Woody plant cell walls: Fundamentals and utilization

Gene regulatory network analysis of silver birch reveals the ancestral state of secondary cell wall biosynthesis in core eudicots

The compact genome and lack of recent whole-genome multiplication (WGM) events make the boreal pioneer tree silver birch (Betula pendula) a promising model for primary and secondary cell wall (PCW and SCW) regulation in forest trees. Here, we constructed regulatory networks through combined co-expression and promoter motif analysis and carried out a tissue-wide analysis of xylan using mass spectrometry. Analyses confirm the evolutionarily conserved model of superimposed layers of regulation and suggest a relatively simple ancestral state still retained in birch. Multispecies network analysis, including birch, poplar, and eucalyptus, identified conserved regulatory interactions, highlighting lignin biosynthesis as least conserved. The SCW biosynthesis co-expression module was enriched with WGM duplicates. While regulator genes were under positive selection, others evolved under relaxed purifying selection, possibly linked with diversification, as indicated by expression and regulatory motif differences. Xylan composition varied between PCW and SCW, revealing unique acetylation patterns. PCW xylan biosynthesis genes showed distinct expression and regulatory motifs, with a novel acetyl transferase potentially involved. This work highlights birch as a valuable model for understanding wood formation, vascular development, and cell wall composition in eudicots.

Cellulose nanocrystal-based intelligent hydrogels: Innovations, challenges, and prospective application in advanced wound healing

Cellulose nanocrystals (CNCs) have emerged as a transformative material in biomedical engineering due to their exceptional mechanical properties, high aspect ratio, and biocompatibility. Recent advances in CNC-based smart hydrogels show great potential in wound care through responsive drug delivery, moisture retention, and infection control. This review critically evaluates CNC-reinforced hydrogels' synthesis, functionalization, and biomedical applications, emphasizing their role in addressing chronic wound healing challenges. Despite promising results, clinical use is limited by scalability, cost, and long-term biocompatibility. Future research should optimize sustainable CNC extraction, integrate smart sensing, and explore cellular mechanisms. Collaboration with industry and pilot projects will provide insights, while workshops can train professionals in smart sensing and CNC optimization. Incorporating Internet of Things (IoT) sensors in CNC machines enables real-time monitoring of key parameters like temperature, vibration, and tool wear, facilitating predictive maintenance and process optimization through data analytics. Developing closed-loop control systems to adjust machining parameters based on real-time data enhances precision and reduces waste. CNC extraction optimization should include determining ideal process parameters, exploring advanced tooling materials, and using simulation software to streamline machining processes. This comprehensive analysis underscores the potential of CNC-based hydrogels to redefine regenerative medicine and personalized wound care.

New insights into plant cell wall functions

The plant cell wall is an extremely complicated natural nanoscale structure composed of cellulose microfibrils embedded in a matrix of noncellulosic polysaccharides, further reinforced by the phenolic compound lignins in some cell types. Such network formed by the interactions of multiscale polymers actually reflects functional form of cell wall to meet the requirements of plant cell functionalization. Therefore, how plants assemble cell wall functional structure is fundamental in plant biology and critical for crop trait formation and domestication as well. Due to the lack of effective analytical techniques to characterize this fundamental but complex network, it remains difficult to establish direct links between cell-wall genes and phenotypes. The roles of plant cell walls are often underestimated as indirect. Over the past decades, many genes involved in cell wall biosynthesis, modification, and remodeling have been identified. The application of a variety of state-of-the-art techniques has made it possible to reveal the fine cell wall networks and polymer interactions. Hence, many exciting advances in cell wall biology have been achieved in recent years. This review provides an updated overview of the mechanistic and conceptual insights in cell wall functionality, and prospects the opportunities and challenges in this field.

Comprehensive transcriptome and metabolome analysis deciphers the mechanism underlying rapid xylem growth in the dominant hybrid poplar QB3

MAIN CONCLUSION

Compared with its parents, the heterosis in growth of QB3 is primarily attributed to the upregulation of auxin and brassinosteroid-related genes, as well as the induced expression of numerous xylem and phloem synthesis genes, particularly the accumulation of lignin. Interestingly, QB3 significantly increased resistance to gray mold, which may be related to anthocyanin accumulation. Our findings illuminate the complex interplay of biological mechanisms that govern the regulation of wood growth and resistance. Poplar, as a fast-growing energy species widely distributed in the northern hemisphere, has important ecological and economic value. The hybridization of poplars is very common and often can bring to the progeny superior growth and resilience traits, but the molecular mechanism of heterosis remains to be studied. Through decades of crossbreeding work, a high-growth rate hybrid offspring named QinBai3 (QB3) was selected from P. alba × (P. alba × P. glandulosa), which provided an ideal model for investigating the molecular mechanism of heterosis. We found that the plant height, ground diameter, and xylem thickness of QB3 were much higher than those of I101 and 84 K. Through transcriptome and qRT-PCR analyses, we found that the expression levels of poplar regulatory genes associated with vegetative growth, brassinosteroid (BR), and auxin hormone signaling were significantly elevated in July compared to February. Meanwhile, compared to its parents, QB3 exhibited more specifically up-regulated genes in the processes of xylem and phloem synthesis, notably PalOPS and PalPRX52. However, in response to certain abiotic stresses, such as water deprivation and UV-B exposure, more down-regulated genes were identified. Metabolome analyses indicated that QB3 significantly increased the levels of lignin and anthocyanin, a result that aligns with the transcriptome data. Additionally, chemical assays confirmed the substantial accumulation of lignin and anthocyanin in QB3, suggesting that increased lignin accumulation may enhance the stem growth rate of QB3. Surprisingly, QB3 significantly increased resistance to Botrytis cinerea B05.10, which was accompanied by anthocyanin accumulation. In addition, our study offers detailed insights into the molecular mechanisms underlying rapid growth and stress resistance in hybrid poplar, thereby providing a new theoretical foundation and practical guidance for forest genetic breeding.

The vascular-cambium-specific transcription factor PtrSCZ1 and its homologue regulate cambium activity and affect xylem development in Populus trichocarpa

Introduction

Vascular cambium proliferates and differentiates into the secondary xylem (wood), enabling the perennial increase in stem diameter for wood formation. In our previous study, we identified 95 vascular-cambium-specific (VCS) transcription factors (TFs) in Populus trichocarpa.

Methods

In this study, we characterized the function of the highly vascular cambium-expressed heat shock TF among these VCSs, PtrSCZ1, using PtrSCZ1-overexpressing transgenic lines and gene-edited mutants in P. trichocarpa.

Results

Overexpressing PtrSCZ1 or its homolog PtrSCZ3 (OE-PtrSCZ1, OE-PtrSCZ3) led to enhanced cambium activity, increased stem diameter, and a larger xylem proportion. CRISPR-based mutants of PtrSCZ1 and PtrSCZ3 exhibited phenotypes opposite to the OE-PtrSCZ1 and OE-PtrSCZ3 plants. This suggests that PtrSCZ1 and PtrSCZ3 redundantly promote cambium activity and secondary growth, leading to increased radial growth in P. trichocarpa. Overexpression and knockout of PtrSCZ1 and PtrSCZ3 significantly affected the expression of key regulatory factors of cambium (PtrWOX4a, PtrWOX4b, PtrWOX13a, PtrPXYa, PtrVCM1, and PtrVCM2) and disrupted cell wall-related gene expression. This demonstrates that PtrSCZ1 and PtrSCZ3 may function in cambium division activity by regulating these key cambium-associated transcription factors for wood formation.

Discussion

Our work identifies PtrSCZ1 and PtrSCZ3 as promising target genes for enhancing wood yield through molecular breeding, and illustrates the role of vascular cambium systems in understanding lateral meristem development.

Nitric oxide mitigates cadmium stress by promoting the biosynthesis of cell walls in Robinia pseudoacacia roots

Cadmium (Cd) pollution is a growing concern worldwide, because it threatens human health through the food chain. Woody plants, such as the pioneer species black locust (Robinia pseudoacacia L.), are widely used in phytoremediation of Cd-contaminated soils, but strongly differ in Cd tolerance. Nitric oxide (NO), a highly reactive gas of biogenic and anthropogenic origin, has been shown to protect plants to Cd exposure. We investigated the protective mechanism of NO against Cd toxicity in black locust using physiological, transcriptomic and metabolomic approaches. We studied the correlation between cell wall traits, genes, and metabolites. The findings indicated that NO improved the growth of black locust under Cd exposure and elevated the fraction of Cd in the cell wall. NO increased cell wall thickness by stimulating the biosynthesis of pectin, cellulose, hemicellulose, and lignin. Transcriptomic and metabolomic analyses demonstrated that NO upregulated genes related to root cell wall biosynthesis and increased the accumulation of related metabolites, thereby increasing the Cd resistance of black locust. Our results elucidated a molecular mechanism underlying NO-mediated Cd tolerance in black locust and provided novel insights for phytoremediation of Cd-polluted soils by woody plants.

Advances in lignocellulosic feedstocks for bioenergy and bioproducts

Lignocellulose, an abundant renewable resource, presents a promising alternative for sustainable energy and industrial applications. However, large-scale adoption of lignocellulosic feedstocks faces considerable obstacles, including scalability, bioprocessing efficiency, and resilience to climate change. This Review examines current efforts and future opportunities for leveraging lignocellulosic feedstocks in bio-based energy and products, with a focus on enhancing conversion efficiency and scalability. It also explores emerging biotechnologies such as CRISPR-based genome editing informed by machine learning, aimed at improving feedstock traits and reducing the environmental impact of fossil fuel dependence.

Epigenetic regulation of lignin biosynthesis in wood formation

Lignin, a major wood component, is the key limiting factor for wood conversion efficiency. Its biosynthesis is controlled by transcriptional regulatory networks involving transcription factor (TF)-DNA interactions. However, the epigenetic mechanisms underlying these interactions in lignin biosynthesis remain largely unknown. Here, using yeast one-hybrid, chromatin immunoprecipitation, and electrophoretic mobility shift assays, we identified that PtrbZIP44-A1, a key wood-forming TF, directly interacts with the promoters of PtrCCoAOMT2 and PtrCCR2, genes involved in the monolignol biosynthetic pathway. We used yeast two-hybrid, bimolecular fluorescence complementation, biochemical analyses, transient and CRISPR-mediated transgenesis in Populus trichocarpa to demonstrate that PtrHDA15, a histone deacetylase, acts as an epigenetic inhibitor and is recruited by PtrbZIP44-A1 for chromatin histone modifications to repress PtrCCoAOMT2 and PtrCCR2, leading to reduced lignin deposition. In transgenic lines overexpressing PtrbZIP44-A1 or PtrHDA15, histone acetylation at the promoters of PtrCCoAOMT2 and PtrCCR2 decreased, reducing their expression and lignin content. Conversely, in loss-of-function ptrbzip44-a1 and ptrhda15 mutants, histone acetylation levels at PtrCCoAOMT2 and PtrCCR2 promoters increased, enhancing target gene expression and lignin content. Our study uncovered an epigenetic mechanism that suppresses lignin biosynthesis. This finding may help fill a knowledge gap between epigenetic regulation and lignin biosynthesis during wood formation in Populus.

Bionic cell wall models: Utilizing TEMPO-oxidized cellulose nanofibers for fucoxanthin delivery systems

Fucoxanthin (FX) has various excellent biological properties but suffers from poor bioavailability. In this work, we build a bionic cell wall model using TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy)-oxidized cellulose. The bionic cell wall enhances the environmental stability of the liposomes and serves as a pH-responsive mechanism. The coating processes protect the structure of liposomes and fucoxanthin under the acidic conditions of the stomach. The bionic cell wall disperses and releases the fucoxanthin in simulated intestinal fluid (SIF). Overall, the protective and release capabilities highlight the potential of cellulose in a bionic cell wall model and provide diversity for the structural design of carriers for delivering functional bioactive components.

The PtobZIP55-PtoMYB170 module regulates the wood anatomical and chemical properties of Populus tomentosa in acclimation to low nitrogen availability

Poplar plantations are often established on nitrogen-poor land, and poplar growth and wood formation are constrained by low nitrogen (LN) availability. However, the molecular mechanisms by which specific genes regulate wood formation in acclimation to LN availability remain unclear. Here, we report a previously unrecognized module, basic region/leucine zipper 55 (PtobZIP55)-PtoMYB170, which regulates the wood formation of Populus tomentosa in acclimation to LN availability. PtobZIP55 was highly expressed in poplar wood and induced by LN. Altered wood anatomical properties and increased lignification were detected in PtobZIP55-overexpressing poplars, whereas the opposite results were detected in PtobZIP55-knockout poplars. Molecular and transgenic analyses revealed that PtobZIP55 directly binds to the promoter sequence of PtoMYB170 to activate its transcription. The phenotypes of PtoMYB170 transgenic poplars were similar to those of PtobZIP55 transgenic poplars under LN conditions. Further molecular analyses revealed that PtoMYB170 directly bound the promoter sequences of lignin biosynthetic genes to activate their transcription to increase lignin concentrations in LN-treated poplar wood. These results suggest that PtobZIP55 activates PtoMYB170 transcription, which in turn positively regulates lignin biosynthetic genes, increasing lignin deposition in the wood of P. tomentosa in the context of acclimation to LN availability.

Spatiotemporal bifurcation of HY5-mediated blue-light signaling regulates wood development during secondary growth

Plants have evolved photoreceptors to optimize their development during primary growth, including germination, hypocotyl elongation, cotyledon opening, and root growth, allowing them to adapt to challenging light conditions. The light signaling transduction pathway during seedling establishment has been extensively studied, but little molecular evidence is available for light-regulated secondary growth, and how light regulates cambium-derived tissue production remains largely unexplored. Here, we show that CRYPTOCHROME (CRY)-dependent blue light signaling and the subsequent attenuation of ELONGATED HYPOCOTYL 5 (HY5) movement to hypocotyls are key inducers of xylem fiber differentiation in Arabidopsis thaliana. Using grafted chimeric plants and hypocotyl-specific transcriptome sequencing of light signaling mutants under controlled light conditions, we demonstrate that the perception of blue light by CRYs in shoots drives secondary cell wall (SCW) deposition at xylem fiber cells during the secondary growth of hypocotyls. We propose that HY5 is a blue light-responsive mobile protein that inhibits xylem fiber formation via direct transcriptional repression of NAC SECONDARY WALL THICKENING PROMOTING 3 (NST3). CRYs retain HY5 in the nucleus, impede its long-distance transport from leaf to hypocotyl, and they initiate NST3-driven SCW gene expression, thereby triggering xylem fiber production. Our findings shed light on the long-range CRYs-HY5-NST3 signaling cascade that shapes xylem fiber development, highlighting the activity of HY5 as a transcriptional repressor during secondary growth.

Discovery of genes that positively affect biomass and stress associated traits in poplar

Woody biomass serves as a renewable resource for various industries, including pulp and paper production, construction, biofuels, and electricity generation. However, the molecular mechanisms behind biomass traits are poorly understood, which significantly curtails the speed and efficiency of their improvement. We used activation tagging to discover genes that can positively affect tree biomass-associated traits. We generated and screened under greenhouse conditions a population of 2,700 independent activation tagging lines. A total of 761 lines, which had significantly and positively affected at least one biomass-associated trait, were discovered. The tag was positioned in the genome for forty lines which were affected in multiple traits and activation of proximal genes validated for a subset. For two lines we fully recapitulated the phenotype of the original lines through overexpression. Moreover, the overexpression led to more pronounced and additional improvements, not observed in the original lines. Importantly, the overexpression of a Fasciclin-like gene (PtaFLA10) and a Patatin-like gene (PtaPAT) was found to substantially improve biomass, with a 40% increase in dry-stem weight, and enhance drought tolerance, respectively. Additionally, PtaPAT overexpression increased cellulose content, which is crucial for biofuel production. Our work shows that the activation tagging approach applied even on a non-genome saturation scale in a poplar tree can be successfully used for the discovery of genes positively modify biomass productivity. Such dominant forward genetics approaches can aid in biotechnological manipulation of woody biomass traits and help unravel the functions and mechanisms of individual genes, gene families, and regulatory modules.

Social and biological innovations are essential to deliver transformative forest biotechnologies

Forests make immense contributions to societies in the form of ecological services and sustainable industrial products. However, they face major challenges to their viability and economic use due to climate change and growing biotic and economic threats, for which recombinant DNA (rDNA) technology can sometimes provide solutions. But the application of rDNA technologies to forest trees faces major social and biological obstacles that make its societal acceptance a 'wicked' problem without straightforward solutions. We discuss the nature of these problems, and the social and biological innovations that we consider essential for progress. As case studies of biological challenges, we focus on studies of modifications in wood chemistry and transformation efficiency. We call for major innovations in regulations, and the dissolution of method-based market barriers, that together could lead to greater research investments, enable wide use of field studies, and facilitate the integration of rDNA-modified trees into conventional breeding programs. Without near-term adoption of such innovations, rDNA-based solutions will be largely unavailable to help forests adapt to the growing stresses from climate change and the proliferation of forest pests, nor will they be available to provide economic and environmental benefits from expanded use of wood and related bioproducts as part of an expanding bioeconomy.

Metabolic engineering of Oryza sativa for lignin augmentation and structural simplification

The sustainable production and utilization of lignocellulose biomass are indispensable for establishing sustainable societies. Trees and large-sized grasses are the major sources of lignocellulose biomass, while large-sized grasses greatly surpass trees in terms of lignocellulose biomass productivity. With an overall aim to improve lignocellulose usability, it is important to increase the lignin content and simplify lignin structures in biomass plants via lignin metabolic engineering. Rice (Oryza sativa) is not only a representative and important grass crop, but also is a model for large-sized grasses in biotechnology. This review outlines progress in lignin metabolic engineering in grasses, mainly rice, including characterization of the lignocellulose properties, the augmentation of lignin content and the simplification of lignin structures. These findings have broad applicability for the metabolic engineering of lignin in large-sized grass biomass plants.

PagEXPA1 combines with PagCDKB2;1 to regulate plant growth and the elongation of fibers in Populus alba × Populus glandulosa

Expansins are important plant cell wall proteins. They can loosen and soften the cell walls and lead to wall extension and cell expansion. To investigate their role in wood formation and fiber elongation, the PagEXPA1 that highly expressed in cell differentiation and expansion tissues was cloned from 84K poplar (Populus alba × P. glandulosa). The subcellular localization showed that PagEXPA1 located in the cell wall and it was highly expressed in primary stems and young leaves. Compared with non-transgenic 84K poplar, overexpression of PagEXPA1 can promote plant-growth, lignification, and fiber cell elongation, while PagEXPA1 Cas9-editing mutant lines exhibited the opposite phenotype. Transcriptome analysis revealed that DEGs were mainly enriched in some important processes, which are associated with cell wall formation and cellulose synthesis. The protein interaction prediction and expression analysis showed that PagCDKB2:1 and PagEXPA1 might have an interaction relationship. The luciferase complementary assay and bimolecular fluorescence complementary assay validated that PagEXPA1 can combined with PagCDKB2;1. So they promoted the expansion of xylem vascular tissues and the development of poplar though participating in the regulation of cell division and differentiation by programming the cell-cycle. It provides good foundation for molecular breeding of fast-growing and high-quality poplar varieties.

Ectopic Expression of PtrLBD39 Retarded Primary and Secondary Growth in Populus trichocarpa

Primary and secondary growth of trees are needed for increments in plant height and stem diameter, respectively, affecting the production of woody biomass for applications in timber, pulp/paper, and related biomaterials. These two types of growth are believed to be both regulated by distinct transcription factor (TF)-mediated regulatory pathways. Notably, we identified PtrLBD39, a highly stem phloem-specific TF in Populus trichocarpa and found that the ectopic expression of PtrLBD39 in P. trichocarpa markedly retarded both primary and secondary growth. In these overexpressing plants, the RNA-seq, ChIP-seq, and weighted gene co-expression network analysis (WGCNA) revealed that PtrLBD39 directly or indirectly regulates TFs governing vascular tissue development, wood formation, hormonal signaling pathways, and enzymes responsible for wood components. This regulation led to growth inhibition, decreased fibrocyte secondary cell wall thickness, and reduced wood production. Therefore, our study indicates that, following ectopic expression in P. trichocarpa, PtrLBD39 functions as a repressor influencing both primary and secondary growth.