Xylan is a major hemicellulosic component of plant cell walls, particularly in hardwood species like poplar. It plays a crucial role in determining the structural integrity and properties of the cell wall by interacting with cellulose and lignin. The biosynthesis of xylan involves a complex pathway where various glycosyltransferases (GTs) are key players. These enzymes facilitate the transfer of sugar moieties to form the xylan backbone and its side chains, which are essential for the proper assembly and function of the cell wall (Biswal et al., 2015; Pawar et al., 2017; Ratke et al., 2018).

Glycosyltransferases are pivotal in the biosynthesis of xylan. Members of the GT43 family, for instance, are involved in the elongation of the xylan backbone. Downregulation of these genes in poplar has been shown to affect xylan content and consequently alter wood properties (Ratke et al., 2015; Ratke et al., 2018). Additionally, other GTs like GAUT12 are implicated in the synthesis of glucuronoxylan and pectin, further highlighting their multifaceted roles in cell wall biosynthesis (Biswal et al., 2015). The precise regulation of these enzymes is crucial for maintaining the balance and composition of cell wall components, which directly impacts the plant's growth and biomass recalcitrance (Biswal et al., 2015; Ratke et al., 2018).

Wood quality in poplar is of significant interest due to its implications for both industrial applications and biofuel production. High-quality wood with optimal cellulose, xylan, and lignin content is desirable for efficient saccharification and subsequent biofuel production. Modifications in the biosynthesis pathways of these components can lead to improved wood properties, such as reduced recalcitrance and enhanced growth, making poplar a more viable feedstock for bioenergy (Biswal et al., 2015; Pawar et al., 2017; Li et al., 2021). Moreover, understanding the genetic regulation of these pathways can aid in the development of poplar varieties with tailored wood qualities for specific industrial needs (Ratke et al., 2018; Hassane et al., 2022).

This systematic review aims to offer insights into the genetic and biochemical pathways that can be targeted to improve wood quality in poplar, thereby advancing its utility in biofuel production and other industrial uses.

1 Glycosyltransferases Involved in Xylan Biosynthesis

1.1 GT47C

1.1.1 Role in xylan backbone synthesis

The GT47 family, particularly the GT47C subfamily, plays a crucial role in the synthesis of the xylan backbone. Xylan, a major hemicellulose component in plant cell walls, is synthesized by a complex of glycosyltransferases, including members of the GT47 family. These enzymes are responsible for adding xylose residues to the growing xylan chain, forming the β-1,4-linked xylose backbone essential for xylan structure and function (Anders et al., 2023).

1.1.2 Gene expression and regulation in poplar

In poplar, the expression of GT47 genes is tightly regulated during wood formation. Studies have shown that the downregulation of GT47 genes in hybrid aspen leads to significant changes in xylan content and cell wall properties. For instance, the suppression of GT47 genes resulted in reduced xylan content and altered cellulose orientation, which in turn affected the overall growth and wood quality of the plants (Ratke etal., 2018). This indicates that GT47 genes are crucial for maintaining proper xylan biosynthesis and cell wall integrity in poplar.

1.2 GT43

1.2.1 Contribution to xylan chain elongation

The GT43 family, including the IRX9 and IRX14 clades, is essential for the elongation of the xylan chain. These enzymes work in concert to add xylose units to the xylan backbone, facilitating the formation of long xylan chains necessary for robust cell wall structure. The GT43 family has been shown to be involved in both primary and secondary cell wall xylan biosynthesis, with distinct sets of GT43 members contributing to each process (Ratke et al., 2015; Anders et al., 2023).

1.2.2 Genetic regulation and associated pathways

In poplar, the expression of GT43 genes is regulated by key transcription factors involved in secondary cell wall formation. For example, the GT43B gene is activated by the transcription factors PtxtMYB021 and PNAC085, which are master regulators of secondary wall biosynthesis. This regulation ensures that GT43 enzymes are expressed in the appropriate tissues and developmental stages to facilitate proper xylan biosynthesis (Ratke et al., 2015). Additionally, the downregulation of GT43 genes in poplar has been shown to stimulate growth and reprogram the transcriptome, indicating a complex regulatory network that balances xylan biosynthesis with overall plant development (Ratke et al., 2018).

1.3 GT8

1.3.1 Function in xylan branching and modification

The GT8 family, particularly the GAUT12/IRX8 subfamily, is involved in the branching and modification of xylan. These enzymes add side chains and other modifications to the xylan backbone, which are crucial for the functional properties of xylan in the cell wall. In poplar, the downregulation of GAUT12 genes leads to reduced xylan and pectin content, highlighting their role in xylan modification and overall cell wall architecture (Biswal et al., 2015).

1.3.2 Regulatory mechanisms in poplar

The regulation of GT8 genes in poplar involves multiple layers of control, including transcriptional and post-transcriptional mechanisms. For instance, the downregulation of GAUT12.1 in poplar results in significant changes in cell wall composition and increased growth, suggesting that GT8 genes are tightly regulated to balance xylan biosynthesis with other aspects of plant development (Biswal et al., 2015). Additionally, the expression of GT8 genes is influenced by various environmental and developmental cues, ensuring that xylan modification is coordinated with the overall needs of the plant.

2 Genetic Regulation of Glycosyltransferases

2.1 Transcriptional control

2.1.1 Key transcription factors involved in glycosyltransferase gene expression

Transcription factors (TFs) play a crucial role in regulating glycosyltransferase (GT) gene expression. In Populus, several TFs have been identified that influence the expression of genes involved in secondary cell wall biosynthesis, including those encoding GTs. For instance, the R2R3 MYB transcription factor MYB189 has been shown to negatively regulate the biosynthesis of lignin, cellulose, and hemicelluloses by directly binding to the promoters of secondary wall biosynthetic genes, thereby repressing their expression (Jiao et al., 2019). Similarly, PtoMYB156, another R2R3-MYB transcription factor, represses phenylpropanoid biosynthetic genes and secondary wall biosynthetic genes, leading to reduced secondary wall thickness and decreased cellulose, lignin, and xylose content (Yang et al., 2017). Additionally, PdMYB221, a poplar ortholog of the Arabidopsis R2R3-MYB transcription factor AtMYB4, directly regulates secondary wall biosynthesis by repressing the expression of key genes involved in the synthesis of cellulose, xylan, and lignin (Tang et al., 2015).

2.1.2 Regulatory networks influencing GT gene expression

The regulatory networks influencing GT gene expression are complex and involve multiple layers of control. For example, the transcription factors PtNST1 and PtMYB21 have been shown to activate the expression of RWA-A and RWA-B genes, which are involved in xylan acetylation in developing wood (Pawar et al., 2017). These regulatory networks ensure the coordinated expression of genes necessary for proper cell wall formation and modification. Furthermore, systems genetics analysis in Eucalyptus has revealed that xylan modification genes are part of expression modules that are co-regulated with genes involved in nucleotide sugar interconversion and phenylalanine biosynthesis, highlighting the interconnected nature of these pathways (Wierzbicki et al., 2019).

2.2 Post-transcriptional regulation

2.2.1 RNA processing and stability

Post-transcriptional regulation, including RNA processing and stability, plays a significant role in the regulation of GT genes. RNA interference (RNAi) has been used to downregulate specific GT genes in Populus, leading to changes in xylan biosynthesis and wood properties. For instance, downregulation of GAUT12.1 in Populus deltoides resulted in reduced xylan and pectin content, increased sugar release, and enhanced growth (Biswal et al., 2015). This indicates that RNA processing mechanisms can be targeted to modulate GT gene expression and influence wood quality.

2.2.2 Role of microRNAs in regulating glycosyltransferase genes

MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression post-transcriptionally by targeting mRNAs for degradation or translational repression. In Populus, miRNAs have been implicated in the regulation of lignin biosynthesis, which is closely linked to GT activity. For example, a study identified 36 miRNA genes associated with lignin biosynthesis, suggesting their potential role in regulating genes involved in cell wall formation, including GTs (Quan et al., 2018). These miRNAs form part of a broader genetic network that coordinates the expression of genes necessary for wood formation.

2.3 Epigenetic regulation

2.3.1 DNA methylation and histone modification

Epigenetic regulation, including DNA methylation and histone modification, can significantly impact GT gene expression. Although specific studies on the epigenetic regulation of GT genes in Populus are limited, it is well-established that these mechanisms play a crucial role in gene expression regulation in plants. DNA methylation and histone modifications can alter chromatin structure, thereby influencing the accessibility of transcription factors to GT gene promoters and affecting their transcriptional activity.

2.3.2 Impact on glycosyltransferase gene expression

Epigenetic modifications can lead to stable and heritable changes in gene expression without altering the DNA sequence. These modifications can either activate or repress GT gene expression, thereby influencing the biosynthesis of cell wall components. For example, histone modifications such as acetylation and methylation can either promote or inhibit the transcription of genes involved in xylan biosynthesis, ultimately affecting wood quality and properties.

In summary, the genetic regulation of glycosyltransferases in Populus involves a complex interplay of transcriptional, post-transcriptional, and epigenetic mechanisms. Key transcription factors, regulatory networks, RNA processing, miRNAs, and epigenetic modifications all contribute to the precise control of GT gene expression, which is essential for proper cell wall biosynthesis and wood formation.

3 Impact of Glycosyltransferases on Wood Mechanical Properties

3.1 Xylan's role in wood structure

3.1.1 Contribution to cell wall integrity and strength

Xylan is a crucial hemicellulose component in the secondary cell walls of hardwood species, significantly contributing to the structural integrity and mechanical strength of wood. The xylan backbone, synthesized by glycosyltransferases (GTs) such as GT43, GT47, and GT8, plays a pivotal role in maintaining cell wall rigidity and resilience. Downregulation of GT43 genes in hybrid aspen has shown to reduce xylan content, leading to thinner cell walls and altered cellulose orientation, which impacts the overall mechanical properties of the wood (Figure 1) (Ratke et al., 2018). Additionally, the acetylation and glucuronidation of xylan, mediated by specific GTs, influence its interaction with cellulose and lignin, further affecting the wood's structural properties (Pawar et al., 2017; Lyczakowski et al., 2021).

Figure 1 Transcript profiling in transgenic hybrid aspen developing wood with reduced GT43 clade B and C expression levels (Adopted from Ratke et al., 2018) Image caption: (a, b) Venn diagrams of up- and downregulated genes, respectively, and analysis of their expression profiles in wood developing tissues using the ASPWOOD database (Sundell et al., 2017). The heatmaps show that the upregulated genes are mostly expressed in the primary-walled developing wood tissues (cambium and radial expansion zone, CA-RE), and only three genes are expressed in maturation zone (M), whereas the downregulated genes are mostly expressed in the secondary wall formation zone (SW), with few genes expressed in the CA-RE or in the phloem and at the annual ring border (PH-AN). The clusters of genes with similar expression profiles are shown beside the heatmaps, and genes associated with these clusters that are discussed in the text are listed. PCD, programmed cell death; TW, tension wood (Adopted from Ratke et al., 2018)

3.1.2 Impact on wood density and mechanical properties

The density and mechanical properties of wood are directly influenced by the composition and structure of its cell walls. Studies have demonstrated that modifications in xylan biosynthesis can lead to significant changes in wood density and mechanical strength. For instance, the downregulation of GAUT12 in Populus resulted in reduced xylan and pectin content, which correlated with increased growth and altered wood density (Biswal et al., 2015). Similarly, the manipulation of xylan acetylation through the downregulation of RWA genes in hybrid aspen has been shown to enhance wood saccharification efficiency without compromising growth, indicating potential improvements in wood density and mechanical properties (Pawar et al., 2017).

3.2 Influence of GT47C, GT43, and GT8 on wood quality

3.2.1 Comparative analysis of wood properties in genetically modified poplar

Genetic modifications targeting GT47C, GT43, and GT8 have provided insights into their roles in wood quality. For example, the suppression of GT43 genes in hybrid aspen led to reduced xylan content and altered cell wall properties, resulting in increased growth and improved lignocellulose saccharification (Ratke et al., 2018). Similarly, the downregulation of GAUT12, a member of the GT8 family, in Populus deltoides resulted in reduced xylan and pectin content, leading to increased sugar release and growth (Biswal et al., 2015). These findings highlight the significant impact of these GTs on wood quality, particularly in terms of mechanical properties and biomass recalcitrance.

3.2.2 Case studies and experimental data

Experimental data from various studies have demonstrated the effects of GT manipulation on wood properties. For instance, the downregulation of UGT72B37 in poplar mutants resulted in a 10% increase in lignin content, suggesting a role in xylem lignification and potential implications for wood strength and durability (Hassane et al., 2022). Additionally, the study of conifer GUX enzymes, which are involved in xylan glucuronidation, revealed distinct patterns of xylan decoration that influence wood recalcitrance and mechanical properties (Lyczakowski et al., 2021). These case studies underscore the importance of GTs in determining wood quality and provide valuable data for further research and biotechnological applications.

3.3 Biotechnological applications

3.3.1 Genetic engineering approaches to enhance wood quality

Biotechnological approaches targeting GTs offer promising avenues for enhancing wood quality. Genetic engineering techniques such as RNA interference (RNAi) and CRISPR/Cas9 have been employed to manipulate the expression of specific GTs, resulting in modified wood properties. For example, RNAi-mediated downregulation of GAUT12 in Populus led to reduced recalcitrance and increased growth, demonstrating the potential for improving wood quality through targeted genetic modifications (Biswal et al., 2015). Similarly, the use of specific promoters, such as the GT43B promoter, has shown efficacy in overexpressing or downregulating genes to engineer wood acetylation and other properties (Ratke et al., 2015).

3.3.2 Potential industrial applications

The biotechnological manipulation of GTs holds significant potential for various industrial applications. Enhanced wood quality through genetic engineering can lead to improved biomass processability, making it more suitable for biofuel production and other bioproducts. For instance, the reduction of wood acetylation through the downregulation of RWA genes has been shown to increase glucose and xylose yields during enzymatic hydrolysis, facilitating more efficient biofuel production (Pawar et al., 2017). Additionally, the modification of lignin content and composition through the manipulation of UGT72B37 and other GTs can result in wood with desirable properties for the paper and pulp industry (Hassane et al., 2022). These applications highlight the potential of GT-targeted biotechnological approaches in enhancing wood quality for various industrial uses.

4 Future Perspectives and Research Directions

4.1 Advancements in genetic engineering

Emerging technologies for gene editing and regulation, such as CRISPR/Cas9, offer promising avenues for enhancing the efficiency of glycosyltransferases involved in xylan biosynthesis. Recent studies have demonstrated the potential of RNA interference (RNAi) to downregulate specific glycosyltransferase genes, leading to significant changes in wood properties. For instance, the downregulation of GT43 genes in hybrid aspen resulted in reduced xylan content and increased lignocellulose saccharification efficiency, highlighting the role of these genes in xylan backbone biosynthesis and their potential as targets for genetic engineering (Ratke et al., 2018). Similarly, RNAi-mediated knockdown of GAUT12.1 in Populus deltoides led to reduced xylan and pectin content, increased sugar release, and enhanced growth, suggesting that GAUT12.1 is a critical player in cell wall architecture and recalcitrance (Biswal et al., 2015). These findings underscore the importance of advancing gene editing technologies to improve glycosyltransferase efficiency and optimize wood quality.

4.2 Integrative approaches

Combining genetic, biochemical, and computational methods is essential for a holistic understanding of xylan biosynthesis and its impact on wood quality. Systems genetics analyses, such as those conducted in Eucalyptus, have revealed the coordination of metabolic pathways associated with xylan modification, identifying key regulatory genes and expression modules (Wierzbicki et al., 2019). Integrative approaches that leverage transcriptome profiling, glycoproteome analysis, and bioinformatics tools can provide comprehensive insights into the complex regulatory networks governing xylan biosynthesis. For example, the identification of glycoproteins involved in wood cell wall synthesis and modification through lectin affinity-based glycoproteome analysis in poplar has highlighted the significance of protein glycosylation in wood formation (Cheng et al., 2022). By integrating these diverse methodologies, researchers can develop more effective strategies for manipulating xylan biosynthesis and improving wood quality.

4.3 Environmental and economic implications

Sustainable forestry and wood production are critical considerations in the context of genetic modifications aimed at enhancing wood quality. The ability to engineer trees with reduced recalcitrance and improved growth characteristics can lead to more efficient biomass processing and reduced environmental impact. For instance, the downregulation of GAUT12.1 in Populus deltoides not only resulted in increased sugar release but also promoted plant growth, suggesting potential economic benefits through enhanced biomass yield and reduced processing costs (Biswal et al., 2015). Additionally, the stimulation of growth observed in hybrid aspen with suppressed GT43 genes indicates that targeted genetic modifications can lead to both improved wood properties and increased biomass production (Ratke et al., 2018). These advancements hold promise for developing sustainable and economically viable wood production systems that meet the growing demand for renewable biofuels and other wood-derived products.

In conclusion, the future of research on glycosyltransferases and xylan biosynthesis in poplar lies in the continued advancement of genetic engineering technologies, the integration of multidisciplinary approaches, and the consideration of environmental and economic impacts. By addressing these key areas, researchers can unlock new possibilities for enhancing wood quality and promoting sustainable forestry practices.

5 Concluding Remarks

Research on glycosyltransferases, particularly the GT43 family, has revealed their critical role in xylan biosynthesis in poplar. Downregulation of GT43 genes in hybrid aspen has shown that these genes are essential for the synthesis of the xylan backbone, which is a major component of wood cell walls. Specifically, the suppression of the B (IRX9) and C (IRX14) clades of GT43 resulted in reduced xylan content and increased lignocellulose saccharification efficiency, indicating their direct involvement in xylan backbone biosynthesis (Ratke et al., 2018). Additionally, glycosyltransferases such as UGT72B37 have been implicated in the glycosylation of monolignols, which are crucial for lignin biosynthesis. Mutations in UGT72B37 led to increased lignin content in the xylem, suggesting a significant role in the lignification process (Cheng et al., 2022).

The genetic regulation of glycosyltransferases and their impact on xylan biosynthesis have profound implications for wood quality in poplar. The downregulation of GT43 genes not only altered xylan content but also stimulated overall growth and modified cell wall properties, such as cellulose orientation and xylem cell wall thinning. These changes can potentially enhance wood quality by improving lignocellulose saccharification efficiency, which is beneficial for biofuel production (Ratke et al., 2018). Furthermore, the role of UGT72B37 in lignin biosynthesis highlights the importance of glycosylation in regulating lignin content, which affects the mechanical properties and durability of wood (Cheng et al., 2022). The identification of glycoproteins involved in wood cell wall synthesis and modification further underscores the complexity of genetic regulation in wood formation (Biswal et al., 2015).

Future research should focus on elucidating the detailed mechanisms by which glycosyltransferases and other glycoproteins regulate xylan and lignin biosynthesis. Advanced genetic tools such as CRISPR/Cas9 can be employed to create targeted mutations and study their effects on wood properties. Additionally, exploring the interactions between different glycosyltransferases and their substrates could provide deeper insights into the regulation of wood formation. The application of these findings could lead to the development of genetically modified poplar with enhanced wood quality, optimized for various industrial uses, including biofuel production and construction materials. Continued research in this field holds promise for sustainable forestry and improved utilization of wood resources.

Acknowledgements

The author would like to express her gratitude to Dr. Fang, the director of the Hainan Institute of Tropical Agricultural Resources, for reading the draft of this paper and providing valuable feedback. The author also thanks the two anonymous peer reviewers for their critical assessment and constructive suggestions on our manuscript.

Funding

This research was supported by the Opening Project of State Key Laboratory of Tree Genetics and Breeding of China (K2018205). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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