Xyloglucan for generating tensile stress to bend tree stem

Conserved autophagy and diverse cell wall composition: unifying features of vascular tissues in evolutionarily distinct plants

BACKGROUND AND AIMS

The formation of multifunctional vascular tissues represents a significant advancement in plant evolution. Differentiation of conductive cells is specific, involving two main pathways, namely protoplast clearance and cell wall modification. In xylogenesis, autophagy is a crucial process for complete protoplast elimination in tracheary elements, whose cell wall also undergoes strong changes. Knowledge pertaining to living sieve elements, which lose most of their protoplast during phloemogenesis, remains limited. We hypothesized that autophagy plays a crucial role, not only in complete cytoplasmic clearance in xylem but also in partial degradation in phloem. Cell wall elaborations of mature sieve elements are not so extensive. These analyses performed on evolutionarily diverse model species potentially make it possible to understand phloemogenesis to an equal extent to xylogenesis.

METHODS

We investigated the distribution of ATG8 protein, which is an autophagy marker, and cell wall components in the roots of ferns, gymnosperms and angiosperms (monocots, dicot herbaceous plants and trees). Furthermore, we conducted a bioinformatic analysis of complete data on ATG8 isoforms for Ceratopteris richardii.

KEY RESULTS

The presence of ATG8 protein was confirmed in both tracheary elements and sieve elements; however, the composition of cell wall components varied considerably among vascular tissues in the selected plants. Arabinogalactan proteins and β-1,4-galactan were detected in the roots of all studied species, suggesting their potential importance in phloem formation or function. In contrast, no evolutionary pattern was observed for xyloglucan, arabinan or homogalacturonan.

CONCLUSIONS

Our findings indicate that the involvement of autophagy in plants is universal during the development of tracheary elements that are dead at maturity and sieve elements that remain alive. Given the conserved nature of autophagy and its function in protoplast degradation for uninterrupted flow, autophagy might have played a vital role in the development of increasingly complex biological organizations, including the formation of vascular tissues. However, different cell wall compositions of xylem and phloem in different species might indicate diverse functionality and potential for substance transport, which is crucial in plant evolution.

Comparative transcriptomics of tropical woody plants supports fast and furious strategy along the leaf economics spectrum in lianas

Lianas, climbing woody plants, influence the structure and function of tropical forests. Climbing traits have evolved multiple times, including ancestral groups such as gymnosperms and pteridophytes, but the genetic basis of the liana strategy is largely unknown. Here, we use a comparative transcriptomic approach for 47 tropical plant species, including ten lianas of diverse taxonomic origins, to identify genes that are consistently expressed or downregulated only in lianas. Our comparative analysis of full-length transcripts enabled the identification of a core interactomic network common to lianas. Sets of transcripts identified from our analysis reveal features related to functional traits pertinent to leaf economics spectrum in lianas, include upregulation of genes controlling epidermal cuticular properties, cell wall remodeling, carbon concentrating mechanism, cell cycle progression, DNA repair and a large suit of downregulated transcription factors and enzymes involved in ABA-mediated stress response as well as lignin and suberin synthesis. All together, these genes are known to be significant in shaping plant morphologies through responses such as gravitropism, phyllotaxy and shade avoidance.

Summary: The full-length fraction of liana transcriptomes mapped on a protein–protein interactome revealed the nature of their convergence through distinct sets of expressed and downregulated genes not observed in free-standing plants.

Do woody vines use gelatinous fibers to climb?

Many plant movements are facilitated by contractile cells called gelatinous fibers (G-fibers), but how G-fibers function in the climbing movements of woody vines remains underexplored. In this Insight, we compare the presence and distribution of G-fibers in the stems of stem-twiners, which wrap around supports, with non-stem-twiners, which attach to supports via tendrils or adventitious roots. An examination of 164 species spanning the vascular plant phylogeny reveals that G-fibers are common in stem-twiners but scarce in non-stem-twiners, suggesting that G-fibers are preferentially formed in the organ responsible for movement. When present, G-fibers are in the xylem, phloem, pericycle, and/or cortex. We discuss the hypothesis that G-fibers are foundational to plant movement and highlight research opportunities concerning G-fiber development and function.

Prior secondary cell wall formation is required for gelatinous layer deposition and posture control in gravi-stimulated aspen

Cell walls, especially secondary cell walls (SCWs), maintain cell shape and reinforce wood, but their structure and shape can be altered in response to gravity. In hardwood trees, tension wood is formed along the upper side of a bending stem and contains wood fiber cells that have a gelatinous layer (G-layer) inside the SCW. In a previous study, we generated nst/snd quadruple-knockout aspens (Populus tremula × Populus tremuloides), in which SCW formation was impaired in 99% of the wood fiber cells. In the present study, we produced nst/snd triple-knockout aspens, in which a large number of wood fibers had thinner SCWs than the wild type (WT) and some had no SCW. Because SCW layers are always formed prior to G-layer deposition, the nst/snd mutants raise interesting questions of whether the mutants can form G-layers without SCW and whether they can control their postures in response to changes in gravitational direction. The nst/snd mutants and the WT plants showed growth eccentricity and vessel frequency reduction when grown on an incline, but the triple mutants recovered their upright growth only slightly, and the quadruple mutants were unable to maintain their postures. The mutants clearly showed that the G-layers were formed in SCW-containing wood fibers but not in those lacking the SCW. Our results indicate that SCWs are essential for G-layer formation and posture control. Furthermore, each wood fiber cell may be able to recognize its cell wall developmental stage to initiate the formation of the G-layer as a response to gravistimulation.

Tailoring renewable materials via plant biotechnology

Plants inherently display a rich diversity in cell wall chemistry, as they synthesize an array of polysaccharides along with lignin, a polyphenolic that can vary dramatically in subunit composition and interunit linkage complexity. These same cell wall chemical constituents play essential roles in our society, having been isolated by a variety of evolving industrial processes and employed in the production of an array of commodity products to which humans are reliant. However, these polymers are inherently synthesized and intricately packaged into complex structures that facilitate plant survival and adaptation to local biogeoclimatic regions and stresses, not for ease of deconstruction and commercial product development. Herein, we describe evolving techniques and strategies for altering the metabolic pathways related to plant cell wall biosynthesis, and highlight the resulting impact on chemistry, architecture, and polymer interactions. Furthermore, this review illustrates how these unique targeted cell wall modifications could significantly extend the number, diversity, and value of products generated in existing and emerging biorefineries. These modifications can further target the ability for processing of engineered wood into advanced high performance materials. In doing so, we attempt to illuminate the complex connection on how polymer chemistry and structure can be tailored to advance renewable material applications, using all the chemical constituents of plant-derived biopolymers, including pectins, hemicelluloses, cellulose, and lignins.

Various effects of the expression of the xyloglucanase gene from Penicillium canescens in transgenic aspen under semi-natural conditions

BACKGROUND

Recombinant carbohydrases genes are used to produce transgenic woody plants with improved phenotypic traits. However, cultivation of such plants in open field is challenging due to a number of problems. Therefore, additional research is needed to alleviate them.

RESULTS

Results of successful cultivation of the transgenic aspens (Populus tremula) carrying the recombinant xyloglucanase gene (sp-Xeg) from Penicillium canescens in semi-natural conditions are reported in this paper for the first time. Change of carbohydrate composition of wood was observed in transgenic aspens carrying the sp-Xeg gene. The transformed transgenic line Xeg-2-1b demonstrated accelerated growth and increased content of cellulose in wood of trees growing in both greenhouse and outside in comparison with the control untransformed line Pt. The accelerated growth was observed also in the transgenic line Xeg-1-1c. Thicker cell-wall and longer xylem fiber were also observed in both these transgenic lines. Undescribed earlier considerable reduction in the wood decomposition rate of the transgenic aspen stems was also revealed for the transformed transgenic lines. The decomposition rate was approximately twice as lower for the transgenic line Xeg-2-3b in comparison with the control untransformed line Pt.

CONCLUSION

A direct dependence of the phenotypic and biochemical traits on the expression of the recombinant gene sp-Xeg was demonstrated. The higher was the level of the sp-Xeg gene expression, the more pronounced were changes in the phenotypic and biochemical traits. All lines showed phenotypic changes in the leave traits. Our results showed that the plants carrying the recombinant sp-Xeg gene do not demonstrate a decrease in growth parameters in semi-natural conditions. In some transgenic lines, a change in the carbohydrate composition of the wood, an increase in the cell wall thickness, and a decrease in the rate of decomposition of wood were observed.

Immunolocalization of β-(1-4)-D-galactan, xyloglucans and xylans in the reaction xylem fibres of Leucaena leucocephala (Lam.) de Wit

Cell wall architecture of tension wood fibres represents a suitable biological system to study the mechanism of growth and maintenance of posture of trees growing under various physical and physiological growth constraints. In the present study, we investigated the spatial distributions of β-(1-4)-D-galactan, xyloglucan and xylans (both less and highly substituted) in the opposite and tension wood fibres of bent Leucaena leucocephala by immunolabelling with monoclonal antibodies LM5, CCRCM1, LM10 and LM11 specific to these carbohydrate epitopes. The presence of non-lignified, tertiary wall layer is the typical tension wood characteristic associated with the reaction xylem fibres in Leucaena. LM5 labelling of opposite fibres showed weak labelling in the cell walls indicating less concentration of β-(1-4)-D-galactans while tension wood showed strong labelling in the tertiary wall layer suggesting the gelatinous layer (G-layer) has a strong cross linking with β-(1-4)-D-galactans. Xyloglucan distribution was more in the compound middle lamellae and the primary wall-S1 layer boundary of tension wood fibres as compared to that of opposite wood. A weak labelling was also evident near the boundary between the G-layer and the secondary wall of tension wood fibres. The secondary wall of opposite and tension wood fibres showed a strong distribution of both ls ACG Xs (LM10) and hs ACG Xs (LM11) while a weak labelling was noticed in the compound middle lamella. Tension wood fibres also showed strong xylan labelling mainly confined to the lignified secondary walls while the G-layer showed weak xylan labelling. In conclusion, our results suggest that β-(1-4)-D-galactans and xyloglucans could be implicated in the tensile stress generation within the G-layer of tension wood fibres of Leucaena leucocephala.

Engineering Non-cellulosic Polysaccharides of Wood for the Biorefinery

Non-cellulosic polysaccharides constitute approximately one third of usable woody biomass for human exploitation. In contrast to cellulose, these substances are composed of several different types of unit monosaccharides and their backbones are substituted by various groups. Their structural diversity and recent examples of their modification in transgenic plants and mutants suggest they can be targeted for improving wood-processing properties, thereby facilitating conversion of wood in a biorefinery setting. Critical knowledge on their structure-function relationship is slowly emerging, although our understanding of molecular interactions responsible for observed phenomena is still incomplete. This review: (1) provides an overview of structural features of major non-cellulosic polysaccharides of wood, (2) describes the fate of non-cellulosic polysaccharides during biorefinery processing, (3) shows how the non-cellulosic polysaccharides impact lignocellulose processing focused on yields of either sugars or polymers, and (4) discusses outlooks for the improvement of tree species for biorefinery by modifying the structure of non-cellulosic polysaccharides.

Plant 'muscles': fibers with a tertiary cell wall

Plants, although sessile organisms, are nonetheless able to move their body parts; for example, during root contraction of geophytes or in the gravitropic reaction by woody stems. One of the major mechanisms enabling these movements is the development of specialized structures that possess contractile properties. Quite unlike animal muscles, for which the action is driven by protein-protein interactions in the protoplasma, the action of plant 'muscles' is polysaccharide-based and located in the uniquely designed, highly cellulosic cell wall that is deposited specifically in fibers. This review describes the development of such cell walls as a widespread phenomenon in the plant kingdom, gives reasons why it should be considered as a tertiary cell wall, and discusses the mechanism of action of the 'muscles'. The origin of the contractile properties lies in the tension of the axially oriented cellulose microfibrils due to entrapment of rhamnogalacturonan-I aggregates that limits the lateral interaction of microfibrils. Long side chains of the nascent rhamnogalacturonan-I are trimmed off during cell wall maturation leading to tension development. Similarities in the tertiary cell wall design in fibers of different plant origin indicate that the basic principles of tension creation may be universal in various ecophysiological situations.

Tension wood structure and morphology conducive for better enzymatic digestion

BACKGROUND

Tension wood is a type of reaction wood in response to bending or leaning stem as a corrective growth process. Tension wood is formed by both natural and man-made processes. Most attractively, tension wood contains higher glucan content and undergoes higher enzymatic conversion to fermentable sugars. Here, we have employed structural techniques, small-angle neutron scattering (SANS) and wide-angle X-ray diffraction (WAXD) to elucidate structural and morphological aspects of tension wood conducive to higher sugar yields.

RESULTS

Small-angle neutron scattering data exhibited a tri-modal distribution of the fibril cross-sectional dimension. The smallest size, 22 Å observed in all samples concurred with the WAXD results of the control and opposite side samples. This smallest and the most abundant occurring size was interpreted as the cellulose elementary microfibril diameter. The intermediate size of 45 Å, which is most pronounced in the tension side sample and consistent with WAXD results for tension side sample, indicates association of neighboring elementary microfibrils to form larger crystallite bundles. The largest size 61 Å observed by SANS was however not observed by WAXD and therefore associated to mesopores.

CONCLUSIONS

Structure and morphology of tension wood is different from control wood. Cellulose crystallinity increases, lignin content is lower and the appearance of mesopores with 61 Å diameter is observed. Despite the presence of higher crystalline cellulose content in tension side, the lower lignin content and may be combined with the abundance of mesopores, substantially improves enzyme accessibility leading to higher yields in cellulose digestion.

A Cell Wall Proteome and Targeted Cell Wall Analyses Provide Novel Information on Hemicellulose Metabolism in Flax

Experimentally-generated (nanoLC-MS/MS) proteomic analyses of four different flax organs/tissues (inner-stem, outer-stem, leaves and roots) enriched in proteins from 3 different sub-compartments (soluble-, membrane-, and cell wall-proteins) was combined with publically available data on flax seed and whole-stem proteins to generate a flax protein database containing 2996 nonredundant total proteins. Subsequent multiple analyses (MapMan, CAZy, WallProtDB and expert curation) of this database were then used to identify a flax cell wall proteome consisting of 456 nonredundant proteins localized in the cell wall and/or associated with cell wall biosynthesis, remodeling and other cell wall related processes. Examination of the proteins present in different flax organs/tissues provided a detailed overview of cell wall metabolism and highlighted the importance of hemicellulose and pectin remodeling in stem tissues. Phylogenetic analyses of proteins in the cell wall proteome revealed an important paralogy in the class IIIA xyloglucan endo-transglycosylase/hydrolase (XTH) family associated with xyloglucan endo-hydrolase activity.Immunolocalisation, FT-IR microspectroscopy, and enzymatic fingerprinting indicated that flax fiber primary/S1 cell walls contained xyloglucans with typical substituted side chains as well as glucuronoxylans in much lower quantities. These results suggest a likely central role of xyloglucans and endotransglucosylase/hydrolase activity in flax fiber formation and cell wall remodeling processes.

Protein expression in tension wood formation monitored at high tissue resolution in Populus

Tension wood (TW) is a specialized tissue with contractile properties that is formed by the vascular cambium in response to gravitational stimuli. We quantitatively analysed the proteomes of Populus tremula cambium and its xylem cell derivatives in stems forming normal wood (NW) and TW to reveal the mechanisms underlying TW formation. Phloem-, cambium-, and wood-forming tissues were sampled by tangential cryosectioning and pooled into nine independent samples. The proteomes of TW and NW samples were similar in the phloem and cambium samples, but diverged early during xylogenesis, demonstrating that reprogramming is an integral part of TW formation. For example, 14-3-3, reactive oxygen species, ribosomal and ATPase complex proteins were found to be up-regulated at early stages of xylem differentiation during TW formation. At later stages of xylem differentiation, proteins involved in the biosynthesis of cellulose and enzymes involved in the biosynthesis of rhamnogalacturonan-I, rhamnogalacturonan-II, arabinogalactan-II and fasciclin-like arabinogalactan proteins were up-regulated in TW. Surprisingly, two isoforms of exostosin family proteins with putative xylan xylosyl transferase function and several lignin biosynthesis proteins were also up-regulated, even though xylan and lignin are known to be less abundant in TW than in NW. These data provided new insight into the processes behind TW formation.

Gravitropisms and reaction woods of forest trees - evolution, functions and mechanisms

Contents 790 I. 790 II. 792 III. 795 IV. 797 V. 798 VI. 800 VII. 800 800 References 800 SUMMARY: The woody stems of trees perceive gravity to determine their orientation, and can produce reaction woods to reinforce or change their position. Together, graviperception and reaction woods play fundamental roles in tree architecture, posture control, and reorientation of stems displaced by wind or other environmental forces. Angiosperms and gymnosperms have evolved strikingly different types of reaction wood. Tension wood of angiosperms creates strong tensile force to pull stems upward, while compression wood of gymnosperms creates compressive force to push stems upward. In this review, the general features and evolution of tension wood and compression wood are presented, along with descriptions of how gravitropisms and reaction woods contribute to the survival and morphology of trees. An overview is presented of the molecular and genetic mechanisms underlying graviperception, initial graviresponse and the regulation of tension wood development in the model angiosperm, Populus. Critical research questions and new approaches are discussed.

Investigating the molecular underpinnings underlying morphology and changes in carbon partitioning during tension wood formation in Eucalyptus

Tension wood has distinct physical and chemical properties, including altered fibre properties, cell wall composition and ultrastructure. It serves as a good system for investigating the genetic regulation of secondary cell wall biosynthesis and wood formation. The reference genome sequence for Eucalyptus grandis allows investigation of the global transcriptional reprogramming that accompanies tension wood formation in this global wood fibre crop. We report the first comprehensive analysis of physicochemical wood property changes in tension wood of Eucalyptus measured in a hybrid (E. grandis × Eucalyptus urophylla) clone, as well as genome-wide gene expression changes in xylem tissues 3 wk post-induction using RNA sequencing. We found that Eucalyptus tension wood in field-grown trees is characterized by an increase in cellulose, a reduction in lignin, xylose and mannose, and a marked increase in galactose. Gene expression profiling in tension wood-forming tissue showed corresponding down-regulation of monolignol biosynthetic genes, and differential expression of several carbohydrate active enzymes. We conclude that alterations of cell wall traits induced by tension wood formation in Eucalyptus are a consequence of a combination of down-regulation of lignin biosynthesis and hemicellulose remodelling, rather than the often proposed up-regulation of the cellulose biosynthetic pathway.

Glycoside Hydrolase Activities in Cell Walls of Sclerenchyma Cells in the Inflorescence Stems of Arabidopsis thaliana Visualized in Situ

Techniques for in situ localization of gene products provide indispensable information for understanding biological function. In the case of enzymes, biological function is directly related to activity, and therefore, knowledge of activity patterns is central to understanding the molecular controls of plant development. We have previously developed a novel type of fluorogenic substrate for revealing glycoside hydrolase activity in planta, based on resorufin β-glycosides Here, we explore a wider range of such substrates to visualize glycoside hydrolase activities in Arabidopsis inflorescence stems in real time, especially highlighting distinct distribution patterns of these activities in the secondary cell walls of sclerenchyma cells. The results demonstrate that β-1,4-glucosidase, β-1,4-glucanase and β-1,4-galactosidase activities accompany secondary wall deposition. In contrast, xyloglucanase activity follows a different pattern, with the highest signal observed in mature cells, concentrated in the middle lamella. These data further the understanding of the process of cell wall deposition and function in sclerenchymatic tissues of plants.

Comprehensive transcriptome analysis of developing xylem responding to artificial bending and gravitational stimuli in Betula platyphylla

Betula platyphylla Suk (birch) is a fast-growing woody species that is important in pulp industries and the biofuels. However, as an important pulp species, few studies had been performed on its wood formation. In the present study, we investigated the molecular responses of birch xylem to artificial bending and gravitational stimuli. After trunks of birch trees were subjected to bending for 8 weeks, the cellulose content was significantly greater in tension wood (TW) than in opposite wood (OW) or normal wood (NW), whereas the lignin content in TW was significantly lower than that in OW and NW. In addition, TW grew more rapidly than OW and generated TW-specific fibers with an additional G-layer. Three transcriptome libraries were constructed from TW, OW and NW of B. platyphylla, respectively, after the plants were subjected to artificial bending. Overall, 80,909 nonredundant unigenes with a mean size of 768 nt were assembled. Expression profiles were generated, and 9,684 genes were found to be significantly differentially expressed among the TW, OW and NW libraries. These included genes involved in secondary cell wall structure, wood composition, and cellulose or lignin biosynthesis. Our study showed that during TW formation, genes involved in cellulose synthesis were induced, while the expression of lignin synthesis-related genes decreased, resulting in increased cellulose content and decreased lignin levels in TW. In addition, fasciclin-like arabinogalactan proteins play important role in TW formation. These findings may provide important insights into wood formation at the molecular level.

Deposition and organisation of cell wall polymers during maturation of poplar tension wood by FTIR microspectroscopy

To advance our understanding of the formation of tension wood, we investigated the macromolecular arrangement in cell walls by Fourier transform infrared microspectroscopy (FTIR) during maturation of tension wood in poplar (Populus tremula x P. alba, clone INRA 717-1B4). The relation between changes in composition and the deposition of the G-layer in tension wood was analysed. Polarised FTIR measurements indicated that in tension wood, already before G-layer formation, a more ordered structure of carbohydrates at an angle more parallel to the fibre axis exists. This was clearly different from the behaviour of opposite wood. With the formation of the S₂ layer in opposite wood and the G-layer in tension wood, the orientation signals from the amorphous carbohydrates like hemicelluloses and pectins were different between opposite wood and tension wood. For tension wood, the orientation for these bands remains the same all along the cell wall maturation process, probably reflecting a continued deposition of xyloglucan or xylan, with an orientation different to that in the S₂ wall throughout the whole process. In tension wood, the lignin was more highly oriented in the S₂ layer than in opposite wood.

Increase in cellulose accumulation and improvement of saccharification by overexpression of arabinofuranosidase in rice

Cellulosic biomass is available for the production of biofuel, with saccharification of the cell wall being a key process. We investigated whether alteration of arabinoxylan, a major hemicellulose in monocots, causes an increase in saccharification efficiency. Arabinoxylans have β-1,4-D-xylopyranosyl backbones and 1,3- or 1,4-α-l-arabinofuranosyl residues linked to O-2 and/or O-3 of xylopyranosyl residues as side chains. Arabinose side chains interrupt the hydrogen bond between arabinoxylan and cellulose and carry an ester-linked feruloyl substituent. Arabinose side chains are the base point for diferuloyl cross-links and lignification. We analyzed rice plants overexpressing arabinofuranosidase (ARAF) to study the role of arabinose residues in the cell wall and their effects on saccharification. Arabinose content in the cell wall of transgenic rice plants overexpressing individual ARAF full-length cDNA (OsARAF1-FOX and OsARAF3-FOX) decreased 25% and 20% compared to the control and the amount of glucose increased by 28.2% and 34.2%, respectively. We studied modifications of cell wall polysaccharides at the cellular level by comparing histochemical cellulose staining patterns and immunolocalization patterns using antibodies raised against α-(1,5)-linked l-Ara (LM6) and β-(1,4)-linked d-Xyl (LM10 and LM11) residues. However, they showed no visible phenotype. Our results suggest that the balance between arabinoxylan and cellulose might maintain the cell wall network. Moreover, ARAF overexpression in rice effectively leads to an increase in cellulose accumulation and saccharification efficiency, which can be used to produce bioethanol.

Functional characterization of a serine-threonine protein kinase from Bambusa balcooa that implicates in cellulose overproduction and superior quality fiber formation

BACKGROUND

Molecular markers allow rapid identification of biologically important germplasm/s having desired character. Previously we have reported a genotype specific molecular marker, Balco1128 [GenBank ID EU258678] of Bambusa balcooa containing an ORF (375 bp) having high similarity with receptor like cytoplasmic kinase of Arabidopsis and Oryza. Balco1128 was found to be associated only with bamboo genotypes endowed with high cellulose and low lignin contents of fibers. Under the above backdrop, it was necessitated to characterize this genetic marker for better understanding of its biological significance in context of superior quality fiber development.

RESULTS

The full length cDNA (3342 bp) of BbKst, a serine-threonine protein kinase was isolated from B. balcooa comprising of six LRR domains at the N-terminal end and a kinase domain at the C-terminal end. Bacteria-expressed BbKst-kinase domain (3339 bp long) showed Mg(2+) dependent kinase activity at pH 7.0, 28°C. Bioinformatics study followed by phospho-amino analysis further confirmed that BbKst-kinase belongs to the serine/threonine protein kinase family. Transcript analysis of the BbKst gene following RNA slot blot hybridization and qPCR revealed higher expression of BbKst during initiation and elongation stages of fiber development. Tissue specific expression studies showed much higher expression of BbKst transcript in stems and internodes of B. balcooa than in leaves and rhizomes. Southern analysis revealed single copy insertion of BbKst in most of the Agrobacterium mediated transgenic tobacco plants. Real-time PCR detected 150-200 fold enhanced expression of BbKst in different T1 tobacco lines than that of the vector transformed plants. Heterologous expression of BbKst under control of 35S promoter in transgenic tobacco showed high cellulose deposition in the xylem fibers. Number of xylary fibers was higher in transgenic T0 and T1 plants than that of empty-vector transformed tobacco plants offering enhanced mechanical strength to the transgenic plants, which was also substantiated by their strong upright phenotypes, significantly higher cellulose contents, flexibility coefficient, slenderness ratio, and lower Runkel ratio of the fibers.

CONCLUSIONS

This finding clearly demonstrated that BbKst gene (GenBank ID JQ432560) encodes a serine/threonine protein kinase. BbKst induced higher cellulose deposition/synthesis in transgenic tobacco plants, an important attribute of fiber quality bestowing additional strength to the plant.

Comprehensive signal assignment of 13C-labeled lignocellulose using multidimensional solution NMR and 13C chemical shift comparison with solid-state NMR

A multidimensional solution NMR method has been developed using various pulse programs including HCCH-COSY and (13)C-HSQC-NOESY for the structural characterization of commercially available (13)C labeled lignocellulose from potatoes (Solanum tuberosum L.), chicory (Cichorium intybus), and corn (Zea mays). This new method allowed for 119 of the signals in the (13)C-HSQC spectrum of lignocelluloses to be assigned and was successfully used to characterize the structures of lignocellulose samples from three plants in terms of their xylan and xyloglucan structures, which are the major hemicelluloses in angiosperm. Furthermore, this new method provided greater insight into fine structures of lignin by providing a high resolution to the aromatic signals of the β-aryl ether and resinol moieties, as well as the diastereomeric signals of the β-aryl ether. Finally, the (13)C chemical shifts assigned in this study were compared with those from solid-state NMR and indicated the presence of heterogeneous dynamics in the polysaccharides where rigid cellulose and mobile hemicelluloses moieties existed together.

Group III-A XTH genes of Arabidopsis encode predominant xyloglucan endohydrolases that are dispensable for normal growth

The molecular basis of primary wall extension endures as one of the central enigmas in plant cell morphogenesis. Classical cell wall models suggest that xyloglucan endo-transglycosylase activity is the primary catalyst (together with expansins) of controlled cell wall loosening through the transient cleavage and religation of xyloglucan-cellulose cross links. The genome of Arabidopsis (Arabidopsis thaliana) contains 33 phylogenetically diverse XYLOGLUCAN ENDO-TRANSGLYCOSYLASE/HYDROLASE (XTH) gene products, two of which were predicted to be predominant xyloglucan endohydrolases due to clustering into group III-A. Enzyme kinetic analysis of recombinant AtXTH31 confirmed this prediction and indicated that this enzyme had similar catalytic properties to the nasturtium (Tropaeolum majus) xyloglucanase1 responsible for storage xyloglucan hydrolysis during germination. Global analysis of Genevestigator data indicated that AtXTH31 and the paralogous AtXTH32 were abundantly expressed in expanding tissues. Microscopy analysis, utilizing the resorufin β-glycoside of the xyloglucan oligosaccharide XXXG as an in situ probe, indicated significant xyloglucan endohydrolase activity in specific regions of both roots and hypocotyls, in good correlation with transcriptomic data. Moreover, this hydrolytic activity was essentially completely eliminated in AtXTH31/AtXTH32 double knockout lines. However, single and double knockout lines, as well as individual overexpressing lines, of AtXTH31 and AtXTH32 did not demonstrate significant growth or developmental phenotypes. These results suggest that although xyloglucan polysaccharide hydrolysis occurs in parallel with primary wall expansion, morphological effects are subtle or may be compensated by other mechanisms. We hypothesize that there is likely to be an interplay between these xyloglucan endohydrolases and recently discovered apoplastic exo-glycosidases in the hydrolytic modification of matrix xyloglucans.

Transcriptional analysis of differentially expressed genes in response to stem inclination in young seedlings of pine

The gravitropic response in trees is a widely studied phenomenon, however understanding of the molecular mechanism involved remains unclear. The purpose of this work was to identify differentially expressed genes in response to inclination using a comparative approach for two conifer species. Young seedlings were subjected to inclination and samples were collected at four different times points. First, suppression subtractive hybridisation (SSH) was used to identify differentially regulated genes in radiata pine (Pinus radiata D. Don). cDNA libraries were constructed from the upper and lower part of inclined stems in a time course experiment, ranging from 2.5 h to 1 month. From a total of 3092 sequences obtained, 2203 elements were assembled, displaying homology to a public database. A total of 942 unigene elements were identified using bioinformatic tools after redundancy analysis. Of these, 614 corresponded to known function genes and 328 to unknown function genes, including hypothetical proteins. Comparative analysis between radiata pine and maritime pine (Pinus pinaster Ait.) was performed to validate the differential expression of relevant candidate genes using qPCR. Selected genes were involved in several functional categories: hormone regulation, phenylpropanoid pathway and signal transduction. This comparative approach for the two conifer species helped determine the molecular gene pattern generated by inclination, providing a set of Pinus gene signatures that may be involved in the gravitropic stress response. These genes may also represent relevant candidate genes involved in the gravitropic response and potentially in wood formation.

Tensional stress generation in gelatinous fibres: a review and possible mechanism based on cell-wall structure and composition

Gelatinous fibres are specialized fibres, distinguished by the presence of an inner, gelatinous cell-wall layer. In recent years, they have attracted increasing interest since their walls have a desirable chemical composition (low lignin, low pentosan, and high cellulose contents) for applications such as saccharification and biofuel production, and they have interesting mechanical properties, being capable of generating high tensional stress. However, the unique character of gelatinous layer has not yet been widely recognized. The first part of this review presents a model of gelatinous-fibre organization and stresses the unique character of the gelatinous layer as a separate type of cell-wall layer, different from either primary or secondary wall layers. The second part discusses major current models of tensional stress generation by these fibres and presents a novel unifying model based on recent advances in knowledge of gelatinous wall structure. Understanding this mechanism could potentially lead to novel biomimetic developments in material sciences.

Association of hemicellulose- and pectin-modifying gene expression with Eucalyptus globulus secondary growth

Wood properties are ultimately related to the morphology and biophysical properties of the xylem cell wall. Although the cellulose and lignin biosynthetic pathways have been extensively studied, modifications of other wall matrix components during secondary growth have attracted relatively less attention. In this work, thirty-eight new Eucalyptus cDNAs encoding cell wall-modifying proteins from nine candidate families that act on the cellulose-hemicellulose and pectin networks were cloned and their gene expression was investigated throughout the developing stem. Semi-quantitative RT-PCR revealed distinct, gene-specific transcription patterns for each clone, allowing the identification of genes up-regulated in xylem or phloem of stem regions undergoing secondary growth. Some genes, namely an endo-1,4-beta-glucanase, one mannan-hydrolase and three pectin methylesterases showed transcription in juvenile and also in mature stages of wood development. The patterns of gene expression using samples from tension and opposite wood disclosed a general trend for up-regulation in tension wood and/or down-regulation in opposite wood. Localised gene expression of two selected representative clones, EGl-XTH1 and EGl-XTH4, obtained through in situ hybridization confirms the RT-PCR results and association with secondary xylem formation. Likewise, immunolocalisation studies with the anti-pectin antibody (JIM5) also supported the idea that the development of tissue-specific pectin characteristics is important during secondary growth. These results emphasize an involvement of hemicellulose and pectin biochemistry in wood formation, suggesting that the controlled and localised modification of these polysaccharides may define cell properties and architecture and thus, contribute to determining different biophysical characteristics of Eucalyptus wood.

Functions of xyloglucan in plant cells

While an increase in the number of xyloglucan tethers between the cellulose microfibrils in plant cell walls increases the walls' rigidity, the degradation of these tethers causes the walls to loosen. Degradation can occur either through the integration of xyloglucan oligosaccharides due to the action of xyloglucan endotransglucosylase or through direct hydrolysis due to the action of xyloglucanase. This is why the addition of xyloglucan and its fragment oligosaccharides causes plant tissue tension to increase and decrease so dramatically. Experiments involving the overexpression of xyloglucanase and cellulase have revealed the roles of xyloglucans in the walls. The degradation of wall xyloglucan in poplar by the transgenic expression of xyloglucanase, for example, not only accelerated stem elongation in the primary wall, but also blocked upright-stem gravitropism in the secondary wall. Overexpression of cellulase also reduced xyloglucan content in the walls as cellulose microfibrils were trimmed at their amorphous region, resulting in increased cell volume in Arabidopsis leaves and in sengon with disturbed leaf movements. The hemicellulose xyloglucan, in its function as a tether, plays a key role in the loosening and tightening of cellulose microfibrils: it enables the cell to change its shape in growth and differentiation zones and to retain its final shape after cell maturation.

Xyloglucan endo-transglycosylase-mediated xyloglucan rearrangements in developing wood of hybrid aspen

Xyloglucan endo-transglycosylases (XETs) encoded by xyloglucan endo-transglycosylases/hydrolase (XTH) genes modify the xyloglucan-cellulose framework of plant cell walls, thereby regulating their expansion and strength. To evaluate the importance of XET in wood development, we studied xyloglucan dynamics and XTH gene expression in developing wood and modified XET activity in hybrid aspen (Populus tremula × tremuloides) by overexpressing PtxtXET16-34. We show that developmental modifications during xylem differentiation include changes from loosely to tightly bound forms of xyloglucan and increases in the abundance of fucosylated xyloglucan epitope recognized by the CCRC-M1 antibody. We found that at least 16 Populus XTH genes, all likely encoding XETs, are expressed in developing wood. Five genes were highly and ubiquitously expressed, whereas PtxtXET16-34 was expressed more weakly but specifically in developing wood. Transgenic up-regulation of XET activity induced changes in cell wall xyloglucan, but its effects were dependent on developmental stage. For instance, XET overexpression increased abundance of the CCRC-M1 epitope in cambial cells and xylem cells in early stages of differentiation but not in mature xylem. Correspondingly, an increase in tightly bound xyloglucan content was observed in primary-walled xylem but a decrease was seen in secondary-walled xylem. Thus, in young xylem cells, XET activity limits xyloglucan incorporation into the tightly bound wall network but removes it from cell walls in older cells. XET overexpression promoted vessel element growth but not fiber expansion. We suggest that the amount of nascent xyloglucan relative to XET is an important determinant of whether XET strengthens or loosens the cell wall.

Plasticity of xyloglucan composition in bean (Phaseolus vulgaris)-cultured cells during habituation and dehabituation to lethal concentrations of dichlobenil

Bean cells that have been habituated to grow in a lethal concentration (12 μM) of 2,6-dichlorobenzonitrile (dichlobenil or DCB, a cellulose biosynthesis inhibitor) are known to have decreased cellulose content in their cell walls. Xyloglucan, which is bound to cellulose and together with it forms the main loading network of plant cell walls, has also been described to decrease in habituated cells, but whether the change on cellulose affects the xyloglucan structure besides its abundance has not been analyzed. Fragmentation analysis with xyloglucan-specific endoglucanase (XEG) and endocellulase revealed that habituation to DCB caused a change in the fine structure of xyloglucan, namely a decrease in fucosyl residues attached to the galactosyl-xylosyl residues along the glucan backbone. After the removal of herbicide from the medium (dehabituated cells), xyloglucan recovered its fucosyl residues. In addition, some cello-oligosaccharides could be detected only in habituated cells' xyloglucan digested by XEG and endocellulase, corresponding to a glucan covalently bound or co-precipitated with the hemicelluloses. These results show that structural flexibility of cell walls relies in part on the plasticity of xyloglucan composition and opens up new perspectives to further research in this field.

Loosening xyloglucan accelerates the enzymatic degradation of cellulose in wood

In order to create trees in which cellulose, the most abundant component in biomass, can be enzymatically hydrolyzed highly for the production of bioethanol, we examined the saccharification of xylem from several transgenic poplars, each overexpressing either xyloglucanase, cellulase, xylanase, or galactanase. The level of cellulose degradation achieved by a cellulase preparation was markedly greater in the xylem overexpressing xyloglucanase and much greater in the xylems overexpressing xylanase and cellulase than in the xylem of the wild-type plant. Although a high degree of degradation occurred in all xylems at all loci, the crystalline region of the cellulose microfibrils was highly degraded in the xylem overexpressing xyloglucanase. Since the complex between microfibrils and xyloglucans could be one region that is particularly resistant to cellulose degradation, loosening xyloglucan could facilitate the enzymatic hydrolysis of cellulose in wood.