Inactivation of LACCASE8 and LACCASE5 genes in Brachypodium distachyon leads to severe decrease in lignin content and high increase in saccharification yield without impacting plant integrity

BACKGROUND

Dedicated lignocellulosic feedstock from grass crops for biofuel production is extensively increasing. However, the access to fermentable cell wall sugars by carbohydrate degrading enzymes is impeded by lignins. These complex polymers are made from reactive oxidized monolignols in the cell wall. Little is known about the laccase-mediated oxidation of monolignols in grasses, and inactivation of the monolignol polymerization mechanism might be a strategy to increase the yield of fermentable sugars.

RESULTS

LACCASE5 and LACCASE8 are inactivated in a Brachypodium double mutant. Relative to the wild type, the lignin content of extract-free mature culms is decreased by 20-30% and the saccharification yield is increased by 140%. Release of ferulic acid by mild alkaline hydrolysis is also 2.5-fold higher. Interfascicular fibers are mainly affected while integrity of vascular bundles is not impaired. Interestingly, there is no drastic impact of the double mutation on plant growth.

CONCLUSION

This work shows that two Brachypodium laccases with clearly identified orthologs in crops are involved in lignification of this model plant. Lignification in interfascicular fibers and metaxylem cells is partly uncoupled in Brachypodium. Orthologs of these laccases are promising targets for improving grass feedstock for cellulosic biofuel production.

Altered lignification in mur1-1 a mutant deficient in GDP-L-fucose synthesis with reduced RG-II cross linking

In the plant cell wall, boron links two pectic domain rhamnogalacturonan II (RG-II) chains together to form a dimer and thus contributes to the reinforcement of cell adhesion. We studied the mur1-1 mutant of Arabidopsis thaliana which has lost the ability to form GDP-fucose in the shoots and show that the extent of RG-II cross-linking is reduced in the lignified stem of this mutant. Surprisingly, MUR1 mutation induced an enrichment of resistant interunit bonds in lignin and triggered the overexpression of many genes involved in lignified tissue formation and in jasmonic acid signaling. The defect in GDP-fucose synthesis induced a loss of cell adhesion at the interface between stele and cortex, as well as between interfascicular fibers. This led to the formation of regenerative xylem, where tissue detachment occurred, and underlined a loss of resistance to mechanical forces. Similar observations were also made on bor1-3 mutant stems which are altered in boron xylem loading, leading us to suggest that diminished RG-II dimerization is responsible for regenerative xylem formation.

Expression atlas and comparative coexpression network analyses reveal important genes involved in the formation of lignified cell wall in Brachypodium distachyon

While Brachypodium distachyon (Brachypodium) is an emerging model for grasses, no expression atlas or gene coexpression network is available. Such tools are of high importance to provide insights into the function of Brachypodium genes. We present a detailed Brachypodium expression atlas, capturing gene expression in its major organs at different developmental stages. The data were integrated into a large-scale coexpression database ( www.gene2function.de), enabling identification of duplicated pathways and conserved processes across 10 plant species, thus allowing genome-wide inference of gene function. We highlight the importance of the atlas and the platform through the identification of duplicated cell wall modules, and show that a lignin biosynthesis module is conserved across angiosperms. We identified and functionally characterised a putative ferulate 5-hydroxylase gene through overexpression of it in Brachypodium, which resulted in an increase in lignin syringyl units and reduced lignin content of mature stems, and led to improved saccharification of the stem biomass. Our Brachypodium expression atlas thus provides a powerful resource to reveal functionally related genes, which may advance our understanding of important biological processes in grasses.

Structural Redesigning Arabidopsis Lignins into Alkali-Soluble Lignins through the Expression of p-Coumaroyl-CoA:Monolignol Transferase PMT

Grass lignins can contain up to 10% to 15% by weight of p-coumaric esters. This acylation is performed on monolignols under the catalysis of p-coumaroyl-coenzyme A monolignol transferase (PMT). To study the impact of p-coumaroylation on lignification, we first introduced the Brachypodium distachyon Bradi2g36910 (BdPMT1) gene into Arabidopsis (Arabidopsis thaliana) under the control of the constitutive maize (Zea mays) ubiquitin promoter. The resulting p-coumaroylation was far lower than that of lignins from mature grass stems and had no impact on stem lignin content. By contrast, introducing either the BdPMT1 or the Bradi1g36980 (BdPMT2) gene into Arabidopsis under the control of the Arabidopsis cinnamate-4-hydroxylase promoter boosted the p-coumaroylation of mature stems up to the grass lignin level (8% to 9% by weight), without any impact on plant development. The analysis of purified lignin fractions and the identification of diagnostic products confirmed that p-coumaric acid was associated with lignins. BdPMT1-driven p-coumaroylation was also obtained in the fah1 (deficient for ferulate 5-hydroxylase) and ccr1g (deficient for cinnamoyl-coenzyme A reductase) lines, albeit to a lower extent. Lignins from BdPMT1-expressing ccr1g lines were also found to be feruloylated. In Arabidopsis mature stems, substantial p-coumaroylation of lignins was achieved at the expense of lignin content and induced lignin structural alterations, with an unexpected increase of lignin units with free phenolic groups. This higher frequency of free phenolic groups in Arabidopsis lignins doubled their solubility in alkali at room temperature. These findings suggest that the formation of alkali-leachable lignin domains rich in free phenolic groups is favored when p-coumaroylated monolignols participate in lignification in a grass in a similar manner.

Mutation in Brachypodium caffeic acid O-methyltransferase 6 alters stem and grain lignins and improves straw saccharification without deteriorating grain quality

Cereal crop by-products are a promising source of renewable raw material for the production of biofuel from lignocellulose. However, their enzymatic conversion to fermentable sugars is detrimentally affected by lignins. Here the characterization of the Brachypodium Bd5139 mutant provided with a single nucleotide mutation in the caffeic acid O-methyltransferase BdCOMT6 gene is reported. This BdCOMT6-deficient mutant displayed a moderately altered lignification in mature stems. The lignin-related BdCOMT6 gene was also found to be expressed in grains, and the alterations of Bd5139 grain lignins were found to mirror nicely those evidenced in stem lignins. The Bd5139 grains displayed similar size and composition to the control. Complementation experiments carried out by introducing the mutated gene into the AtCOMT1-deficient Arabidopsis mutant demonstrated that the mutated BdCOMT6 protein was still functional. Such a moderate down-regulation of lignin-related COMT enzyme reduced the straw recalcitrance to saccharification, without compromising the vegetative or reproductive development of the plant.

Use of food and packaging model matrices to investigate the antioxidant properties of biorefinery grass lignins

The antioxidant properties of grass lignins recovered from an alkaline industrial process and from different ethanol organosolv pretreatment processes were compared using two types of tests: (i) classical radical 2,2'-diphenyl-1-picrylhydrazyl (DPPH(•)) scavenging tests in dioxane/water or ethanol and (ii) tests involving multiphasic systems (lipid dispersion in water or cellulose film suspended in ethanol). These multiphasic systems were representative of food and packaging matrices in view of high-value applications. All lignins, in solution or in the film, effectively scavenged radicals. Moreover, they were competitive with a food commercial rosemary extract to protect linoleic acid against oxidation. Whereas the DPPH(•) test in dioxane was not discriminant, differences appeared between lignins when the test was performed in ethanol or with the multiphasic systems. Moreover, radical scavenging activity was preserved in the film even after its immersion in ethanol. Structural analysis of lignins revealed that low-molar-mass phenolics, namely p-hydroxycinnamic acids and lignin depolymerization products, governed lignin antioxidant properties in the multiphasic systems.

LACCASE5 is required for lignification of the Brachypodium distachyon Culm

The oxidation of monolignols is a required step for lignin polymerization and deposition in cell walls. In dicots, both peroxidases and laccases are known to participate in this process. Here, we provide evidence that laccases are also involved in the lignification of Brachypodium distachyon, a model plant for temperate grasses. Transcript quantification data as well as in situ and immunolocalization experiments demonstrated that at least two laccases (LACCASE5 and LACCASE6) are present in lignifying tissues. A mutant with a misspliced LACCASE5 messenger RNA was identified in a targeting-induced local lesion in genome mutant collection. This mutant shows 10% decreased Klason lignin content and modification of the syringyl-to-guaiacyl units ratio. The amount of ferulic acid units ester linked to the mutant cell walls is increased by 40% when compared with control plants, while the amount of ferulic acid units ether linked to lignins is decreased. In addition, the mutant shows a higher saccharification efficiency. These results provide clear evidence that laccases are required for B. distachyon lignification and are promising targets to alleviate the recalcitrance of grass lignocelluloses.

Impact of the brown-midrib bm5 mutation on maize lignins

We have investigated the impact of the brown-midrib bm5 mutation on lignins and on p-coumaric acid and ferulic acid ester-linked to maize (Zea mays L.) cell walls. Lignified stalks or plant aerial parts (without ears) collected at grain maturity were studied in three genetic backgrounds. Relative to the control, bm5 mutants displayed lower levels of lignins and of p-coumarate esters but increased levels of ferulate esters. Thioacidolysis revealed that bm5 lignins display an increased frequency of free-phenolic guaiacyl units. More importantly, thioacidolysis provided unusual amounts of 1,2,2-trithioethyl ethylguaiacol, a marker compound diagnostic for the incorporation of free ferulic acid into lignins by bis 8-O-4 cross-coupling. As the resulting acetal bonding pattern is a chemically labile branch point introduced in maize lignins by the bm5 mutation, this alteration is prone to facilitate the delignification pretreatments used in the cellulose-to-ethanol process.

Disrupting the cinnamyl alcohol dehydrogenase 1 gene (BdCAD1) leads to altered lignification and improved saccharification in Brachypodium distachyon

Brachypodium distachyon (Brachypodium) has been proposed as a model for grasses, but there is limited knowledge regarding its lignins and no data on lignin-related mutants. The cinnamyl alcohol dehydrogenase (CAD) genes involved in lignification are promising targets to improve the cellulose-to-ethanol conversion process. Down-regulation of CAD often induces a reddish coloration of lignified tissues. Based on this observation, we screened a chemically induced population of Brachypodium mutants (Bd21-3 background) for red culm coloration. We identified two mutants (Bd4179 and Bd7591), with mutations in the BdCAD1 gene. The mature stems of these mutants displayed reduced CAD activity and lower lignin content. Their lignins were enriched in 8-O-4- and 4-O-5-coupled sinapaldehyde units, as well as resistant inter-unit bonds and free phenolic groups. By contrast, there was no increase in coniferaldehyde end groups. Moreover, the amount of sinapic acid ester-linked to cell walls was measured for the first time in a lignin-related CAD grass mutant. Functional complementation of the Bd4179 mutant with the wild-type BdCAD1 allele restored the wild-type phenotype and lignification. Saccharification assays revealed that Bd4179 and Bd7591 lines were more susceptible to enzymatic hydrolysis than wild-type plants. Here, we have demonstrated that BdCAD1 is involved in lignification of Brachypodium. We have shown that a single nucleotide change in BdCAD1 reduces the lignin level and increases the degree of branching of lignins through incorporation of sinapaldehyde. These changes make saccharification of cells walls pre-treated with alkaline easier without compromising plant growth.