H-lignin can be deposited independently of CINNAMYL ALCOHOL DEHYDROGENASE C and D in Arabidopsis

Simultaneous Down-Regulation of Dominant Cinnamoyl CoA Reductase and Cinnamyl Alcohol Dehydrogenase Dramatically Altered Lignin Content in Mulberry

Mulberry (Morus alba L.) is a significant economic tree species in China. The lignin component serves as a critical limiting factor that impacts both the forage quality and the conversion efficiency of mulberry biomass into biofuel. Cinnamoyl CoA reductase (CCR; EC 1.21.1.44) and cinnamyl alcohol dehydrogenase (CAD; EC 1.1.1.95) are the key enzymes that catalyze the final two reductive steps in the biosynthesis of monolignols. In this study, we conducted a comprehensive functional analysis to validate the predominant CCR genes involved in monolignol biosynthesis. In this study, we initially validated the predominant CCR genes implicated in monolignol biosynthesis through an extensive functional analysis. Phylogenetic analysis, tissue-specific expression profiling and enzymatic assays indicated that MaCCR1 is the authentic CCR involved in lignin biosynthesis. Furthermore, the expression level of MaCCR1 exhibited a significant positive correlation with lignin content, and the down-regulation of MaCCR1 via virus-induced gene silencing resulted in altered lignin content in mulberry. The down-regulation of MaCCR1 and MaCAD3/4, both individually and concurrently, exhibited markedly different effects on lignin content and mulberry growth. Specifically, the simultaneous down-regulation of MaCCR1 and MaCAD3/4 significantly altered lignin content in mulberry, resulting in dwarfism of the plants. Conversely, the down-regulation of MaCAD3/4 alone not only decreased lignin content but also led to an increase in biomass. These findings offer compelling evidence elucidating the roles of MaCCRs in mulberry and identify specific target genes, thereby providing a crucial foundation for the genetic modification of lignin biosynthesis.

Systematic synthesis and identification of monolignol pathway metabolites

Monolignol serves as the building blocks to constitute lignin, the second abundant polymer on Earth. Despite two decades of diligent efforts, complete identification of all metabolites in the currently proposed monolignol biosynthesis pathway has proven elusive. This limitation also hampers their potential application. One of the primary obstacles is the challenge of assembling a collection of all molecules, because many are commercially unavailable or prohibitively costly. In this study, we established systematic pipelines to synthesize all 24 molecules through the conversions between functional groups on a core structure followed by the application to other core structures. We successfully identified all of them in Populus trichocarpa and Eucalyptus grandis, two representative species respectively from malpighiales and myrtales in angiosperms. Knowledge about monolignol metabolite chemosynthesis and identification will form the foundation for future studies.

Transcriptome and Metabolomic Analyses Reveal Tissue-Specific Glycosylation of Phenylpropanoids and Flavonoids in Toxicodendron vernicifluum

Toxicodendron vernicifluum (Stokes) F. A. Barkley is a tree species used primarily for lacquer production. Our study utilized transcriptome and metabolomic analysis to investigate the biosynthesis of phenylpropanoids and flavonoids, specifically the glycosylated forms, in T. vernicifluum roots, stems, and leaves. HPLC-QTOF-MS/MS identified 186 compounds, with tissue-specific distributions revealed by PCA. Flavonoids and phenylpropanoids glycosides were significantly more abundant in leaves compared with roots and stems. Full-length sequencing uncovered 17,266 transcripts in T. vernicifluum. Gene expression analysis showed higher activity of phenylpropanoid and flavonoid biosynthesis pathways in leaves. Certain genes, such as CYP73A, 4CL, CRR, CYP84A/F5H, and CYP93C, displayed associations with compound content distributions. Root tissue exhibited a higher concentration of isoflavones. Notably, glycosyltransferase expression demonstrated significant correlations with glycosylated compounds' content. Biochemical validation confirmed the involvement of TvPB_c0_g2904, encoding a UDP-glucosyltransferase, in genistin biosynthesis in T. vernicifluum.

Genome-wide characterization and expression analysis of the CINNAMYL ALCOHOL DEHYDROGENASE gene family in Triticum aestivum

BACKGROUND

CINNAMYL ALCOHOL DEHYDROGENASE (CAD) catalyzes the NADPH-dependent reduction of cinnamaldehydes into cinnamyl alcohols and is a key enzyme found at the final step of the monolignol pathway. Cinnamyl alcohols and their conjugates are subsequently polymerized in the secondary cell wall to form lignin. CAD genes are typically encoded by multi-gene families and thus traditionally organized into general classifications of functional relevance.

RESULTS

In silico analysis of the hexaploid Triticum aestivum genome revealed 47 high confidence TaCAD copies, of which three were determined to be the most significant isoforms (class I) considered bone fide CADs. Class I CADs were expressed throughout development both in RNAseq data sets as well as via qRT-PCR analysis. Of the 37 class II TaCADs identified, two groups were observed to be significantly co-expressed with class I TaCADs in developing tissue and under chitin elicitation in RNAseq data sets. These co-expressed class II TaCADs were also found to be phylogenetically unrelated to a separate clade of class II TaCADs previously reported to be an influential resistance factor to pathogenic fungal infection. Lastly, two groups were phylogenetically identified as class III TaCADs, which possess distinct conserved gene structures. However, the lack of data supporting their catalytic activity for cinnamaldehydes and their bereft transcriptional presence in lignifying tissues challenges their designation and function as CADs.

CONCLUSIONS

Taken together, our comprehensive transcriptomic analyses suggest that TaCAD genes contribute to overlapping but nonredundant functions during T. aestivum growth and development across a wide variety of agroecosystems and provide tolerance to various stressors.

Enabling Lignin Valorization Through Integrated Advances in Plant Biology and Biorefining

Despite lignin having long been viewed as an impediment to the processing of biomass for the production of paper, biofuels, and high-value chemicals, the valorization of lignin to fuels, chemicals, and materials is now clearly recognized as a critical element for the lignocellulosic bioeconomy. However, the intended application for lignin will likely require a preferred lignin composition and form. To that end, effective lignin valorization will require the integration of plant biology, providing optimal feedstocks, with chemical process engineering, providing efficient lignin transformations. Recent advances in our understanding of lignin biosynthesis have shown that lignin structure is extremely diverse and potentially tunable, while simultaneous developments in lignin refining have resulted in the development of several processes that are more agnostic to lignin composition. Here, we review the interface between in planta lignin design and lignin processing and discuss the advances necessary for lignin valorization to become a feature of advanced biorefining.

Pinoresinol rescues developmental phenotypes of Arabidopsis phenylpropanoid mutants overexpressing FERULATE 5-HYDROXYLASE

Most phenylpropanoid pathway flux is directed toward the production of monolignols, but this pathway also generates multiple bioactive metabolites. The monolignols coniferyl and sinapyl alcohol polymerize to form guaiacyl (G) and syringyl (S) units in lignin, components that are characteristic of plant secondary cell walls. Lignin negatively impacts the saccharification potential of lignocellulosic biomass. Although manipulation of its content and composition through genetic engineering has reduced biomass recalcitrance, in some cases, these genetic manipulations lead to impaired growth. The reduced-growth phenotype is often attributed to poor water transport due to xylem collapse in low-lignin mutants, but alternative models suggest that it could be caused by the hyper- or hypoaccumulation of phenylpropanoid intermediates. In Arabidopsis thaliana, overexpression of FERULATE 5-HYDROXYLASE (F5H) shifts the normal G/S lignin ratio to nearly pure S lignin and does not result in substantial changes to plant growth. In contrast, when we overexpressed F5H in the low-lignin mutants cinnamyl dehydrogenase c and d (cadc cadd), cinnamoyl-CoA reductase 1, and reduced epidermal fluorescence 3, plant growth was severely compromised. In addition, cadc cadd plants overexpressing F5H exhibited defects in lateral root development. Exogenous coniferyl alcohol (CA) and its dimeric coupling product, pinoresinol, rescue these phenotypes. These data suggest that mutations in the phenylpropanoid pathway limit the biosynthesis of pinoresinol, and this effect is exacerbated by overexpression of F5H, which further draws down cellular pools of its precursor, CA. Overall, these genetic manipulations appear to restrict the synthesis of pinoresinol or a downstream metabolite that is necessary for plant growth.

Systematic approaches to C-lignin engineering in Medicago truncatula

BACKGROUND

C-lignin is a homopolymer of caffeyl alcohol present in the seed coats of a variety of plant species including vanilla orchid, various cacti, and the ornamental plant Cleome hassleriana. Because of its unique chemical and physical properties, there is considerable interest in engineering C-lignin into the cell walls of bioenergy crops as a high-value co-product of bioprocessing. We have used information from a transcriptomic analysis of developing C. hassleriana seed coats to suggest strategies for engineering C-lignin in a heterologous system, using hairy roots of the model legume Medicago truncatula.

RESULTS

We systematically tested strategies for C-lignin engineering using a combination of gene overexpression and RNAi-mediated knockdown in the caffeic acid/5-hydroxy coniferaldehyde 3/5-O-methyltransferase (comt) mutant background, monitoring the outcomes by analysis of lignin composition and profiling of monolignol pathway metabolites. In all cases, C-lignin accumulation required strong down-regulation of caffeoyl CoA 3-O-methyltransferase (CCoAOMT) paired with loss of function of COMT. Overexpression of the Selaginella moellendorffii ferulate 5-hydroxylase (SmF5H) gene in comt mutant hairy roots resulted in lines that unexpectedly accumulated high levels of S-lignin.

CONCLUSION

C-Lignin accumulation of up to 15% of total lignin in lines with the greatest reduction in CCoAOMT expression required the strong down-regulation of both COMT and CCoAOMT, but did not require expression of a heterologous laccase, cinnamyl alcohol dehydrogenase (CAD) or cinnamoyl CoA reductase (CCR) with preference for 3,4-dihydroxy-substituted substrates in M. truncatula hairy roots. Cell wall fractionation studies suggested that the engineered C-units are not present in a heteropolymer with the bulk of the G-lignin.

Strigolactones positively regulate Verticillium wilt resistance in cotton via crosstalk with other hormones

Verticillium wilt caused by Verticillium dahliae is a serious vascular disease in cotton (Gossypium spp.). V. dahliae induces the expression of the CAROTENOID CLEAVAGE DIOXYGENASE 7 (GauCCD7) gene involved in strigolactone (SL) biosynthesis in Gossypium australe, suggesting a role for SLs in Verticillium wilt resistance. We found that the SL analog rac-GR24 enhanced while the SL biosynthesis inhibitor TIS108 decreased cotton resistance to Verticillium wilt. Knock-down of GbCCD7 and GbCCD8b genes in island cotton (Gossypium barbadense) decreased resistance, whereas overexpression of GbCCD8b in upland cotton (Gossypium hirsutum) increased resistance to Verticillium wilt. Additionally, Arabidopsis (Arabidopsis thaliana) SL mutants defective in CCD7 and CCD8 putative orthologs were susceptible, whereas both Arabidopsis GbCCD7- and GbCCD8b-overexpressing plants were more resistant to Verticillium wilt than wild-type (WT) plants. Transcriptome analyses showed that several genes related to the jasmonic acid (JA)- and abscisic acid (ABA)-signaling pathways, such as MYELOCYTOMATOSIS 2 (GbMYC2) and ABA-INSENSITIVE 5, respectively, were upregulated in the roots of WT cotton plants in responses to rac-GR24 and V. dahliae infection but downregulated in the roots of both GbCCD7- and GbCCD8b-silenced cotton plants. Furthermore, GbMYC2 suppressed the expression of GbCCD7 and GbCCD8b by binding to their promoters, which might regulate the homeostasis of SLs in cotton through a negative feedback loop. We also found that GbCCD7- and GbCCD8b-silenced cotton plants were impaired in V. dahliae-induced reactive oxygen species (ROS) accumulation. Taken together, our results suggest that SLs positively regulate cotton resistance to Verticillium wilt through crosstalk with the JA- and ABA-signaling pathways and by inducing ROS accumulation.