Modification of plant cell walls with hydroxycinnamic acids by BAHD acyltransferases

Metabolites in Cellular Leachate as Biomarkers for the Nature and Severity of Injury Following a Short vs. Prolonged Freezing Stress

Freeze injury is typically quantified by measuring electrolyte leakage from plant tissues exposed to a freeze-thaw cycle, whereby higher leakage indicates greater injury. The severity of stress is also assessed by the ability (or lack thereof) of the injured tissue to recover, wherein reduced leakage during the post-thaw period relative to right after thaw indicates recoverable injury. In this study, spinach leaves were injured to various degrees by freezing them for different durations at a fixed sub-zero temperature. We explored the cellular nature of freeze-thaw injury by comparing the profiles of cellular metabolites that were leaked from reversibly and/or irreversibly injured leaves. A total of 47 metabolites were, thus, detected in the leachate, including 8 sugars, 20 amino acids, GABA, uracil, adenine, palmitic acid, ferulic, coumaric acid, and other organic/inorganic acids involved in primary and secondary metabolism (e.g., citric acid, phosphoric acid, coumaric acid, etc.). Combined multivariate metabolome analysis, including principal component analysis, hierarchical clustering, and partial least square discriminant analysis, together identified 14 key metabolites mainly responsible for distinguishing the reversible vs. irreversible injury. These metabolites are also indicators for various lesions, including structural/functional perturbations to cellular components (cell wall, chloroplast, vacuole, plasma membrane, nucleic acids), disruption of metabolic homeostasis (antioxidant system, cytoplasmic pH, Kreb's cycle), protein degradation, and hypoxia.

Essential yet dispensable: The role of CINNAMATE 4-HYDROXYLASE in rice cell wall lignification

A comprehensive understanding of the intricate lignin biosynthesis in grasses could contribute to enhancing our ability to utilize grass biomass. CINNAMATE 4-HYDROXYLASE (C4H), in conjunction with PHENYLALANINE AMMONIA-LYASE (PAL), initiates the entry of phenylalanine into the cinnamate/monolignol pathway, leading to the production of diverse phenylpropanoids, including lignin monomers. Despite extensive research on C4H in eudicots, genetic studies of C4H in grasses remain considerably limited. Notably, the role of C4H in the presence of PHENYLALANINE/TYROSINE AMMONIA-LYASE (PTAL), a grass-specific ammonia-lyase that can bypass the conserved PAL-C4H pathway by recruiting tyrosine into the cinnamate/monolignol pathway, remains unclear. To address this gap, a set of genome-edited rice (Oryza sativa) mutants harboring knockout mutations in rice C4H genes were generated and subjected to the analysis of growth phenotype and cell wall chemotype, alongside isotopic feeding and chemical inhibitor assays to test the contributions of the PAL-C4H and PTAL pathways. The phenotype and chemotype characterizations of C4H-knockout rice mutants demonstrated that Class I (OsC4H1) and Class II (OsC4H2a and OsC4H2b) C4Hs cooperatively contribute to lignin biosynthesis in rice. Nevertheless, the effects of C4H deficiency on plant development and lignin formation in rice appeared to be markedly less prominent compared to those reported in eudicots. The 13C-labeled phenylalanine and tyrosine feeding experiments demonstrated that even with the phenylalanine-derived PAL-C4H pathway completely blocked, C4H-knockout rice could still produce substantial levels of lignin and maintain sound cell walls by utilizing the tyrosine-derived PTAL pathway. Overall, this study demonstrates the essential but dispensable role of C4H in grass cell wall lignification.

Lignin reinforced eco-friendly and functional nanoarchitectonics materials with tailored interfacial barrier performance

Exploring innovative and sustainable routes for the production of biodegradable biomass-based materials is critical to promote a circular carbon economy and carbon neutrality goals. Fossil-based non-biodegradable plastic waste poses a nonnegligible threat to humans and the ecological environment, and biomass-based functional materials are becoming increasingly viable alternatives. Lignin, a naturally occurring macromolecular polymer, is green and renewable resource rich in aromatic rings, with biodegradability, biocompatibility, and excellent processability for eco-friendly composites. Moreover, versatile and high tunable lignins can be valorized into functional materials, which are crucial building blocks in the fabrication of biomass-derived composites. Lignin's unique chemical structure, solvent resistance, anti-aging, and anti-ultraviolet functional properties make it highly potential for the fabrication of sustainable biobased barrier materials. This review systematically summarizes the progress in the fabrication and application of lignin-based functional composites, with a particular focus on barrier materials. First, the structural diversity of lignins from different sources and fractionation methods, and the structural modification strategies of lignins are briefly introduced. Then, the multiple barrier performances of lignin-based composites are listed, and the fabrication methods of different composites based on the polymer matrix systems are elaborated. In terms of diverse applications, this review highlights the multifaceted barrier properties of lignin-based composites in oxygen barrier, water vapor barrier, ultraviolet barrier, flame retardant and antibacterial applications. These functional barrier materials are widely used in food/pharmaceutical packaging, agricultural protection, construction, etc., providing an excellent option for sustainable materials with high barrier performance requirements. Finally, the main challenges faced by lignin-based barrier materials and the future directions are proposed.

Structural Variations of Broccoli Polyphenolics and Their Antioxidant Capacity as a Function of Growing Temperature

Polyphenolics in plants exist in free, soluble-bound, and insoluble-bound structural forms. The concentration of these structural forms depends on the plant's developmental stage, tissue type, soil water availability, and food preparation methods. In this study, for the first time, the effects of growth temperature (RT-room temperature-23 °C day/18 °C night, HT-high temperature-38 °C day/33 °C night, LT-low temperature-12 °C day/7 °C night) on variations of polyphenolic structural forms-free, soluble-bound (esterified and glycosylated), and insoluble-bound-in broccoli (Brassica oleracea L. convar. botrytis (L.) Alef. var. cymosa Duch.) microgreens were investigated. Using spectrophotometric, RP-HPLC, and statistical analyses, it was found that the highest amount of total phenolics (TP) in broccoli microgreens was present in the esterified form, regardless of the temperature at which they were grown (63.21 ± 3.49 mg GAE/g dw in RT, 65.55 ± 8.33 mg GAE/g dw in HT, 77.44 ± 7.82 mg GAE/g dw in LT). LT significantly increased the amount of free (from 13.30 ± 2.22 mg GAE/g dw in RT to 18.33 ± 3.85 mg GAE/g dw) and esterified soluble TP (from 63.21 ± 3.49 mg GAE/g dw in RT to 77.44 ± 7.82 mg GAE/g dw), while HT significantly increased the amount of TP glycosylated forms (from 14.85 ± 1.45 mg GAE/g dw in RT to 17.84 ± 1.20 mg GAE/g dw). LT also enhanced free and esterified forms of total flavonoids, tannins, hydroxycinnamic acids, and flavonols. HT, on the other hand, increased glycosylated forms of TP, flavonoids, tannins, hydroxycinnamic acids, flavonols, and phenolic acids, and decreased insoluble-bound tannins. According to the ABTS method, HT induced antioxidant potential of free and glycosylated forms, while LT increased antioxidant capacity of free forms only. According to the FRAP method, LT increased antioxidant potential of free and esterified polyphenolic forms. Also, based on ABTS and FRAP assays, esterified polyphenolics showed significantly higher antioxidant capacity than any other form. Principal component analysis showed that structural form had a greater impact than temperature. Hierarchical clustering showed that RT-, HT- and LT-broccoli microgreens were most similar in their glycosylated polyphenolics, but differed the most in esterified forms, which were also the most distinct overall. In conclusion, HT and LT induced specific shifts in the structural forms of broccoli polyphenolics and their antioxidant capacity. Based on the results, we recommend applying LT to increase the amount of free and esterified polyphenolics in broccoli microgreens, while HT may be used to enhance glycosylated forms.

Identification of universal grass genes and estimates of their monocot-/commelinid-/grass-specificity

Motivation

Where experiments identify sets of grass genes of unknown function, e.g. underlying a QTL or co-expressed in a transcriptome, it is useful to know which of these genes are common to all grasses (universal) and whether they likely have monocot-/commelinid-/grass-specific function.

Results

A pipeline used data on 16 grass full genomes from Ensembl Plants to generate 13 312 highly conserved, universal groups of grass protein-coding genes. Validation steps showed that 98.8% of these groups also had gene matches in recently sequenced genomes from two major grass clades not used in the pipeline. Comparison with many non-grass genomes identified 4609 of these groups as likely of monocot-/commelinid-/grass-specific function. Both grouping of genes and specificity were defined using hidden Markov model (HMM) profiles of the groups. The HMM-based approach performed better than simple percentage identity in discriminating between test sets of known specific and non-specific genes. The results give novel insight into the nature of monocot-/commelinid-/grass-specific genes. Researchers can use the universal_grass_peps database to gain evidence for their experimentally identified grass genes being involved in monocot-/commelinid-/grass-specific traits.

Availability and implementation

The universal_grass_peps database is available for download at https://data.rothamsted.ac.uk/dataset/universal_grass_peps.

The Occurrence, Uses, Biosynthetic Pathway, and Biotechnological Production of Plumbagin, a Potent Antitumor Naphthoquinone

Plumbagin is an important naphthoquinone with potent anticancer properties besides multitudinous uses in healthcare. It is produced in a limited number of species and families but mostly in the roots of Plumbaginaceae family members. The biosynthetic pathway and the genes that regulate plumbagin synthesis are not completely known, but details of these are being revealed. Several species, including Plumbago, Drosera, and others, are being uprooted for the extraction of plumbagin by pharmaceutical industries, leading to the destruction of natural habitats. The pharmaceutical industry is therefore facing an acute shortage of plant material. This necessitates enhancing the accumulation of plumbagin using suspensions and hairy roots to meet market demands. Many factors, such as the aggregate size of the inoculum, stability of the culture, and the sequential effects of elicitors, immobilization, and permeabilization, have been demonstrated to act synergistically and markedly augment plumbagin accumulation. Hairy root cultures can be used for the large-scale production, growth, and plumbagin accumulation, and the exploration of their efficacy is now imperative. The secretion of compounds into the spent medium and their in situ adsorption via resin has remarkable potential, but this has not been thoroughly exploited. Improvements in the quality of biomass, selection of cell lines, and production of plumbagin in bioreactors have thus far been sporadic, and these parameters need to be further exploited. In this review, we report the advances made relating to the importance of stable cell line selection for the accumulation of compounds in long-term cultures, hairy root cultures for the accumulation of plumbagin, and its semicontinuous production via total cell recycling in different types of bioreactors. Such advances might pave the way for industrial exploitation. The steps in the biosynthetic pathway that are currently understood might also aid us in isolating the relevant genes in order to examine the effects of their overexpression or heterologous downregulation or to edit the genome using CRISPR-Cas9 technology in order to enhance the accumulation of plumbagin. Its potential as an anticancer molecule and its mode of action have been amply demonstrated, but plumbagin has not been exploited in clinics due to its insolubility in water and its highly lipophilic nature. Plumbagin-loaded nanoemulsions, plumbagin-silver, or albumin nanoparticle formulations can overcome these problems relating to its solubility and are currently being tried to improve its bioavailability and antiproliferative activities, as discussed in the current paper.

Compositional Analysis of Cutin in Maize Leaves

The cuticle is a lipid barrier that covers the air-exposed surfaces of plants. It consists of waxes and cutin, a cell wall-attached lipid polyester of oxygenated fatty acids and glycerol. Unlike waxes, cutin is insoluble in organic solvents, and its composition is typically studied by chemical depolymerization followed by monomer analysis by gas chromatography (GC). Here, we describe a method for the chemical depolymerization of cutin in maize leaves and subsequent compositional analysis of the constituent lipid monomers. The method has been adapted from protocols for cutin analysis developed for Arabidopsis, by both optimizing the amount of leaf tissue used and including a data analysis process specific to the monomers present in maize cutin. The approach uses base-catalyzed transmethylation, which produces fatty acid methyl esters, and silylation, which gives trimethylsilyl ether derivatives of hydroxyl groups for gas chromatographic analysis. For monomer identification, a few representative samples are first analyzed by GC-mass spectrometry (GC-MS). This is then followed by analysis of all replicates by gas chromatography coupled to a flame ionization detector (GC-FID) for monomer quantification, because the flame ionization detector provides a linear response over a wide mass range, is relatively simple to operate, and is more cost-effective to maintain compared to mass spectrometry detectors. Although the protocol bypasses time-consuming cuticle isolation steps by using whole-leaf samples, this means that a fraction of the compounds in the chromatographic profiles do not derive from cutin. Accordingly, we discuss some considerations for the interpretation of the resulting depolymerization products. Our protocol offers specific guidance on preparing maize leaf samples, ensuring reproducible results, and enabling the detection of subtle variations in cutin monomer composition among plant genotypes or developmental stages.

Jasmonic Acid (JA) Signaling Pathway in Rice Defense Against Chilo suppressalis Infestation

Jasmonic acid (JA) signaling plays a crucial role in rice defense against the striped stem borer, Chilo suppressalis, a notorious pest causing significant yield losses. This review explores the current understanding of JA-mediated defense mechanisms in rice, focusing on the molecular basis, regulatory elements, and practical implications for pest management. JA biosynthesis and signaling pathways are induced upon C. suppressalis infestation, leading to the activation of various defense responses. These include upregulation of JA-responsive genes involved in the production of proteinase inhibitors, volatile organic compounds, and other defensive compounds. The review also discusses the crosstalk between JA and other hormonal pathways, such as salicylic acid and ethylene, in fine-tuning defense responses. Structural modifications in rice plants, such as cell wall reinforcement and accumulation of secondary metabolites, have been highlighted as key components of JA-mediated defense against C. suppressalis. Furthermore, the practical applications of this knowledge in breeding insect-resistant rice varieties and developing sustainable pest management strategies were explored. Future research directions are proposed to further elucidate the complexities of JA signaling in rice-insect interactions and harness this knowledge to enhance crop protection.

Mining of latent feruloyl esterase resources in rumen and insight into dual-functional feruloyl esterase-xylanase from Pecoramyces ruminantium F1

Feruloyl esterase (FAE) has been extensively studied for its crucial auxiliary effect in the biodegradation of lignocellulose. In this study, a FAE database including 15,293 amino acid sequences was established to gain a better understanding of rumen FAEs through multi-omics analysis. The higher expression level of rumen fungal FAEs over bacterial FAEs suggests that rumen fungi may have more important role in the lignocellulose degradation. Analyses of the information acquired through the database showed that the rumen FAEs are mainly derived from anaerobic fungi. One special candidate harboring both feruloyl esterase and endoxylanase modules (Fae00416) from anaerobic fungus Pecoramyces ruminantium F1 was found to have intramolecular synergy between the esterase and xylanase domains, which underpins the importance of this enzymes in heteropolysaccharide degradation. The discovery of novel and efficient FAEs in rumen could contribute to enhancing the production of biofuels and bioproducts.

Mechanisms Involved in Cell Wall Remodeling in Etiolated Rice Shoots Grown Under Osmotic Stress

Osmotic stress impacts the cell wall properties in plants. This study aimed to elucidate the mechanisms involved in cell wall remodeling in etiolated (dark-grown) rice (Oryza sativa L.) shoots grown under polyethylene glycol (PEG)-induced osmotic stress conditions. Shoot growth was inhibited by 70% by the treatment with 60 mM PEG for 2 days. However, when the stressed seedlings were transferred to a solution without PEG, their shoot growth rate increased significantly. A measurement of the cell wall mechanical properties revealed that the cell walls of the stressed shoots became looser and more extensible than those of unstressed shoots. Among the cell wall constituents, the amounts of cell wall-bound phenolic acids, such as ferulic acid (FA), p-coumaric acid (p-CA), and diferulic acid (DFA), per shoot and per unit of matrix polysaccharide content were significantly reduced in the stressed shoots compared to those in the unstressed shoots. Concerning the formation of cell wall-bound phenolic acids, the activity of cell wall-bound peroxidase (CW-PRX) per unit of cell wall content, which is responsible for the coupling reaction of FA to produce DFA, was 3.5 times higher in stressed shoots than in unstressed shoots, while the activity was reduced by 20% on a shoot basis in stressed shoots compared to that in unstressed shoots. The expression levels of the major class III peroxidase genes in stressed shoots were either comparable to or slightly lower than those in unstressed shoots. Conversely, the phenylalanine ammonia-lyase (PAL) activity, which contributes to the biosynthesis of FA and p-CA, was reduced by 55% and 30% on a shoot and unit-of-protein-content basis, respectively, in stressed shoots compared to that in unstressed shoots. The expression levels of abundantly expressed PAL genes decreased by 14-46% under osmotic stress. Moreover, the gene expression levels of specific BAHD acyltransferases, which are responsible for the addition of FA and p-CA to form ester-linked moieties on cell wall constituents, decreased by 15-33% under osmotic stress. These results suggest that the downregulation of the expression of specific PAL and BAHD acyltransferase genes in osmotically stressed rice shoots is responsible for a reduction in the formation of cell wall-bound phenolic acid monomers. This, in turn, may result in a decrease in the levels of DFAs. The reduction in the formation of DFA-mediated cross-linking structures within the cell wall may contribute to an increase in the mechanical extensibility of the cell wall. The remodeling of cell walls in an extensible and loosened state could assist in maintaining the growth capacity of etiolated rice shoots grown under osmotic stress and contribute to rapid growth recovery following the alleviation of osmotic stress.

Altered Lignin Accumulation in Sorghum Mutated in Silicon Uptake Transporter SbLsi1

Sorghum [Sorghum bicolor (L.) Moench] has been receiving attention as a feedstock for lignocellulose biomass energy. During the combustion process, ash-containing silicon (Si) can be produced, which causes problems in furnace maintenance. Hence, lowering Si content in plants is crucial. However, limiting Si supply to crops is difficult in practice because Si is abundant in the soil. Previously, an Si uptake transporter (SbLsi1) has been identified, and an Si-depleted mutant has also been generated in the model sorghum variety BTx623. In this study, we aimed to investigate the changes induced by a mutation in SbLsi1 on the accumulation and structure of lignin in cell walls. Through chemical and NMR analyses, we demonstrated that the lsi1 mutation resulted in a significant increase in lignin accumulation levels as well as a significant reduction in Si content. At least some of the modification was induced by transcriptional changes, as suggested by the upregulation of phenylpropanoid biosynthesis-related genes in the mutant plants. These findings derived from the model variety could be useful for the future development of practical cultivars with high biomass and less Si content for bioenergy applications.

Disruption of aldehyde dehydrogenase decreases cell wall-bound p-hydroxycinnamates and improves cell wall digestibility in rice

In grass cell walls, ferulic acid (FA) serves as an important cross-linker between cell wall polymers, such as arabinoxylan (AX) and lignin, affecting the physicochemical properties of the cell walls as well as the utilization properties of grass lignocellulose for biorefinering. Here, we demonstrate that hydroxycinnamaldehyde dehydrogenase (HCALDH) plays a crucial role in the biosynthesis of the FA used for cell wall feruloylation in rice (Oryza sativa). Bioinformatic and gene expression analyses of aldehyde dehydrogenases (ALDHs) identified two rice ALDH subfamily 2C members, OsHCALDH2 (OsALDH2C2) and OsHCALDH3 (OsALDH2C3), potentially involved in cell wall feruloylation in major vegetative tissues of rice. CRISPR-Cas9 genome editing of OsHCALDH2 and OsHCALDH3 revealed that the contents of AX-bound ferulate were reduced by up to ~45% in the cell walls of the HCALDH-edited mutants, demonstrating their roles in cell wall feruloylation. The abundance of hemicellulosic sugars including arabinosyl units on AX was notably reduced in the cell walls of the HCALDH-edited mutants, whereas cellulose and lignin contents remained unaffected. In addition to reducing cell wall-bound ferulate, the loss of OsHCALDH2 and/or OsHCALDH3 also partially reduced cell wall-bound p-coumarate and sinapate in the vegetative tissues of rice, whereas it did not cause detectable changes in the amount of γ-oryzanol (feruloyl sterols) in rice seeds. Furthermore, the HCALDH-edited mutants exhibited improved cell wall saccharification efficiency, both with and without alkaline pretreatment, plausibly due to the reduction in cell wall cross-linking FA. Overall, HCALDH appears to present a potent bioengineering target for enhancing utilization properties of grass lignocellulose.

In situ imaging of LPMO action on plant tissues

Lytic polysaccharide monooxygenases (LPMOs) are copper-dependent enzymes that oxidatively cleave recalcitrant polysaccharides such as cellulose. Several studies have reported LPMO action in synergy with other carbohydrate-active enzymes (CAZymes) for the degradation of lignocellulosic biomass but direct LPMO action at the plant tissue level remains challenging to investigate. Here, we have developed a MALDI-MS imaging workflow to detect oxidised oligosaccharides released by a cellulose-active LPMO at cellular level on maize tissues. Using this workflow, we imaged LPMO action and gained insight into the spatial variation and relative abundance of oxidised and non-oxidised oligosaccharides. We reveal a targeted action of the LPMO related to the composition and organisation of plant cell walls.

Cuticle development and the underlying transcriptome-metabolome associations during early seedling establishment

The plant cuticle is a complex extracellular lipid barrier that has multiple protective functions. This study investigated cuticle deposition by integrating metabolomics and transcriptomics data gathered from six different maize seedling organs of four genotypes, the inbred lines B73 and Mo17, and their reciprocal hybrids. These datasets captured the developmental transition of the seedling from heterotrophic skotomorphogenic growth to autotrophic photomorphogenic growth, a transition that is highly vulnerable to environmental stresses. Statistical interrogation of these data revealed that the predominant determinant of cuticle composition is seedling organ type, whereas the seedling genotype has a smaller effect on this phenotype. Gene-to-metabolite associations assessed by integrated statistical analyses identified three gene networks associated with the deposition of different elements of the cuticle: cuticular waxes; monomers of lipidized cell wall biopolymers, including cutin and suberin; and both of these elements. These gene networks reveal three metabolic programs that appear to support cuticle deposition, including processes of chloroplast biogenesis, lipid metabolism, and molecular regulation (e.g. transcription factors, post-translational regulators, and phytohormones). This study demonstrates the wider physiological metabolic context that can determine cuticle deposition and lays the groundwork for new targets for modulating the properties of this protective barrier.

Altered Metabolism in Knockdown Lines of Two HXXXD/BAHD Acyltransferases During Wound Healing in Potato Tubers

Suberin biosynthesis involves the coordinated regulation of both phenolic and aliphatic metabolisms. HXXXD/BAHD acyltransferases occupy a unique place in suberization, as they function to crosslink phenolic and aliphatic monomers during suberin assembly. To date, only one suberin-associated HXXXD/BAHD acyltransferase, StFHT, has been described in potatoes, whereas, in Arabidopsis, at least two are implicated in suberin biosynthesis. RNAseq data from wound-induced potato tubers undergoing suberization indicate that transcripts for 28 HXXXD/BAHD acyltransferase genes accumulate in response to wounding. In the present study, we generated RNAi knockdown lines for StFHT and another highly wound-induced HXXXD/BAHD acyltransferase, designated StHCT, and characterized their wound-induced suberin phenotype. StFHT-RNAi and StHCT-RNAi knockdown lines share the same aliphatic suberin phenotype of reduced esterified ferulic acid and ferulates, which is similar to the previously described StFHT-RNAi knockdown suberin phenotype. However, the phenolic suberin phenotype differed between the two knockdown genotypes, with StHCT-RNAi knockdown lines having proportionately more p-hydroxyphenyl-derived moieties than either StFHT-RNAi knockdown or empty vector control lines. Analysis of soluble polar metabolites revealed that StHCT catalyzes a step upstream from StFHT. Overall, our data support the involvement of more than one HXXXD/BAHD acyltransferase in potato suberin biosynthesis.

Breeding for improved digestibility and processing of lignocellulosic biomass in Zea mays

Forage maize is a versatile crop extensively utilized for animal nutrition in agriculture and holds promise as a valuable resource for the production of fermentable sugars in the biorefinery sector. Within this context, the carbohydrate fraction of the lignocellulosic biomass undergoes deconstruction during ruminal digestion and the saccharification process. However, the cell wall's natural resistance towards enzymatic degradation poses a significant challenge during both processes. This so-called biomass recalcitrance is primarily attributed to the presence of lignin and ferulates in the cell walls. Consequently, maize varieties with a reduced lignin or ferulate content or an altered lignin composition can have important beneficial effects on cell wall digestibility. Considerable efforts in genetic improvement have been dedicated towards enhancing cell wall digestibility, benefiting agriculture, the biorefinery sector and the environment. In part I of this paper, we review conventional and advanced breeding methods used in the genetic improvement of maize germplasm. In part II, we zoom in on maize mutants with altered lignin for improved digestibility and biomass processing.

Enhancing monolignol ferulate conjugate levels in poplar lignin via OsFMT1

BACKGROUND

The phenolic polymer lignin is one of the primary chemical constituents of the plant secondary cell wall. Due to the inherent plasticity of lignin biosynthesis, several phenolic monomers have been shown to be incorporated into the polymer, as long as the monomer can undergo radicalization so it can participate in coupling reactions. In this study, we significantly enhance the level of incorporation of monolignol ferulate conjugates into the lignin polymer to improve the digestibility of lignocellulosic biomass.

RESULTS

Overexpression of a rice Feruloyl-CoA Monolignol Transferase (FMT), OsFMT1, in hybrid poplar (Populus alba x grandidentata) produced transgenic trees clearly displaying increased cell wall-bound ester-linked ferulate, p-hydroxybenzoate, and p-coumarate, all of which are in the lignin cell wall fraction, as shown by NMR and DFRC. We also demonstrate the use of a novel UV-Vis spectroscopic technique to rapidly screen plants for the presence of both ferulate and p-hydroxybenzoate esters. Lastly we show, via saccharification assays, that the OsFMT1 transgenic p oplars have significantly improved processing efficiency compared to wild-type and Angelica sinensis-FMT-expressing poplars.

CONCLUSIONS

The findings demonstrate that OsFMT1 has a broad substrate specificity and a higher catalytic efficiency compared to the previously published FMT from Angelica sinensis (AsFMT). Importantly, enhanced wood processability makes OsFMT1 a promising gene to optimize the composition of lignocellulosic biomass.

Metabolic engineering of Oryza sativa for lignin augmentation and structural simplification

The sustainable production and utilization of lignocellulose biomass are indispensable for establishing sustainable societies. Trees and large-sized grasses are the major sources of lignocellulose biomass, while large-sized grasses greatly surpass trees in terms of lignocellulose biomass productivity. With an overall aim to improve lignocellulose usability, it is important to increase the lignin content and simplify lignin structures in biomass plants via lignin metabolic engineering. Rice (Oryza sativa) is not only a representative and important grass crop, but also is a model for large-sized grasses in biotechnology. This review outlines progress in lignin metabolic engineering in grasses, mainly rice, including characterization of the lignocellulose properties, the augmentation of lignin content and the simplification of lignin structures. These findings have broad applicability for the metabolic engineering of lignin in large-sized grass biomass plants.

Xylan-directed cell wall assembly in grasses

Xylan is the most abundant hemicellulosic polysaccharide in the cell walls of grasses and is pivotal for the assembly of distinct cell wall structures that govern various cellular functions. Xylan also plays a crucial role in regulating biomass recalcitrance, ultimately affecting the utilization potential of lignocellulosic materials. Over the past decades, our understanding of the xylan biosynthetic machinery and cell wall organization has substantially improved due to the innovative application of multiple state-of-the-art techniques. Notably, novel xylan-based nanostructures have been revealed in the cell walls of xylem vessels, promoting a more extensive exploration of the role of xylan in the formation of cell wall structures. This Update summarizes recent achievements in understanding xylan biosynthesis, modification, modeling, and compartmentalization in grasses, providing a brief overview of cell wall assembly regarding xylan. We also discuss the potential for tailoring xylan to facilitate the breeding of elite energy and feed crops.

Grass lignin: biosynthesis, biological roles, and industrial applications

Lignin is a phenolic heteropolymer found in most terrestrial plants that contributes an essential role in plant growth, abiotic stress tolerance, and biotic stress resistance. Recent research in grass lignin biosynthesis has found differences compared to dicots such as Arabidopsis thaliana. For example, the prolific incorporation of hydroxycinnamic acids into grass secondary cell walls improve the structural integrity of vascular and structural elements via covalent crosslinking. Conversely, fundamental monolignol chemistry conserves the mechanisms of monolignol translocation and polymerization across the plant phylum. Emerging evidence suggests grass lignin compositions contribute to abiotic stress tolerance, and periods of biotic stress often alter cereal lignin compositions to hinder pathogenesis. This same recalcitrance also inhibits industrial valorization of plant biomass, making lignin alterations and reductions a prolific field of research. This review presents an update of grass lignin biosynthesis, translocation, and polymerization, highlights how lignified grass cell walls contribute to plant development and stress responses, and briefly addresses genetic engineering strategies that may benefit industrial applications.

Disruption of p-coumaroyl-CoA:monolignol transferases in rice drastically alters lignin composition

Grasses are abundant feedstocks that can supply lignocellulosic biomass for production of cell-wall-derived chemicals. In grass cell walls, lignin is acylated with p-coumarate. These p-coumarate decorations arise from the incorporation of monolignol p-coumarate conjugates during lignification. A previous biochemical study identified a rice (Oryza sativa) BAHD acyltransferase (AT) with p-coumaroyl-CoA:monolignol transferase (PMT) activity in vitro. In this study, we determined that that enzyme, which we name OsPMT1 (also known as OsAT4), and the closely related OsPMT2 (OsAT3) harbor similar catalytic activity toward monolignols. We generated rice mutants deficient in either or both OsPMT1 and OsPMT2 by CRISPR/Cas9-mediated mutagenesis and subjected the mutants' cell walls to analysis using chemical and nuclear magnetic resonance methods. Our results demonstrated that OsPMT1 and OsPMT2 both function in lignin p-coumaroylation in the major vegetative tissues of rice. Notably, lignin-bound p-coumarate units were undetectable in the ospmt1 ospmt2-2 double-knockout mutant. Further, in-depth structural analysis of purified lignins from the ospmt1 ospmt2-2 mutant compared with control lignins from wild-type rice revealed stark changes in polymer structures, including alterations in syringyl/guaiacyl aromatic unit ratios and inter-monomeric linkage patterns, and increased molecular weights. Our results provide insights into lignin polymerization in grasses that will be useful for the optimization of bioengineering approaches for the effective use of biomass in biorefineries.

Altered cell wall hydroxycinnamate composition impacts leaf- and canopy-level CO2 uptake and water use in rice

Cell wall properties play a major role in determining photosynthetic carbon uptake and water use through their impact on mesophyll conductance (CO2 diffusion from substomatal cavities into photosynthetic mesophyll cells) and leaf hydraulic conductance (water movement from xylem, through leaf tissue, to stomata). Consequently, modification of cell wall (CW) properties might help improve photosynthesis and crop water use efficiency (WUE). We tested this using 2 independent transgenic rice (Oryza sativa) lines overexpressing the rice OsAT10 gene (encoding a "BAHD" CoA acyltransferase), which alters CW hydroxycinnamic acid content (more para-coumaric acid and less ferulic acid). Plants were grown under high and low water levels, and traits related to leaf anatomy, CW composition, gas exchange, hydraulics, plant biomass, and canopy-level water use were measured. Alteration of hydroxycinnamic acid content led to statistically significant decreases in mesophyll CW thickness (-14%) and increased mesophyll conductance (+120%) and photosynthesis (+22%). However, concomitant increases in stomatal conductance negated the increased photosynthesis, resulting in no change in intrinsic WUE (ratio of photosynthesis to stomatal conductance). Leaf hydraulic conductance was also unchanged; however, transgenic plants showed small but statistically significant increases in aboveground biomass (AGB) (+12.5%) and canopy-level WUE (+8.8%; ratio of AGB to water used) and performed better under low water levels than wild-type plants. Our results demonstrate that changes in CW composition, specifically hydroxycinnamic acid content, can increase mesophyll conductance and photosynthesis in C3 cereal crops such as rice. However, attempts to improve photosynthetic WUE will need to enhance mesophyll conductance and photosynthesis while maintaining or decreasing stomatal conductance.

Highly differentiated genomic properties underpin the different cell walls of Poaceae and eudicots

Plant cell walls of Poaceae and eudicots differ substantially, both in the content and composition of their components. However, the genomic and genetic basis underlying these differences is not fully resolved. In this research, we analyzed multiple genomic properties of 150 cell wall gene families across 169 angiosperm genomes. The properties analyzed include gene presence/absence, copy number, synteny, occurrence of tandem gene clusters, and phylogenetic gene diversity. Results revealed a profound genomic differentiation of cell wall genes between Poaceae and eudicots, often associated with the cell wall diversity between these plant groups. For example, overall patterns of gene copy number variation and synteny were clearly divergent between Poaceae and eudicot species. Moreover, differential Poaceae-eudicot copy number and genomic contexts were observed for all the genes within the BEL1-like HOMEODOMAIN 6 regulatory pathway, which respectively induces and represses secondary cell wall synthesis in Poaceae and eudicots. Similarly, divergent synteny, copy number, and phylogenetic gene diversification were observed for the major biosynthetic genes of xyloglucans, mannans, and xylans, potentially contributing to the differences in content and types of hemicellulosic polysaccharides differences in Poaceae and eudicot cell walls. Additionally, the Poaceae-specific tandem clusters and/or higher copy number of PHENYLALANINE AMMONIA-LYASE, CAFFEIC ACID O-METHYLTRANSFERASE, or PEROXIDASE genes may underly the higher content and larger variety of phenylpropanoid compounds observed in Poaceae cell walls. All these patterns are discussed in detail in this study, along with their evolutionary and biological relevance for cell wall (genomic) diversification between Poaceae and eudicots.

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.