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• The woody stems of coniferous gymnosperms produce specialised compression wood to adjust the stem growth orientation in response to gravitropic stimulation. During this process, tracheids develop a compression-wood-specific S₂L cell wall layer with lignins highly enriched with p-hydroxyphenyl (H)-type units derived from H-type monolignol, whereas lignins produced in the cell walls of normal wood tracheids are exclusively composed of guaiacyl (G)-type units from G-type monolignol with a trace amount of H-type units. We show that laccases, a class of lignin polymerisation enzymes, play a crucial role in the spatially organised polymerisation of H-type and G-type monolignols during compression wood formation in Japanese cypress (Chamaecyparis obtusa). • We performed a series of chemical-probe-aided imaging analysis on C. obtusa compression wood cell walls, together with gene expression, protein localisation and enzymatic assays of C. obtusa laccases. • Our data indicated that CoLac1 and CoLac3 with differential oxidation activities towards H-type and G-type monolignols were precisely localised to distinct cell wall layers in which H-type and G-type lignin units were preferentially produced during the development of compression wood tracheids. • We propose that, not only the spatial localisation of laccases, but also their biochemical characteristics dictate the spatial patterning of lignin polymerisation in gymnosperm compression wood.

Summary 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.

Lignin is a polyphenolic polymer that strengthens and waterproofs the cell wall of specialized plant cell types. Lignification is part of the normal differentiation programme and functioning of specific cell types, but can also be triggered as a response to various biotic and abiotic stresses in cells that would not otherwise be lignifying. Cell wall lignification exhibits specific characteristics depending on the cell type being considered. These characteristics include the timing of lignification during cell differentiation, the palette of associated enzymes and substrates, the sub-cellular deposition sites, the monomeric composition and the cellular autonomy for lignin monomer production. This review provides an overview of the current understanding of lignin biosynthesis and polymerization at the cell biology level. The lignification process ranges from full autonomy to complete co-operation depending on the cell type. The different roles of lignin for the function of each specific plant cell type are clearly illustrated by the multiple phenotypic defects exhibited by knock-out mutants in lignin synthesis, which may explain why no general mechanism for lignification has yet been defined. The range of phenotypic effects observed include altered xylem sap transport, loss of mechanical support, reduced seed protection and dispersion, and/or increased pest and disease susceptibility.

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.

The leaf micromorphology and ultrastructure of Hydrocharis morsus-ranae leaves were examined using light microscopy, scanning electron microscopy, and laser confocal microscopy. We used also the Pascal program (LSM 5) for detected of fluorescence intensity chlorophylls and intensity of fluorescence intensity of monolignols, and also for to calculate the area of the periclinal walls of the epidermis as well as the volume of air occupied by the deepening cushions formed by the main cells of the lower epidermis. The study found that buoyancy affects the micromorphology of the upper and lower epidermis, the formation of aerenchyma between the mesophyll and epidermis, and the presence of air space under the cells of the lower epidermis. Differences in the ultrastructure of the upper and lower epidermis walls and the presence of silicon only on the upper epidermis suggest different functions for these tissues. The content of polysaccharides and monolignols in the leaves depends on the type of epidermal cells and the walls that form air cushions under the leaf blade. The study discusses the role of the air cushion and the involvement of monolignols and callose in plant adaptation to buoyancy. The article also explores the functional role of anticlinal wall projections on the leaf surfaces, which help the leaves float and stay on the water.

The flavone tricin (Fig. 1) has become of significant interest recently for a number of reasons, not the least of which is that it is now found to be a monomer for the abundant lignin polymer in all grasses (Poaceae) in the commelinid monocots. Tricin is scarcely found in dicots, but is known in alfalfa, Medicago sativa.

• 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.

Verticillium wilt, caused by the soil-borne fungus Verticillium dahliae , is one of the most devastating diseases in cotton ( Gossypium spp.). Lignin in the cell wall forms a physical barrier to inhibit pathogen invasion, and defense-induced lignification reinforces secondary cell wall to prevent pathogens from further spreading. Cinnamyl alcohol dehydrogenases (CADs) catalyze the production of three main monolignols, p -coumaryl- (H), coniferyl- (G), and sinapyl-alcohols (S), which are the fundamental blocks of lignin. Here, we identified CAD genes in G. hirsutum , analyzed their expression profiles in cotton leaf, stem, and root from different developmental stages, and selected GhCAD35 , GhCAD45 , and GhCAD43 , which were consistently induced by V. dahliae inoculation in G. hirsutum cultivars resistant or susceptible to V. dahliae . On the basis of confirmation of the in vitro enzymatic activity of the three proteins in generation of the three monolignols, we used virus-induced gene silencing (VIGS) to investigate the effects of silencing of GhCAD35 , GhCAD45 , or GhCAD43 on resistance to V. dahliae as well as on deposition and the composition of lignin. Silencing each of the three CAD s impaired the defense-induced lignification and salicylic acid biosynthesis in stem, and compromised resistance to V. dahliae . Moreover, our study showed that silencing the three GhCAD s severely affected the biosynthesis of S-lignin, leading to a decrease of the syringyl/guaiacyl (S/G) ratio. Heterogeneous overexpression of GhCAD35 , GhCAD45 , or GhCAD43 in Arabidopsis enhanced disease resistance. Taken together, our study demonstrates a role of the three GhCAD s in defense-induced lignin biosynthesis and resistance to V. dahliae in G. hirsutum .

Pathogens hitting the plant cell wall is the first impetus that triggers the phenylpropanoid pathway for plant defense. The phenylpropanoid pathway bifurcates into the production of an enormous array of compounds based on the few intermediates of the shikimate pathway in response to cell wall breaches by pathogens. The whole metabolomic pathway is a complex network regulated by multiple gene families and it exhibits refined regulatory mechanisms at the transcriptional, post-transcriptional, and post-translational levels. The pathway genes are involved in the production of anti-microbial compounds as well as signaling molecules. The engineering in the metabolic pathway has led to a new plant defense system of which various mechanisms have been proposed including salicylic acid and antimicrobial mediated compounds. In recent years, some key players like phenylalanine ammonia lyases (PALs) from the phenylpropanoid pathway are proposed to have broad spectrum disease resistance (BSR) without yield penalties. Now we have more evidence than ever, yet little understanding about the pathway-based genes that orchestrate rapid, coordinated induction of phenylpropanoid defenses in response to microbial attack. It is not astonishing that mutants of pathway regulator genes can show conflicting results. Therefore, precise engineering of the pathway is an interesting strategy to aim at profitably tailored plants. Here, this review portrays the current progress and challenges for phenylpropanoid pathway-based resistance from the current prospective to provide a deeper understanding.

Being an abundant renewable source of aromatic compounds, lignin is an important component of future bio-based economy. Currently, biotechnological processing of lignin through low molecular weight compounds is one of the conceptually promising ways for its valorization. To obtain lignin fragments suitable for further inclusion into microbial metabolism, it is proposed to use a ligninolytic system of white-rot fungi, which mainly comprises laccases and peroxidases. However, laccase and peroxidase genes are almost always represented by many non-allelic copies that form multigene families within the genome of white-rot fungi, and the contributions of exact family members to the overall process of lignin degradation has not yet been determined. In this article, the response of the Trametes hirsuta LE-BIN 072 ligninolytic system to the presence of various monolignol-related phenolic compounds (veratryl alcohol, p-coumaric acid, vanillic acid, and syringic acid) in culture media was monitored at the level of gene transcription and protein secretion. By showing which isozymes contribute to the overall functioning of the ligninolytic system of the T. hirsuta LE-BIN 072, the data obtained in this study will greatly contribute to the possible application of this fungus and its ligninolytic enzymes in lignin depolymerization processes.