-mediated editing of MsC3H successfully reduced shoot lignin content, improved digestibility, and nutritional values without sacrificing plant growth and biomass yield. These lines could be used in alfalfa breeding programs to generate elite transgene-free alfalfa cultivars with reduced lignin and improved forage quality.

Wood of trees: Cellular structure, molecular formation, and genetic engineering

Wood is an invaluable asset to human society due to its renewable nature, making it suitable for both sustainable energy production and material manufacturing. Additionally, wood derived from forest trees plays a crucial role in sequestering a significant portion of the carbon dioxide fixed during photosynthesis by terrestrial plants. Nevertheless, with the expansion of the global population and ongoing industrialization, forest coverage has been substantially decreased, resulting in significant challenges for wood production and supply. Wood production practices have changed away from natural forests toward plantation forests. Thus, understanding the underlying genetic mechanisms of wood formation is the foundation for developing high-quality, fast-growing plantation trees. Breeding ideal forest trees for wood production using genetic technologies has attracted the interest of many. Tremendous studies have been carried out in recent years on the molecular, genetic, and cell-biological mechanisms of wood formation, and considerable progress and findings have been achieved. These studies and findings indicate enormous possibilities and prospects for tree improvement. This review will outline and assess the cellular and molecular mechanisms of wood formation, as well as studies on genetically improving forest trees, and address future development prospects.

Simultaneous suppression of lignin, tricin and wall-bound phenolic biosynthesis via the expression of monolignol 4-O-methyltransferases in rice

Grass lignocelluloses feature complex compositions and structures. In addition to the presence of conventional lignin units from monolignols, acylated monolignols and flavonoid tricin also incorporate into lignin polymer; moreover, hydroxycinnamates, particularly ferulate, cross-link arabinoxylan chains with each other and/or with lignin polymers. These structural complexities make grass lignocellulosics difficult to optimize for effective agro-industrial applications. In the present study, we assess the applications of two engineered monolignol 4-O-methyltransferases (MOMTs) in modifying rice lignocellulosic properties. Two MOMTs confer regiospecific para-methylation of monolignols but with different catalytic preferences. The expression of MOMTs in rice resulted in differential but drastic suppression of lignin deposition, showing more than 50% decrease in guaiacyl lignin and up to an 90% reduction in syringyl lignin in transgenic lines. Moreover, the levels of arabinoxylan-bound ferulate were reduced by up to 50%, and the levels of tricin in lignin fraction were also substantially reduced. Concomitantly, up to 11 μmol/g of the methanol-extractable 4-O-methylated ferulic acid and 5-7 μmol/g 4-O-methylated sinapic acid were accumulated in MOMT transgenic lines. Both MOMTs in vitro displayed discernible substrate promiscuity towards a range of phenolics in addition to the dominant substrate monolignols, which partially explains their broad effects on grass phenolic biosynthesis. The cell wall structural and compositional changes resulted in up to 30% increase in saccharification yield of the de-starched rice straw biomass after diluted acid-pretreatment. These results demonstrate an effective strategy to tailor complex grass cell walls to generate improved cellulosic feedstocks for the fermentable sugar-based production of biofuel and bio-chemicals.

Woody plant cell walls: Fundamentals and utilization

Cell walls in plants, particularly forest trees, are the major carbon sink of the terrestrial ecosystem. Chemical and biosynthetic features of plant cell walls were revealed early on, focusing mostly on herbaceous model species. Recent developments in genomics, transcriptomics, epigenomics, transgenesis, and associated analytical techniques are enabling novel insights into formation of woody cell walls. Here, we review multilevel regulation of cell wall biosynthesis in forest tree species. We highlight current approaches to engineering cell walls as potential feedstock for materials and energy and survey reported field tests of such engineered transgenic trees. We outline opportunities and challenges in future research to better understand cell type biogenesis for more efficient wood cell wall modification and utilization for biomaterials or for enhanced carbon capture and storage.

Identification and Functional Characterization of a Novel Sinapyl Alcohol Acyltransferase from Euphorbia lathyris L

Methoxyeugenol is a phenylpropene compound derived from plants and has various bioactivities. The chemical synthesis of methoxyeugenol is accompanied by pollution issues, whereas extraction from plants is associated with problems such as low yield and high cost. The production of methoxyeugenol can be effectively addressed through an enzymatic approach. In this study, the acyltransferase genes of Euphorbia lathyris L. were screened by homologous alignment of the transcriptome data of E. lathyris in the late growth stage and the acyltransferase genes of the closely related plant species. The results showed that ElBAHD10 had the closest relationship with earlier reported ScCFAT and PhCFAT, which were found to catalyze the reaction of coniferyl alcohol to generate coniferyl acetate. The ElBAHD10 gene was successfully cloned from E. lathyris and subsequently expressed in Escherichia coli. The purified protein ElBAHD10 catalyzed the reaction of sinapyl alcohol with acetyl CoA and cinnamoyl CoA to form sinapyl acetate and sinapyl cinnamate, respectively. In contrast, the crude ElBAHD10 protein could catalyze sinapyl alcohol to directly generate methoxyeugenol. The recombinant E. coli strain expressing ElBAHD10 produced methoxyeugenol through whole-cell transformation. This study provides insights and lays the foundation for methoxyeugenol production through biosynthetic approaches.

Family characteristics, phylogenetic reconstruction, and potential applications of the plant BAHD acyltransferase family

The BAHD acyltransferase family is a class of proteins in plants that can acylate a variety of primary and specialized secondary metabolites. The typically acylated products have greatly improved stability, lipid solubility, and bioavailability and thus show significant differences in their physicochemical properties and pharmacological activities. Here, we review the protein structure, catalytic mechanism, and phylogenetic reconstruction of plant BAHD acyltransferases to describe their family characteristics, acylation reactions, and the processes of potential functional differentiation. Moreover, the potential applications of the BAHD family in human activities are discussed from the perspectives of improving the quality of economic plants, enhancing the efficacy of medicinal plants, improving plant biomass for use in biofuel, and promoting stress resistance of land plants. This review provides a reference for the research and production of plant BAHD acyltransferases.

Lignin Hydrogenolysis: Phenolic Monomers from Lignin and Associated Phenolates across Plant Clades

The chemical complexity of lignin remains a major challenge for lignin valorization into commodity and fine chemicals. A knowledge of the lignin features that favor its valorization and which plants produce such lignins can be used in plant selection or to engineer them to produce lignins that are more ideally suited for conversion. Sixteen biomass samples were compositionally surveyed by NMR and analytical degradative methods, and the yields of phenolic monomers following hydrogenolytic depolymerization were assessed to elucidate the key determinants controlling the depolymerization. Hardwoods, including those incorporating monolignol p-hydroxybenzoates into their syringyl/guaiacyl copolymeric lignins, produced high monomer yields by hydrogenolysis, whereas grasses incorporating monolignol p-coumarates and ferulates gave lower yields, on a lignin basis. Softwoods, with their more condensed guaiacyl lignins, gave the lowest yields. Lignins with a high syringyl unit content released elevated monomer levels, with a high-syringyl polar transgenic being particularly striking. Herein, we distinguish phenolic monomers resulting from the core lignin vs those from pendent phenolate esters associated with the biomass cell wall, acylating either polysaccharides or lignins. The basis for these observations is rationalized as a means to select or engineer biomass for optimal conversion to worthy phenolic monomers.

Novel candidate genes for lignin structure identified through genome-wide association study of naturally varying Populus trichocarpa

Populus is a promising lignocellulosic feedstock for biofuels and bioproducts. However, the cell wall biopolymer lignin is a major barrier in conversion of biomass to biofuels. To investigate the variability and underlying genetic basis of the complex structure of lignin, a population of 409 three-year-old, naturally varying Populus trichocarpa genotypes were characterized by heteronuclear single quantum coherence (HSQC) nuclear magnetic resonance (NMR). A subsequent genome-wide association study (GWAS) was conducted using approximately 8.3 million single nucleotide polymorphisms (SNPs), which identified 756 genes that were significantly associated (-log₁₀(p-value)>6) with at least one lignin phenotype. Several promising candidate genes were identified, many of which have not previously been reported to be associated with lignin or cell wall biosynthesis. These results provide a resource for gaining insights into the molecular mechanisms of lignin biosynthesis and new targets for future genetic improvement in poplar.

Modification of plant cell walls with hydroxycinnamic acids by BAHD acyltransferases

In the last decade it has become clear that enzymes in the "BAHD" family of acyl-CoA transferases play important roles in the addition of phenolic acids to form ester-linked moieties on cell wall polymers. We focus here on the addition of two such phenolics-the hydroxycinnamates, ferulate and p-coumarate-to two cell wall polymers, glucuronoarabinoxylan and to lignin. The resulting ester-linked feruloyl and p-coumaroyl moities are key features of the cell walls of grasses and other commelinid monocots. The capacity of ferulate to participate in radical oxidative coupling means that its addition to glucuronoarabinoxylan or to lignin has profound implications for the properties of the cell wall - allowing respectively oxidative crosslinking to glucuronoarabinoxylan chains or introducing ester bonds into lignin polymers. A subclade of ~10 BAHD genes in grasses is now known to (1) contain genes strongly implicated in addition of p-coumarate or ferulate to glucuronoarabinoxylan (2) encode enzymes that add p-coumarate or ferulate to lignin precursors. Here, we review the evidence for functions of these genes and the biotechnological applications of manipulating them, discuss our understanding of mechanisms involved, and highlight outstanding questions for future research.

Metabolic engineering of p-hydroxybenzoate in poplar lignin

Ester-linked p-hydroxybenzoate occurs naturally in poplar lignin as pendent groups that can be released by mild alkaline hydrolysis. These 'clip-off' phenolics can be separated from biomass and upgraded into diverse high-value bioproducts. We introduced a bacterial chorismate pyruvate lyase gene into transgenic poplar trees with the aim of producing more p-hydroxybenzoate from chorismate, itself a metabolic precursor to lignin. By driving heterologous expression specifically in the plastids of cells undergoing secondary wall formation, this strategy achieved a 50% increase in cell-wall-bound p-hydroxybenzoate in mature wood and nearly 10 times more in developing xylem relative to control trees. Comparable amounts also remained as soluble p-hydroxybenzoate-containing xylem metabolites, pointing to even greater engineering potential. Mass spectrometry imaging showed that the elevated p-hydroxybenzoylation was largely restricted to the cell walls of fibres. Finally, transgenic lines outperformed control trees in assays of saccharification potential. This study highlights the biotech potential of cell-wall-bound phenolate esters and demonstrates the importance of substrate supply in lignin engineering.

Engineering sorghum for higher 4-hydroxybenzoic acid content

Engineering bioenergy crops to accumulate coproducts in planta can increase the value of lignocellulosic biomass and enable a sustainable bioeconomy. In this study, we engineered sorghum with a bacterial gene encoding a chorismate pyruvate-lyase (ubiC) to reroute the plastidial pool of chorismate from the shikimate pathway into the valuable compound 4-hydroxybenzoic acid (4-HBA). A gene encoding a feedback-resistant version of 3-deoxy-d-arabino-heptulonate-7-phosphate synthase (aroG) was also introduced in an attempt to increase the carbon flux through the shikimate pathway. At the full maturity and senesced stage, two independent lines that co-express ubiC and aroG produced 1.5 and 1.7 dw% of 4-HBA in biomass, which represents 36- and 40-fold increases compared to the titer measured in wildtype. The two transgenic lines showed no obvious phenotypes, growth defects, nor alteration of cell wall polysaccharide content when cultivated under controlled conditions. In the field, when harvested before grain maturity, transgenic lines contained 0.8 and 1.2 dw% of 4-HBA, which represent economically relevant titers based on recent technoeconomic analysis. Only a slight reduction (11-15%) in biomass yield was observed in transgenics grown under natural environment. This work provides the first metabolic engineering steps toward 4-HBA overproduction in the bioenergy crop sorghum to improve the economics of biorefineries by accumulating a value-added coproduct that can be recovered from biomass and provide an additional revenue stream.

Lignin engineering in forest trees: From gene discovery to field trials

Wood is an abundant and renewable feedstock for the production of pulp, fuels, and biobased materials. However, wood is recalcitrant toward deconstruction into cellulose and simple sugars, mainly because of the presence of lignin, an aromatic polymer that shields cell-wall polysaccharides. Hence, numerous research efforts have focused on engineering lignin amount and composition to improve wood processability. Here, we focus on results that have been obtained by engineering the lignin biosynthesis and branching pathways in forest trees to reduce cell-wall recalcitrance, including the introduction of exotic lignin monomers. In addition, we draw general conclusions from over 20 years of field trial research with trees engineered to produce less or altered lignin. We discuss possible causes and solutions for the yield penalty that is often associated with lignin engineering in trees. Finally, we discuss how conventional and new breeding strategies can be combined to develop elite clones with desired lignin properties. We conclude this review with priorities for the development of commercially relevant lignin-engineered trees.

Genome assembly and chemogenomic profiling of National Flower of Singapore Papilionanthe Miss Joaquim ‘Agnes’ reveals metabolic pathways regulating floral traits

Singapore’s National Flower, Papilionanthe (Ple.) Miss Joaquim ‘Agnes’ (PMJ) is highly prized as a horticultural flower from the Orchidaceae family. A combination of short-read sequencing, single-molecule long-read sequencing and chromatin contact mapping was used to assemble the PMJ genome, spanning 2.5 Gb and 19 pseudo-chromosomal scaffolds. Genomic resources and chemical profiling provided insights towards identifying, understanding and elucidating various classes of secondary metabolite compounds synthesized by the flower. For example, presence of the anthocyanin pigments detected by chemical profiling coincides with the expression of ANTHOCYANIN SYNTHASE (ANS), an enzyme responsible for the synthesis of the former. Similarly, the presence of vandaterosides (a unique class of glycosylated organic acids with the potential to slow skin aging) discovered using chemical profiling revealed the involvement of glycosyltransferase family enzymes candidates in vandateroside biosynthesis. Interestingly, despite the unnoticeable scent of the flower, genes involved in the biosynthesis of volatile compounds and chemical profiling revealed the combination of oxygenated hydrocarbons, including traces of linalool, beta-ionone and vanillin, forming the scent profile of PMJ. In summary, by combining genomics and biochemistry, the findings expands the known biodiversity repertoire of the Orchidaceae family and insights into the genome and secondary metabolite processes of PMJ.

Genome assembly and metabolomic analysis of Singapore’s national flower, Papilionanthe (Ple.) Miss Joaquim “Agnes” (PMJ), expands the known genetic repertoire of the Orchidaceae family and insights into the biochemical processes of PMJ.

Engineering Curcumin Biosynthesis in Poplar Affects Lignification and Biomass Yield

Lignocellulosic biomass is recalcitrant toward deconstruction into simple sugars mainly due to the presence of lignin. By engineering plants to partially replace traditional lignin monomers with alternative ones, lignin degradability and extractability can be enhanced. Previously, the alternative monomer curcumin has been successfully produced and incorporated into lignified cell walls of Arabidopsis by the heterologous expression of DIKETIDE-CoA SYNTHASE (DCS) and CURCUMIN SYNTHASE2 (CURS2). The resulting transgenic plants did not suffer from yield penalties and had an increased saccharification yield after alkaline pretreatment. Here, we translated this strategy into the bio-energy crop poplar. Via the heterologous expression of DCS and CURS2 under the control of the secondary cell wall CELLULOSE SYNTHASE A8-B promoter (ProCesA8-B), curcumin was also produced and incorporated into the lignified cell walls of poplar. ProCesA8-B:DCS_CURS2 transgenic poplars, however, suffered from shoot-tip necrosis and yield penalties. Compared to that of the wild-type (WT), the wood of transgenic poplars had 21% less cellulose, 28% more matrix polysaccharides, 23% more lignin and a significantly altered lignin composition. More specifically, ProCesA8-B:DCS_CURS2 lignin had a reduced syringyl/guaiacyl unit (S/G) ratio, an increased frequency of p-hydroxyphenyl (H) units, a decreased frequency of p-hydroxybenzoates and a higher fraction of phenylcoumaran units. Without, or with alkaline or hot water pretreatment, the saccharification efficiency of the transgenic lines was equal to that of the WT. These differences in (growth) phenotype illustrate that translational research in crops is essential to assess the value of an engineering strategy for applications. Further fine-tuning of this research strategy (e.g., by using more specific promoters or by translating this strategy to other crops such as maize) might lead to transgenic bio-energy crops with cell walls more amenable to deconstruction without settling in yield.

A new approach to zip-lignin: 3,4-dihydroxybenzoate is compatible with lignification

Renewed interests in the development of bioenergy, biochemicals, and biomaterials have elicited new strategies for engineering the lignin of biomass feedstock plants. This study shows, for the first time, that 3,4-dihydroxybenzoate (DHB) is compatible with the radical coupling reactions that assemble polymeric lignin in plants. We introduced a bacterial 3-dehydroshikimate dehydratase into hybrid poplar (Populus alba × grandidentata) to divert carbon flux away from the shikimate pathway, which lies upstream of lignin biosynthesis. Transgenic poplar wood had up to 33% less lignin with p-hydroxyphenyl units comprising as much as 10% of the lignin. Mild alkaline hydrolysis of transgenic wood released fewer ester-linked p-hydroxybenzoate groups than control trees, and revealed the novel incorporation of cell-wall-bound DHB, as well as glycosides of 3,4-dihydroxybenzoic acid (DHBA). Two-dimensional nuclear magnetic resonance (2D-NMR) analysis uncovered DHBA-derived benzodioxane structures suggesting that DHB moieties were integrated into the lignin polymer backbone. In addition, up to 40% more glucose was released from transgenic wood following ionic liquid pretreatment and enzymatic hydrolysis. This work highlights the potential of diverting carbon flux from the shikimate pathway for lignin engineering and describes a new type of 'zip-lignin' derived from the incorporation of DHB into poplar lignin.

Identification, Molecular Cloning, and Functional Characterization of a Coniferyl Alcohol Acyltransferase Involved in the Biosynthesis of Dibenzocyclooctadiene Lignans in Schisandra chinensis

Schisandra chinensis owes its therapeutic efficacy to the dibenzocyclooctadiene lignans, which are limited to the Schisandraceae family and whose biosynthetic pathway has not been elucidated. Coniferyl alcohol is the synthetic precursor of various types of lignans and can be acetylated to form coniferyl acetate by coniferyl alcohol acyltransferase (CFAT), which belongs to the BAHD acyltransferase family. This catalytic reaction is important because it is the first committed step of the hypothetical biosynthetic pathway in which coniferyl alcohol gives rise to dibenzocyclooctadiene lignans. However, the gene encoding CFAT in S. chinensis has not been identified. In this study, firstly we identified 37 ScBAHD genes from the transcriptome datasets of S. chinensis. According to bioinformatics, phylogenetic, and expression profile analyses, 1 BAHD gene, named ScBAHD1, was cloned from S. chinensis. The heterologous expression in Escherichia coli and in vitro activity assays revealed that the recombinant enzyme of ScBAHD1 exhibits acetyltransferase activity with coniferyl alcohol and some other alcohol substrates by using acetyl-CoA as the acetyl donor, which indicates ScBAHD1 functions as ScCFAT. Subcellular localization analysis showed that ScCFAT is mainly located in the cytoplasm. In addition, we generated a three-dimensional (3D) structure of ScCFAT by homology modeling and explored the conformational interaction between protein and ligands by molecular docking simulations. Overall, this study identified the first enzyme with catalytic activity from the Schisandraceae family and laid foundations for future investigations to complete the biosynthetic pathway of dibenzocyclooctadiene lignans.

Rerouting of the lignin biosynthetic pathway by inhibition of cytosolic shikimate recycling in transgenic hybrid aspen

Lignin is a phenolic polymer deposited in the plant cell wall, and is mainly polymerized from three canonical monomers (monolignols), i.e. p-coumaryl, coniferyl and sinapyl alcohols. After polymerization, these alcohols form different lignin substructures. In dicotyledons, monolignols are biosynthesized from phenylalanine, an aromatic amino acid. Shikimate acts at two positions in the route to the lignin building blocks. It is part of the shikimate pathway that provides the precursor for the biosynthesis of phenylalanine, and is involved in the transesterification of p-coumaroyl-CoA to p-coumaroyl shikimate, one of the key steps in the biosynthesis of coniferyl and sinapyl alcohols. The shikimate residue in p-coumaroyl shikimate is released in later steps, and the resulting shikimate becomes available again for the biosynthesis of new p-coumaroyl shikimate molecules. In this study, we inhibited cytosolic shikimate recycling in transgenic hybrid aspen by accelerated phosphorylation of shikimate in the cytosol through expression of a bacterial shikimate kinase (SK). This expression elicited an increase in p-hydroxyphenyl units of lignin and, by contrast, a decrease in guaiacyl and syringyl units. Transgenic plants with high SK activity produced a lignin content comparable to that in wild-type plants, and had an increased processability via enzymatic saccharification. Although expression of many genes was altered in the transgenic plants, elevated SK activity did not exert a significant effect on the expression of the majority of genes responsible for lignin biosynthesis. The present results indicate that cytosolic shikimate recycling is crucial to the monomeric composition of lignin rather than for lignin content.

pHBMT1, a BAHD-family monolignol acyltransferase, mediates lignin acylation in poplar

Poplar (Populus) lignin is naturally acylated with p-hydroxybenzoate ester moieties. However, the enzyme(s) involved in the biosynthesis of the monolignol-p-hydroxybenzoates have remained largely unknown. Here, we performed an in vitro screen of the Populus trichocarpa BAHD acyltransferase superfamily (116 genes) using a wheatgerm cell-free translation system and found five enzymes capable of producing monolignol-p-hydroxybenzoates. We then compared the transcript abundance of the five corresponding genes with p-hydroxybenzoate concentrations using naturally occurring unrelated genotypes of P. trichocarpa and revealed a positive correlation between the expression of p-hydroxybenzoyl-CoA monolig-nol transferase (pHBMT1, Potri.001G448000) and p-hydroxybenzoate levels. To test whether pHBMT1 is responsible for the biosynthesis of monolignol-p-hydroxybenzoates, we overexpressed pHBMT1 in hybrid poplar (Populus alba × P. grandidentata) (35S::pHBMT1 and C4H::pHBMT1). Using three complementary analytical methods, we showed that there was an increase in soluble monolignol-p-hydroxybenzoates and cell-wall-bound monolignol-p-hydroxybenzoates in the poplar transgenics. As these pendent groups are ester-linked, saponification releases p-hydroxybenzoate, a precursor to parabens that are used in pharmaceuticals and cosmetics. This identified gene could therefore be used to engineer lignocellulosic biomass with increased value for emerging biorefinery strategies.

Lignin p-Hydroxybenzoylation Is Negatively Correlated With Syringyl Units in Poplar

The lignin found in the cell walls of poplar fibres is decorated with ester-linked p-hydroxybenzoate moieties that originate from the participation of acylated monolignols in lignin polymerisation. Although little is known about the biological implications of these cell-wall constituents, it has historically been postulated that acylated monolignols might promote lignification in syringyl lignin-rich species such as poplar. However, cell-wall-bound p-hydroxybenzoate groups were negatively correlated with syringyl units in a collection of 316 unrelated genotypes of black cottonwood (Populus trichocarpa). Based upon this observation, several alternative hypotheses on the occurrence of lignin acylation are presented.

The Inducible Accumulation of Cell Wall-Bound p-Hydroxybenzoates Is Involved in the Regulation of Gravitropic Response of Poplar

Lignin in Populus species is acylated with p-hydroxybenzoate. Monolignol p-hydroxybenzoyltransferase 1 (PHBMT1) mediates p-hydroxybenzoylation of sinapyl alcohol, eventually leading to the modification of syringyl lignin subunits. Angiosperm trees upon gravistimulation undergo the re-orientation of their growth along with the production of specialized secondary xylem, i.e., tension wood (TW), that generates tensile force to pull the inclined stem or leaning branch upward. Sporadic evidence suggests that angiosperm TW contains relatively a high percentage of syringyl lignin and lignin-bound p-hydroxybenzoate. However, whether such lignin modification plays a role in gravitropic response remains unclear. By imposing mechanical bending and/or gravitropic stimuli to the hybrid aspens in the wild type (WT), lignin p-hydroxybenzoate deficient, and p-hydroxybenzoate overproduction plants, we examined the responses of plants to gravitropic/mechanical stress and their cell wall composition changes. We revealed that mechanical bending or gravitropic stimulation not only induced the overproduction of crystalline cellulose fibers and increased the relative abundance of syringyl lignin, but also significantly induced the expression of PHBMT1 and the increased accumulation of p-hydroxybenzoates in TW. Furthermore, we found that although disturbing lignin-bound p-hydroxybenzoate accumulation in the PHBMT1 knockout and overexpression (OE) poplars did not affect the major chemical composition shifts of the cell walls in their TW as occurred in the WT plants, depletion of p-hydroxybenzoates intensified the gravitropic curving of the plantlets in response to gravistimulation, evident with the enhanced stem secant bending angle. By contrast, hyperaccumulation of p-hydroxybenzoates mitigated gravitropic response. These data suggest that PHBMT1-mediated lignin modification is involved in the regulation of poplar gravitropic response and, likely by compromising gravitropism and/or enhancing autotropism, negatively coordinates the action of TW cellulose fibers to control the poplar wood deformation and plant growth.