Deciphering the enigma of lignification: precursor transport, oxidation, and the topochemistry of lignin assembly

The RALF2-FERONIA-MYB63 module orchestrates growth and defense in tomato roots

Plant secreted peptides RAPID ALKALINISATION FACTORs (RALFs), which act through the receptor FERONIA (FER), play important roles in plant growth. However, it remains unclear whether and how RALF-FER contributes to the trade-off of plant growth-defense. Here, we used a variety of techniques such as CRISPR/Cas9, protein-protein interaction and transcriptional regulation methods to investigate the role of RALF2 and its receptor FER in regulating lignin deposition, root growth, and defense against Fusarium oxysporum f. sp. lycopersici (Fol) in tomato (Solanum lycopersicum). The ralf2 and fer mutants show reduced primary root length, elevated lignin accumulation, and enhanced resistance against Fol than the wild-type. FER interacts with and phosphorylates MYB63 to promote its degradation. MYB63 serves as an activator of lignin deposition by regulating the transcription of dirigent protein gene DIR19. Mutation of DIR19 suppresses lignin accumulation, and reverses the short root phenotype and Fol resistance in ralf2 or fer mutant. Collectively, our results demonstrate that the RALF2-FER-MYB63 module fine-tunes root growth and resistance against Fol through regulating the deposition of lignin in tomato roots. The study sheds new light on how plants maintain the growth-defense balance via RALF-FER.

The OsDIR55 gene increases salt tolerance by altering the root diffusion barrier

The increased soil salinity is becoming a major challenge to produce more crops and feed the growing population of the world. In this study, we demonstrated that overexpression of OsDIR55 gene enhances rice salt tolerance by altering the root diffusion barrier. OsDIR55 is broadly expressed in all examined tissues and organs with the maximum expression levels at lignified regions in rice roots. Salt stress upregulates the expression of OsDIR55 gene in an abscisic acid (ABA)-dependent manner. Loss-function and overexpression of OsDIR55 compromised and improved the development of CS and root diffusion barrier, manifested with the decreased and increased width of CS, respectively, and ultimately affected the permeability of the apoplastic diffusion barrier in roots. OsDIR55 deficiency resulted in Na+ accumulation, ionic imbalance, and growth arrest, whereas overexpression of OsDIR55 enhances salinity tolerance and provides an overall benefit to plant growth and yield potential. Collectively, we propose that OsDIR55 is crucial for ions balance control and salt stress tolerance through regulating lignification-mediated root barrier modifications in rice.

Adaptation mechanism of three Impatiens species to different habitats based on stem morphology, lignin and MYB4 gene

BACKGROUND

Impatiens is an important genus with rich species of garden plants, and its distribution is extremely extensive, which is reflected in its diverse ecological environment. However, the specific mechanisms of Impatiens' adaptation to various environments and the mechanism related to lignin remain unclear.

RESULTS

Three representative Impatiens species,Impatiens chlorosepala (wet, low degree of lignification), Impatiens uliginosa (aquatic, moderate degree of lignification) and Impatiens rubrostriata (terrestrial, high degree of lignification), were selected and analyzed for their anatomical structures, lignin content and composition, and lignin-related gene expression. There are significant differences in anatomical parameters among the stems of three Impatiens species, and the anatomical structure is consistent with the determination results of lignin content. Furthermore, the thickness of the xylem and cell walls, as well as the ratio of cell wall thickness to stem diameter have a strong correlation with lignin content. The anatomical structure and degree of lignification in Impatiens can be attributed to the plant's growth environment, morphology, and growth rate. Our analysis of lignin-related genes revealed a negative correlation between the MYB4 gene and lignin content. The MYB4 gene may control the lignin synthesis in Impatiens by controlling the structural genes involved in the lignin synthesis pathway, such as HCT, C3H, and COMT. Nonetheless, the regulation pathway differs between species of Impatiens.

CONCLUSIONS

This study demonstrated consistency between the stem anatomy of Impatiens and the results obtained from lignin content and composition analyses. It is speculated that MYB4 negatively regulates the lignin synthesis in the stems of three Impatiens species by regulating the expression of structural genes, and its regulation mechanism appears to vary across different Impatiens species. This study analyses the variations among different Impatiens plants in diverse habitats, and can guide further molecular investigations of lignin biosynthesis in Impatiens.

A density functional theory study on interactions in water-bridged dimeric complexes of lignin

Lignin is the main plant cell wall component responsible for recalcitrance in the process of lignocellulosic biomass conversion into biofuels. The recalcitrance and insolubility of lignin in different reaction media are due in part to the hydrogen bonds and π interactions that hold syringyl (S) and guaiacyl (G) units together and promote the formation of stable water-bridged dimeric complexes (WBDCs): S⋯G and S⋯S, in native lignin. The current understanding of how each type of interaction influences the stability of these complexes within lignin native cell walls is still limited. Here, we found by DFT calculations that hydrogen bonding is more dominant than π-stacking interaction between aromatic rings of WBDCs. Although there is a stronger interaction of hydrogen bonds between subunits and water and higher π-stacking interaction in the S⋯S complex compared to the S⋯G complex, the former complex is less thermodynamically stable than the latter due to the entropic contribution coming from the methoxy substituents in the S-unit. Our results demonstrate that the methoxylation degree of lignin units does not significantly influence the structural geometries of WBDCs; if anything, an enhanced dispersion interaction between ring aromatics results in quasi-sandwich geometries as found in "coiled" lignin structures in the xylem tissue of wood. In the same way as that with ionic liquids, polar solvents can dissolve S-lignin by favorable interactions with the aliphatic hydroxyl group in the α-position as the key site or the aromatic hydroxyl group as the secondary site.

Reactive oxygen species: Multidimensional regulators of plant adaptation to abiotic stress and development

Reactive oxygen species (ROS) are produced as undesirable by-products of metabolism in various cellular compartments, especially in response to unfavorable environmental conditions, throughout the life cycle of plants. Stress-induced ROS production disrupts normal cellular function and leads to oxidative damage. To cope with excessive ROS, plants are equipped with a sophisticated antioxidative defense system consisting of enzymatic and non-enzymatic components that scavenge ROS or inhibit their harmful effects on biomolecules. Nonetheless, when maintained at relatively low levels, ROS act as signaling molecules that regulate plant growth, development, and adaptation to adverse conditions. Here, we provide an overview of current approaches for detecting ROS. We also discuss recent advances in understanding ROS signaling, ROS metabolism, and the roles of ROS in plant growth and responses to various abiotic stresses.

Transcription factor PbMYB80 regulates lignification of stone cells and undergoes RING finger protein PbRHY1-mediated degradation in pear fruit

The Chinese white pear (Pyrus bretschneideri) fruit carries a high proportion of stone cells, adversely affecting fruit quality. Lignin is a main component of stone cells in pear fruit. In this study, we discovered that a pear MYB transcription factor, PbMYB80, binds to the promoters of key lignin biosynthesis genes and inhibits their expression. Stable overexpression of PbMYB80 in Arabidopsis showed that lignin deposition and secondary wall thickening were inhibited, and the expression of the lignin biosynthesis genes in transgenic Arabidopsis was decreased. Transient overexpression of PbMYB80 in pear fruit inhibited lignin metabolism and stone cell development, and the expression of some genes in the lignin metabolism pathway was reduced. In contrast, silencing PbMYB80 with VIGS increased the lignin and stone cell content in pear fruit, and increased expression of genes in the lignin metabolism pathway. By screening a pear fruit cDNA library in yeast, we found that PbMYB80 binds to a RING finger (PbRHY1) protein. We also showed that PbRHY1 exhibits E3 ubiquitin ligase activity and degrades ubiquitinated PbMYB80 in vivo and in vitro. This investigation contributes to a better understanding of the regulation of lignin biosynthesis in pear fruit, and provides a theoretical foundation for increasing pear fruit quality at the molecular level.

Applications and opportunities of click chemistry in plant science

The Nobel Prize in Chemistry for 2022 was awarded to the pioneers of Lego-like 'click chemistry': combinatorial chemistry with remarkable modularity and diversity. It has been applied to a wide variety of biological systems, from microorganisms to plants and animals, including humans. Although click chemistry is a powerful chemical biology tool, comparatively few studies have examined its potential in plant science. Here, we review click chemistry reactions and their applications in plant systems, highlighting the activity-based probes and metabolic labeling strategies combined with bioorthogonal click chemistry to visualize plant biological processes. These applications offer new opportunities to explore and understand the underlying molecular mechanisms regulating plant composition, growth, metabolism, defense, and immune responses.

Berberine bridge enzyme-like oxidases of cellodextrins and mixed-linked β-glucans control seed coat formation

Plants have evolved various resistance mechanisms to cope with biotic stresses that threaten their survival. The BBE23 member (At5g44360/BBE23) of the Arabidopsis berberine bridge enzyme-like (BBE-l) protein family (Arabidopsis thaliana) has been characterized in this paper in parallel with the closely related and previously described CELLOX (At4g20860/BBE22). In addition to cellodextrins, both enzymes, renamed here as CELLODEXTRIN OXIDASE 2 and 1 (CELLOX2 and CELLOX1), respectively, oxidize the mixed-linked β-1→3/β-1→4-glucans (MLGs), recently described as capable of activating plant immunity, reinforcing the view that the BBE-l family includes members that are devoted to the control of the homeostasis of potential cell wall-derived damage-associated molecular patterns (DAMPs). The 2 putatively paralogous genes display different expression profiles. Unlike CELLOX1, CELLOX2 is not expressed in seedlings or adult plants and is not involved in immunity against Botrytis cinerea. Both are instead expressed in a concerted manner in the seed coat during development. Whereas CELLOX2 is expressed mainly during the heart stage, CELLOX1 is expressed at the immediately later stage, when the expression of CELLOX2 decreases. Analysis of seeds of cellox1 and cellox2 knockout mutants shows alterations in the coat structure: the columella area is smaller in cellox1, radial cell walls are thicker in both cellox1 and cellox2, and the mucilage halo is reduced in cellox2. However, the coat monosaccharide composition is not significantly altered, suggesting an alteration of the organization of the cell wall, thus reinforcing the notion that the architecture of the cell wall in specific organs is determined not only by the dynamics of the synthesis/degradation of the main polysaccharides but also by its enzymatic oxidation.

A new family of proteins is required for tethering of Casparian strip membrane domain and nutrient homoeostasis in rice

Cell-cell junctions are essential for multicellular organisms to maintain nutrient homoeostasis. A plant-type tight junction, the Casparian strip (CS)-Casparian strip membrane domain (CSD) that seals the paracellular space between adjacent endodermal cells, has been known for more than one hundred years. However, the molecular basis of this structure remains unknown. Here we report that a new family of proteins containing a glycine/alanine/proline-rich domain, a lectin domain and a secretory signal peptide (GAPLESS) mediates tethering of the plasma membrane to the CS in rice. The GAPLESS proteins are specifically localized in the CS of root endodermal cells, and loss of their functions results in a disabled cell-cell junction and disrupted nutrient homoeostasis. The GAPLESS protein forms a tight complex with OsCASP1 in the plasma membrane, thereby mediating the CS-CSD junction. This study provides valuable insights into the junctional complex of plant endodermal cells, shedding light on our understanding of nutrient homoeostasis in crops and the cell junctions of eukaryotes.

Metabolome and Transcriptome Analysis of Sulfur-Induced Kiwifruit Stem Laccase Gene Involved in Syringyl Lignin Synthesis against Bacterial Canker

Kiwifruit canker is caused by Pseudomonas syringae pv. actinidiae and is one of the most destructive diseases of kiwifruit worldwide. Sulfur can improve the deposit of lignin in kiwifruit stems and induce disease resistance, but the action mechanism at the molecular level remains unclear. This omics-based study revealed that sulfur-induced S lignin synthesis contributes to disease resistance. Histological staining verified sulfur-enhanced total lignin deposition in kiwifruit stems. High-performance liquid chromatography and confocal Raman microscopy showed that sulfur-activated S lignin was mainly deposited in the cell corner. Metabolome and transcriptome analysis revealed that the levels of phenylpropanoid pathway S lignin precursors sinapic acid and sinapyl alcohol were significantly increased and 16 laccase genes were upregulated. Sulfur-induced resistance defense promoted elevated laccase activity by activating the laccase genes, participating in sinapic acid and sinapyl alcohol substance synthesis, and ultimately polymerizing S lignin at cell corner against kiwifruit canker disease.

Mitigation of Acetamiprid Residue Disruption on Pea Seed Germination by Selenium Nanoparticles and Lentinans

The use of pesticides for pest control during the storage period of legume seeds is a common practice. This study evaluated the disruptive effects on pea seed germination and the repair effects of selenium nanoparticles (SeNPs) and lentinans (LNTs) This study examined the biomass, nutrient content, antioxidant indicators, plant hormones, phenolic compounds, and metabolites associated with the lignin biosynthesis pathway in pea sprouts. The application of acetamiprid resulted in a significant decrease in yield, amino-acid content, and phenolic compound content of pea sprouts, along with observed lignin deposition. Moreover, acetamiprid residue exerted a notable level of stress on pea sprouts, as evidenced by changes in antioxidant indicators and plant hormones. During pea seed germination, separate applications of 5 mg/L SeNPs or 20 mg/L LNTs partially alleviated the negative effects induced by acetamiprid. When used in combination, these treatments restored most of the aforementioned indicators to levels comparable to the control group. Correlation analysis suggested that the regulation of lignin content in pea sprouts may involve lignin monomer levels, reactive oxygen species (ROS) metabolism, and plant hormone signaling mediation. This study provides insight into the adverse impact of acetamiprid residues on pea sprout quality and highlights the reparative mechanism of SeNPs and LNTs, offering a quality assurance method for microgreens, particularly pea sprouts. Future studies can validate the findings of this study from the perspective of gene expression.

Transcriptome analysis of gene expression profiles reveals wood formation mechanisms in Chinese fir at different stand ages

Forests are crucial sustainable sources of natural ecosystems and contribute to human welfare. Cunninghamia lanceolata (Chinese fir) is an economically important conifer and occupies the largest area in China that produces global wood resources. Although Chinese fir has high economic value in China, little information is known regarding its mechanisms of wood formation. Therefore, transcriptome analysis was conducted to study the gene expression patterns and associated timber formation mechanisms in Chinese fir at different stand ages. In the present study, a total of 837,156 unigenes were identified in 84 samples from Chinese fir (pith and root) at different stand ages via RNA-Seq. Among them, most of the differentially expressed genes (DEGs) were significantly enrichment in plant hormone signal transduction, flavonoid metabolism pathway, starch and sucrose metabolism, and MAPK signal transduction pathway, which might be associated with the diameter formation in Chinese fir. The DEGs in these pathways were analyzed in Chinese fir and were related to lignin synthesis, cell wall formation and cell wall reinforcement/thickening. These genes might play an important role in regulating timber formation/growth in Chinese fir. In addition, certain transcriptome factors (TFs) related to Chinese fir timber formation were identified, including WRKY33, WRKY22, PYR/PYL, and MYC2. Weighted co-expression network analysis (WGCNA) showed that glucan endo-1,3-beta-d-glucosidase was a hub gene significantly correlated with the growth-related genes in Chinese fir. Sixteen key genes that related to diameter regulation in Chinese fir were verified by qRT-PCR analysis. These key genes might have a fine regulatory role in timber formation in Chinese fir. Our results pave the way for research on the regulatory mechanisms of wood formation, and provide an insight for improving the quality production of Chinese fir.

Lignin-Based Materials for Additive Manufacturing: Chemistry, Processing, Structures, Properties, and Applications

The utilization of lignin, the most abundant aromatic biomass component, is at the forefront of sustainable engineering, energy, and environment research, where its abundance and low-cost features enable widespread application. Constructing lignin into material parts with controlled and desired macro- and microstructures and properties via additive manufacturing has been recognized as a promising technology and paves the way to the practical application of lignin. Considering the rapid development and significant progress recently achieved in this field, a comprehensive and critical review and outlook on three-dimensional (3D) printing of lignin is highly desirable. This article fulfils this demand with an overview on the structure of lignin and presents the state-of-the-art of 3D printing of pristine lignin and lignin-based composites, and highlights the key challenges. It is attempted to deliver better fundamental understanding of the impacts of morphology, microstructure, physical, chemical, and biological modifications, and composition/hybrids on the rheological behavior of lignin/polymer blends, as well as, on the mechanical, physical, and chemical performance of the 3D printed lignin-based materials. The main points toward future developments involve hybrid manufacturing, in situ polymerization, and surface tension or energy driven molecular segregation are also elaborated and discussed to promote the high-value utilization of lignin.

Pathogen-induced autophagy regulates monolignol transport and lignin formation in plant immunity

The evolutionary plant-pathogen arms race has equipped plants with the immune system that can defend against pathogens. Pattern-triggered immunity and effector-triggered immunity are two major branches of innate immunity that share immune responses, including oxidative bursts, transcriptional reprogramming, and cell wall modifications such as lignin deposition. In a previous study, we reported that lignin rapidly accumulates in pathogen-infected Arabidopsis leaves and acts as a mechanical barrier, spatially restricting pathogens and cell death. Lignin deposition into the cell wall is a three-step process: monolignol biosynthesis, transport, and polymerization. While monolignol biosynthesis and polymerization are relatively well understood, the mechanism of monolignol transport remains unclear. In this study, we show that macroautophagy/autophagy modulates pathogen-induced lignin formation. Lignification and other immune responses were impaired in autophagy-defective atg (autophagy-related) mutants. In microscopy analyses, monolignols formed punctate structures in response to pathogen infection and colocalized with autophagic vesicles. Furthermore, autophagic activity and lignin accumulation were both enhanced in dnd1 (defense, no death 1) mutant with elevated disease resistance but no cell death and crossing dnd1-1 with atg mutants resulted in a lignin deficit, further supporting that lignin formation requires autophagy. Collectively, these findings demonstrate that lignification, particularly monolignol transport, is achieved through autophagic membrane trafficking in plant immunity.Abbreviations: ABC transporter: ATP-binding cassette transporter; ACD2/AT4G37000: accelerated cell death 2; ATG: autophagy-related; C3'H/AT2G40890: p-coumaroyl shikimate 3-hydroxylase; C4H/AT2G30490: cinnamate 4-hydroxylase; CA: coniferyl alcohol; CaMV: cauliflower mosaic virus; CASP: Casparian strip membrane domain protein; CASPL: CASP-like protein; CBB: Coomassie Brilliant Blue; CCoAOMT1/AT4G34050: caffeoyl-CoA O-methyltransferase 1; CCR1/AT1G15950: cinnamoyl-CoA reductase 1; CFU: colony-forming unit; COMT1/AT5G54160: caffeic acid O-methyltransferase 1; Con A: concanamycin A; DMAC: dimethylaminocoumarin; DND1/AT5G15410: defense, no death 1; CNGC2: cyclic nucleotide-gated channel 2; ER: endoplasmic reticulum; ESB1/AT2G28670/DIR10: enhanced suberin 1; ETI: effector-triggered immunity; EV: extracellular vesicle; F5H/AT4G36220: ferulate-5-hydroxylase; Fluo-3 AM: Fluo-3 acetoxymethyl ester; GFP: green fluorescent protein; HCT/AT5G48930: p-hydroxycinnamoyl-CoA:quinate/shikimate p-hydroxycinnamoyltransferase; HR: hypersensitive response; LAC: laccase; LTG: LysoTracker Green; LSD1/AT4G200380: lesion stimulating disease 1; PAL1/AT2G37040: phenylalanine ammonia-lyase 1; PAMP: pathogen-associated molecular patterns; PCD: programmed cell death; PE: phosphatidylethanolamine; PRX: peroxidase; Pst DC3000: Pseudomonas syringe pv. tomato DC3000; PTI: pattern-triggered immunity; SA: salicylic acid; SD: standard deviation; SID2/AT1G7410: SA induction-deficient 2; UGT: UDP-glucosyltransferase; UPLC: ultraperformance liquid chromatography; UPS: unconventional protein secretion; V-ATPase: vacuolar-type H⁺-translocating ATPase.

Comprehensive expression analyses of the ABCG subfamily reveal SvABCG17 as a potential transporter of lignin monomers in the model C4 grass Setaria viridis

Although several aspects of lignin metabolism have been extensively characterized, the mechanism(s) by which lignin monomers are transported across the plasma membrane remains largely unknown. Biochemical, proteomic, expression and co-expression analyses from several plant species support the involvement of active transporters, mainly those belonging to the ABC superfamily. Here, we report on the genome-wide characterization of the ABCG gene subfamily in the model C4 grass Setaria viridis and further identification of the members potentially involved in monolignol transport. A total of 48 genes encoding SvABCGs were found in the S. viridis genome, from which 21 SvABCGs were classified as full-size transporters and 27 as half-size transporters. Comprehensive analysis of the ABCG subfamily in S. viridis based on expression and co-expression analyses support a role for SvABCG17 in monolignol transport: (i) SvABCG17 is orthologous to AtABCG29, a monolignol transporter in Arabidopsis thaliana; (ii) SvABCG17 displays a similar expression profile to that of lignin biosynthetic genes in a set of different S. viridis tissues and along the elongating internode; (iii) SvABCG17 is highly co-expressed with lignin-related genes in a public transcriptomic database; (iv) SvABCG17displays particularly high expression in the top of the S. viridis elongating internode, a tissue undergoing active lignification; (v) SvABCG17 mRNA localization coincides with the histochemical pattern of lignin deposition; and (vi) the promoter of SvABCG17 is activated by secondary cell wall-associated transcription factors, especially by lignin-specific activators of the MYB family. Further studies might reveal further aspects of this potential monolignol transporter, including its real substrate specificity and whether it works redundantly with other ABC members during S. viridis lignification.

Genetic Relationship, SPAD Reading, and Soluble Sugar Content as Indices for Evaluating the Graft Compatibility of Citrus Interstocks

Simple Summary

Grafting is a critical agricultural practice in citrus growing. The effectiveness of grafting not only depends on the technique but also on the stock–scion combinations. In this study, we investigated the grafting compatibility of five interstock combinations based on physiological and biochemical traits. The results revealed that the grafting compatibility in the early stages of grafting mediated by interstocks was related to the genetic relationship. The leaf chlorophyll content (SPAD reading, soil plant analysis development) and soluble sugar could be employed as preselected indicators to assess compatibility in the late stage of grafting. Our findings lay the foundation for the further research on rootstock–scion interaction mechanism.

Abstract

The interstock, a stock between the rootstock and scion, has a significant regulatory effect on the stock and scion, and its function is highly dependent on graft compatibility. To assess the graft compatibility of the interstock and scion, ‘Yuanxiaochun’ was top grafted onto ‘Ponkan’, ‘Shiranuhi’, ‘Harumi’, ‘Tarocco’, and ‘Kumquat’. The results showed significant differences in the survival ratio and preservation ratio among different combinations. Grafting compatibility in the early stages of grafting was associated with the genetic relationship. The biomass accumulation revealed that the interstock could influence both the rootstock and the scion. The physiological and biochemical traits analysis suggested that SPAD reading and soluble sugar could be employed as preselected indices to evaluate graft compatibility in the late stage of grafting. These results indicated that the evaluation of graft compatibility was a dynamic process. The findings provided a new approach for studying the stock and scion interaction mechanisms mediated by interstock, and directly provided a theoretical and practical basis for the high-grafting of ‘Yuanxiaochun’ citrus.

Comparative Multi-Omics Analysis Reveals Lignin Accumulation Affects Peanut Pod Size

Pod size is one of the important factors affecting peanut yield. However, the metabolites relating to pod size and their biosynthesis regulatory mechanisms are still unclear. In the present study, two peanut varieties (Tif and Lps) with contrasting pod sizes were used for a comparative metabolome and transcriptome analysis. Developing peanut pods were sampled at 10, 20 and 30 days after pegging (DAP). A total of 720 metabolites were detected, most of which were lipids (20.3%), followed by phenolic acids (17.8%). There were 43, 64 and 99 metabolites identified as differentially accumulated metabolites (DAMs) at 10, 20 and 30 DAP, respectively, and flavonoids were the major DAMs between Tif and Lps at all three growth stages. Multi-omics analysis revealed that DAMs and DEGs (differentially expressed genes) were significantly enriched in the phenylpropanoid biosynthesis (ko00940) pathway, the main pathway of lignin biosynthesis, in each comparison group. The comparisons of the metabolites in the phenylpropanoid biosynthesis pathway accumulating in Tif and Lps at different growth stages revealed that the accumulation of p-coumaryl alcohol (H-monolignol) in Tif was significantly greater than that in Lps at 30 DAP. The differential expression of gene-LOC112771695, which is highly correlated with p-coumaryl alcohol and involved in the biosynthesis of monolignols, between Tif and Lps might explain the differential accumulation of p-coumaryl alcohol. The content of H-lignin in genetically diverse peanut varieties demonstrated that H-lignin content affected peanut pod size. Our findings would provide insights into the metabolic factors influencing peanut pod size and guidance for the genetic improvement of the peanut.

Comparative Transcriptome and Anatomic Characteristics of Stems in Two Alfalfa Genotypes

Stems are more important to forage quality than leaves in alfalfa. To understand lignin formation at different stages in alfalfa, lignin distribution, anatomical characteristics and transcriptome profile were employed using two alfalfa cultivars. The results showed that the in vitro true digestibility (IVTD) of stems in WL168 was significantly higher than that of Zhungeer, along with the significantly lower neutral detergent fiber (NDF), acid detergent fiber (ADF) and lignin contents. In addition, Zhungeer exhibited increased staining of the xylem areas in the stems of different developmental stages compared to WL168. Interestingly, the stems of WL168 appeared intracellular space from the stage 3, while Zhungeer did not. The comparative transcriptome analysis showed that a total of 1993 genes were differentially expressed in the stem between the cultivars, with a higher number of expressed genes in the stage 4. Of the differentially expressed genes, starch and sucrose metabolism as well as phenylpropanoid biosynthesis pathways were the most significantly enriched pathways. Furthermore, expression of genes involved in lignin biosynthesis such as PAL, 4CL, HCT, CAD, COMT and POD coincides with the anatomic characteristics and lignin accumulation. These results may help elucidate the regulatory mechanisms of lignin biosynthesis and improve forage quality in alfalfa.

The calcium-mediated homogalacturonan pectin complexation in cell walls contributes the firmness increase in loquat fruit during postharvest storage

INTRODUCTION

Postharvest textural changes in fruit are mainly divided into softening and lignification. Loquat fruit could have severe lignification with increased firmness during postharvest storage. Pectin is mainly associated with the postharvest softening of fruit, but some studies also found that pectin could be involved in strengthening the mechanical properties of the plant.

OBJECTIVES

This study focused on characterizing the dynamics of pectin and its complexation in the cell wall of lignified loquat fruit during postharvest storage, and how these changes could influence fruit firmness.

METHODS

The homogalacturonan (HG) pectin in the cell wall of loquat fruit was identified using monoclonal antibodies. An oligogalacturonide (OG) probe was used to label the egg-box structure formed by Ca²⁺ cross-linking with low-methylesterified HG. An exogenous injection was used to verify the role of egg-box structures in the firmness increase in loquat fruit.

RESULTS

The JIM5 antibody revealed that low-methylesterified HG accumulated in the tricellular junctions and middle lamella of loquat fruit that had severe lignification symptoms. The pectin methylesterase (PME) activity increased during the early stages of storage at 0°C, and the calcium-pectate content and flesh firmness constantly increased during storage. The OG probe demonstrated the accumulation of egg-box structures at the cellular level. The exogenous injection of PME and Ca²⁺ into the loquat flesh led to an increase in firmness with more low-methylesterified HG and egg-box structure signals.

CONCLUSION

PME-mediated demethylesterification generated large amounts of low-methylesterified HG in the cell wall. This low-methylesterified HG further cross-linked with Ca²⁺ to form egg-box structures. The pectin-involved complexations then contributed to the increased firmness in loquat fruit. Overall, besides being involved in fruit softening, pectin could also be involved in strengthening the mechanical properties of postharvest fruit. This study provides new ideas for obtaining a better texture of postharvest loquat fruits based on pectin regulation.

Reactive oxygen species function as signaling molecules in controlling plant development and hormonal responses

Reactive oxygen species (ROS) serve as second messengers in plant signaling pathways to remodel plant growth and development. New insights into how enzymatic ROS-producing machinery is regulated by hormones or localized during development have provided a framework for understanding the mechanisms that control ROS accumulation patterns. Signaling-mediated increases in ROS can then modulate the activity of proteins through reversible oxidative modification of specific cysteine residues. Plants also control the synthesis of antioxidants, including plant-specialized metabolites, to further define when, where, and how much ROS accumulate. The availability of sophisticated imaging capabilities, combined with a growing tool kit of ROS detection technologies, particularly genetically encoded biosensors, sets the stage for improved understanding of ROS as signaling molecules.

Role of lignin-based nanoparticles in anticancer drug delivery and bioimaging: An up-to-date review

Lignin, an aromatic biopolymer, is the second most abundant naturally occurring one after cellulose that has drawn a great deal of interest over the years for its potential uses owing to the presence of high content of phenolic compounds, ecofriendly feature and cost-efficiency in comparison to the synthetic polymers. Nevertheless, with the intention of advancing its development, several efforts have been performed in the direction of utilizing lignin on the nanoscale due to its inimitable properties. The notable absorption capacity, fluorescence emission, biodegradability and non-toxicity of lignin nanoparticles permit its appropriateness as a vehicle for drugs and as a bioimaging material. Moreover, lignin nanoparticles have shown plausible therapeutic effects, such as anticancer, antimicrobial, and antioxidant. The current review sheds light on the recent development in the formulation and anticancer applications of lignin nanoparticles as a drug carrier and as a diagnostic tool. The surface properties of the nanomaterial affect the end product characteristics, hence, factors namely; lignin source, isolation technique, purification and quantitation methods, are discussed in this review. This study represents original work that has not been published elsewhere and that has not been submitted simultaneously for publication elsewhere. The manuscript has been read, revised, and approved by the authors.

Origins of covalent linkages within the lignin-carbohydrate network of biomass

Covalent linkages between lignin and the surrounding carbohydrate network, often referred to as lignin-carbohydrate complexes (LCCs), have been proposed to affect the organization of the biomass microstructure and directly correlate with the recalcitrant nature of biomass. However, the existence and frequency of these LCC linkages remain controversial and largely unknown, primarily due to the harsh experimental techniques available to characterize them. During the predominant lignin polymerization pathway a reactive intermediate is formed. Though this intermediate can covalently bind to the surrounding cellulose/hemicellulose matrix, it has been traditionally assumed to react exclusively with water, leading to purely physical interactions between lignin and cellulose/hemicellulose in the cell wall. This work, for the first time, provides direct evidence of the molecular mechanism of the formation of benzyl ether and benzyl ester LCC linkages via the speculated lignin polymerization pathway. The formation of these LCC linkages showed thermodynamic favorability, while remaining kinetically facile, compared to the previously assumed mechanism of the lignin intermediate reacting with water. The present work suggests that the surrounding carbohydrate matrix could play a role in the organization of lignin deposition and these covalent linkages could be a key factor in biomass recalcitrance.

PbUGT72AJ2-Mediated Glycosylation Plays an Important Role in Lignin Formation and Stone Cell Development in Pears (Pyrus bretschneideri)

Glycosylation is necessary for many processes of plant secondary metabolism. It can maintain plant homeostasis and is of great significance to normal plant growth and development. At present, the significance of glycosylation for lignin biosynthesis has been proven in some plants, but it has not yet been reported in pears. We used in situ hybridization, in vitro expression, substrate catalysis, transgenic Arabidopsisthaliana, and transient transformation of pear fruit in our investigation, which was predicated on the identification of a gene PbUGT72AJ2 that may be involved in lignin monolignol glycosylation according to our previous work. These results revealed that PbUGT72AJ2 transcripts were localized to some pulp cell walls, lignin deposition, and stone cell areas of pear fruit. The recombinant PbUGT72AJ2-pGEX4T-1 protein had activity against coniferyl alcohol and sinapyl alcohol, and its catalytic efficiency against coniferyl alcohol was higher than that against sinapyl alcohol. When PbUGT72AJ2 was transferred into Arabidopsisthaliana mutants, it was found that some characteristics of Arabidopsisthalianaugt72e3 mutants were restored. In Arabidopsisthaliana, overexpression of PbUGT72AJ2 enhanced the contents of coniferin and syringin, whereas lignification did not change significantly. Transient transformation of pear fruit showed that when PbUGT72AJ2 in pear fruit was silenced by RNA interference, the content of lignin and stone cells in pear fruit increased, whereas the gene PbUGT72AJ2 was overexpressed in pear fruit, and there was almost no change in the pear fruit compared with the control. Lignin deposition in pear fruit was closely related to stone cell development. In this study, we proved that PbUGT72AJ2 plays an important role in lignin deposition and stone cell development in pear fruit, which provides a molecular biological basis for improving pear fruit quality at the molecular level.

Analysis of the β-Glucosidase Family Reveals Genes Involved in the Lignification of Stone Cells in Chinese White Pear (Pyrus bretschneideri Rehd.)

BGLU β-glucosidases in glycoside hydrolase family 1 (GH1) are involved in many processes of plant secondary metabolism. In particular, its de-glycosylation function plays an important role in the transport of lignin monolignols. No comprehensive study of the BGLU family in Chinese pear (Pyrus bretschneideri Rehd.) has been reported yet. In this study, the 50 BGLU family members from Chinese white pear were identified. Three candidate genes, PbBGLU1, PbBGLU15, and PbBGLU16, that may be involved in lignin synthesis were screened by bioinformatics analysis and qRT-PCR. Subcellular localization showed that all three of these candidate genes were expressed in the extracellular region. Then, we analyzed the functions of PbBGLU1 and PbBGLU16. In situ hybridization analysis showed that PbBGLU1 transcripts were not only localized to some pulp cell walls, lignin deposition, and stone cell areas of a pear fruit, but that was also a small amount of enrichment in normal pear flesh cells. PbBGLU16 transcripts were only enriched in lignin deposition and stone cell areas of pear fruit. Enzyme activity analysis revealed that GST-PbBGLU1 and GST-PbBGLU16 had a stronger activity and higher catalytic efficiency for coniferin than syringin. In addition, GST-PbBGLU16 exhibited the higher activity and catalytic efficiency for the two substrates compared with GST-PbBGLU1. The transformation of PbBGLU1 and PbBGLU16 into Arabidopsis identified that the lignin contents of Arabidopsis BGLU-45 mutant, PbBGLU1-RE, and PbBGLU16-RE were not changed than that of wild-type. However, compared with wild-type Arabidopsis, the overexpression of the plant's lignin increased in varying degrees. The effect of PbBGLU16 on the lignin increment was greater than that of PbBGLU1 in Arabidopsis. In pear fruits, with transient overexpression of PbBGLU1, the contents of lignin and stone cells were significantly higher (0.01 < P < 0.05) than those with empty vector injection pear fruits. After transient expression of PbBGLU16, lignin in pear fruit increased significantly (0.01 < P < 0.05) and stone cells showed a very significant difference (P < 0.01) compared with the control group. However, RNA interference silenced these two genes in pear fruit, which seemed to have no impression on lignin and stone cells. This study provides a molecular biological basis for improving pear fruit quality at the molecular level.

ACORBA: Automated workflow to measure Arabidopsis thaliana root tip angle dynamics

The ability of plants to sense and orient their root growth towards gravity is studied in many laboratories. It is known that manual analysis of image data is subjected to human bias. Several semi-automated tools are available for analysing images from flatbed scanners, but there is no solution to automatically measure root bending angle over time for vertical-stage microscopy images. To address these problems, we developed ACORBA, which is an automated software that can measure root bending angle over time from vertical-stage microscope and flatbed scanner images. ACORBA also has a semi-automated mode for camera or stereomicroscope images. It represents a flexible approach based on both traditional image processing and deep machine learning segmentation to measure root angle progression over time. As the software is automated, it limits human interactions and is reproducible. ACORBA will support the plant biologist community by reducing labour and increasing reproducibility of image analysis of root gravitropism.

Physico-chemical interactions within lignocellulosic biomass and their importance in developing solvent based deconstruction methods

Fundamental understanding of physico-chemical interactions among the biopolymers in lignocellulosic biomass is crucial to develop atom-efficient deconstruction methods.

pH-Dependent interaction mechanism of lignin nanofilms

Lignin has been spotlighted as an abundant renewable bioresource for use in material technologies and applications such as biofuels, binders, composites, and nanomaterials for drug delivery. However, owing to its complex and irregular structure, it is difficult to investigate its fundamental interaction mechanism, which is necessary to promote its use. In this study, a surface forces apparatus (SFA) was used to investigate the pH-dependent molecular interactions between a lignin nanofilm and five functionalized self-assembled monolayers (SAMs). The lignin nanofilm adhered most strongly to the amine-functionalized SAM, indicating that the molecular interactions with lignin were mainly electrostatic and cation-π interactions. The force-distance profile between lignin and a methyl-functionalized SAM revealed pH-dependent interactions similar to those between two lignin nanofilms. This finding indicates that the dominant cohesion mechanism is hydrophobic interactions. A quartz crystal microbalance with dissipation was used to investigate the adsorption of free lignin molecules on functionalized SAMs. Lignin molecules, which were free in solution, were most effectively adsorbed to the phenyl-functionalized SAM. To investigate whether the nanoscopic interaction forces could be extended to macroscopic properties, the compressive strength of activated carbon-lignin composites prepared at different pH values was evaluated. As the pH increased, the compressive strength decreased owing to the reduced hydrophobic interactions between the activated carbon and lignin, consistent with the SFA results. These quantitative results regarding lignin interactions can advance the potential use of lignin as an eco-friendly biomaterial.

Comparative transcriptomic analysis of genes involved in stem lignin biosynthesis in woody and herbaceous Paeonia species

Paeonia is recognized globally due to its ornamental value. However, the mechanisms behind the formation of distinct levels of lignification in Paeonia stems remain largely unknown. In this study, we selected three representative Paeonia species, namely P. ostii (shrub), P. lactiflora (herb), and P. × 'Hexie' (semi-shrub), to evaluate and contrast their respective anatomical structure, phytochemical composition and transcriptomic profile. Our results showed that the degree of lignin deposition on the cell wall, along with the total amount of lignin and its monomers (especially G-lignin) were higher in P. ostii stems compared to the other two species at almost all development stages except 80 days after flowering. Furthermore, we estimated a total number of unigenes of 60,238 in P. ostii, 43,563 in P. × 'Hexie', and 40,212 in P. lactiflora from stem transcriptome. We then built a co-expression network of 25 transcription factors and 21 enzyme genes involved in lignin biosynthesis and identified nine key candidate genes. The expression patterns of these genes were positively correlated with the transcription levels of PAL, C4H, 4CL2, CCR, and COMT, as well as lignin content. Moreover, the highest relative expression levels of CCR, 4CL2, and C4H were found in P. ostii. This study provides an explanation for the observed differences in lignification between woody and herbaceous Paeonia stems, and constitutes a novel reference for molecular studies of stem-specific lignification process and lignin biosynthesis that can impact the ornamental industry.

Exogenous proanthocyanidins improve tolerance of Cu-toxicity by amelioration of oxidative damage and re-programming of gene expression in Medicago sativa

Excess copper (Cu) in soil due to industrial and agricultural practices can result in reduced plant growth. Excess Cu resulted in severely retarded root growth with severe discoloration of Alfalfa (Medicago sativa) and Medicago truncatula. Growth in the presence of hydrogen peroxide resulted in similar symptoms that could be partially recovered by the addition of the reductant ascorbic acid revealing damage was likely due to oxidative stress. The addition of proanthocyanidins (PAs) in the presence of Cu prevented much of the damage, including plant growth and restoration of lignin synthesis which was inhibited in the presence of excess Cu. Transcriptome analyses of the impact of excess Cu and the amelioration after PAs treatment revealed that changes were enriched in functions associated with the cell wall and extracellular processes, indicating that inhibition of cell wall synthesis was likely the reason for retarded growth. Excess Cu appeared to induce a strong defense response, along with alterations in the expression of a number of genes encoding transcription factors, notably related to ethylene signaling. The addition of PAs greatly reduced this response, and also induced novel genes that likely help ameliorate the effects of excess Cu. These included induction of genes involved in the last step of ascorbic acid biosynthesis and of enzymes involved in cell wall synthesis. Combined, these results show that excess Cu causes severe oxidative stress damage and inhibition of cell wall synthesis, which can be relieved by the addition of PAs.

Enzyme Complexes of Ptr4CL and PtrHCT Modulate Co-enzyme A Ligation of Hydroxycinnamic Acids for Monolignol Biosynthesis in Populus trichocarpa

Co-enzyme A (CoA) ligation of hydroxycinnamic acids by 4-coumaric acid:CoA ligase (4CL) is a critical step in the biosynthesis of monolignols. Perturbation of 4CL activity significantly impacts the lignin content of diverse plant species. In Populus trichocarpa, two well-studied xylem-specific Ptr4CLs (Ptr4CL3 and Ptr4CL5) catalyze the CoA ligation of 4-coumaric acid to 4-coumaroyl-CoA and caffeic acid to caffeoyl-CoA. Subsequently, two 4-hydroxycinnamoyl-CoA:shikimic acid hydroxycinnamoyl transferases (PtrHCT1 and PtrHCT6) mediate the conversion of 4-coumaroyl-CoA to caffeoyl-CoA. Here, we show that the CoA ligation of 4-coumaric and caffeic acids is modulated by Ptr4CL/PtrHCT protein complexes. Downregulation of PtrHCTs reduced Ptr4CL activities in the stem-differentiating xylem (SDX) of transgenic P. trichocarpa. The Ptr4CL/PtrHCT interactions were then validated in vivo using biomolecular fluorescence complementation (BiFC) and protein pull-down assays in P. trichocarpa SDX extracts. Enzyme activity assays using recombinant proteins of Ptr4CL and PtrHCT showed elevated CoA ligation activity for Ptr4CL when supplemented with PtrHCT. Numerical analyses based on an evolutionary computation of the CoA ligation activity estimated the stoichiometry of the protein complex to consist of one Ptr4CL and two PtrHCTs, which was experimentally confirmed by chemical cross-linking using SDX plant protein extracts and recombinant proteins. Based on these results, we propose that Ptr4CL/PtrHCT complexes modulate the metabolic flux of CoA ligation for monolignol biosynthesis during wood formation in P. trichocarpa.

CRISPR-Knockout of CSE Gene Improves Saccharification Efficiency by Reducing Lignin Content in Hybrid Poplar

Caffeoyl shikimate esterase (CSE) has been shown to play an important role in lignin biosynthesis in plants and is, therefore, a promising target for generating improved lignocellulosic biomass crops for sustainable biofuel production. Populus spp. has two CSE genes (CSE1 and CSE2) and, thus, the hybrid poplar (Populus alba × P. glandulosa) investigated in this study has four CSE genes. Here, we present transgenic hybrid poplars with knockouts of each CSE gene achieved by CRISPR/Cas9. To knockout the CSE genes of the hybrid poplar, we designed three single guide RNAs (sg1-sg3), and produced three different transgenic poplars with either CSE1 (CSE1-sg2), CSE2 (CSE2-sg3), or both genes (CSE1/2-sg1) mutated. CSE1-sg2 and CSE2-sg3 poplars showed up to 29.1% reduction in lignin deposition with irregularly shaped xylem vessels. However, CSE1-sg2 and CSE2-sg3 poplars were morphologically indistinguishable from WT and showed no significant differences in growth in a long-term living modified organism (LMO) field-test covering four seasons. Gene expression analysis revealed that many lignin biosynthetic genes were downregulated in CSE1-sg2 and CSE2-sg3 poplars. Indeed, the CSE1-sg2 and CSE2-sg3 poplars had up to 25% higher saccharification efficiency than the WT control. Our results demonstrate that precise editing of CSE by CRISPR/Cas9 technology can improve lignocellulosic biomass without a growth penalty.

Transcriptome-Wide Identification and Quantification of Caffeoylquinic Acid Biosynthesis Pathway and Prediction of Its Putative BAHDs Gene Complex in A. spathulifolius

The phenylpropanoid pathway is a major secondary metabolite pathway that helps plants overcome biotic and abiotic stress and produces various byproducts that promote human health. Its byproduct caffeoylquinic acid is a soluble phenolic compound present in many angiosperms. Hydroxycinnamate-CoA shikimate/quinate transferase is a significant enzyme that plays a role in accumulating CQA biosynthesis. This study analyzed transcriptome-wide identification of the phenylpropanoid to caffeoylquinic acid biosynthesis candidate genes in A. spathulifolius flowers and leaves. Transcriptomic analyses of the flowers and leaves showed a differential expression of the PPP and CQA biosynthesis regulated unigenes. An analysis of PPP-captive unigenes revealed a major duplication in the following genes: PAL, 120 unigenes in leaves and 76 in flowers; C3′H, 169 unigenes in leaves and 140 in flowers; 4CL, 41 unigenes in leaves and 27 in flowers; and C4H, 12 unigenes in leaves and 4 in flowers. The phylogenetic analysis revealed 82 BAHDs superfamily members in leaves and 72 in flowers, among which five unigenes encode for HQT and three for HCT. The three HQT are common to both leaves and flowers, whereas the two HQT were specialized for leaves. The pattern of HQT synthesis was upregulated in flowers, whereas HCT was expressed strongly in the leaves of A. spathulifolius. Overall, 4CL, C4H, and HQT are expressed strongly in flowers and CAA and HCT show more expression in leaves. As a result, the quantification of HQT and HCT indicates that CQA biosynthesis is more abundant in the flowers and synthesis of caffeic acid in the leaves of A. spathulifolius.

Multi-responses of O-methyltransferase genes to salt stress and fiber development of Gossypium species

BACKGROUND

O-methyltransferases (OMTs) are an important group of enzymes that catalyze the transfer of a methyl group from S-adenosyl-L-methionine to their acceptor substrates. OMTs are divided into several groups according to their structural features. In Gossypium species, they are involved in phenolics and flavonoid pathways. Phenolics defend the cellulose fiber from dreadful external conditions of biotic and abiotic stresses, promoting strength and growth of plant cell wall.

RESULTS

An OMT gene family, containing a total of 192 members, has been identified and characterized in three main Gossypium species, G. hirsutum, G. arboreum and G. raimondii. Cis-regulatory elements analysis suggested important roles of OMT genes in growth, development, and defense against stresses. Transcriptome data of different fiber developmental stages in Chromosome Substitution Segment Lines (CSSLs), Recombination Inbred Lines (RILs) with excellent fiber quality, and standard genetic cotton cultivar TM-1 demonstrate that up-regulation of OMT genes at different fiber developmental stages, and abiotic stress treatments have some significant correlations with fiber quality formation, and with salt stress response. Quantitative RT-PCR results revealed that GhOMT10_Dt and GhOMT70_At genes had a specific expression in response to salt stress while GhOMT49_At, GhOMT49_Dt, and GhOMT48_At in fiber elongation and secondary cell wall stages.

CONCLUSIONS

Our results indicate that O-methyltransferase genes have multi-responses to salt stress and fiber development in Gossypium species and that they may contribute to salt tolerance or fiber quality formation in Gossypium.

Reactive Oxygen Species Link Gene Regulatory Networks During Arabidopsis Root Development

Plant development under altered nutritional status and environmental conditions and during attack from invaders is highly regulated by plant hormones at the molecular level by various signaling pathways. Previously, reactive oxygen species (ROS) were believed to be harmful as they cause oxidative damage to cells; however, in the last decade, the essential role of ROS as signaling molecules regulating plant growth has been revealed. Plant roots accumulate relatively high levels of ROS, and thus, maintaining ROS homeostasis, which has been shown to regulate the balance between cell proliferation and differentiation at the root tip, is important for proper root growth. However, when the balance is disturbed, plants are unable to respond to the changes in the surrounding conditions and cannot grow and survive. Moreover, ROS control cell expansion and cell differentiation processes such as root hair formation and lateral root development. In these processes, the transcription factor-mediated gene expression network is important downstream of ROS. Although ROS can independently regulate root growth to some extent, a complex crosstalk occurs between ROS and other signaling molecules. Hormone signals are known to regulate root growth, and ROS are thought to merge with these signals. In fact, the crosstalk between ROS and these hormones has been elucidated, and the central transcription factors that act as a hub between these signals have been identified. In addition, ROS are known to act as important signaling factors in plant immune responses; however, how they also regulate plant growth is not clear. Recent studies have strongly indicated that ROS link these two events. In this review, we describe and discuss the role of ROS signaling in root development, with a particular focus on transcriptional regulation. We also summarize the crosstalk with other signals and discuss the importance of ROS as signaling molecules for plant root development.

Towards lignin derived thermoplastic polymers

Lignin is the second most abundant biobased material found on earth. It is produced mainly as a byproduct of pulp and paper industry and biorefineries. Despite its abundance, lignin valorization is not achieved on a large scale. Recently, there has been a growing demand for using the renewable and biodegradable raw materials in the commodity polymers. Potential use of lignin as a component in thermoplastic polymers is a promising approach for its value-added utilization. Given the vast applications of thermoplastic materials, there is lack of comprehensive review on lignin based thermoplastic polymers in literature. This review focuses on the utilization of lignin as functional and structural component of the thermoplastic polymers which requires structural modifications of lignin pertaining to the polymeric system. First, various lignin modifications were discussed in view of controlling the homogeneity, reactivity, processability and compatibility of lignin for successful thermoplastic copolymer synthesis and blend processing. Then, various copolymerization methodologies of lignin applicable for thermoplastic monomers are reviewed. Lastly, the lignin based thermoplastic blends are discussed which covers the lignin blends with various thermoplastic polymers and the chemical modifications required to improve its compatibility in polymer matrix. Some of the promising potential applications and future perspectives to achieve the goal of lignin-based commercial thermoplastics polymers are addressed.

Molecular and physiological mechanisms underlying magnesium-deficiency-induced enlargement, cracking and lignification of Citrus sinensis leaf veins

Little is known about the physiological and molecular mechanisms underlying magnesium (Mg)-deficiency-induced enlargement, cracking and lignification of midribs and main lateral veins of Citrus leaves. Citrus sinensis (L.) Osbeck seedlings were irrigated with nutrient solution at a concentration of 0 (Mg-deficiency) or 2 (Mg-sufficiency) mM Mg(NO3)2 for 16 weeks. Enlargement, cracking and lignification of veins occurred only in lower leaves, but not in upper leaves. Total soluble sugars (glucose + fructose + sucrose), starch and cellulose concentrations were less in Mg-deficiency veins of lower leaves (MDVLL) than those in Mg-sufficiency veins of lower leaves (MSVLL), but lignin concentration was higher in MDVLL than that in MSVLL. However, all four parameters were similar between Mg-deficiency veins of upper leaves (MDVUL) and Mg-sufficiency veins of upper leaves (MSVUL). Using label-free, liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis, we identified 1229 and 492 differentially abundant proteins (DAPs) in MDVLL vs MSVLL and MDVUL vs MSVUL, respectively. Magnesium-deficiency-induced alterations of Mg, nonstructural carbohydrates, cell wall components, and protein profiles were greater in veins of lower leaves than those in veins of upper leaves. The increased concentration of lignin in MDVLL vs MSVLL might be caused by the following factors: (i) repression of cellulose and starch accumulation promoted lignin biosynthesis; (ii) abundances of proteins involved in phenylpropanoid biosynthesis pathway, hormone biosynthesis and glutathione metabolism were increased; and (iii) the abundances of the other DAPs [viz., copper/zinc-superoxide dismutase, ascorbate oxidase (AO) and ABC transporters] involved in lignin biosynthesis were elevated. Also, the abundances of several proteins involved in cell wall metabolism (viz., expansins, Rho GTPase-activating protein gacA, AO, monocopper oxidase-like protein and xyloglucan endotransglucosylase/hydrolase) were increased in MDVLL vs MSVLL, which might be responsible for the enlargement and cracking of leaf veins.

Characterization of the UDP-glycosyltransferase UGT72 Family in Poplar and Identification of Genes Involved in the Glycosylation of Monolignols

Monolignols are the building blocks for lignin polymerization in the apoplastic domain. Monolignol biosynthesis, transport, storage, glycosylation, and deglycosylation are the main biological processes partaking in their homeostasis. In Arabidopsis thaliana, members of the uridine diphosphate-dependent glucosyltransferases UGT72E and UGT72B subfamilies have been demonstrated to glycosylate monolignols. Here, the poplar UGT72 family, which is clustered into four groups, was characterized: Group 1 UGT72AZ1 and UGT72AZ2, homologs of Arabidopsis UGT72E1-3, as well as group 4 UGT72B37 and UGT72B39, homologs of Arabidopsis UGT72B1-3, glycosylate monolignols. In addition, promoter-GUS analyses indicated that poplar UGT72 members are expressed within vascular tissues. At the subcellular level, poplar UGT72s belonging to group 1 and group 4 were found to be associated with the nucleus and the endoplasmic reticulum. However, UGT72A2, belonging to group 2, was localized in bodies associated with chloroplasts, as well as possibly in chloroplasts. These results show a partial conservation of substrate recognition between Arabidopsis and poplar homologs, as well as divergent functions between different groups of the UGT72 family, for which the substrates remain unknown.

VviUCC1 Nucleotide Diversity, Linkage Disequilibrium and Association with Rachis Architecture Traits in Grapevine

Cluster compactness is a trait with high agronomic relevance, affecting crop yield and grape composition. Rachis architecture is a major component of cluster compactness determinism, and is a target trait toward the breeding of grapevine varieties less susceptible to pests and diseases. Although its genetic basis is scarcely understood, a preliminary result indicated a possible involvement of the VviUCC1 gene. The aim of this study was to characterize the VviUCC1 gene in grapevine and to test the association between the natural variation observed for a series of rachis architecture traits and the polymorphisms detected in the VviUCC1 sequence. This gene encodes an uclacyanin plant-specific cell-wall protein involved in fiber formation and/or lignification processes. A high nucleotide diversity in the VviUCC1 gene promoter and coding regions was observed, but no critical effects were predicted in the protein domains, indicating a high level of conservation of its function in the cultivated grapevine. After correcting statistical models for genetic stratification and linkage disequilibrium effects, marker-trait association results revealed a series of single nucleotide polymorphisms (SNPs) significantly associated with cluster compactness and rachis traits variation. Two of them (Y-984 and K-88) affected two common cis-transcriptional regulatory elements, suggesting an effect on phenotype via gene expression regulation. This work reinforces the interest of further studies aiming to reveal the functional effect of the detected VviUCC1 variants on grapevine rachis architecture.

SCHENGEN receptor module drives localized ROS production and lignification in plant roots

Production of reactive oxygen species (ROS) by NADPH oxidases (NOXs) impacts many processes in animals and plants, and many plant receptor pathways involve rapid, NOX‐dependent increases of ROS. Yet, their general reactivity has made it challenging to pinpoint the precise role and immediate molecular action of ROS. A well‐understood ROS action in plants is to provide the co‐substrate for lignin peroxidases in the cell wall. Lignin can be deposited with exquisite spatial control, but the underlying mechanisms have remained elusive. Here, we establish a kinase signaling relay that exerts direct, spatial control over ROS production and lignification within the cell wall. We show that polar localization of a single kinase component is crucial for pathway function. Our data indicate that an intersection of more broadly localized components allows for micrometer‐scale precision of lignification and that this system is triggered through initiation of ROS production as a critical peroxidase co‐substrate.

The developmental dynamics of the sweet sorghum root transcriptome elucidate the differentiation of apoplastic barriers

Apoplastic barriers in the endodermis, such as Casparian strips and suberin lamellae, control the passage of water and minerals into the stele. Apoplastic barriers are thus thought to contribute to salt exclusion in salt-excluding plants such as sweet sorghum (Sorghum bicolor). However, little is known about the genes involved in the development of the apoplastic barrier. Here, we identified candidate genes involved in Casparian strip and suberin lamella development in the roots of a sweet sorghum line (M-81E). Three distinct developmental regions (no differentiation, developing, and mature) were identified based on Casparian strip and suberin lamella staining in root cross sections. Sequencing of RNA extracted from these distinct sections identified key genes participating in the differentiation of the apoplastic barrier. The different sections were structurally distinct, presumably due to differences in gene expression. Genes controlling the phenylpropanoid pathway, fatty acid elongation, and fatty acid ω-hydroxylation appeared to be directly responsible for the formation of the apoplastic barrier. Our dataset elucidates the molecular processes underpinning apoplastic barrier development and provides a basis for future research on molecular mechanisms of apoplastic barrier formation and salt exclusion.Abbreviations: SHR, SHORTROOT; MYB, MYB DOMAIN PROTEIN; CIFs, Casparian strip integrity factors; CASP, Casparian strip domain proteins; PER, peroxidase; ESB1, ENHANCED SUBERIN1; CS, Casparian strip; RPKM, reads per kilobase per million reads; DEGs, differentially expressed genes; FDR, false discovery rate; GO, Gene Ontology; KEGG, Kyoto Encyclopedia of Genes and Genomes; RNA-seq, RNA sequencing; PAL, phenylalanine ammonia-lyase; CYP, cytochrome P450 monooxygenases; 4CL, 4-coumarate-CoA ligase; AAE5, ACYL-ACTIVATING ENZYME5; CCR, cinnamoyl CoA reductase; TKPR, TETRAKETIDE ALPHA-PYRONE REDUCTASE1; CAD, cinnamyl alcohol dehydrogenase; HST, shikimate O-hydroxycinnamoyltransferase; PMAT2, PHENOLIC GLUCOSIDE MALONYLTRANSFERASE2; CCOAOMT, caffeoyl-CoA O-methyltransferase; KCS, β-ketoacyl-CoA synthase; CUT1, CUTICULAR PROTEIN1; DET2, 5-alpha-reductase; TAX, 3'-N-debenzoyl-2'-deoxytaxol N-benzoyltransferase; CER1, ECERIFERUM1; FAR, fatty acyl reductase; AF-CoA, alcohol-forming fatty acyl-CoA reductase; ABCG, ATP-binding cassette, subfamily G; ERF, ethylene-responsive transcription factor; HSF, heat stress transcription factor; NTF, NUCLEAR TRANSCRIPTION FACTOR Y SUBUNIT B-5; GPAT, glycerol 3-phosphate acyltransferase.

Genome-wide characterization of the laccase gene family in Setaria viridis reveals members potentially involved in lignification

Five laccase genes are potentially involved in developmental lignification in the model C4 grass Setaria viridis and their different tissue specificities suggest subfunctionalization events. Plant laccases are copper-containing glycoproteins involved in monolignol oxidation and, therefore, their activity is essential for lignin polymerization. Although these enzymes belong to large multigene families with highly redundant members, not all of them are thought to be involved in lignin metabolism. Here, we report on the genome-wide characterization of the laccase gene family in the model C4 grass Setaria viridis and further identification of the members potentially involved in monolignol oxidation. A total of 52 genes encoding laccases (SvLAC1 to SvLAC52) were found in the genome of S. viridis, and phylogenetic analyses showed that these genes were heterogeneously distributed among the characteristic six subclades of the family and are under relaxed selective constraints. The observed expansion in the total number of genes in this species was mainly caused by tandem duplications within subclade V, which accounts for 68% of the whole family. Comparative phylogenetic analyses showed that the expansion of subclade V is specifically observed for the Paniceae tribe within the Panicoideae subfamily in grasses. Five SvLAC genes (SvLAC9, SvLAC13, SvLAC15, SvLAC50, and SvLAC52) fulfilled the criteria established to identify lignin-related candidates: (1) phylogenetic proximity to previously characterized lignin-related laccases from other species, (2) similar expression pattern to that observed for lignin biosynthetic genes in the S. viridis elongating internode, and (3) high expression in S. viridis tissues undergoing active lignification. In addition, in situ hybridization experiments not only confirmed that these selected SvLAC genes were expressed in lignifying cells, but also that their expression showed different tissue specificities, suggesting subfunctionalization events within the family. These five laccase genes are strong candidates to be involved in lignin polymerization in S. viridis and might be good targets for lignin bioengineering strategies.

Mini Review: Transport of Hydrophobic Polymers Into the Plant Apoplast

The plant apoplast contains the four hydrophobic polymer, lignin, suberin, cutin, and cutan, that are crucial for stress resistance, controlling solute diffusion, and strengthening the cell wall. Some of these polymers are widely used in industry and daily life products, such as all wood-containing goods (lignin) and wine cork (suberin). Despite the importance of these polymers, several aspects of their formation remain unknown. This mini review highlights technical bottlenecks in the current research and summarizes recent insights into the precursor transmembrane transport, an essential step in the polymer formation. We also briefly discuss how some of the remaining knowledge gaps can be closed and how a better understanding of these biopolymers will benefit other research fields.

Mechanisms Underlying Graft Union Formation and Rootstock Scion Interaction in Horticultural Plants

Grafting is a common practice for vegetative propagation and trait improvement in horticultural plants. A general prerequisite for successful grafting and long term survival of grafted plants is taxonomic proximity between the root stock and scion. For the success of a grafting operation, rootstock and scion should essentially be closely related. Interaction between the rootstock and scion involves complex physiological-biochemical and molecular mechanisms. Successful graft union formation involves a series of steps viz., lining up of vascular cambium, generation of a wound healing response, callus bridge formation, followed by vascular cambium formation and subsequent formation of the secondary xylem and phloem. For grafted trees compatibility between the rootstock/scion is the most essential factor for their better performance and longevity. Graft incompatibility occurs on account of a number of factors including of unfavorable physiological responses across the graft union, transmission of virus or phytoplasma and anatomical deformities of vascular tissue at the graft junction. In order to avoid the incompatibility problems, it is important to predict the same at an early stage. Phytohormones, especially auxins regulate key events in graft union formation between the rootstock and scion, while others function to facilitate the signaling pathways. Transport of macro as well as micro molecules across long distances results in phenotypic variation shown by grafted plants, therefore grafting can be used to determine the pattern and rate of recurrence of this transport. A better understanding of rootstock scion interactions, endogenous growth substances, soil or climatic factors needs to be studied, which would facilitate efficient selection and use of rootstocks in the future. Protein, hormones, mRNA and small RNA transport across the junction is currently emerging as an important mechanism which controls the stock/scion communication and simultaneously may play a crucial role in understanding the physiology of grafting more precisely. This review provides an understanding of the physiological, biochemical and molecular basis underlying grafting with special reference to horticultural plants.

Lignin biosynthesis: old roads revisited and new roads explored

Lignin is a major component of secondarily thickened plant cell walls and is considered to be the second most abundant biopolymer on the planet. At one point believed to be the product of a highly controlled polymerization procedure involving just three potential monomeric components (monolignols), it is becoming increasingly clear that the composition of lignin is quite flexible. Furthermore, the biosynthetic pathways to the major monolignols also appear to exhibit flexibility, particularly as regards the early reactions leading to the formation of caffeic acid from coumaric acid. The operation of parallel pathways to caffeic acid occurring at the level of shikimate esters or free acids may help provide robustness to the pathway under different physiological conditions. Several features of the pathway also appear to link monolignol biosynthesis to both generation and detoxification of hydrogen peroxide, one of the oxidants responsible for creating monolignol radicals for polymerization in the apoplast. Monolignol transport to the apoplast is not well understood. It may involve passive diffusion, although this may be targeted to sites of lignin initiation/polymerization by ordered complexes of both biosynthetic enzymes on the cytosolic side of the plasma membrane and structural anchoring of proteins for monolignol oxidation and polymerization on the apoplastic side. We present several hypothetical models to illustrate these ideas and stimulate further research. These are based primarily on studies in model systems, which may or may not reflect the major lignification process in forest trees.

Transcriptomic analysis of early fruit development in Chinese white pear (Pyrus bretschneideri Rehd.) and functional identification of PbCCR1 in lignin biosynthesis

BACKGROUND

The content of stone cells and lignin is one of the key factors affecting the quality of pear fruit. In a previous study, we determined the developmental regularity of stone cells and lignin in 'Dangshan Su' pear fruit 15-145 days after pollination (DAP). However, the development of fruit stone cells and lignin before 15 DAP has not been heavily researched.

RESULTS

In this study, we found that primordial stone cells began to appear at 7 DAP and that the fruit had formed a large number of stone cells at 15 DAP. Subsequently, transcriptome sequencing was performed on fruits at 0, 7, and 15 DAP and identified 3834 (0 vs. 7 DAP), 4049 (7 vs. 15 DAP) and 5763 (0 vs. 15 DAP) DEGs. During the 7-15 DAP period, a large number of key enzyme genes essential for lignin biosynthesis are gradually up-regulated, and their expression pattern is consistent with the accumulation of lignin in this period. Further analysis found that the biosynthesis of S-type lignin in 'Dangshan Su' pear does not depend on the catalytic activity of PbSAD but is primarily generated by the catalytic activity of caffeoyl-CoA through CCoAOMT, CCR, F5H, and CAD. We cloned PbCCR1, 2 and analysed their functions in Chinese white pear lignin biosynthesis. PbCCR1 and 2 have a degree of functional redundancy; both demonstrate the ability to participate in lignin biosynthesis. However, PbCCR1 may be the major gene for lignin biosynthesis, while PbCCR2 has little effect on lignin biosynthesis.

CONCLUSIONS

Our results revealed that 'Dangshan Su' pear began to form a large number of stone cells and produce lignin after 7 DAP and mainly accumulated materials from 0 to 7 DAP. PbCCR1 is mainly involved in the biosynthesis of lignin in 'Dangshan Su' pear and plays a positive role in lignin biosynthesis.

The lignin toolbox of the model grass Setaria viridis

The core set of biosynthetic genes potentially involved in developmental lignification was identified in the model C4 grass Setaria viridis. Lignin has been recognized as a major recalcitrant factor negatively affecting the processing of plant biomass into bioproducts. However, the efficient manipulation of lignin deposition in order to generate optimized crops for the biorefinery requires a fundamental knowledge of several aspects of lignin metabolism, including regulation, biosynthesis and polymerization. The current availability of an annotated genome for the model grass Setaria viridis allows the genome-wide characterization of genes involved in the metabolic pathway leading to the production of monolignols, the main building blocks of lignin. Here we performed a comprehensive study of monolignol biosynthetic genes as an initial step into the characterization of lignin metabolism in S. viridis. A total of 56 genes encoding bona fide enzymes catalyzing the consecutive ten steps of the monolignol biosynthetic pathway were identified in the S. viridis genome. A combination of comparative phylogenetic studies, high-throughput expression analysis and quantitative RT-PCR analysis was further employed to identify the family members potentially involved in developmental lignification. Accordingly, 14 genes clustered with genes from closely related species with a known function in lignification and showed an expression pattern that correlates with lignin deposition. These genes were considered the "core lignin toolbox" responsible for the constitutive, developmental lignification in S. viridis. These results provide the basis for further understanding lignin deposition in C4 grasses and will ultimately allow the validation of biotechnological strategies to produce crops with enhanced processing properties.

GhbHLH18 negatively regulates fiber strength and length by enhancing lignin biosynthesis in cotton fibers

Cotton fibers are developed epidermal cells of the seed coat and contain large amounts of cellulose and minor lignin-like components. Lignin in the cell walls of cotton fibers effectively provides mechanical strength and is also presumed to restrict fiber elongation and secondary cell wall synthesis. To analyze the effect of lignin and lignin-like phenolics on fiber quality and the transcriptional regulation of lignin synthesis in cotton fibers, we characterized the function of a bHLH transcription factor, GhbHLH18, during fiber elongation stage. GhbHLH18 knock-down plants have longer and stronger fibers, and accumulate less lignin-like phenolics in mature cotton fibers than control plants. By mining public transcriptomic data for developing fibers, we discovered that GhbHLH18 is coexpressed with most lignin synthesis pathway genes. Furthermore, we showed that GhbHLH18 strongly binds to the E-box in the promoter region of GhPER8 and activates its expression. Transient over expression of GhPER8 protein in tobacco leaves significantly decreased the content of coniferyl alcohol and sinapic alcohol-the substrate respectively for G-lignin and S-lignin biosynthesis. These results suggest that GhbHLH18 is negatively associated with fiber quality by activating peroxidase-mediated lignin metabolism, thus the paper represents an alternative strategy to improve fiber quality.