MYB-mediated upregulation of lignin biosynthesis in Oryza sativa towards biomass refinery

Lignin Biosynthesis in Postharvest Water Bamboo (Zizania latifolia) Shoots during Cold Storage Is Regulated by RBOH-Mediated Reactive Oxygen Species Signaling

Lignification is a major cause of senescence in fresh shoots of water bamboo (Zizania latifolia), which is a popular vegetable in southeast Asia; however, its physiological and molecular mechanisms is less understood. In the present study, lignin content and transcriptome change in postharvested water bamboo shoots under cold storage were investigated. We found that lignin significantly accumulated in the epidermis of the shoots with the increase of firmness. In the cold storage shoots, the major up-regulated genes were involved in phenylpropanoid biosynthesis, plant-pathogen interactions, and starch and sucrose metabolism. The lignin biosynthesis genes PAL, 4CL, C4H, CCoAOMT, CCR, F5H, CAD, and POD family were up-regulated during cold storage, while HCT and C3H were down-regulated. The MAPK signaling pathway was also up-regulated and respiratory burst oxidase homologue (RBOH) genes were strongly up-regulated. Therefore, we investigated the RBOH gene family and their expression profile in water bamboo shoots. The results indicated that 10 ZlRBOHs were up-regulated in cold storage shoots. Diphenyleneiodonium chloride (DPI), an inhibitor of RBOH oxidase, significantly inhibited the expression of genes involved in lignin deposition and biosynthesis, while H2O2 enhanced these processes. These results suggest that lignification of water bamboo shoots is regulated by RBOH-mediated ROS signaling.

Genome-Wide Identification and Evolution Analysis of R2R3-MYB Gene Family Reveals S6 Subfamily R2R3-MYB Transcription Factors Involved in Anthocyanin Biosynthesis in Carrot

The taproot of purple carrot accumulated rich anthocyanin, but non-purple carrot did not. MYB transcription factors (TFs) condition anthocyanin biosynthesis in many plants. Currently, genome-wide identification and evolution analysis of R2R3-MYB gene family and their roles involved in conditioning anthocyanin biosynthesis in carrot is still limited. In this study, a total of 146 carrot R2R3-MYB TFs were identified based on the carrot transcriptome and genome database and were classified into 19 subfamilies on the basis of R2R3-MYB domain. These R2R3-MYB genes were unevenly distributed among nine chromosomes, and Ka/Ks analysis suggested that they evolved under a purified selection. The anthocyanin-related S6 subfamily, which contains 7 MYB TFs, was isolated from R2R3-MYB TFs. The anthocyanin content of rhizodermis, cortex, and secondary phloem in ‘Black nebula’ cultivar reached the highest among the 3 solid purple carrot cultivars at 110 days after sowing, which was approximately 4.20- and 3.72-fold higher than that in the ‘Deep purple’ and ‘Ziwei’ cultivars, respectively. The expression level of 7 MYB genes in purple carrot was higher than that in non-purple carrot. Among them, DcMYB113 (DCAR_008994) was specifically expressed in rhizodermis, cortex, and secondary phloem tissues of ‘Purple haze’ cultivar, with the highest expression level of 10,223.77 compared with the control ‘DPP’ cultivar at 70 days after sowing. DcMYB7 (DCAR_010745) was detected in purple root tissue of ‘DPP’ cultivar and its expression level in rhizodermis, cortex, and secondary phloem was 3.23-fold higher than that of secondary xylem at 110 days after sowing. Our results should be useful for determining the precise role of S6 subfamily R2R3-MYB TFs participating in anthocyanin biosynthesis in carrot.

Limiting silicon supply alters lignin content and structures of sorghum seedling cell walls

Sorghum has been recognized as a promising energy crop. The composition and structure of lignin in the cell wall are important factors that affect the quality of plant biomass as a bioenergy feedstock. Silicon (Si) supply may affect the lignin content and structure, as both Si and lignin are possibly involved in plant mechanical strength. However, our understanding regarding the interaction between Si and lignin in sorghum is limited. Therefore, in this study, we analyzed the lignin in the cell walls of sorghum seedlings cultured hydroponically with or without Si supplementation. Limiting the Si supply significantly increased the thioglycolic acid lignin content and thioacidolysis-derived syringyl/guaiacyl monomer ratio. At least part of the modification may be attributable to the change in gene expression, as suggested by the upregulation of phenylpropanoid biosynthesis-related genes under -Si conditions. The cell walls of the -Si plants had a higher mechanical strength and calorific value than those of the +Si plants. These results provide some insights into the enhancement of the value of sorghum biomass as a feedstock for energy production by limiting Si uptake.

Comprehensive evaluation of Chinese peanut mini-mini core collection and QTL mapping for aflatoxin resistance

BACKGROUND

Aflatoxin contamination caused by Aspergillus fungi has been a serious factor affecting food safety of peanut (Arachis hypogaea L.) because aflatoxins are highly harmful for human and animal health. As three mechanisms of resistance to aflatoxin in peanut including shell infection resistance, seed infection resistance and aflatoxin production resistance exist among naturally evolved germplasm stocks, it is highly crucial to pyramid these three resistances for promoting peanut industry development and protecting consumers' health. However, less research effort has been made yet to investigate the differentiation and genetic relationship among the three resistances in diversified peanut germplasm collections.

RESULTS

In this study, the Chinese peanut mini-mini core collection selected from a large basic collection was systematically evaluated for the three resistances against A. flavus for the first time. The research revealed a wide variation among the diversified peanut accessions for all the three resistances. Totally, 14 resistant accessions were identified, including three with shell infection resistance, seven with seed infection resistance and five with aflatoxin production resistance. A special accession, Zh.h1312, was identified with both seed infection and aflatoxin production resistance. Among the five botanic types of A. hypogaea, the var. vulgaris (Spanish type) belonging to subspecies fastigiata is the only one which possessed all the three resistances. There was no close correlation between shell infection resistance and other two resistances, while there was a significant positive correlation between seed infection and toxin production resistance. All the three resistances had a significant negative correlation with pod or seed size. A total of 16 SNPs/InDels associated with the three resistances were identified through genome-wide association study (GWAS). Through comparative analysis, Zh.h1312 with seed infection resistance and aflatoxin production resistance was also revealed to possess all the resistance alleles of associated loci for seed infection index and aflatoxin content.

CONCLUSIONS

This study provided the first comprehensive understanding of differentiation of aflatoxin resistance in diversified peanut germplasm collection, and would further contribute to the genetic enhancement for resistance to aflatoxin contamination.

Co-expression network analyses of anthocyanin biosynthesis genes in Ruellia (Wild Petunias; Acanthaceae)

Background

Anthocyanins are major pigments contributing to flower coloration and as such knowledge of molecular architecture underlying the anthocyanin biosynthetic pathway (ABP) is key to understanding flower color diversification. To identify ABP structural genes and associated regulatory networks, we sequenced 16 transcriptomes generated from 10 species of Ruellia and then conducted co-expression analyses among resulting data.

Results

Complete coding sequences for 12 candidate structural loci representing eight genes plus nine candidate regulatory loci were assembled. Analysis of non-synonymous/synonymous (dn/ds) mutation rates indicated all identified loci are under purifying selection, suggesting overall selection to prevent the accumulation of deleterious mutations. Additionally, upstream enzymes have lower rates of molecular evolution compared to downstream enzymes. However, site-specific tests of selection yielded evidence for positive selection at several sites, including four in F3'H2 and five in DFR3, and these sites are located in protein binding regions. A species-level phylogenetic tree constructed using a newly implemented hybrid transcriptome–RADseq approach implicates several flower color transitions among the 10 species. We found evidence of both regulatory and structural mutations to F3′5'H in helping to explain the evolution of red flowers from purple-flowered ancestors.

Conclusions

Sequence comparisons and co-expression analyses of ABP loci revealed that mutations in regulatory loci are likely to play a greater role in flower color transitions in Ruellia compared to mutations in underlying structural genes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01955-x.

Identification and Functional Analysis of the CgNAC043 Gene Involved in Lignin Synthesis from Citrusgrandis "San Hong"

Overaccumulation of lignin (a physiological disorder known as granulation) often occurs during fruit ripening and postharvest storage in pomelo (Citrus grandis). It causes an unpleasant fruit texture and taste. Previous studies have shown that lignin metabolism is closely associated with the process of juice sacs granulation. At present, the underlying transcriptional regulatory mechanisms remain unclear. In this study, we identified and isolated a candidate NAC transcription factor, CgNAC043, that is involved in the regulation of lignin biosynthesis in Citrus grandis, which has homologs in Arabidopsis and other plants. We used the fruit juice sacs of 'San hong' as the material, the staining for lignin with HCl-phloroglucinol of fruit juice sacs became dark red from the various developmental stages at 172 to 212 days post anthesis (DPA). The RT-qPCR was used to analyze the gene expression of CgNAC043 and its target gene CgMYB46 in fruit sacs, it was found that the expression trend of CgNAC043 was basically same as CgMYB46, which increased gradually and peaked at 212 DPA. The expression level of CgNAC043 in juice sacs obtained away from the core was the lowest, while those near the core and granulated area were highly expressed. The transcriptional activation activity of CgNAC043 and CgMYB46 was analyzed by a yeast two-hybrid system, with only CgNAC043 showing transcriptional activation activity in Y2H Gold yeast. A transformation vector, p1301- CgNAC043, was transformed into the mesocarp of 'San hong' by Agrobacterium-mediated transformation. Results showed that the expression of transcription factors CgMYB58 and CgMYB46 are all upregulated. Further experiments proved that CgNAC043 not only can directly trans-activate the promoter of CgMYB46 but also trans-activate the promoters for the lignin biosynthesis-related genes CgCCoAOMT and CgC3H by dual luciferase assay. We isolated the CgNAC043 gene in pomelo and found CgNAC043 regulates target genes conferring the regulation of juice sacs granulation.

Nitrogen deficiency results in changes to cell wall composition of sorghum seedlings

Sorghum [Sorghum bicolor (L.) Moench] has been gaining attention as a feedstock for biomass energy production. While it is obvious that nitrogen (N) supply significantly affects sorghum growth and biomass accumulation, our knowledge is still limited regarding the effect of N on the biomass quality of sorghum, such as the contents and structures of lignin and other cell wall components. Therefore, in this study, we investigated the effects of N supply on the structure and composition of sorghum cell walls. The cell walls of hydroponically cultured sorghum seedlings grown under sufficient or deficient N conditions were analyzed using chemical, two-dimensional nuclear magnetic resonance, gene expression, and immunohistochemical methods. We found that the level of N supply considerably affected the cell wall structure and composition of sorghum seedlings. Limitation of N led to a decrease in the syringyl/guaiacyl lignin unit ratio and an increase in the amount and alteration of tissue distribution of several hemicelluloses, including mixed linkage (1 → 3), (1 → 4)-β-d-glucan, and arabinoxylan. At least some of these cell wall alterations could be associated with changes in gene expression. Nitrogen status is thus one of the factors affecting the cell wall properties of sorghum seedlings.

Transcriptomic and Proteomic Profiling Reveal the Key Role of AcMYB16 in the Response of Pseudomonas syringae pv. actinidiae in Kiwifruit

Kiwifruit bacterial canker caused by Pseudomonas syringae pv. actinidiae (Psa), is an important disease of kiwifruit (Actinidia Lind.). Plant hormones may induce various secondary metabolites to resist pathogens via modulation of hormone-responsive transcription factors (TFs), as reported in past studies. In this study, we showed that JA accumulated in the susceptible cultivar Actinidia chinensis 'Hongyang' but decreased in the resistant cultivar of A. chinensis var. deliciosa 'Jinkui' in response to Psa. Integrated transcriptomic and proteomic analyses were carried out using the resistant cultivar 'Jinkui'. A total of 5,045 differentially expressed genes (DEGs) and 1,681 differentially expressed proteins (DEPs) were identified after Psa infection. Two pathways, 'plant hormone signal transduction' and 'phenylpropanoid biosynthesis,' were activated at the protein and transcript levels. In addition, a total of 27 R2R3-MYB transcription factors (TFs) were involved in the response to Psa of 'Jinkui,' including the R2R3-MYB TF subgroup 4 gene AcMYB16, which was downregulated in 'Jinkui' but upregulated in 'Hongyang.' The promoter region of AcMYB16 has a MeJA responsiveness cis-acting regulatory element (CRE). Transient expression of the AcMYB16 gene in the leaves of 'Jinkui' induced Psa infection. Together, these data suggest that AcMYB16 acts as a repressor to regulate the response of kiwifruit to Psa infection. Our work will help to unravel the processes of kiwifruit resistance to pathogens and will facilitate the development of varieties with resistance against bacterial pathogens.

Double knockout of OsWRKY36 and OsWRKY102 boosts lignification with altering culm morphology of rice

Breeding to enrich lignin, a major component of lignocelluloses, in plants contributes to enhanced applications of lignocellulosic biomass into solid biofuels and valuable aromatic chemicals. To collect information on enhancing lignin deposition in grass species, important lignocellulose feedstocks, we generated rice (Oryza sativa) transgenic lines deficient in OsWRKY36 and OsWRKY102, which encode putative transcriptional repressors for secondary cell wall formation. We used CRISPR/Cas9-mediated targeted mutagenesis and closely characterized their altered cell walls using chemical and nuclear magnetic resonance (NMR) methods. Both OsWRKY36 and OsWRKY102 mutations significantly increased lignin content by up to 28 % and 32 %, respectively. Additionally, OsWRKY36/OsWRKY102-double-mutant lines displayed lignin enrichment of cell walls (by up to 41 %) with substantially altered culm morphology over the single-mutant lines as well as the wild-type controls. Our chemical and NMR analyses showed that relative abundances of guaiacyl and p-coumarate units were slightly higher and lower, respectively, in the WRKY mutant lignins compared with those in the wild-type lignins. Our results provide evidence that both OsWRKY36 and OsWRKY102 are associated with repression of rice lignification.

Expression Atlas of Selaginella moellendorffii Provides Insights into the Evolution of Vasculature, Secondary Metabolism, and Roots

Selaginella moellendorffii is a representative of the lycophyte lineage that is studied to understand the evolution of land plant traits such as the vasculature, leaves, stems, roots, and secondary metabolism. However, only a few studies have investigated the expression and transcriptional coordination of Selaginella genes, precluding us from understanding the evolution of the transcriptional programs behind these traits. We present a gene expression atlas comprising all major organs, tissue types, and the diurnal gene expression profiles for S. moellendorffii We show that the transcriptional gene module responsible for the biosynthesis of lignocellulose evolved in the ancestor of vascular plants and pinpoint the duplication and subfunctionalization events that generated multiple gene modules involved in the biosynthesis of various cell wall types. We demonstrate how secondary metabolism is transcriptionally coordinated and integrated with other cellular pathways. Finally, we identify root-specific genes and show that the evolution of roots did not coincide with an increased appearance of gene families, suggesting that the development of new organs does not coincide with increased fixation of new gene functions. Our updated database at conekt.plant.tools represents a valuable resource for studying the evolution of genes, gene families, transcriptomes, and functional gene modules in the Archaeplastida kingdom.

Identification of Homeobox Genes Associated with Lignification and Their Expression Patterns in Bamboo Shoots

: Homeobox (HB) genes play critical roles in regulating various aspects of plant growth and development. However, little is known about HB genes in bamboo. In this study, a total of 115 HB genes (PeHB001‒PeHB115) were identified from moso bamboo (Phyllostachys edulis) and grouped into 13 distinct classes (BEL, DDT, HD-ZIP I‒IV, KNOX, NDX, PHD, PINTOX, PLINC, SAWADEE, and WOX) based on the conserved domains and phylogenetic analysis. The number of members in the different classes ranged from 2 to 24, and they usually varied in terms of exon‒intron distribution pattern and length. There were 20 conserved motifs found in 115 PeHBs, with motif 1 being the most common. Gene ontology (GO) analysis showed that PeHBs had diverse molecular functions, with 19 PeHBs being annotated as having xylem development, xylem, and phloem pattern formation functions. Co-expression network analysis showed that 10 of the 19 PeHBs had co-expression correlations, and three members of the KNOX class were hub proteins that interacted with other transcription factors (TFs) such as MYB, bHLH, and OVATE, which were associated with lignin synthesis. Yeast two-hybridization results further proved that PeHB037 (BEL class) interacted with PeHB057 (KNOX class). Transcriptome expression profiling indicated that all PeHBs except PeHB017 were expressed in at least one of the seven tissues of moso bamboo, and 90 PeHBs were expressed in all the tissues. The qRT-PCR results of the 19 PeHBs showed that most of them were upregulated in shoots as the height increased. Moreover, a KNOX binding site was found in the promoters of the key genes involved in lignin synthesis such as Pe4CL, PeC3H, PeCCR, and PeCOMT, which had positive expression correlations with five KNOX genes. Similar results were found in winter bamboo shoots with prolonged storage time, which was consistent with the degree of lignification. These results provide basic data on PeHBs in moso bamboo, which will be helpful for future functional research on PeHBs with positive regulatory roles in the process of lignification.

OsMYB108 loss-of-function enriches p-coumaroylated and tricin lignin units in rice cell walls

Breeding approaches to enrich lignins in biomass could be beneficial to improving the biorefinery process because lignins increase biomass heating value and represent a potent source of valuable aromatic chemicals. However, despite the fact that grasses are promising lignocellulose feedstocks, limited information is yet available for molecular-breeding approaches to upregulate lignin biosynthesis in grass species. In this study, we generated lignin-enriched transgenic rice (Oryza sativa), a model grass species, via targeted mutagenesis of the transcriptional repressor OsMYB108 using CRISPR/Cas9-mediated genome editing. The OsMYB108-knockout rice mutants displayed increased expressions of lignin biosynthetic genes and enhanced lignin deposition in culm cell walls. Chemical and two-dimensional nuclear magnetic resonance (NMR) analyses revealed that the mutant cell walls were preferentially enriched in γ-p-coumaroylated and tricin lignin units, both of which are typical and unique components in grass lignins. NMR analysis also showed that the relative abundances of major lignin linkage types were altered in the OsMYB108 mutants.

Citrus sinensis MYB Transcription Factor CsMYB85 Induce Fruit Juice Sac Lignification Through Interaction With Other CsMYB Transcription Factors

Varieties of Citrus are commercially important fruits that are cultivated worldwide and are valued for being highly nutritious and having an appealing flavor. Lignification of citrus fruit juice sacs is a serious physiological disorder that occurs during postharvest storage, for which the underlying transcriptional regulatory mechanisms remain unclear. In this study, we identified and isolated a candidate MYB transcription factor, CsMYB85, that is involved in the regulation of lignin biosynthesis in Citrus sinensis, which has homologs in Arabidopsis and other plants. We found that during juice sac lignification, CsMYB85 expression levels increase significantly, and therefore, suspected that this gene may control lignin biosynthesis during the lignification process. Our results indicated that CsMYB85 binds the CsMYB330 promoter, regulates its expression, and interacts with CsMYB308 in transgenic yeast and tobacco. A transient expression assay indicated that Cs4CL1 expression levels and lignin content significantly increased in fruit juice sacs overexpressing CsMYB85. At4CL1 expression levels and lignin content were also significantly increased in Arabidopsis overexpressing CsMYB85. We accordingly present convincing evidence for the participation of the CsMYB85 transcription factor in fruit juice sac lignification, and thereby provide new insights into the transcriptional regulation of this process in citrus fruits.

Lignin characterization of rice CONIFERALDEHYDE 5-HYDROXYLASE loss-of-function mutants generated with the CRISPR/Cas9 system

The aromatic composition of lignin is an important trait that greatly affects the usability of lignocellulosic biomass. We previously identified a rice (Oryza sativa) gene encoding coniferaldehyde 5-hydroxylase (OsCAld5H1), which was effective in modulating syringyl (S)/guaiacyl (G) lignin composition ratio in rice, a model grass species. Previously characterized OsCAld5H1-knockdown rice lines, which were produced via an RNA-interference approach, showed augmented G lignin units yet contained considerable amounts of residual S lignin units. In this study, to further investigate the effect of suppression of OsCAld5H1 on rice lignin structure, we generated loss-of-function mutants of OsCAld5H1 using the CRISPR/Cas9-mediated genome editing system. Homozygous OsCAld5H1-knockout lines harboring anticipated frame-shift mutations in OsCAld5H1 were successfully obtained. A series of wet-chemical and two-dimensional NMR analyses on cell walls demonstrated that although lignins in the mutant were predictably enriched in G units all the tested mutant lines produced considerable numbers of S units. Intriguingly, lignin γ-p-coumaroylation analysis by the derivatization followed by reductive cleavage method revealed that enrichment of G units in lignins of the mutants was limited to the non-γ-p-coumaroylated units, whereas grass-specific γ-p-coumaroylated lignin units were almost unaffected. Gene expression analysis indicated that no homologous genes of OsCAld5H1 were overexpressed in the mutants. These data suggested that CAld5H is mainly involved in the production of non-γ-p-coumaroylated S lignin units, common in both eudicots and grasses, but not in the production of grass-specific γ-p-coumaroylated S units in rice.

Rice Genome-Scale Network Integration Reveals Transcriptional Regulators of Grass Cell Wall Synthesis

Grasses have evolved distinct cell wall composition and patterning relative to dicotyledonous plants. However, despite the importance of this plant family, transcriptional regulation of its cell wall biosynthesis is poorly understood. To identify grass cell wall-associated transcription factors, we constructed the Rice Combined mutual Ranked Network (RCRN). The RCRN covers >90% of annotated rice (Oryza sativa) genes, is high quality, and includes most grass-specific cell wall genes, such as mixed-linkage glucan synthases and hydroxycinnamoyl acyltransferases. Comparing the RCRN and an equivalent Arabidopsis network suggests that grass orthologs of most genetically verified eudicot cell wall regulators also control this process in grasses, but some transcription factors vary significantly in network connectivity between these divergent species. Reverse genetics, yeast-one-hybrid, and protoplast-based assays reveal that OsMYB61a activates a grass-specific acyltransferase promoter, which confirms network predictions and supports grass-specific cell wall synthesis genes being incorporated into conserved regulatory circuits. In addition, 10 of 15 tested transcription factors, including six novel Wall-Associated regulators (WAP1, WACH1, WAHL1, WADH1, OsMYB13a, and OsMYB13b), alter abundance of cell wall-related transcripts when transiently expressed. The results highlight the quality of the RCRN for examining rice biology, provide insight into the evolution of cell wall regulation, and identify network nodes and edges that are possible leads for improving cell wall composition.

Downregulation of p-COUMAROYL ESTER 3-HYDROXYLASE in rice leads to altered cell wall structures and improves biomass saccharification

p-Coumaroyl ester 3-hydroxylase (C3'H) is a key enzyme involved in the biosynthesis of lignin, a phenylpropanoid polymer that is the major constituent of secondary cell walls in vascular plants. Although the crucial role of C3'H in lignification and its manipulation to upgrade lignocellulose have been investigated in eudicots, limited information is available in monocotyledonous grass species, despite their potential as biomass feedstocks. Here we address the pronounced impacts of C3'H deficiency on the structure and properties of grass cell walls. C3'H-knockdown lines generated via RNA interference (RNAi)-mediated gene silencing, with about 0.5% of the residual expression levels, reached maturity and set seeds. In contrast, C3'H-knockout rice mutants generated via CRISPR/Cas9-mediated mutagenesis were severely dwarfed and sterile. Cell wall analysis of the mature C3'H-knockdown RNAi lines revealed that their lignins were largely enriched in p-hydroxyphenyl (H) units while being substantially reduced in the normally dominant guaiacyl (G) and syringyl (S) units. Interestingly, however, the enrichment of H units was limited to within the non-acylated lignin units, with grass-specific γ-p-coumaroylated lignin units remaining apparently unchanged. Suppression of C3'H also resulted in relative augmentation in tricin residues in lignin as well as a substantial reduction in wall cross-linking ferulates. Collectively, our data demonstrate that C3'H expression is an important determinant not only of lignin content and composition but also of the degree of cell wall cross-linking. We also demonstrated that C3'H-suppressed rice displays enhanced biomass saccharification.

Change in lignin structure, but not in lignin content, in transgenic poplar overexpressing the rice master regulator of secondary cell wall biosynthesis

We previously succeeded in enhancing wood formation of wood in transgenic poplar plants by overexpressing secondary wall NAM/ATAF/CUC (NAC) domain protein 1 from Oryza sativa (OsSWN1), a transcription factor 'master regulator' of secondary cell wall formation in rice, under control of the fiber preferential NST3/SND1 promoter from Arabidopsis. Transgenic plants had an increased cell wall thickness and cell wall density of individual cells in the secondary xylem of stems as well as an increased wood density. OsSWN1 triggers the induction of polysaccharide and lignin biosynthetic gene expressions, however, resulting in no significant impact on the lignin content in the transgenic plants. In contrast, wet and dry chemical analyses of lignin revealed changes in S/G ratio and in the composition of lignin interunit linkages in transgenic lines. The results from gene expression analysis suggest that the structural changes in lignin were due to an unbalanced induction of lignin biosynthetic genes in transgenic lines. Our present data indicate that the overexpression of the chimeric transcription factor causes accelerated deposition of secondary cell wall components including lignin and polysaccharides through an acquired mechanism.

A comparative study of the biomass properties of Erianthus and sugarcane: lignocellulose structure, alkaline delignification rate, and enzymatic saccharification efficiency

A comprehensive understanding of the structure and properties of gramineous lignocelluloses is needed to facilitate their uses in biorefinery. In this study, lignocelluloses from fractionated internode tissues of two taxonomically close species, Erianthus arundinaceus and sugarcane (Saccharum spp.), were characterized. Our analyses determined that syringyl (S) lignins were predominant over guaiacyl (G) or p-hydroxyphenyl (H) lignins in sugarcane tissues; on the other hand, S lignin levels were similar to those of G lignin in Erianthus tissues. In addition, tricin units were detected in sugarcane tissues, but not in Erianthus tissues. Distributions of lignin inter-monomeric linkage types were also different in Erianthus and sugarcane tissues. Alkaline treatment removed lignins from sugarcane tissues more efficiently than Erianthus tissues, resulting in a higher enzymatic digestibility of sugarcane tissues compared with Erianthus tissues. Our data indicate that Erianthus biomass displayed resistance to alkaline delignification and enzymatic digestion.

Regulation of CONIFERALDEHYDE 5-HYDROXYLASE expression to modulate cell wall lignin structure in rice

Regulation of a gene encoding coniferaldehyde 5-hydroxylase leads to substantial alterations in lignin structure in rice cell walls, identifying a promising genetic engineering target for improving grass biomass utilization. The aromatic composition of lignin greatly affects utilization characteristics of lignocellulosic biomass and, therefore, has been one of the primary targets of cell wall engineering studies. Limited information is, however, available regarding lignin modifications in monocotyledonous grasses, despite the fact that grass lignocelluloses have a great potential for feedstocks of biofuel production and various biorefinery applications. Here, we report that manipulation of a gene encoding coniferaldehyde 5-hydroxylase (CAld5H, or ferulate 5-hydroxylase, F5H) leads to substantial alterations in syringyl (S)/guaiacyl (G) lignin aromatic composition in rice (Oryza sativa), a major model grass and commercially important crop. Among three CAld5H genes identified in rice, OsCAld5H1 (CYP84A5) appeared to be predominantly expressed in lignin-producing rice vegetative tissues. Down-regulation of OsCAld5H1 produced altered lignins largely enriched in G units, whereas up-regulation of OsCAld5H1 resulted in lignins enriched in S units, as revealed by a series of wet-chemical and NMR structural analyses. Our data collectively demonstrate that OsCAld5H1 expression is a major factor controlling S/G lignin composition in rice cell walls. Given that S/G lignin composition affects various biomass properties, we contemplate that manipulation of CAld5H gene expression represents a promising strategy to upgrade grass biomass for biorefinery applications.