The poplar glycosyltransferase GT47C is functionally conserved with Arabidopsis Fragile fiber8

Impact of xylan on field productivity and wood saccharification properties in aspen

Xylan that comprises roughly 25% of hardwood biomass is undesirable in biorefinery applications involving saccharification and fermentation. Efforts to reduce xylan levels have therefore been made in many species, usually resulting in improved saccharification. However, such modified plants have not yet been tested under field conditions. Here we evaluate the field performance of transgenic hybrid aspen lines with reduced xylan levels and assess their usefulness as short-rotation feedstocks for biorefineries. Three types of transgenic lines were tested in four-year field tests with RNAi constructs targeting either Populus GT43 clades B and C (GT43BC) corresponding to Arabidopsis clades IRX9 and IRX14, respectively, involved in xylan backbone biosynthesis, GATL1.1 corresponding to AtGALT1 involved in xylan reducing end sequence biosynthesis, or ASPR1 encoding an atypical aspartate protease. Their productivity, wood quality traits, and saccharification efficiency were analyzed. The only lines differing significantly from the wild type with respect to growth and biotic stress resistance were the ASPR1 lines, whose stems were roughly 10% shorter and narrower and leaves showed increased arthropod damage. GT43BC lines exhibited no growth advantage in the field despite their superior growth in greenhouse experiments. Wood from the ASPR1 and GT43BC lines had slightly reduced density due to thinner cell walls and, in the case of ASPR1, larger cell diameters. The xylan was less extractable by alkali but more hydrolysable by acid, had increased glucuronosylation, and its content was reduced in all three types of transgenic lines. The hemicellulose size distribution in the GALT1.1 and ASPR1 lines was skewed towards higher molecular mass compared to the wild type. These results provide experimental evidence that GATL1.1 functions in xylan biosynthesis and suggest that ASPR1 may regulate this process. In saccharification without pretreatment, lines of all three constructs provided 8-11% higher average glucose yields than wild-type plants. In saccharification with acid pretreatment, the GT43BC construct provided a 10% yield increase on average. The best transgenic lines of each construct are thus predicted to modestly outperform the wild type in terms of glucose yields per hectare. The field evaluation of transgenic xylan-reduced aspen represents an important step towards more productive feedstocks for biorefineries.

PtrWOX13A Promotes Wood Formation and Bioactive Gibberellins Biosynthesis in Populus trichocarpa

WUSCHEL-related homeobox (WOX) genes are plant-specific transcription factors (TFs) involved in multiple processes of plant development. However, there have hitherto no studies on the WOX TFs involved in secondary cell wall (SCW) formation been reported. In this study, we identified a Populus trichocarpa WOX gene, PtrWOX13A, which was predominantly expressed in SCW, and then characterized its functions through generating PtrWOX13A overexpression poplar transgenic lines; these lines exhibited not only significantly enhanced growth potential, but also remarkably increased SCW thicknesses, fiber lengths, and lignin and hemicellulose contents. However, no obvious change in cellulose content was observed. We revealed that PtrWOX13A directly activated its target genes through binding to two cis-elements, ATTGATTG and TTAATSS, in their promoter regions. The fact that PtrWOX13A responded to the exogenous GAs implies that it is responsive to GA homeostasis caused by GA inactivation and activation genes (e.g., PtrGA20ox4, PtrGA2ox1, and PtrGA3ox1), which were regulated by PtrWOX13A directly or indirectly. Since the master switch gene of SCW formation, PtrWND6A, and lignin biosynthesis regulator, MYB28, significantly increased in PtrWOX13A transgenic lines, we proposed that PtrWOX13A, as a higher hierarchy TF, participated in SCW formation through controlling the genes that are components of the known hierarchical transcription regulation network of poplar SCW formation, and simultaneously triggering a gibberellin-mediated signaling cascade. The discovery of PtrWOX13A predominantly expressed in SCW and its regulatory functions in the poplar wood formation has important implications for improving the wood quality of trees via genetic engineering.

PtrHAT22, as a higher hierarchy regulator, coordinately regulates secondary cell wall component biosynthesis in Populus trichocarpa

Homeodomain-leucine zipper (HD-Zip) II transcription factors (TFs) have been reported to play vital roles in diverse biological processes of plants. However, it remains unclear whether HD-Zip II TFs regulate secondary cell wall (SCW) in woody plants. In this study, we performed the functional characterization of a Populus trichocarpa HD-Zip II TF, PtrHAT22, which encodes a nuclear localized transcription repressor predominantly expressing in secondary developing tissues. Overexpression of PtrHAT22 showed arrested growths, including reduced heights and diameters above the ground, small leaves, and decreased biomass. Meanwhile, the contents of lignin, cellulose, and thickness of SCW significantly decreased, whilst the content of hemicellulose obviously increased in PtrHAT22 transgenic poplar. The expressions of some wood-associated TFs and structural genes significantly changed accordingly with the alternations of SCW characteristics in PtrHAT22 transgenic poplar. Furthermore, PtrHAT22 directly repressed the promoter activities of PtrMYB20, PtrMYB28, and PtrCOMT2, and bind two cis-acting elements that were specifically enriched in their promoter regions. Taken together, our results suggested that PtrHAT22, as a higher hierarchy TF like PtrWNDs, exerted coordination regulation of poplar SCW component biosynthesis through directly and indirectly regulating structural genes and different hierarchy TFs of SCW formation network.

An improved and efficient method of Agrobacterium syringe infiltration for transient transformation and its application in the elucidation of gene function in poplar

BACKGROUND

Forest trees have important economic and ecological value. As a model tree, poplar has played a significant role in elucidating the molecular mechanisms underlying tree biology. However, a lack of mutant libraries and time-consuming stable genetic transformation processes severely limit progress into the functional characterization of poplar genes. A convenient and fast transient transformation method is therefore needed to enhance progress on functional genomics in poplar.

METHODS

A total of 11 poplar clones were screened for amenability to syringe infiltration. Syringe infiltration was performed on the lower side of the leaves of young soil-grown plants. Transient expression was evaluated by visualizing the reporters β-glucuronidase (GUS) and green fluorescent protein (GFP). The experimental parameters of the syringe agroinfiltration were optimized based on the expression levels of the reporter luciferase (LUC). Stably transformed plants were regenerated from transiently transformed leaf explants through callus-induced organogenesis. The functions of Populus genes in secondary cell wall-thickening were characterized by visualizing lignin deposition therein after staining with basic fuchsin.

RESULTS

We greatly improved the transient transformation efficiency of syringe Agrobacterium infiltration in poplar through screening for a suitable poplar clone from a variety of clones and optimizing the syringe infiltration procedure. The selected poplar clone, Populus davidiana × P. bolleana, is amenable to Agrobacterium syringe infiltration, as indicated by the easy diffusion of the bacterial suspension inside the leaf tissues. Using this technique, we localized a variety of poplar proteins in specific intracellular organelles and illustrated the protein-protein and protein-DNA interactions. The transiently transformed leaves could be used to generate stably transformed plants with high efficiency through callus induction and differentiation processes. Furthermore, transdifferentiation of the protoxylem-like vessel element and ectopic secondary wall thickening were induced in the agroinfiltrated leaves via the transient overexpression of genes associated with secondary wall formation.

CONCLUSIONS

The application of P. davidiana × P. bolleana in Agrobacterium syringe infiltration provides a foundation for the rapid and high-throughput functional characterization of Populus genes in intact poplar plants, including those involved in wood formation, and provides an effective alternative to Populus stable genetic transformation.

Characterization of a High Hierarchical Regulator, PtrGATA12, Functioning in Differentially Regulating Secondary Wall Component Biosynthesis in Populus trichocarpa

In plants, GATA transcription factors (TFs) have been reported to play vital roles in to a wide range of biological processes. To date, there is still no report about the involvement and functions of woody plant GATA TFs in wood formation. In this study, we described the functional characterization of a Populus trichocarpa GATA TF, PtrGATA12, which encodes a nuclear-localized transcriptional activator predominantly expressing in developing xylem tissues. Overexpression of PtrGATA12 not only inhibited growths of most phenotypic traits and biomass accumulation, but also altered the expressions of some master TFs and pathway genes involved in secondary cell wall (SCW) and programmed cell death, leading to alternated SCW components and breaking forces of stems of transgenic lines. The significant changes occurred in the contents of hemicellulose and lignin and SCW thicknesses of fiber and vessel that increased by 13.5 and 10.8%, and 20.83 and 11.83%, respectively. Furthermore, PtrGATA12 bound directly to the promoters of a battery of TFs and pathway genes and activated them; the binding sites include two cis-acting elements that were specifically enriched in their promoter regions. Taken together, our results suggest PtrGATA12, as a higher hierarchical TF on the top of PtrWND6A, PtrWND6B, PtrMYB152, and PtrMYB21, exert a coordinated regulation of SCW components biosynthesis pathways through directly and indirectly controlling master TFs, middle-level TFs, and further downstream pathway genes of the currently known hierarchical transcription network that governs SCW formation.

The Osmotin-Like Protein Gene PdOLP1 Is Involved in Secondary Cell Wall Biosynthesis during Wood Formation in Poplar

Osmotin-like proteins (OLPs) mediate defenses against abiotic and biotic stresses and fungal pathogens in plants. However, no OLPs have been functionally elucidated in poplar. Here, we report an osmotin-like protein designated PdOLP1 from Populus deltoides (Marsh.). Expression analysis showed that PdOLP1 transcripts were mainly present in immature xylem and immature phloem during vascular tissue development in P. deltoides. We conducted phenotypic, anatomical, and molecular analyses of PdOLP1-overexpressing lines and the PdOLP1-downregulated hybrid poplar 84K (Populus alba × Populus glandulosa) (Hybrid poplar 84K PagOLP1, PagOLP2, PagOLP3 and PagOLP4 are highly homologous to PdOLP1, and are downregulated in PdOLP1-downregulated hybrid poplar 84K). The overexpression of PdOLP1 led to a reduction in the radial width and cell layer number in the xylem and phloem zones, in expression of genes involved in lignin biosynthesis, and in the fibers and vessels of xylem cell walls in the overexpressing lines. Additionally, the xylem vessels and fibers of PdOLP1-downregulated poplar exhibited increased secondary cell wall thickness. Elevated expression of secondary wall biosynthetic genes was accompanied by increases in lignin content, dry weight biomass, and carbon storage in PdOLP1-downregulated lines. A PdOLP1 coexpression network was constructed and showed that PdOLP1 was coexpressed with a large number of genes involved in secondary cell wall biosynthesis and wood development in poplar. Moreover, based on transcriptional activation assays, PtobZIP5 and PtobHLH7 activated the PdOLP1 promoter, whereas PtoBLH8 and PtoWRKY40 repressed it. A yeast one-hybrid (Y1H) assay confirmed interaction of PtoBLH8, PtoMYB3, and PtoWRKY40 with the PdOLP1 promoter in vivo. Together, our results suggest that PdOLP1 is a negative regulator of secondary wall biosynthesis and may be valuable for manipulating secondary cell wall deposition to improve carbon fixation efficiency in tree species.

Genome-Wide Identification and Characterization of Glycosyltransferase Family 47 in Cotton

The glycosyltransferase (GT) 47 family is involved in the biosynthesis of xylose, pectin and xyloglucan and plays a significant role in maintaining the normal morphology of the plant cell wall. However, the functions of GT47s are less well known in cotton. In the present study, a total of 53, 53, 105 and 109 GT47 genes were detected by genome-wide identification in Gossypium arboreum, G. raimondii, G. hirsutum and G. barbadense, respectively. All the GT47s were classified into six major groups via phylogenetic analysis. The exon/intron structure and protein motifs indicated that each branch of the GT47 genes was highly conserved. Collinearity analysis showed that GT47 gene family expansion occurred in Gossypium spp. mainly through whole-genome duplication and that segmental duplication mainly promoted GT47 gene expansion within the A and D subgenomes. The Ka/Ks values suggested that the GT47 gene family has undergone purifying selection during the long-term evolutionary process. Transcriptomic data and qRT-PCR showed that GhGT47 genes exhibited different expression patterns in each tissue and during fiber development. Our results suggest that some genes in the GhGT47 family might be associated with fiber development and the abiotic stress response, which could promote further research involving functional analysis of GT47 genes in cotton.

Suppression of PtrDUF579-3 Expression Causes Structural Changes of the Glucuronoxylan in Populus

DUF579 (domain unknown function 579) genes have been reported to play diverse roles in cell wall biosynthesis, such as in glucuronoxylan (GX) synthesis. As GX is a major type of hemicelluloses in hard wood species, how DUF579 genes function in wood formation remains to be demonstrated in planta. This study reports a Populus DUF579 gene, PtrDUF579-3, which is characterized for its function in wood cell wall formation. PtrDUF579-3 is localized in Golgi apparatus and catalyzes methylation of the glucuronic acid (GlcA) in GX biosynthesis. Suppression of PtrDUF579-3 expression in Populus caused a reduction in both the GlcA side chain number and GlcA side chain methylation on the GX backbone. The modified GX polymer through PtrDUF579-3 suppression was more susceptible to acid treatment and the PtrDUF579-3 suppressed plants displayed enhanced cellulose digestibility. These results suggest that PtrDUF579-3 is involved in GX biosynthesis and GX structure can be modified through PtrDUF579-3 suppression in Populus.

Functional Characterization of NAC and MYB Transcription Factors Involved in Regulation of Biomass Production in Switchgrass (Panicum virgatum)

Switchgrass is a promising biofuel feedstock due to its high biomass production and low agronomic input requirements. Because the bulk of switchgrass biomass used for biofuel production is lignocellulosic secondary walls, studies on secondary wall biosynthesis and its transcriptional regulation are imperative for designing strategies for genetic improvement of biomass production in switchgrass. Here, we report the identification and functional characterization of a group of switchgrass transcription factors, including several NACs (PvSWNs) and a MYB (PvMYB46A), for their involvement in regulating secondary wall biosynthesis. PvSWNs and PvMYB46A were found to be highly expressed in stems and their expression was closely associated with sclerenchyma cells. Overexpression of PvSWNs and PvMYB46A in Arabidopsis was shown to result in activation of the biosynthetic genes for cellulose, xylan and lignin and ectopic deposition of secondary walls in normally parenchymatous cells. Transactivation and complementation studies demonstrated that PvSWNs were able to activate the SNBE-driven GUS reporter gene and effectively rescue the secondary wall defects in the Arabidopsis snd1 nst1 double mutant, indicating that they are functional orthologs of Arabidopsis SWNs. Furthermore, we showed that PvMYB46A could activate the SMRE-driven GUS reporter gene and complement the Arabidopsis myb46 myb83 double mutant, suggesting that it is a functional ortholog of Arabidopsis MYB46/MYB83. Together, these results indicate that PvSWNs and PvMYB46A are transcriptional switches involved in regulating secondary wall biosynthesis, which provides molecular tools for genetic manipulation of biomass production in switchgrass.

Populus GT43 family members group into distinct sets required for primary and secondary wall xylan biosynthesis and include useful promoters for wood modification

The plant GT43 protein family includes xylosyltransferases that are known to be required for xylan backbone biosynthesis, but have incompletely understood specificities. RT-qPCR and histochemical (GUS) analyses of expression patterns of GT43 members in hybrid aspen, reported here, revealed that three clades of the family have markedly differing specificity towards secondary wall-forming cells (wood and extraxylary fibres). Intriguingly, GT43A and B genes (corresponding to the Arabidopsis IRX9 clade) showed higher specificity for secondary-walled cells than GT43C and D genes (IRX14 clade), although both IRX9 and IRX14 are required for xylosyltransferase activity. The remaining genes, GT43E, F and G (IRX9-L clade), showed broad expression patterns. Transient transactivation analyses of GT43A and B reporters demonstrated that they are activated by PtxtMYB021 and PNAC085 (master secondary wall switches), mediated in PtxtMYB021 activation by an AC element. The high observed secondary cell wall specificity of GT43B expression prompted tests of the efficiency of its promoter (pGT43B), relative to the CaMV 35S (35S) promoter, for overexpressing a xylan acetyl esterase (CE5) or downregulating REDUCED WALL ACETYLATION (RWA) family genes and thus engineering wood acetylation. CE5 expression was weaker when driven by pGT43B, but it reduced wood acetyl content substantially more efficiently than the 35S promoter. RNAi silencing of the RWA family, which was ineffective using 35S, was achieved when using GT43B promoter. These results show the utility of the GT43B promoter for genetically engineering properties of wood and fibres.

Plant biotechnology for lignocellulosic biofuel production

Lignocelluloses from plant cell walls are attractive resources for sustainable biofuel production. However, conversion of lignocellulose to biofuel is more expensive than other current technologies, due to the costs of chemical pretreatment and enzyme hydrolysis for cell wall deconstruction. Recalcitrance of cell walls to deconstruction has been reduced in many plant species by modifying plant cell walls through biotechnology. These results have been achieved by reducing lignin content and altering its composition and structure. Reduction of recalcitrance has also been achieved by manipulating hemicellulose biosynthesis and by overexpression of bacterial enzymes in plants to disrupt linkages in the lignin-carbohydrate complexes. These modified plants often have improved saccharification yield and higher ethanol production. Cell wall-degrading (CWD) enzymes from bacteria and fungi have been expressed at high levels in plants to increase the efficiency of saccharification compared with exogenous addition of cellulolytic enzymes. In planta expression of heat-stable CWD enzymes from bacterial thermophiles has made autohydrolysis possible. Transgenic plants can be engineered to reduce recalcitrance without any yield penalty, indicating that successful cell wall modification can be achieved without impacting cell wall integrity or plant development. A more complete understanding of cell wall formation and structure should greatly improve lignocellulosic feedstocks and reduce the cost of biofuel production.

Secondary cell wall composition and candidate gene expression in developing willow (Salix purpurea) stems

The properties of the secondary cell wall (SCW) in willow largely determine the suitability of willow biomass feedstock for potential bioenergy and biofuel applications. SCW development has been little studied in willow and it is not known how willow compares with model species, particularly the closely related genus Populus. To address this and relate SCW synthesis to candidate genes in willow, a tractable bud culture-derived system was developed in Salix purpurea, and cell wall composition and RNA-Seq transcriptome were followed in stems during early development. A large increase in SCW deposition in the period 0-2 weeks after transfer to soil was characterised by a big increase in xylan content, but no change in the frequency of substitution of xylan with glucuronic acid, and increased abundance of putative transcripts for synthesis of SCW cellulose, xylan and lignin. Histochemical staining and immunolabeling revealed that increased deposition of lignin and xylan was associated with xylem, xylem fibre cells and phloem fibre cells. Transcripts orthologous to those encoding xylan synthase components IRX9 and IRX10 and xylan glucuronyl transferase GUX1 in Arabidopsis were co-expressed, and showed the same spatial pattern of expression revealed by in situ hybridisation at four developmental stages, with abundant expression in proto-xylem, xylem fibre and ray parenchyma cells and some expression in phloem fibre cells. The results show a close similarity with SCW development in Populus species, but also give novel information on the relationship between spatial and temporal variation in xylan-related transcripts and xylan composition.

Functional conservation of the glycosyltransferase gene GT47A in the monocot rice

Glucuronoarabinoxylan is the major hemicellulose in grass cell walls, yet the mechanism of xylan synthesis in monocot plants is still unclear. Unraveling the genes involved in the biosynthesis of xylan in rice will be very important for the utilization of rice straw as a source of bioenergy in the future. In this report, we investigated the functional role of a rice gene homologous to Arabidopsis IRREGULAR XYLEM10 (IRX10), belonging to the glycosyl transferase (GT) gene family 47 (GT47), in the biosynthesis of xylan. The protein sequence of OsGT47A from rice exhibits a 93.49% similarity to IRX10, which is involved in the biosynthesis of glucuronoxylan in Arabidopsis. Phylogenetic analysis of the GT47 glycosyl transferase family in the rice genome revealed that OsGT47A is a closely related homolog of IRX10 and IRX10L. Expression pattern analysis showed that the OsGT47A gene is highly expressed in the rice stem. Overexpression of OsGT47A in the irx10 irx10L double mutant rescued the plant growth phenotype and restored secondary wall thickness. Analysis of monosaccharides indicated that the rescued plants had levels of xylose identical to those of the wild type plants, and the fluorescence signals were restored in the complementation plants by xylan immunolocalization. The OsGT47A complementation under the native promoter of Arabidopsis IRX10L (ProIRX10L) partially rescued the double mutant, indicating that OsGT47A is functionally equivalent to IRX10L. Together, these results suggest that the IRX10 homolog OsGT47A exhibits functional conservation and is most likely involved in xylan synthesis in rice.

Identification and biochemical characterization of four wood-associated glucuronoxylan methyltransferases in Populus

Wood is one of the promising bioenergy feedstocks for lignocellulosic biofuel production. Understanding how wood components are synthesized will help us design strategies for better utilization of wood for biofuel production. One of the major wood components is xylan, in which about 10% of xylosyl residues are substituted with glucuronic acid (GlcA) side chains. All the GlcA side chains of xylan in wood of Populus trichocarpa are methylated, which is different from Arabidopsis xylan in which about 60% of GlcA side chains are methylated. Genes responsible for methylation of GlcA side chains in Populus xylan have not been identified. Here, we report genetic and biochemical analyses of four DUF579 domain-containing proteins, PtrGXM1, PtrGXM2, PtrGXM3 and PtrGXM4, from Populus trichocarpa and their roles in GlcA methylation in xylan. The PtrGXM genes were found to be highly expressed in wood-forming cells and their encoded proteins were shown to be localized in the Golgi. When overexpressed in the Arabidopsis gxm1/2/3 triple mutant, PtrGXMs were able to partially complement the mutant phenotypes including defects in glucuronoxylan methyltransferase activity and GlcA methylation in xylan, indicating that PtrGXMs most likely function as glucuronoxylan methyltransferases. Direct evidence was provided by enzymatic analysis of recombinant PtrGXM proteins showing that they possessed a methyltransferase activity capable of transferring the methyl group onto GlcA-substituted xylooligomers. Kinetic analysis showed that PtrGXMs exhibited differential affinities toward the GlcA-substituted xylooligomer acceptor with PtrGXM3 and PtrGXM4 having 10 times higher K_m values than PtrGXM1 and PtrGXM2. Together, these findings indicate that PtrGXMs are methyltransferases mediating GlcA methylation in Populus xylan during wood formation.

Golgi enrichment and proteomic analysis of developing Pinus radiata xylem by free-flow electrophoresis

Our understanding of the contribution of Golgi proteins to cell wall and wood formation in any woody plant species is limited. Currently, little Golgi proteomics data exists for wood-forming tissues. In this study, we attempted to address this issue by generating and analyzing Golgi-enriched membrane preparations from developing xylem of compression wood from the conifer Pinus radiata. Developing xylem samples from 3-year-old pine trees were harvested for this purpose at a time of active growth and subjected to a combination of density centrifugation followed by free flow electrophoresis, a surface charge separation technique used in the enrichment of Golgi membranes. This combination of techniques was successful in achieving an approximately 200-fold increase in the activity of the Golgi marker galactan synthase and represents a significant improvement for proteomic analyses of the Golgi from conifers. A total of thirty known Golgi proteins were identified by mass spectrometry including glycosyltransferases from gene families involved in glucomannan and glucuronoxylan biosynthesis. The free flow electrophoresis fractions of enriched Golgi were highly abundant in structural proteins (actin and tubulin) indicating a role for the cytoskeleton during compression wood formation. The mass spectrometry proteomics data associated with this study have been deposited to the ProteomeXchange with identifier PXD000557.

Three Novel Rice Genes Closely Related to the Arabidopsis IRX9, IRX9L, and IRX14 Genes and Their Roles in Xylan Biosynthesis

Xylan is the second most abundant polysaccharide on Earth, and represents a major component of both dicot wood and the cell walls of grasses. Much knowledge has been gained from studies of xylan biosynthesis in the model plant, Arabidopsis. In particular, the irregular xylem (irx) mutants, named for their collapsed xylem cells, have been essential in gaining a greater understanding of the genes involved in xylan biosynthesis. In contrast, xylan biosynthesis in grass cell walls is poorly understood. We identified three rice genes Os07g49370 (OsIRX9), Os01g48440 (OsIRX9L), and Os06g47340 (OsIRX14), from glycosyltransferase family 43 as putative orthologs to the putative β-1,4-xylan backbone elongating Arabidopsis IRX9, IRX9L, and IRX14 genes, respectively. We demonstrate that the over-expression of the closely related rice genes, in full or partly complement the two well-characterized Arabidopsis irregular xylem (irx) mutants: irx9 and irx14. Complementation was assessed by measuring dwarfed phenotypes, irregular xylem cells in stem cross sections, xylose content of stems, xylosyltransferase (XylT) activity of stems, and stem strength. The expression of OsIRX9 in the irx9 mutant resulted in XylT activity of stems that was over double that of wild type plants, and the stem strength of this line increased to 124% above that of wild type. Taken together, our results suggest that OsIRX9/OsIRX9L, and OsIRX14, have similar functions to the Arabidopsis IRX9 and IRX14 genes, respectively. Furthermore, our expression data indicate that OsIRX9 and OsIRX9L may function in building the xylan backbone in the secondary and primary cell walls, respectively. Our results provide insight into xylan biosynthesis in rice and how expression of a xylan synthesis gene may be modified to increase stem strength.

Partial functional conservation of IRX10 homologs in physcomitrella patens and Arabidopsis thaliana indicates an evolutionary step contributing to vascular formation in land plants

BACKGROUND

Plant cell walls are complex multicomponent structures that have evolved to fulfil an essential function in providing strength and protection to cells. Hemicelluloses constitute a key component of the cell wall and recently a number of the genes thought to encode the enzymes required for its synthesis have been identified in Arabidopsis. The acquisition of hemicellulose synthesis capability is hypothesised to have been an important step in the evolution of higher plants.

RESULTS

Analysis of the Physcomitrella patens genome has revealed the presence of homologs for all of the Arabidopsis glycosyltransferases including IRX9, IRX10 and IRX14 required for the synthesis of the glucuronoxylan backbone. The Physcomitrella IRX10 homolog is expressed in a variety of moss tissues which were newly formed or undergoing expansion. There is a high degree of sequence conservation between the Physcomitrella IRX10 and Arabidopsis IRX10 and IRX10-L. Despite this sequence similarity, the Physcomitrella IRX10 gene is only able to partially rescue the Arabidopsis irx10 irx10-L double mutant indicating that there has been a neo- or sub-functionalisation during the evolution of higher plants. Analysis of the monosaccharide composition of stems from the partially rescued Arabidopsis plants does not show any significant change in xylose content compared to the irx10 irx10-L double mutant. Likewise, knockout mutants of the Physcomitrella IRX10 gene do not result in any visible phenotype and there is no significant change in monosaccharide composition of the cell walls.

CONCLUSIONS

The fact that the Physcomitrella IRX10 (PpGT47A) protein can partially complement an Arabidopsis irx10 irx10-L double mutant suggests that it shares some function with the Arabidopsis proteins, but the lack of a phenotype in knockout lines shows that the function is not required for growth or development under normal conditions in Physcomitrella. In contrast, the Arabidopsis irx10 and irx10 irx10-L mutants have strong phenotypes indicating an important function in growth and development. We conclude that the evolution of vascular plants has been associated with a significant change or adaptation in the function of the IRX10 gene family.

Constitutive expression of a fungal glucuronoyl esterase in Arabidopsis reveals altered cell wall composition and structure

A family 15 carbohydrate esterase (CE15) from the white-rot basidiomycete, Phanerochaete carnosa (PcGCE), was transformed into Arabidopsis thaliana Col-0 and was expressed from the constitutive cauliflower mosaic virus 35S promoter. Like other CE15 enzymes, PcGCE hydrolyzed methyl-4-O-methyl-d-glucopyranuronate and could target ester linkages that contribute to lignin-carbohydrate complexes that form in plant cell walls. Three independently transformed Arabidopsis lines were evaluated in terms of nine morphometric parameters, total sugar and lignin composition, cell wall anatomy, enzymatic saccharification and xylan extractability. The transgenic lines consistently displayed a leaf-yellowing phenotype, as well as reduced glucose and xylose content by as much as 30% and 35%, respectively. Histological analysis revealed 50% reduction in cell wall thickness in the interfascicular fibres of transgenic plants, and FT-IR microspectroscopy of interfascicular fibre walls indicated reduction in lignin cross-linking in plants overexpressing PcGCE. Notably, these characteristics could be correlated with improved xylose recovery in transgenic plants, up to 15%. The current analysis represents the first example whereby a fungal glucuronoyl esterase is expressed in Arabidopsis and shows that the promotion of glucuronoyl esterase activity in plants can alter the extent of intermolecular cross-linking within plant cell walls.

Engineering of plants with improved properties as biofuels feedstocks by vessel-specific complementation of xylan biosynthesis mutants

BACKGROUND

Cost-efficient generation of second-generation biofuels requires plant biomass that can easily be degraded into sugars and further fermented into fuels. However, lignocellulosic biomass is inherently recalcitrant toward deconstruction technologies due to the abundant lignin and cross-linked hemicelluloses. Furthermore, lignocellulosic biomass has a high content of pentoses, which are more difficult to ferment into fuels than hexoses. Engineered plants with decreased amounts of xylan in their secondary walls have the potential to render plant biomass a more desirable feedstock for biofuel production.

RESULTS

Xylan is the major non-cellulosic polysaccharide in secondary cell walls, and the xylan deficient irregular xylem (irx) mutants irx7, irx8 and irx9 exhibit severe dwarf growth phenotypes. The main reason for the growth phenotype appears to be xylem vessel collapse and the resulting impaired transport of water and nutrients. We developed a xylan-engineering approach to reintroduce xylan biosynthesis specifically into the xylem vessels in the Arabidopsis irx7, irx8 and irx9 mutant backgrounds by driving the expression of the respective glycosyltransferases with the vessel-specific promoters of the VND6 and VND7 transcription factor genes. The growth phenotype, stem breaking strength, and irx morphology was recovered to varying degrees. Some of the plants even exhibited increased stem strength compared to the wild type. We obtained Arabidopsis plants with up to 23% reduction in xylose levels and 18% reduction in lignin content compared to wild-type plants, while exhibiting wild-type growth patterns and morphology, as well as normal xylem vessels. These plants showed a 42% increase in saccharification yield after hot water pretreatment. The VND7 promoter yielded a more complete complementation of the irx phenotype than the VND6 promoter.

CONCLUSIONS

Spatial and temporal deposition of xylan in the secondary cell wall of Arabidopsis can be manipulated by using the promoter regions of vessel-specific genes to express xylan biosynthetic genes. The expression of xylan specifically in the xylem vessels is sufficient to complement the irx phenotype of xylan deficient mutants, while maintaining low overall amounts of xylan and lignin in the cell wall. This engineering approach has the potential to yield bioenergy crop plants that are more easily deconstructed and fermented into biofuels.

Immunolocalization of hemicelluloses in Arabidopsis thaliana stem. Part I: temporal and spatial distribution of xylans

We investigated the microdistribution of xylans in different cell types of Arabidopsis stem using immunolocalization methods with LM10 and LM11 antibodies. Xylan labeling in xylary fibers (fibers) was initially detected at the cell corner of the S(1) layer and increased gradually during fiber maturation, showing correlation between xylan labeling and general secondary cell wall formation processes in fibers. Metaxylem vessels (vessels) showed earlier development of secondary cell walls than fibers, but revealed almost identical labeling patterns to fibers during maturation. No difference in labeling patterns and intensity was detected in the cell wall of fibers, vessels and protoxylem vessels (proto-vessels) between LM10 and LM11, indicating that vascular bundle cells may be chemically composed of a highly homogeneous xylan type. Interestingly, interfascicular fibers (If-fibers) showed different labeling patterns between the two antibodies and also between different developmental stages. LM10 showed no labeling in primary cell walls and intercellular layers of If-fibers at the S(1) formation stage, but some labeling was detected in middle lamella cell corner regions at the S(2) formation stage. In contrast, LM11 revealed uniform labeling across the If-fiber cell wall during all developmental stages. These results suggest that If-fibers have different xylan deposition processes and patterns from vascular bundle cells. The presence of xylan was also confirmed in parenchyma cells following pectinase treatment. Together our results indicate that there are temporal and spatial differences in xylan labeling between cell types in Arabidopsis stem. Differences in xylan labeling between Arabidopsis stem and poplar are also discussed.

A simple and efficient transient transformation for hybrid aspen (Populus tremula × P. tremuloides)

BACKGROUND

The genus Populus is accepted as a model system for molecular tree biology. To investigate gene functions in Populus spp. trees, generating stable transgenic lines is the common technique for functional genetic studies. However, a limited number of genes have been targeted due to the lengthy transgenic process. Transient transformation assays complementing stable transformation have significant advantages for rapid in vivo assessment of gene function. The aim of this study is to develop a simple and efficient transient transformation for hybrid aspen and to provide its potential applications for functional genomic approaches.

RESULTS

We developed an in planta transient transformation assay for young hybrid aspen cuttings using Agrobacterium-mediated vacuum infiltration. The transformation conditions such as the infiltration medium, the presence of a surfactant, the phase of bacterial growth and bacterial density were optimized to achieve a higher transformation efficiency in young aspen leaves. The Agrobacterium infiltration assay successfully transformed various cell types in leaf tissues. Intracellular localization of four aspen genes was confirmed in homologous Populus spp. using fusion constructs with the green fluorescent protein. Protein-protein interaction was detected in transiently co-transformed cells with bimolecular fluorescence complementation technique. In vivo promoter activity was monitored over a few days in aspen cuttings that were transformed with luciferase reporter gene driven by a circadian clock promoter.

CONCLUSIONS

The Agrobacterium infiltration assay developed here is a simple and enhanced throughput method that requires minimum handling and short transgenic process. This method will facilitate functional analyses of Populus genes in a homologous plant system.

Over-expression of a putative poplar glycosyltransferase gene, PtGT1, in tobacco increases lignin content and causes early flowering

Family 1 glycosyltransferases catalyse the glycosylation of small molecules and play an important role in maintaining cell homeostasis and regulating plant growth and development. In this study, a putative glycosyltransferase gene of family 1, PtGT1, was cloned from poplar (Populus tomentosa Carr.). Sequence analysis showed that this gene encodes a protein of 481 amino acid residues with a conserved PSPG box at its C-terminal, suggesting that it is active in the glycosylation of plant secondary products. The PtGT1 gene was expressed in poplar stems and leaves, with a particularly high expression level in elongating stems. Transgenic tobacco plants ectopically over-expressing PtGT1 were obtained and phenotypes were analysed. Wiesner and Mäule staining showed that stem xylem of transgenic tobacco plants stained more strongly than controls. Measurement of the Klason lignins showed much higher lignin content in the transgenic lines than in control plants. Furthermore, the ectopic over-expression of PtGT1 in tobacco resulted in an early flowering phenotype. These findings offer a possible starting point towards better understanding of the function of poplar PtGT1, and provide a novel strategy for lignin engineering and flowering control in plants through the genetic manipulation of a poplar glycosyltransferase gene.

Biochemical characterization of xylan xylosyltransferases involved in wood formation in poplar

The major polysaccharides in dicot wood biomass are cellulose and xylan. Although wood-associated cellulose synthase genes responsible for cellulose biosynthesis have been characterized, wood-associated xylan synthase genes have not been biochemically identified. A recent report by Lee et al. (2012) provides the first biochemical evidence that two functionally non-redundant Arabidopsis GT43 members are xylosyltransferases (XylTs) that function cooperatively in the elongation of the xylan backbone. We further extend this finding in the current report demonstrating that two poplar (Populus trichocarpa) GT43 glycosyltransferases, PtrGT43B and PtrGT43C, are xylan XylTs involved in wood formation. We show that microsomes from transgenic tobacco BY2 cells coexpressing PtrGT43B and PtrGT43C exhibited a high XylT activity capable of generating β-(1,4)-linked xylooligosaccharides, whereas little XylT activity was detected in microsomes with expression of PtrGT43B or PtrGT43C alone. These findings indicate that poplar GT43 members are XylTs that act cooperatively in catalyzing the successive transfer of xylosyl residues during xylan backbone biosynthesis, which provides further support of the hypothesis that the biochemical functions of GT43 members in vascular plants are evolutionarily conserved.

Overexpression of constitutively active Arabidopsis RabG3b promotes xylem development in transgenic poplars

An Arabidopsis small GTPase, RabG3b, was previously characterized as a component of autophagy and as a positive regulator for xylem development in Arabidopsis. In this work, we assessed whether RabG3b modulates xylem-associated traits in poplar in a similar way as in Arabidopsis. We generated transgenic poplars (Populus alba × Populus tremula var. glandulosa) overexpressing a constitutively active form of RabG3b (RabG3bCA) and performed a range of morphological, histochemical and molecular analyses to examine xylogenesis. RabG3bCA transgenic poplars showed increased stem growth due to enhanced xylem development. Autophagic structures were observed in differentiating xyelm cells undergoing programmed cell death (PCD) in wild-type poplar, and were more abundant in RabG3bCA transgenic poplar plants and cultured cells. Xylogenic activation was also accompanied by the expression of secondary wall-, PCD- and autophagy-related genes. Collectively, our results suggest that Arabidopsis RabG3b functions to regulate xylem growth through the activation of autophagy during wood formation in Populus, as does the same in Arabidopsis.

The ability of land plants to synthesize glucuronoxylans predates the evolution of tracheophytes

Glucuronoxylans with a backbone of 1,4-linked β-D-xylosyl residues are ubiquitous in the secondary walls of gymnosperms and angiosperms. Xylans have been reported to be present in hornwort cell walls, but their structures have not been determined. In contrast, the presence of xylans in the cell walls of mosses and liverworts remains a subject of debate. Here we present data that unequivocally establishes that the cell walls of leafy tissue and axillary hair cells of the moss Physcomitrella patens contain a glucuronoxylan that is structurally similar to glucuronoxylans in the secondary cell walls of vascular plants. Some of the 1,4-linked β-D-xylopyranosyl residues in the backbone of this glucuronoxylan bear an α-D-glucosyluronic acid (GlcpA) sidechain at O-2. In contrast, the lycopodiophyte Selaginella kraussiana synthesizes a glucuronoxylan substituted with 4-O-Me-α-D-GlcpA sidechains, as do many hardwood species. The monilophyte Equisetum hyemale produces a glucuronoxylan with both 4-O-Me-α-D-GlcpA and α-D-GlcpA sidechains, as does Arabidopsis. The seedless plant glucuronoxylans contain no discernible amounts of the reducing-end sequence that is characteristic of gymnosperm and eudicot xylans. Phylogenetic studies showed that the P. patens genome contains genes with high sequence similarity to Arabidopsis CAZy family GT8, GT43 and GT47 glycosyltransferases that are likely involved in xylan synthesis. We conclude that mosses synthesize glucuronoxylan that is structurally similar to the glucuronoxylans present in the secondary cell walls of lycopodiophytes, monilophytes, and many seed-bearing plants, and that several of the glycosyltransferases required for glucuronoxylan synthesis evolved before the evolution of tracheophytes.

The four Arabidopsis reduced wall acetylation genes are expressed in secondary wall-containing cells and required for the acetylation of xylan

Xylan is one of the major polysaccharides in cellulosic biomass, and understanding the mechanisms underlying xylan biosynthesis will potentially help us design strategies to produce cellulosic biomass better suited for biofuel production. Although a number of genes have been shown to be essential for xylan biosynthesis, genes involved in the acetylation of xylan have not yet been identified. Here, we report the comprehensive genetic and functional studies of four Arabidopsis REDUCED WALL ACETYLATION (RWA) genes and demonstrate their involvement in the acetylation of xylan during secondary wall biosynthesis. It was found that the RWA genes were expressed in cells undergoing secondary wall thickening and their expression was regulated by SND1, a transcriptional master switch of secondary wall biosynthesis. The RWA proteins were shown to be localized in the Golgi, where xylan biosynthesis occurs. Analyses of a suite of single, double, triple and quadruple rwa mutants revealed a significant reduction in the secondary wall thickening and the stem mechanical strength in the quadruple rwa1/2/3/4 mutant but not in other mutants. Further chemical and structural analyses of xylan demonstrated that the rwa1/2/3/4 mutations resulted in a reduction in the amount of acetyl groups on xylan. In addition, the ratio of non-methylated to methylated glucuronic acid side chains was altered in the rwa1/2/3/4 mutant. Together, our results demonstrate that the four Arabidopsis RWA genes function redundantly in the acetylation of xylan during secondary wall biosynthesis.

Molecular dissection of xylan biosynthesis during wood formation in poplar

Xylan, being the second most abundant polysaccharide in dicot wood, is considered to be one of the factors contributing to wood biomass recalcitrance for biofuel production. To better utilize wood as biofuel feedstock, it is crucial to functionally characterize all the genes involved in xylan biosynthesis during wood formation. In this report, we investigated roles of poplar families GT43 and GT8 glycosyltransferases in xylan biosynthesis during wood formation. There exist seven GT43 genes in the genome of poplar (Populus trichocarpa), five of which, namely PtrGT43A, PtrGT43B, PtrGT43C, PtrGT43D, and PtrGT43E, were shown to be highly expressed in the developing wood and their encoded proteins were localized in the Golgi. Comprehensive genetic complementation coupled with chemical analyses demonstrated that overexpression of PtrGT43A/B/E but not PtrGT43C/D was able to rescue the xylan defects conferred by the Arabidopsis irx9 mutant, whereas overexpression of PtrGT43C/D but not PtrGT43A/B/E led to a complementation of the xylan defects in the Arabidopsis irx14 mutant. The essential roles of poplar GT43 members in xylan biosynthesis was further substantiated by RNAi down-regulation of GT43B in the hybrid poplar (Populus alba x tremula) leading to reductions in wall thickness and xylan content in wood, and an elevation in the abundance of the xylan reducing end sequence. Wood digestibility analysis revealed that cellulase digestion released more glucose from the wood of poplar GT43B RNAi lines than the control wood, indicating a decrease in wood biomass recalcitrance. Furthermore, RNAi down-regulation of another poplar wood-associated glycosyltransferase, PoGT8D, was shown to cause decreases in wall thickness and xylan content as well as in the abundance of the xylan reducing end sequence. Together, these findings demonstrate that the poplar GT43 members form two functionally non-redundant groups, namely PtrGT43A/B/E as functional orthologs of Arabidopsis IRX9 and PtrGT43C/D as functional orthologs of Arabidopsis IRX14, all of which are involved in the biosynthesis of xylan backbones, and that the poplar GT8D is essential for the biosynthesis of the xylan reducing end sequence.

Down-regulation of glycosyltransferase 8D genes in Populus trichocarpa caused reduced mechanical strength and xylan content in wood

Members of glycosyltransferase protein families GT8, GT43 and GT47 are implicated in the biosynthesis of xylan in the secondary cell walls of Arabidopsis. The Arabidopsis mutant irx8 has a 60% reduction in xylan. However, over-expression of an ortholog of Arabidopsis IRX8, poplar PoGT8D, in Arabidopsis irx8 mutant could not restore xylan synthesis. The functions of tree GT8D genes remain unclear. We identified two GT8 gene homologs, PtrGT8D1 and PtrGT8D2, in Populus trichocarpa. They are the only two GT8D members and are abundantly and specifically expressed in the differentiating xylem of P. trichocarpa. PtrGT8D1 transcript abundance was >7 times that of PtrGT8D2. To elucidate the genetic function of GT8D in P. trichocarpa, the expression of PtrGT8D1 and PtrGT8D2 was simultaneously knocked down through RNAi. Four transgenic lines had 85-94% reduction in transcripts of PtrGT8D1 and PtrGT8D2, resulting in 29-36% reduction in stem wood xylan content. Xylan reduction had essentially no effect on cellulose quantity but caused an 11-25% increase in lignin. These transgenics exhibit a brittle wood phenotype, accompanied by increased vessel diameter and thinner fiber cell walls in stem xylem. Stem modulus of elasticity and modulus of rupture were reduced by 17-29% and 16-23%, respectively, and were positively correlated with xylan content but negatively correlated with lignin quantity. These results suggest that PtrGT8Ds play key roles in xylan biosynthesis in wood. Xylan may be a more important factor than lignin affecting the stiffness and fracture strength of wood.

The Arabidopsis family GT43 glycosyltransferases form two functionally nonredundant groups essential for the elongation of glucuronoxylan backbone

There exist four members of family GT43 glycosyltransferases in the Arabidopsis (Arabidopsis thaliana) genome, and mutations of two of them, IRX9 and IRX14, have previously been shown to cause a defect in glucuronoxylan (GX) biosynthesis. However, it is currently unknown whether IRX9 and IRX14 perform the same biochemical function and whether the other two GT43 members are also involved in GX biosynthesis. In this report, we performed comprehensive genetic analysis of the functional roles of the four Arabidopsis GT43 members in GX biosynthesis. The I9H (IRX9 homolog) and I14H (IRX14 homolog) genes were shown to be specifically expressed in cells undergoing secondary wall thickening, and their encoded proteins were targeted to the Golgi, where GX is synthesized. Overexpression of I9H but not IRX14 or I14H rescued the GX defects conferred by the irx9 mutation, whereas overexpression of I14H but not IRX9 or I9H complemented the GX defects caused by the irx14 mutation. Double mutant analyses revealed that I9H functioned redundantly with IRX9 and that I14H was redundant with IRX14 in their functions. In addition, double mutations of IRX9 and IRX14 were shown to cause a loss of secondary wall thickening in fibers and a much more severe reduction in GX amount than their single mutants. Together, these results provide genetic evidence demonstrating that all four Arabidopsis GT43 members are involved in GX biosynthesis and suggest that they form two functionally nonredundant groups essential for the normal elongation of GX backbone.

Arabidopsis - a powerful model system for plant cell wall research

Plant cell walls are composites of various carbohydrates, proteins and other compounds. Cell walls provide plants with strength and protection, and also represent the most abundant source of renewable biomass. Despite the importance of plant cell walls, comparatively little is known about the identities of genes and functions of proteins involved in their biosynthesis. The model plant Arabidopsis and the availability of its genome sequence have been invaluable for the identification and functional characterization of genes encoding enzymes involved in plant cell-wall biosynthesis. This review covers recent progress in the identification and characterization of genes encoding proteins involved in the biosynthesis of Arabidopsis cell-wall polysaccharides and arabinogalactan proteins. These studies have improved our understanding of both the mechanisms of cell-wall biosynthesis and the functions of various cell-wall polymers, and have highlighted areas where further research is needed.

The poplar GT8E and GT8F glycosyltransferases are functional orthologs of Arabidopsis PARVUS involved in glucuronoxylan biosynthesis

The poplar GT8E and GT8F glycosyltransferases have previously been shown to be associated with wood formation, but their roles in the biosynthesis of wood components are not known. Here, we show that PoGT8E and PoGT8F are expressed in vessels and fibers during wood formation and their encoded proteins are predominantly located in the endoplasmic reticulum. We demonstrate that expression of PoGT8E and PoGT8F in the Arabidopsis parvus mutant rescues the defects in the content and structure of glucuronoxylan conferred by the parvus mutation. These findings suggest that PoGT8E and PoGT8F are involved in glucuronoxylan biosynthesis during wood formation in poplar.

Two poplar glycosyltransferase genes, PdGATL1.1 and PdGATL1.2, are functional orthologs to PARVUS/AtGATL1 in Arabidopsis

Several genes in Arabidopsis, including PARVUS/AtGATL1, have been implicated in xylan synthesis. However, the biosynthesis of xylan in woody plants, where this polysaccharide is a major component of wood, is poorly understood. Here, we characterize two Populus genes, PdGATL1.1 and PdGATL1.2, the closest orthologs to the Arabidopsis PARVUS/GATL1 gene, with respect to their gene expression in poplar, their sub-cellular localization, and their ability to complement the parvus mutation in Arabidopsis. Overexpression of the two poplar genes in the parvus mutant rescued most of the defects caused by the parvus mutation, including morphological changes, collapsed xylem, and altered cell wall monosaccharide composition. Quantitative RT-PCR showed that PdGATL1.1 is expressed most strongly in developing xylem of poplar. In contrast, PdGATL1.2 is expressed much more uniformly in leaf, shoot tip, cortex, phloem, and xylem, and the transcript level of PdGATL1.2 is much lower than that of PdGATL1.1 in all tissues examined. Sub-cellular localization experiments showed that these two proteins are localized to both ER and Golgi in comparison with marker proteins resident to these sub-cellular compartments. Our data indicate that PdGATL1.1 and PdGATL1.2 are functional orthologs of PARVUS/GATL1 and can play a role in xylan synthesis, but may also have role(s) in the synthesis of other wall polymers.

Down-regulation of PoGT47C expression in poplar results in a reduced glucuronoxylan content and an increased wood digestibility by cellulase

Xylan is the second most abundant polysaccharide in dicot wood. Unraveling the biosynthetic pathway of xylan is important not only for our understanding of the process of wood formation but also for our rational engineering of wood for biofuel production. Although several glycosyltransferases are implicated in glucuronoxylan (GX) biosynthesis in Arabidopsis, whether their close orthologs in woody tree species are essential for GX biosynthesis during wood formation has not been investigated. In fact, no studies have been reported to evaluate the effects of alterations in secondary wall-associated glycosyltransferases on wood formation in tree species. In this report, we demonstrate that PoGT47C, a poplar glycosyltransferase belonging to family GT47, is essential for the normal biosynthesis of GX and the normal secondary wall thickening in the wood of the hybrid poplar Populus alba x tremula. RNA interference (RNAi) inhibition of PoGT47C resulted in a drastic reduction in the thickness of secondary walls, a deformation of vessels and a decreased amount of GX in poplar wood. Structural analysis of GX using nuclear magnetic resonance (NMR) spectroscopy demonstrated that the reducing end of GX from poplar wood contains the tetrasaccharide sequence, beta-d-Xylp-(1-->3)-alpha-l-Rhap-(1-->2)-alpha-d-GalpA-(1-->4)-d-Xylp, and that its abundance was significantly decreased in the GX from the wood of the GT47C-RNAi lines. The transgenic wood was found to yield more glucose by cellulase digestion than the wild-type wood, indicating that the GX reduction in wood reduces the recalcitrance of wood to cellulase digestion. Together, these results provide direct evidence demonstrating that the PoGT47C glycosyltransferase is essential for normal GX biosynthesis in poplar wood and that GX modification could improve the digestibility of wood cellulose by cellulase.

Enzymatic treatments reveal differential capacities for xylan recognition and degradation in primary and secondary plant cell walls

The capacity of four xylan-directed probes (carbohydrate-binding modules CfCBM2b-1-2 and CjCBM15; monoclonal antibodies LM10 and LM11) to recognize xylan polysaccharides in primary and secondary cell walls of tobacco stem sections has been determined. Enzymatic removal of pectic homogalacturonan revealed differential recognition of xylans in restricted regions of cortical primary cell walls. Monoclonal antibody binding to these exposed xylans was more sensitive to xylanase action than carbohydrate-binding module (CBM) binding. In contrast, the recognition of xylans by CBMs in secondary cell walls of the same organ was more sensitive to xylanase action than the recognition of xylans by the monoclonal antibodies. A methodology was developed to quantify indirect immunofluorescence intensities, and to evaluate xylanase impacts. The four xylan probes were also used to detect xylan populations in chromatographic separations of solubilized cell wall materials from tobacco stems. Altogether, these observations reveal the heterogeneity of the xylans in plant cell walls. They indicate that although CBM and antibody probes can exhibit similar specificities against solubilized polymers, they can have differential capacities for xylan recognition in muro, and that the access of molecular probes and enzymes to xylan epitopes/ligands also varies between primary and secondary cell walls that are present in the same organ.

Cloning and expression analysis of a wood-associated xylosidase gene (PtaBXL1) in poplar tension wood

In stems of woody angiosperms responding to mechanical stress, imposed for instance by tilting the stem or formation of a branch, tension wood (TW) forms above the affected part, while anatomically distinct opposite wood (OW) forms below it. In poplar TW the S3 layer of the secondary walls is substituted by a "gelatinous layer" that is almost entirely composed of cellulose and has much lower hemicellulose contents than unstressed wood. However, changes in xylan contents (the predominant hemicelluloses), their interactions with other wall components and the mechanisms involved in TW formation have been little studied. Therefore, in the study reported here we determined the structure and distribution of xylans, cloned the genes encoding the xylan remodeling enzymes beta-xylosidases (PtaBXLi), and examined their expression patterns during tension wood, normal wood and opposite wood xylogenesis in poplar. We confirm that poplar wood xylans are substituted solely by 4-O-methylglucuronic acid in both TW and OW. However, although glucuronoxylans are strongly represented in both primary and secondary layers of OW, no 4-O-methylGlcA xylan was found in G-layers of TW. Four full-length BXL cDNAs encoding putative beta-xylosidases were cloned. One, PtaBXL1, for which xylosidase activity was confirmed by heterologous expression in Escherichia coli, exhibited a wood-specific expression pattern in TW. In conclusion, xylan as PtaBXL1, encoding beta4-xylosidase activity, are down-regulated in TW.

In situ analysis of cell wall polymers associated with phloem fibre cells in stems of hemp, Cannabis sativa L

A study of stem anatomy and the sclerenchyma fibre cells associated with the phloem tissues of hemp (Cannabis sativa L.) plants is of interest for both understanding the formation of secondary cell walls and for the enhancement of fibre utility as industrial fibres and textiles. Using a range of molecular probes for cell wall polysaccharides we have surveyed the presence of cell wall components in stems of hemp in conjunction with an anatomical survey of stem and phloem fibre development. The only polysaccharide detected to occur abundantly throughout the secondary cell walls of phloem fibres was cellulose. Pectic homogalacturonan epitopes were detected in the primary cell walls/intercellular matrices between the phloem fibres although these epitopes were present at a lower level than in the surrounding parenchyma cell walls. Arabinogalactan-protein glycan epitopes displayed a diversity of occurrence in relation to fibre development and the JIM14 epitope was specific to fibre cells, binding to the inner surface of secondary cell walls, throughout development. Xylan epitopes were found to be present in the fibre cells (and xylem secondary cell walls) and absent from adjacent parenchyma cell walls. Analysis of xylan occurrence in the phloem fibre cells of hemp and flax indicated that xylan epitopes were restricted to the primary cell walls of fibre cells and were not present in the secondary cell walls of these cells.

Cell wall modifications in Arabidopsis plants with altered alpha-L-arabinofuranosidase activity

Although cell wall remodeling is an essential feature of plant growth and development, the underlying molecular mechanisms are poorly understood. This work describes the characterization of Arabidopsis (Arabidopsis thaliana) plants with altered expression of ARAF1, a bifunctional alpha-L-arabinofuranosidase/beta-D-xylosidase (At3g10740) belonging to family 51 glycosyl-hydrolases. ARAF1 was localized in several cell types in the vascular system of roots and stems, including xylem vessels and parenchyma cells surrounding the vessels, the cambium, and the phloem. araf1 T-DNA insertional mutants showed no visible phenotype, whereas transgenic plants that overexpressed ARAF1 exhibited a delay in inflorescence emergence and altered stem architecture. Although global monosaccharide analysis indicated only slight differences in cell wall composition in both mutant and overexpressing lines, immunolocalization experiments using anti-arabinan (LM6) and anti-xylan (LM10) antibodies indicated cell type-specific alterations in cell wall structure. In araf1 mutants, an increase in LM6 signal intensity was observed in the phloem, cambium, and xylem parenchyma in stems and roots, largely coinciding with ARAF1 expression sites. The ectopic overexpression of ARAF1 resulted in an increase in LM10 labeling in the secondary walls of interfascicular fibers and xylem vessels. The combined ARAF1 gene expression and immunolocalization studies suggest that arabinan-containing pectins are potential in vivo substrates of ARAF1 in Arabidopsis.

The PARVUS gene is expressed in cells undergoing secondary wall thickening and is essential for glucuronoxylan biosynthesis

Xylan, cellulose and lignin are the three major components of secondary walls in wood, and elucidation of the biosynthetic pathway of xylan is of importance for potential modification of secondary wall composition to produce wood with improved properties. So far, three Arabidopsis glycosyltransferases, FRAGILE FIBER8, IRREGULAR XYLEM8 and IRREGULAR XYLEM9, have been implicated in glucuronoxylan (GX) biosynthesis. In this study, we demonstrate that PARVUS, which is a member of family GT8, is required for the biosynthesis of the tetrasaccharide primer sequence, beta-D-Xyl-(1 --> 3)-alpha-l-Rha-(1 --> 2)-alpha-D-GalA-(1 --> 4)-D-Xyl, located at the reducing end of GX. The PARVUS gene is expressed during secondary wall biosynthesis in fibers and vessels, and its encoded protein is predominantly localized in the endoplasmic reticulum. Mutation of the PARVUS gene leads to a drastic reduction in secondary wall thickening and GX content. Structural analysis of GX using (1)H-nuclear magnetic resonance (NMR) spectroscopy revealed that the parvus mutation causes a loss of the tetrasaccharide primer sequence at the reducing end of GX and an absence of glucuronic acid side chains in GX. Activity assay showed that the xylan xylosyltransferase and glucuronyltransferase activities were not affected in the parvus mutant. Together, these findings implicate a possible role for PARVUS in the initiation of biosynthesis of the GX tetrasaccharide primer sequence and provide novel insights into the mechanisms of GX biosynthesis.

Disruption of ATCSLD5 results in reduced growth, reduced xylan and homogalacturonan synthase activity and altered xylan occurrence in Arabidopsis

Members of a large family of cellulose synthase-like genes (CSLs) are predicted to encode glycosyl transferases (GTs) involved in the biosynthesis of plant cell walls. The CSLA and CSLF families are known to contain mannan and glucan synthases, respectively, but the products of other CSLs are unknown. Here we report the effects of disrupting ATCSLD5 expression in Arabidopsis. Both stem and root growth were significantly reduced in ATCSLD5 knock-out plants, and these plants also had increased susceptibility to the cellulose synthase inhibitor isoxaben. Antibody and carbohydrate-binding module labelling indicated a reduction in the level of xylan in stems, and in vitro GT assays using microsomes from stems revealed that ATCSLD5 knock-out plants also had reduced xylan and homogalacturonan synthase activity. Expression in Nicotiana benthamiana of ATCSLD5 and ATCSLD3, fluorescently tagged at either the C- or the N-terminal, indicated that these GTs are likely to be localized in the Golgi apparatus. However, the position of the fluorescent tag affected the subcellular localization of both proteins. The work presented provides a comprehensive analysis of the effects of disrupting ATCSLD5 in planta, and the possible role(s) of this gene and other ATCSLDs in cell wall biosynthesis are discussed.

The irregular xylem9 mutant is deficient in xylan xylosyltransferase activity

Xylan is the second most abundant polysaccharide in dicot wood, and thus elucidation of the xylan biosynthetic pathway is required to understand the mechanisms controlling wood formation. Genetic and chemical studies in Arabidopsis have implicated three genes, FRAGILE FIBER8 (FRA8), IRREGULAR XYLEM8 (IRX8) and IRREGULAR XYLEM9 (IRX9), in the biosynthesis of glucuronoxylan (GX), but the biochemical functions of the encoded proteins are not known. In this study, we determined the effect of the fra8, irx8 and irx9 mutations on the activities of xylan xylosyltransferase (XylT) and glucuronyltransferase (GlcAT). We show that microsomes isolated from the stems of wild-type Arabidopsis exhibit XylT and GlcAT activities in the presence of exogenous 1,4-linked beta-d-xylooligomers. Xylooligomers ranging in size from two to six can be used as acceptors by XylT to form xylooligosaccharides with up to 12 xylosyl residues. We provide evidence that the irx9 mutation results in a substantial reduction in XylT activity but has no discernible effect on GlcAT activity. In contrast, neither XylT nor GlcAT activity is affected by fra8 and irx8 mutations. Our results provide biochemical evidence that the irx9 mutation results in a deficiency in xylan XylT activity, thus leading to a defect in the elongation of the xylan backbone.

Two NAC domain transcription factors, SND1 and NST1, function redundantly in regulation of secondary wall synthesis in fibers of Arabidopsis

Secondary walls are the major component of wood, and studies of the mechanisms regulating secondary wall synthesis is important for understanding the process of wood formation. We have previously shown that the NAC domain transcription factor SECONDARY WALL-ASSOCIATED NAC DOMAIN PROTEIN1 (SND1) is a key regulator of secondary wall synthesis in fibers of Arabidopsis thaliana stems and dominant repression of SND1 leads to a reduction in secondary wall thickening in fibers. However, T-DNA knockout of the SND1 gene did not cause an alteration in secondary wall thickness, suggesting that other SND1 homologs may compensate for the loss of SND1 expression. Here, we studied the effects of simultaneous inhibition of SND1 and its homolog, NAC SECONDARY WALL THICKENING PROMOTING FACTOR1 (NST1), on secondary wall synthesis in fibers. We show that simultaneous RNA interference (RNAi) inhibition of the expression of both SND1 and NST1 genes results in loss of secondary wall formation in fibers of stems. The fiber cells in the stems of SND1/NST1-RNAi plants lack all three major secondary wall components, including cellulose, xylan, and lignin, which is accompanied by a severe reduction in the expression of genes involved in their biosynthesis. In addition, inhibition of SND1 and NST1 leads to down-regulation of several fiber-associated transcription factor genes. Double T-DNA knockout mutations of SND1 and NST1 genes cause the same effects, as does simultaneous RNAi inhibition of SND1 and NST1. Our results provide first line evidence demonstrating that SND1 and NST1 function redundantly in the regulation of secondary wall synthesis in fibers.

Molecular characterization of PoGT8D and PoGT43B, two secondary wall-associated glycosyltransferases in poplar

Dicot wood is mainly composed of cellulose, lignin and glucuronoxylan (GX). Although the biosynthetic genes for cellulose and lignin have been studied intensively, little is known about the genes involved in the biosynthesis of GX during wood formation. Here, we report the molecular characterization of two genes, PoGT8D and PoGT43B, which encode putative glycosyltransferases, in the hybrid poplar Populus alba x tremula. The predicted amino acid sequences of PoGT8D and PoGT43B exhibit 89 and 75% similarity to the Arabidopsis thaliana IRREGULAR XYLEM8 (IRX8) and IRX9, respectively, both of which have been shown to be required for GX biosynthesis. The PoGT8D and PoGT43B genes were found to be expressed in cells undergoing secondary wall thickening, including the primary xylem, secondary xylem and phloem fibers in stems, and the secondary xylem in roots. Both PoGT8D and PoGT43B are predicted to be type II membrane proteins and shown to be targeted to Golgi. Overexpression of PoGT43B in the irx9 mutant was able to rescue the defects in plant size and secondary wall thickness and partially restore the xylose content. Taken together, our results demonstrate that PoGT8D and PoGT43B are Golgi-localized, secondary wall-associated proteins, and PoGT43B is a functional ortholog of IRX9 involved in GX biosynthesis during wood formation.

Arabidopsis irregular xylem8 and irregular xylem9: implications for the complexity of glucuronoxylan biosynthesis

Mutations of Arabidopsis thaliana IRREGULAR XYLEM8 (IRX8) and IRX9 were previously shown to cause a collapsed xylem phenotype and decreases in xylose and cellulose in cell walls. In this study, we characterized IRX8 and IRX9 and performed chemical and structural analyses of glucuronoxylan (GX) from irx8 and irx9 plants. IRX8 and IRX9 are expressed specifically in cells undergoing secondary wall thickening, and their encoded proteins are targeted to the Golgi, where GX is synthesized. 1H-NMR spectroscopy showed that the reducing end of Arabidopsis GX contains the glycosyl sequence 4-beta-D-Xylp-(1-->4)-beta-D-Xylp-(1-->3)-alpha-L-Rhap-(1-->2)-alpha-D-GalpA-(1-->4)-D-Xylp, which was previously identified in birch (Betula verrucosa) and spruce (Picea abies) GX. This indicates that the reducing end structure of GXs is evolutionarily conserved in woody and herbaceous plants. This sequence is more abundant in irx9 GX than in the wild type, whereas irx8 and fragile fiber8 (fra8) plants are nearly devoid of it. The number of GX chains increased and the GX chain length decreased in irx9 plants. Conversely, the number of GX chains decreased and the chain length heterodispersity increased in irx8 and fra8 plants. Our results suggest that IRX9 is required for normal GX elongation and indicate roles for IRX8 and FRA8 in the synthesis of the glycosyl sequence at the GX reducing end.