Methyl esterification divergently affects the degradability of pectic uronosyls in nonlignified and lignified maize cell walls

Phenolic cross-links: building and de-constructing the plant cell wall

Covering: Up to 2019Phenolic cross-links and phenolic inter-unit linkages result from the oxidative coupling of two hydroxycinnamates or two molecules of tyrosine. Free dimers of hydroxycinnamates, lignans, play important roles in plant defence. Cross-linking of bound phenolics in the plant cell wall affects cell expansion, wall strength, digestibility, degradability, and pathogen resistance. Cross-links mediated by phenolic substituents are particularly important as they confer strength to the wall via the formation of new covalent bonds, and by excluding water from it. Four biopolymer classes are known to be involved in the formation of phenolic cross-links: lignins, extensins, glucuronoarabinoxylans, and side-chains of rhamnogalacturonan-I. Lignins and extensins are ubiquitous in streptophytes whereas aromatic substituents on xylan and pectic side-chains are commonly assumed to be particular features of Poales sensu lato and core Caryophyllales, respectively. Cross-linking of phenolic moieties proceeds via radical formation, is catalyzed by peroxidases and laccases, and involves monolignols, tyrosine in extensins, and ferulate esters on xylan and pectin. Ferulate substituents, on xylan in particular, are thought to be nucleation points for lignin polymerization and are, therefore, of paramount importance to wall architecture in grasses and for the development of technology for wall disassembly, e.g. for the use of grass biomass for production of 2^nd generation biofuels. This review summarizes current knowledge on the intra- and extracellular acylation of polysaccharides, and inter- and intra-molecular cross-linking of different constituents. Enzyme mediated lignan in vitro synthesis for pharmaceutical uses are covered as are industrial exploitation of mutant and transgenic approaches to control cell wall cross-linking.

Structural features of alternative lignin monomers associated with improved digestibility of artificially lignified maize cell walls

Plant biologists are seeking new approaches for modifying lignin to improve the digestion and utilization of structural polysaccharides in crop cultivars for the production of biofuels, biochemicals, and livestock. To identify promising targets for lignin bioengineering, we artificially lignified maize (Zea mays L.) cell walls with normal monolignols plus 21 structurally diverse alternative monomers to assess their suitability for lignification and for improving fiber digestibility. Lignin formation and structure were assessed by mass balance, Klason lignin, acetyl bromide lignin, gel-state 2D-NMR and thioacidolysis procedures, and digestibility was evaluated with rumen microflora and from glucose production by fungal enzymes following mild acid or base pretreatments. Highly acidic or hydrophilic monomers proved unsuitable for lignin modification because they severely depressed cell wall lignification. By contrast, monomers designed to moderately alter hydrophobicity or introduce cleavable acetal, amide, or ester functionalities into the polymer often readily formed lignin, but most failed to improve digestibility, even after chemical pretreatment. Fortunately, several types of phenylpropanoid derivatives containing multiple ester-linked catechol or pyrogallol units were identified as desirable genetic engineering targets because they readily formed wall-bound polymers and improved digestibility, presumably by blocking cross-linking of lignin to structural polysaccharides and promoting lignin fragmentation during mild acidic and especially alkaline pretreatment.

Activation tagging of Arabidopsis POLYGALACTURONASE INVOLVED IN EXPANSION2 promotes hypocotyl elongation, leaf expansion, stem lignification, mechanical stiffening, and lodging

Pectin is the most abundant component of primary cell walls in eudicot plants. The modification and degradation of pectin affects multiple processes during plant development, including cell expansion, organ initiation, and cell separation. However, the extent to which pectin degradation by polygalacturonases affects stem development and secondary wall formation remains unclear. Using an activation tag screen, we identified a transgenic Arabidopsis thaliana line with longer etiolated hypocotyls, which overexpresses a gene encoding a polygalacturonase. We designated this gene as POLYGALACTURONASE INVOLVED IN EXPANSION2 (PGX2), and the corresponding activation tagged line as PGX2^AT . PGX2 is widely expressed in young seedlings and in roots, stems, leaves, flowers, and siliques of adult plants. PGX2-GFP localizes to the cell wall, and PGX2^AT plants show higher total polygalacturonase activity and smaller pectin molecular masses than wild-type controls, supporting a function for this protein in apoplastic pectin degradation. A heterologously expressed, truncated version of PGX2 also displays polygalacturonase activity in vitro. Like previously identified PGX1^AT plants, PGX2^AT plants have longer hypocotyls and larger rosette leaves, but they also uniquely display early flowering, earlier stem lignification, and lodging stems with enhanced mechanical stiffness that is possibly due to decreased stem thickness. Together, these results indicate that PGX2 both functions in cell expansion and influences secondary wall formation, providing a possible link between these two developmental processes.

Investigating Biochemical and Developmental Dependencies of Lignification with a Click-Compatible Monolignol Analog in Arabidopsis thaliana Stems

Lignin is a key structural component of plant cell walls that provides rigidity, strength, and resistance against microbial attacks. This hydrophobic polymer also serves a crucial role in water transport. Despite its abundance and essential functions, several aspects of lignin biosynthesis and deposition remain cryptic. Lignin precursors are known to be synthesized in the cytoplasm by complex biosynthetic pathways, after which they are transported to the apoplastic space, where they are polymerized via free radical coupling reactions into polymeric lignin. However, the lignin deposition process and the factors controlling it are unclear. In this study, the biochemical and developmental dependencies of lignification were investigated using a click-compatible monolignol analog, 3-O-propargylcaffeyl alcohol (3-OPC), which can incorporate into both in vitro polymerized lignin and Arabidopsis thaliana tissues. Fluorescence labeling of 3-OPC using click chemistry followed by confocal fluorescence microscopy enabled the detection and imaging of 3-OPC incorporation patterns. These patterns were consistent with endogenous lignification observed in different developmental stages of Arabidopsis stems. However, the concentration of supplied monolignols influenced where lignification occurred at the subcellular level, with low concentrations being deposited in cell corners and middle lamellae and high concentrations also being deposited in secondary walls. Experimental inhibition of multiple lignification factors confirmed that 3-OPC incorporation proceeds via a free radical coupling mechanism involving peroxidases/laccases and reactive oxygen species (ROS). Finally, the presence of peroxide-producing enzymes determined which cell walls lignified: adding exogenous peroxide and peroxidase caused cells that do not naturally lignify in Arabidopsis stems to lignify. In summary, 3-OPC accurately mimics natural lignification patterns in different developmental stages of Arabidopsis stems and allows for the dissection of key biochemical and enzymatic factors controlling lignification.

Ploidy effect and genetic architecture exploration of stalk traits using DH and its corresponding haploid populations in maize

BACKGROUND

Doubled haploid (DH) lines produced via in vivo haploid induction have become indispensable in maize research and practical breeding, so it is important to understand traits characteristics in DH and its corresponding haploids which derived from each DH lines. In this study, a DH population derived from Zheng58 × Chang7-2 and a haploid population, were developed, genotyped and evaluated to investigate genetic architecture of eight stalk traits, especially rind penetrometer resistance (RPR) and in vitro dry matter digestion (IVDMD), which affecting maize stalk lodging-resistance and feeding values, respectively.

RESULTS

Phenotypic correlation coefficients ranged from 0.38 to 0.69 between the two populations for eight stalk traits. Heritability values of all stalk traits ranged from 0.49 to 0.81 in the DH population, and 0.58 to 0.89 in the haploid population. Quantitative trait loci (QTL) mapping study showed that a total of 47 QTL for all traits accounting for genetic variations ranging from 1.6 to 36.5% were detected in two populations. One or more QTL sharing common region for each trait were detected between two different ploidy populations. Potential candidate genes predicated from the four QTL support intervals for RPR and IVDMD were indirectly or directly involved with cellulose and lignin biosynthesis, which participated in cell wall formation. The increased expression levels of lignin and cellulose synthesis key genes in the haploid situation illustrated that dosage compensation may account for genome dosage effect in our study.

CONCLUSIONS

The current investigation extended understanding about the genetic basis of stalk traits and correlations between DH and its haploid populations, which showed consistence and difference between them in phenotype, QTL characters, and gene expression. The higher heritabilities and partly higher QTL detection power were presented in haploid population than in DH population. All of which described above could lay a preliminary foundation for genetic architecture study with haploid population and may benefit selection in haploid-stage to reduce cost in DH breeding.

Metabolic engineering of novel lignin in biomass crops

Lignin, a phenolic polymer in the secondary wall, is the major cause of lignocellulosic biomass recalcitrance to efficient industrial processing. From an applications perspective, it is desirable that second-generation bioenergy crops have lignin that is readily degraded by chemical pretreatments but still fulfill its biological role in plants. Because plants can tolerate large variations in lignin composition, often without apparent adverse effects, substitution of some fraction of the traditional monolignols by alternative monomers through genetic engineering is a promising strategy to tailor lignin in bioenergy crops. However, successful engineering of lignin incorporating alternative monomers requires knowledge about phenolic metabolism in plants and about the coupling properties of these alternative monomers. Here, we review the current knowledge about lignin biosynthesis and the pathways towards the main phenolic classes. In addition, the minimal requirements are defined for molecules that, upon incorporation into the lignin polymer, make the latter more susceptible to biomass pretreatment. Numerous metabolites made by plants meet these requirements, and several have already been tested as monolignol substitutes in biomimetic systems. Finally, the status of detection and identification of compounds by phenolic profiling is discussed, as phenolic profiling serves in pathway elucidation and for the detection of incorporation of alternative lignin monomers.

Identifying new lignin bioengineering targets: impact of epicatechin, quercetin glycoside, and gallate derivatives on the lignification and fermentation of maize cell walls

Apoplastic targeting of secondary metabolites compatible with monolignol polymerization may provide new avenues for designing lignins that are less inhibitory toward fiber fermentation. To identify suitable monolignol substitutes, primary maize cell walls were artificially lignified with normal monolignols plus various epicatechin, quercetin glycoside, and gallate derivatives added as 0 or 45% by weight of the precursor mixture. The flavonoids and gallates had variable effects on peroxidase activity, but all dropped lignification pH. Epigallocatechin gallate, epicatechin gallate, epicatechin vanillate, epigallocatechin, galloylhyperin, and pentagalloylglucose formed wall-bound lignin at moderate to high concentrations, and their incorporation increased 48 h in vitro ruminal fiber fermentability by 20-33% relative to lignified controls. By contrast, ethyl gallate and corilagin severely depressed lignification and increased 48 h fermentability by about 50%. The results suggest several flavonoid and gallate derivatives are promising lignin bioengineering targets for improving the inherent fermentability of nonpretreated cell walls.

Hydroxycinnamate conjugates as potential monolignol replacements: in vitro lignification and cell wall studies with rosmarinic acid

The plasticity of lignin biosynthesis should permit the inclusion of new compatible phenolic monomers, such as rosmarinic acid (RA) and analogous catechol derivatives, into cell-wall lignins that are consequently less recalcitrant to biomass processing. In vitro lignin polymerization experiments revealed that RA readily underwent peroxidase-catalyzed copolymerization with monolignols and lignin oligomers to form polymers with new benzodioxane inter-unit linkages. Incorporation of RA permitted extensive depolymerization of synthetic lignins by mild alkaline hydrolysis, presumably by cleavage of ester intra-unit linkages within RA. Copolymerization of RA with monolignols into maize cell walls by in situ peroxidases significantly enhanced alkaline lignin extractability and promoted subsequent cell wall saccharification by fungal enzymes. Incorporating RA also improved cell wall saccharification by fungal enzymes and by rumen microflora even without alkaline pretreatments, possibly by modulating lignin hydrophobicity and/or limiting cell wall cross-linking. Consequently, we anticipate that bioengineering approaches for partial monolignol substitution with RA and analogous plant hydroxycinnamates would permit more efficient utilization of plant fiber for biofuels or livestock production.

Iridium-catalyzed oxidative methyl esterification of primary alcohols and diols with methanol

Oxidative methyl esterification of primary alcohols and diols with methanol was successfully achieved, using acetone as a hydrogen acceptor, under the influence of an iridium complex combined with 2-(methylamino)ethanol (MAE) as catalyst.

Identifying new lignin bioengineering targets: 1. Monolignol-substitute impacts on lignin formation and cell wall fermentability

BACKGROUND

Recent discoveries highlighting the metabolic malleability of plant lignification indicate that lignin can be engineered to dramatically alter its composition and properties. Current plant biotechnology efforts are primarily aimed at manipulating the biosynthesis of normal monolignols, but in the future apoplastic targeting of phenolics from other metabolic pathways may provide new approaches for designing lignins that are less inhibitory toward the enzymatic hydrolysis of structural polysaccharides, both with and without biomass pretreatment. To identify promising new avenues for lignin bioengineering, we artificially lignified cell walls from maize cell suspensions with various combinations of normal monolignols (coniferyl and sinapyl alcohols) plus a variety of phenolic monolignol substitutes. Cell walls were then incubated in vitro with anaerobic rumen microflora to assess the potential impact of lignin modifications on the enzymatic degradability of fibrous crops used for ruminant livestock or biofuel production.

RESULTS

In the absence of anatomical constraints to digestion, lignification with normal monolignols hindered both the rate and extent of cell wall hydrolysis by rumen microflora. Inclusion of methyl caffeate, caffeoylquinic acid, or feruloylquinic acid with monolignols considerably depressed lignin formation and strikingly improved the degradability of cell walls. In contrast, dihydroconiferyl alcohol, guaiacyl glycerol, epicatechin, epigallocatechin, and epigallocatechin gallate readily formed copolymer-lignins with normal monolignols; cell wall degradability was moderately enhanced by greater hydroxylation or 1,2,3-triol functionality. Mono- or diferuloyl esters with various aliphatic or polyol groups readily copolymerized with monolignols, but in some cases they accelerated inactivation of wall-bound peroxidase and reduced lignification; cell wall degradability was influenced by lignin content and the degree of ester group hydroxylation.

CONCLUSION

Overall, monolignol substitutes improved the inherent degradability of non-pretreated cell walls by restricting lignification or possibly by reducing lignin hydrophobicity or cross-linking to structural polysaccharides. Furthermore some monolignol substitutes, chiefly readily cleaved bi-phenolic conjugates like epigallocatechin gallate or diferuloyl polyol esters, are expected to greatly boost the enzymatic degradability of cell walls following chemical pretreatment. In ongoing work, we are characterizing the enzymatic saccharification of intact and chemically pretreated cell walls lignified by these and other monolignol substitutes to identify promising genetic engineering targets for improving plant fiber utilization.

Degradation of forage chicory by ruminal fibrolytic bacteria

AIMS

Determine the susceptibility of forage chicory (Cichorium intybus L.) to degradation by ruminal fibrolytic bacteria and measure the effects on cell-wall pectic polysaccharides.

METHODS AND RESULTS

Large segments of fresh forage chicory were degraded in vitro by Lachnospira multiparus and Fibrobacter succinogenes, but not by Ruminococcus flavefaciens or Butyrivibrio hungatei. Cell-wall pectins were degraded extensively (95%) and rapidly by L. multiparus with a simultaneous release of uronic acids and the pectin-derived neutral monosaccharides arabinose, galactose and rhamnose. Fibrobacter succinogenes also degraded cell-wall pectins extensively, but at a slower rate than L. multiparus. Immunofluorescence microscopy using monoclonal antibodies revealed that, after incubation, homogalacturonans with both low and high degrees of methyl esterification were almost completely lost from walls of all cell types and from the middle lamella between cells.

CONCLUSIONS

Only two of the four ruminal bacteria with pectinolytic activity degraded fresh chicory leaves, and each showed a different pattern of pectin breakdown. Degradation was greatest for F. succinogenes which also had cellulolytic activity.

SIGNIFICANCE AND IMPACT OF THE STUDY

The finding of extensive removal of pectic polysaccharides from the middle lamella and the consequent decrease in particle size may explain the decreased rumination and the increased intake observed in ruminants grazing forage chicory.