Lignin synthesis and accumulation in barley cultivars differing in their resistance to lodging

Exogenous silicon improved the cell wall stability by activating non-structural carbohydrates and structural carbohydrates metabolism in salt and drought stressed Glycyrrhiza uralensis stem

The plant cell wall is a crucial barrier against environmental stress, mainly composed of lignin and carbohydrates such as cellulose, hemicellulose, and pectin. This study explored the direct regulatory mechanism of silicon (Si) on cell wall components of Glycyrrhiza uralensis (G. uralensis) stems under salt and drought (S + D) stress and the indirect regulatory mechanism of non-structural carbohydrates on structural carbohydrates, mediated by uridine diphosphate glucose (UDPG), through joint physiological, biochemical, and transcriptomic analyses. Under S + D stress, Si increased the contents of cell wall components, altered the structure of cell wall, and directly promoted cell wall re-construction by regulating gene expression levels and enzyme activities related to cell wall biosynthesis. Meanwhile, Si facilitated the accumulation of carbohydrates by regulating enzyme activities and gene expression levels in the anabolic pathway of polysaccharides, thereby promoting UDPG conversion and indirectly providing substrates for cell wall synthesis. In conclusion, Si directly and indirectly facilitates the synthesis of cell wall components by regulating both cell wall metabolism and non-structural carbohydrates metabolism, thus reinforcing the cell wall, enhancing its stability, and improving the salt and drought tolerance of G. uralensis stems.

Regulation of lignin biosynthesis by GhCAD37 affects fiber quality and anther vitality in upland cotton

Cotton stands as a pillar in the textile industry due to its superior natural fibers. Lignin, a complex polymer synthesized from phenylalanine and deposited in mature cotton fibers, is believed to be essential for fiber quality, although the precise effects remain largely unclear. In this study, we characterized two ubiquitously expressed cinnamyl alcohol dehydrogenases (CAD), GhCAD37A and GhCAD37D (GhCAD37A/D), in Gossypium hirsutum. GhCAD37A/D possess CAD enzymatic activities, to catalyze the generation of monolignol products during lignin biosynthesis. Analysis of transgenic cotton knockout and overexpressing plants revealed that GhCAD37A/D are important regulators of fiber quality, positively impacting breaking strength but negatively affecting fiber length and elongation percentage by modulating lignin biosynthesis in fiber cells. Moreover, GhCAD37A/D are shown to modulate anther vitality and affect stem lodging trait in cotton by influencing lignin biosynthesis in the vascular bundles of anther and stem, respectively. Additionally, our study revealed that Ghcad37A/D knockout plants displayed red stem xylem, likely due to the overaccumulation of aldehyde intermediates in the phenylpropanoid metabolism pathway, as indicated by metabolomics analysis. Thus, our work illustrates that GhCAD37A/D are two important enzymes of lignin biosynthesis in different cotton organs, influencing fiber quality, anther vitality, and stem lodging.

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.

Transcriptome analysis identifies differentially expressed genes involved in lignin biosynthesis in barley

Lignin is an essential metabolite for plant growth but negatively affects the quality of forage barley. Genetic modification of quality traits to improve the forage digestibility requires an understanding of the molecular mechanism of lignin biosynthesis. RNA-Seq was used to quantify transcripts differentially expressed among leaf, stem and spike tissues from two barley genotypes. A total of 13,172 differentially expressed genes (DEGs) were identified, of which much more up-regulated DEGs were detected from the contrasting groups of leaf vs spike (L-S) and stem vs spike (S-S), and down-regulated DEGs were dominant in the group of stem vs leaf (S-L). 47 DEGs were successfully annotated to the monolignol pathway and six of them were candidate genes regulating the lignin biosynthesis. The qRT-PCR assay verified the expression profiles of the six candidate genes. Among them, four genes might positively regulate the lignin biosynthesis during forage barley development in terms of the consistency of their expression levels and changes of lignin content among the tissues, while the other two genes may have the reverse effects. These findings provide target genes for further investigations on molecular regulatory mechanisms of lignin biosynthesis and genetic resources for improvement of forage quality in barley molecular breeding programme.

Comprehensive Analyses of Simple Sequence Repeat (SSR) in Bamboo Genomes and Development of SSR Markers with Peroxidase Genes

Simple sequence repeats (SSRs) are one of the most important molecular markers, which are widespread in plants. Bamboos are important forest resources worldwide. Here, the comprehensive identification and comparative analysis of SSRs were performed in three woody and two herbaceous bamboo species. Altogether 567,175 perfect SSRs and 71,141 compound SSRs were identified from 5737.8 Mb genome sequences of five bamboo species. Di-nucleotide SSRs were the most predominant type, with an average of ~50,152.2 per species. Most SSRs were located in intergenic regions, while those located in genic regions were relatively less. Moreover, the results of annotation distribution indicated that terms with P450, peroxidase and ATP-binding cassette transporter related to lignin biosynthesis might play important roles in woody and herbaceous bamboos under the mediation of SSRs. Furthermore, the peroxidase gene family consisted of a large number of genes containing SSRs was selected for the evolutionary relationship analysis and SSR markers development. Fifteen SSR markers derived from peroxidase family genes of Phyllostachys edulis were identified as polymorphic in 34 accessions belonging to seven genera in Bambusoideae. These results provided a comprehensive insight of SSR markers into bamboo genomes, which would facilitate bamboo research related to comparative genomics, evolution and marker-assisted selection.

The MicroRNA397a-LACCASE17 module regulates lignin biosynthesis in Medicago ruthenica (L.)

Mechanical strength is essential for the upright growth habit, which is one of the most important characteristics of terrestrial plants. Lignin, a phenylpropanoid-derived polymer mainly present in secondary cell walls plays critical role in providing mechanical support. Here, we report that the prostrate-stem cultivar of the legume forage Medicago ruthenica cultivar 'Mengnong No. 1' shows compromised mechanical strength compared with the erect-stem cultivar 'Zhilixing'. The erect-stem cultivar, 'Zhilixing' has significantly higher lignin content, leading to higher mechanical strength than the prostrate-stem cultivar. The low abundance of miRNA397a in the Zhiixing cultivar causes reduced cleavage of MrLAC17 transcript, which results in enhanced expression level of MrLAC17 compared to that in the prostrate-stem cultivar Mengnong No. 1. Complementation of the Arabidopsis lac4 lac17 double mutants with MrLAC17 restored the lignin content to wild-type levels, confirming that MrLAC17 perform an exchangeable role with Arabidopsis laccases. LAC17-mediated lignin polymerization is therefore increased in the 'Zhilixing', causing the erect stem phenotype. Our data reveal the importance of the miR397a in the lignin biosynthesis and suggest a strategy for molecular breeding targeting plant architecture in legume forage.

Characterization, Expression Profiling, and Biochemical Analyses of the Cinnamoyl-CoA Reductase Gene Family for Lignin Synthesis in Alfalfa Plants

Cinnamoyl-CoA reductase (CCR) is a pivotal enzyme in plant lignin synthesis, which has a role in plant secondary cell wall development and environmental stress defense. Alfalfa is a predominant legume forage with excellent quality, but the lignin content negatively affects fodder digestibility. Currently, there is limited information on CCR characteristics, gene expression, and its role in lignin metabolism in alfalfa. In this study, we identified 30 members in the CCR gene family of Medicago sativa. In addition, gene structure, conserved motif, and evolution analysis suggested MsCCR1–7 presumably functioned as CCR, while the 23 MsCCR-likes fell into three categories. The expression patterns of MsCCRs/MsCCR-likes suggested their role in plant development, response to environmental stresses, and phytohormone treatment. These results were consistent with the cis-elements in their promoters. Histochemical staining showed that lignin accumulation gradually deepened with the development, which was consistent with gene expression results. Furthermore, recombinant MsCCR1 and MsCCR-like1 were purified and the kinetic parameters were tested under four substrates. In addition, three-dimensional structure models of MsCCR1 and MsCCR-like1 proteins showed the difference in the substrate-binding motif H212(X)2K215R263. These results will be useful for further application for legume forage quality modification and biofuels industry engineering in the future.

QTL Identification for Stem Fiber, Strength and Rot Resistance in a DH Population from an Alien Introgression of Brassica napus

Stem fiber, stem strength and stem-rot resistance are important agronomic traits in Brassica napus. To understand the molecular mechanism that controls the stem-related traits, we investigated the stem lignin (ADL), cellulose (Cel), hemicellulose (Hem) content, S/G monolignol ratio (SG), stem breaking force (BF), breaking strength (F) and Sclerotinia sclerotiorum resistance (SSR). Each trait was significantly positively or negatively correlated with more than three of the other six traits. QTL mapping for ADL, Cel, Hem, SG, BF, F and SSR were performed using a doubled haploid population derived from an intertribal B. napus introgression line 'Y689' crossed with B. napus cv. 'Westar'. A total of 67 additive QTL were identified and integrated into 55 consensus QTL by meta-analysis. Among the 55 consensus QTL, 23 (41.8%) QTL were co-located and were integrated into 11 unique QTL. The QTL by environment (Q × E) interactions were analyzed and 22 combined QTL were identified. In addition, candidate genes within the QTL intervals were proposed based on the known function of Arabidopsis orthologs. These results provided valuable information for improving lodging resistance, S. sclerotiorum resistance and mechanized harvesting of B. napus.

Silicon enhances stem strength by promoting lignin accumulation in herbaceous peony (Paeonia lactiflora Pall.)

Herbaceous peony (Paeonia lactiflora Pall.) is a popular high-end cut flower, but stem bending caused by low stem strength severely decreases its quality. To enhance stem strength, the regulatory effects of exogenous silicon were investigated in P. lactiflora. The results showed that silicon application enhanced stem strength by increasing the thickness of secondary cell walls and the layers of thickened secondary cells. Moreover, more lignin accumulated, particularly G-lignin and S-lignin, and the activities of lignin biosynthetic enzymes increased with silicon application. In addition, based on transcriptome analysis, silicon application induced the expression of genes participating in lignin biosynthesis pathway. Among them, hydroxycinnamoyl-CoA: shikimate hydroxycinnamoyl transferase gene (HCT1) was isolated from P. lactiflora and found to be mainly localized in the cytoplasm of cells. Overexpression of PlHCT1 increased the layers of thickened secondary cells and lignin accumulation in tobacco, resulting in enhanced stem strength and demonstrably straight stems. Finally, silicon content, lignin content and PlHCT1 expression in P. lactiflora cultivars with high stem strengths were totally higher than those in cultivars with low stem strengths. These results indicated that silicon application enhanced stem strength by promoting lignin accumulation in P. lactiflora, which has prospects for stem quality improvement in general.

Altering maize (Zea mays) seedlings’ growth and lignification processes by action of novel synthesized compounds

Effective management of the course of crop vegetation and adaptation to biotic and abiotic stresses is a prerequisite for stable grain production and requires replenishment of the arsenal of plant growth regulators. The effect of novel synthesized cage amides on maize seedlings morphogenesis has been tested. Seeds of a mid-early maize hybrid 'DN Galatea' after the pre-sowing treatment with 0.01% solutions of test compounds were grown in distilled water. The roots and shoots sections of 10-day-old maize seedlings were stained with phloroglucinol solution to reveal the lignin-containing anatomical structures. The effects of nine different test compounds, exceeding the well-known effects of the phytohormone auxin, promoted the maize seedlings’ linear growth, increased wet weight of roots and shoots, and dry biomass accumulation both in seedlings roots and shoots. Several test compounds activated the dry weight accumulation process without significantly affecting the root and shoot length. In the maize seedlings’ roots, an increase in the diameter and number of the xylem vessels was found, as well as an increase in the lignin-containing layer thickness of the endoderm cells in the root cortex. In the maize seedlings’ shoots, the test compounds caused an increase in the thickness of the lignin-containing outer layer of the seedlings’ first leaf. In general, the test compounds’ effect on seedling roots can potentially enhance root formation; increase efficiency of the roots water-conducting system and the tissues’ strength, thus reducing the likelihood of root lodging in maize plants. The effects of the test compounds revealed in the seedlings’ shoots reflect the activation of the shoots’ structure formation and may have a positive value for enhancing the strength of the plant stems and counteracting the stem lodging of the maize plants.

Changes of lignin biosynthesis in tobacco leaves during maturation

Lignin is one of the most valuable renewable industrial materials. To elucidate the mechanism via which lignin is synthesised, we compared the lignin content, leaf hardness, cell wall thickness of palisade tissue, and gene expression patterns of lignin biosynthetic enzymes in three tobacco (Nicotiana tabacum L.) varieties during maturation. The results consistently showed that during maturation, the accumulation of lignin gradually increased in tobacco leaves, reaching a peak at full maturity (45 days after topping), and then gradually decreased. Similarly, the transcript level analysis revealed that the gene expression pattern of NtPAL, NtC4H, NtCCoAOMT and NtCOMT were relatively high, and consistent with the lignin content changes. Thus, the four genes may play regulatory roles in the synthesis of tobacco lignin. Analysis of tissue expression patterns of the lignin synthesis-related gene showed that the NtPAL, NtC4H, Nt4CL, NtHCT, NtCCoAOMT, NtCOMT, NtCCR, NtCAD, and NtPAO were all expressed in stems, roots, and leaves. NtC3H and NtF5H were specifically expressed in stems and roots, and not in leaves. Consistently, the NtC3H promoter induced high GUS expression in stems and petioles, marginal in roots, and no GUS activity in leaves. These results provide insights into molecular regulation of lignin biosynthesis in tobacco.

Evaluation of Pyraclostrobin as an Ingredient for Soybean Seed Treatment by Analyzing its Accumulation-Dissipation Kinetics, Plant-Growth Activation, and Protection Against Phytophthora sojae

Seed treatment with fungicides has been regarded as a principal, effective, and economic technique for soybean [Glycine max (L.) Merr.] against pathogenic microorganisms during seed germination and seedling growth. Investigation of the characteristics of seed-treatment reagents is an indispensable basis for their application. The aim of the present work is to evaluate the use of pyraclostrobin as an ingredient for soybean seed treatment by investigating its accumulation-dissipation kinetics in plants, plant-growth activation, and protection against Phytophthora sojae. The results showed that the pyraclostrobin stimulated the visible growth (root and shoot length) of soybean plants, increased the chlorophyll level and root activity, and lowered the malonaldehyde (MDA) level. The peak level and bioavailability of pyraclostrobin in soybean roots were 19.9- and 33.2-fold those in leaves, respectively, indicating that pyraclostrobin was mainly accumulated in roots. Pyraclostrobin had a continuous positive effect on the flavonoid levels and the phenylalanine ammonialyase (PAL) activity in roots and leaves, which could enhance the plant defense system. Pyraclostrobin showed in vitro toxicity to P. sojae with a half-inhibition concentration (EC50) of 1.59 and 1.24 μg/mL for pyraclostrobin and pyraclostrobin plus salicylhydroxamic acid (SHAM, an inhibitor of the alternative pathway of respiration), respectively. Seed treatment with pyraclostrobin significantly reduced the severity of Phytophthora root rot, with a control efficacy of 60.7%. To the best of our knowledge, this is the first report on the characteristics of pyraclostrobin used in soybean seed treatment and its efficacy against Phytophthora root rot.