BdGT43B2 functions in xylan biosynthesis and is essential for seedling survival in Brachypodium distachyon

XYLAN O-ACETYLTRANSFERASE 6 promotes xylan synthesis by forming a complex with IRX10 and governs wall formation in rice

Xylan, a pivotal polymer with diversified structures, is indispensable for cell wall integrity and contributes to plant growth and biomass recalcitrance. Xylan is synthesized by multienzyme complexes named xylan synthase complexes (XSCs). However, the biochemical mechanism of XSCs and the functions of core components within XSC remain unclear. Here, we report that rice (Oryza sativa) XYLAN O-ACETYLTRANSFERASE 6 (XOAT6) and the xylan synthase IRREGULAR XYLEM10 (IRX10) represent core components of the XSC, acting together to biosynthesize acetyl-xylans. Co-fractionation mass spectrometry and protein-protein interaction analyses revealed that IRX10 and XOAT6 physically interact within XSC, corroborated by similar xylan defects in xoat6 and irx10 mutants. Biochemical assays showed that XOAT6 is an O-acetyltransferase of the xylan backbone and facilitates chain polymerization catalyzed by IRX10. Fluorescence correlation spectroscopy further visualized the xylooligomer polymerization process at a single-molecule level. Solid-state NMR analysis, electron microscopy observations, and nanoindentation examinations identified the altered xylan conformation, disorganized cellulosic structure, and increased wall rigidity and cellulose accessibility in the mutants, leading to brittleness and improved saccharification efficiency. Our findings provide insights into the assembly of XSCs and xylan biosynthesis and offer a framework for tailoring xylans to improve crop traits and biomass.

Xylan structural diversity, biosynthesis, and functional regulation in plants

Xylan is a vital component of plant cell walls, contributing to their structural integrity and flexibility through interactions with other polymers. Its structure varies among plant species, influencing the mechanical properties of cell walls. Xylan also has significant industrial potential, including in biofuels, biomaterials, food, and pharmaceuticals, due to its ability to be converted into valuable bioproducts. However, key aspects of xylan biosynthesis, regulation, and structural impact on plant growth and structures remain unclear. This review highlights current researches on xylan biosynthesis, modification, and applications, identifying critical gaps in knowledge. Meanwhile the review proposes new approaches to regulate xylan synthesis and understand its role in cell wall assembly and interactions with other polymers. Addressing these gaps could unlock the full industrial potential of xylan, leading to more sustainable applications.

Specific protein interactions between rice members of the GT43 and GT47 families form various central cores of putative xylan synthase complexes

Members of the glycosyltransferase (GT)43 and GT47 families have been associated with heteroxylan synthesis in both dicots and monocots and are thought to assemble into central cores of putative xylan synthase complexes (XSCs). Currently, it is unknown whether protein-protein interactions within these central cores are specific, how many such complexes exist, and whether these complexes are functionally redundant. Here, we used gene association network and co-expression approaches in rice to identify four OsGT43s and four OsGT47s that assemble into different GT43/GT47 complexes. Using two independent methods, we showed that (i) these GTs assemble into at least six unique complexes through specific protein-protein interactions and (ii) the proteins interact directly in vitro. Confocal microscopy showed that, when alone, all OsGT43s were retained in the endoplasmic reticulum (ER), while all OsGT47s were localized in the Golgi. co-expression of OsGT43s and OsGT47s displayed complexes that form in the ER but accumulate in Golgi. ER-to-Golgi trafficking appears to require interactions between OsGT43s and OsGT47s. Comparison of the central cores of the three putative rice OsXSCs to wheat, asparagus, and Arabidopsis XSCs, showed great variation in GT43/GT47 combinations, which makes the identification of orthologous central cores between grasses and dicots challenging. However, the emerging picture is that all central cores from these species seem to have at least one member of the IRX10/IRX10-L clade in the GT47 family in common, suggesting greater functional importance for this family in xylan synthesis. Our findings provide a new framework for future investigation of heteroxylan biosynthesis and function in monocots.

Altering the substitution and cross-linking of glucuronoarabinoxylans affects cell wall architecture in Brachypodium distachyon

The Poaceae family of plants provides cereal crops that are critical for human and animal nutrition, and also, they are an important source of biomass. Interacting plant cell wall components give rise to recalcitrance to digestion; thus, understanding the wall molecular architecture is important to improve biomass properties. Xylan is the main hemicellulose in grass cell walls. Recently, we reported structural variation in grass xylans, suggesting functional specialisation and distinct interactions with cellulose and lignin. Here, we investigated the functions of these xylans by perturbing the biosynthesis of specific xylan types. We generated CRISPR/Cas9 knockout mutants in Brachypodium distachyon XAX1 and GUX2 genes involved in xylan substitution. Using carbohydrate gel electrophoresis, we identified biochemical changes in different xylan types. Saccharification, cryo-SEM, subcritical water extraction and ssNMR were used to study wall architecture. BdXAX1A and BdGUX2 enzymes modify different types of grass xylan. Brachypodium mutant walls are likely more porous, suggesting the xylan substitutions directed by both BdXAX1A and GUX2 enzymes influence xylan-xylan and/or xylan-lignin interactions. Since xylan substitutions influence wall architecture and digestibility, our findings open new avenues to improve cereals for food and to use grass biomass for feed and the production of bioenergy and biomaterials.

Xylan-directed cell wall assembly in grasses

Xylan is the most abundant hemicellulosic polysaccharide in the cell walls of grasses and is pivotal for the assembly of distinct cell wall structures that govern various cellular functions. Xylan also plays a crucial role in regulating biomass recalcitrance, ultimately affecting the utilization potential of lignocellulosic materials. Over the past decades, our understanding of the xylan biosynthetic machinery and cell wall organization has substantially improved due to the innovative application of multiple state-of-the-art techniques. Notably, novel xylan-based nanostructures have been revealed in the cell walls of xylem vessels, promoting a more extensive exploration of the role of xylan in the formation of cell wall structures. This Update summarizes recent achievements in understanding xylan biosynthesis, modification, modeling, and compartmentalization in grasses, providing a brief overview of cell wall assembly regarding xylan. We also discuss the potential for tailoring xylan to facilitate the breeding of elite energy and feed crops.

Identification and expression analysis of the glycosyltransferase GT43 family members in bamboo reveal their potential function in xylan biosynthesis during rapid growth

Background

Xylan is one of the most abundant hemicelluloses and can crosslink cellulose and lignin to increase the stability of cell walls. A number of genes encoding glycosyltransferases play vital roles in xylan biosynthesis in plants, such as those of the GT43 family. However, little is known about glycosyltransferases in bamboo, especially woody bamboo which is a good substitute for timber.

Results

A total of 17 GT43 genes (PeGT43–1 ~ PeGT43–17) were identified in the genome of moso bamboo (Phyllostachys edulis), which belong to three subfamilies with specific motifs. The phylogenetic and collinearity analyses showed that PeGT43s may have undergone gene duplication, as a result of collinearity found in 12 pairs of PeGT43s, and between 17 PeGT43s and 10 OsGT43s. A set of cis-acting elements such as hormones, abiotic stress response and MYB binding elements were found in the promoter of PeGT43s. PeGT43s were expressed differently in 26 tissues, among which the highest expression level was found in the shoots, especially in the rapid elongation zone and nodes. The genes coexpressed with PeGT43s were annotated as associated with polysaccharide metabolism and cell wall biosynthesis. qRT–PCR results showed that the coexpressed genes had similar expression patterns with a significant increase in 4.0 m shoots and a peak in 6.0 m shoots during fast growth. In addition, the xylan content and structural polysaccharide staining intensity in bamboo shoots showed a strong positive correlation with the expression of PeGT43s. Yeast one-hybrid assays demonstrated that PeMYB35 could recognize the 5′ UTR/promoter of PeGT43–5 by binding to the SMRE cis-elements.

Conclusions

PeGT43s were found to be adapted to the requirement of xylan biosynthesis during rapid cell elongation and cell wall accumulation, as evidenced by the expression profile of PeGT43s and the rate of xylan accumulation in bamboo shoots. Yeast one-hybrid analysis suggested that PeMYB35 might be involved in xylan biosynthesis by regulating the expression of PeGT43–5 by binding to its 5′ UTR/promoter. Our study provides a comprehensive understanding of PeGT43s in moso bamboo and lays a foundation for further functional analysis of PeGT43s for xylan biosynthesis during rapid growth.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08192-y.

Functional analysis of GT61 glycosyltransferases from grass species in xylan substitutions

Multiple rice GT61 members were demonstrated to be xylan arabinosyltransferases (XATs) mediating 3-O-arabinosylation of xylan and the functions of XATs and xylan 2-O-xylosyltransferases were shown to be conserved in grass species. Xylan is the major hemicellulose in the cell walls of grass species and it is typified by having arabinofuranosyl (Araf) substitutions. In this report, we demonstrated that four previously uncharacterized, Golgi-localized glycosyltransferases residing in clade A or B of the rice GT61 family were able to mediate 3-O-arabinosylation of xylan when heterologously expressed in the Arabidopsis gux1/2/3 triple mutant. Biochemical characterization of their recombinant proteins established that they were xylan arabinosyltransferases (XATs) capable of transferring Araf residues onto xylohexaose acceptors, and thus they were named OsXAT4, OsXAT5, OsXAT6 and OsXAT7. OsXAT5 and the previously identified OsXAT2 were shown to be able to arabinosylate xylooligomers with a degree of polymerization of as low as 3. Furthermore, a number of XAT homologs from maize, sorghum, Brachypodium and switchgrass were found to exhibit activities catalyzing Araf transfer onto xylohexaose, indicating that they are XATs involved in xylan arabinosylation in these grass species. Moreover, we revealed that homologs of another GT61 member, xylan 2-O-xylosyltransferase (XYXT1), from these grass species could mediate 2-O-xylosylation of xylan when expressed in the Arabidopsis gux1/2/3 mutant. Together, our findings indicate that multiple OsXATs are involved in 3-O-arabinosylation of xylan and the functions of XATs and XYXTs are conserved in grass species.

RNAi suppression of xylan synthase genes in wheat starchy endosperm

The xylan backbone of arabinoxylan (AX), the major cell wall polysaccharide in the wheat starchy endosperm, is synthesised by xylan synthase which is a complex of three subunits encoded by the GT43_1, GT43_2 and GT47_2 genes. RNAi knock-down of either GT43_1 or all three genes (triple lines) resulted in decreased AX measured by digestion with endoxylanase (to 33 and 34.9% of the controls) and by monosaccharide analysis (to 45.9% and 47.4% of the controls) with greater effects on the amount of water-extractable AX (to 20.6 and 19.9% of the controls). Both sets of RNAi lines also had greater decreases in the amounts of substituted oligosaccharides released by digestion of AX with endoxylanase than in fragments derived only from the xylan backbone. Although the GT43_1 and triple lines had similar effects on AX they did differ in their contents of soluble sugars (increased in triple only) and on grain size (decreased in triple only). Both sets of transgenic lines had decreased grain hardness, indicating effects on cell wall mechanics. These results, and previously published studies of RNAi suppression of GT43_2 and GT47_2 and of a triple mutant of GT43_2, are consistent with the model of xylan synthase comprising three subunits one of which (GT47_2) is responsible for catalysis with the other two subunits being required for correct functioning but indicate that separate xylan synthase complexes may be responsible for the synthesis of populations of AX which differ in their structure and solubility.

BdFAR4, a root-specific fatty acyl-coenzyme A reductase, is involved in fatty alcohol synthesis of root suberin polyester in Brachypodium distachyon

Suberin is a complex hydrophobic polymer of aliphatic and phenolic compounds which controls the movement of gases, water, and solutes and protects plants from environmental stresses and pathogenic infection. The synthesis and regulatory pathways of suberin remain unknown in Brachypodium distachyon. Here we describe the identification of a B. distachyon gene, BdFAR4, encoding a fatty acyl-coenzyme A reductase (FAR) by a reverse genetic approach, and investigate the molecular relevance of BdFAR4 in the root suberin synthesis of B. distachyon. BdFAR4 is specifically expressed throughout root development. Heterologous expression of BdFAR4 in yeast (Saccharomyces cerevisiae) afforded the production of C20:0 and C22:0 fatty alcohols. The loss-of-function knockout of BdFAR4 by CRISPR/Cas9-mediated gene editing significantly reduced the content of C20:0 and C22:0 fatty alcohols associated with root suberin. In contrast, overexpression of BdFAR4 in B. distachyon and tomato (Solanum lycopersicum) resulted in the accumulation of root suberin-associated C20:0 and C22:0 fatty alcohols, suggesting that BdFAR4 preferentially accepts C20:0 and C22:0 fatty acyl-CoAs as substrates. The BdFAR4 protein was localized to the endoplasmic reticulum in Arabidopsis thaliana protoplasts and Nicotiana benthamiana leaf epidermal cells. BdFAR4 transcript levels can be increased by abiotic stresses and abscisic acid treatment. Furthermore, yeast one-hybrid, dual-luciferase activity, and electrophoretic mobility shift assays indicated that the R2R3-MYB transcription factor BdMYB41 directly binds to the promoter of BdFAR4. Taken together, these results imply that BdFAR4 is essential for the production of root suberin-associated fatty alcohols, especially under stress conditions, and that its activity is transcriptionally regulated by the BdMYB41 transcription factor.