The XTH gene family: an update on enzyme structure, function, and phylogeny in xyloglucan remodeling

Studies on the rice LEAF INCLINATION1 (LC1), an IAA-amido synthetase, reveal the effects of auxin in leaf inclination control

The angle of rice leaf inclination is an important agronomic trait and closely related to the yields and architecture of crops. Although few mutants with altered leaf angles have been reported, the molecular mechanism remains to be elucidated, especially whether hormones are involved in this process. Through genetic screening, a rice gain-of-function mutant leaf inclination1, lc1-D, was identified from the Shanghai T-DNA Insertion Population (SHIP). Phenotypic analysis confirmed the exaggerated leaf angles of lc1-D due to the stimulated cell elongation at the lamina joint. LC1 is transcribed in various tissues and encodes OsGH3-1, an indole-3-acetic acid (IAA) amido synthetase, whose homolog of Arabidopsis functions in maintaining the auxin homeostasis by conjugating excess IAA to various amino acids. Indeed, recombinant LC1 can catalyze the conjugation of IAA to Ala, Asp, and Asn in vitro, which is consistent with the decreased free IAA amount in lc1-D mutant. lc1-D is insensitive to IAA and hypersensitive to exogenous BR, in agreement with the microarray analysis that reveals the altered transcriptions of genes involved in auxin signaling and BR biosynthesis. These results indicate the crucial roles of auxin homeostasis in the leaf inclination control.

Transcriptomic and metabolomic analysis of Yukon Thellungiella plants grown in cabinets and their natural habitat show phenotypic plasticity

BACKGROUND

Thellungiella salsuginea is an important model plant due to its natural tolerance to abiotic stresses including salt, cold, and water deficits. Microarray and metabolite profiling have shown that Thellungiella undergoes stress-responsive changes in transcript and organic solute abundance when grown under controlled environmental conditions. However, few reports assess the capacity of plants to display stress-responsive traits in natural habitats where concurrent stresses are the norm.

RESULTS

To determine whether stress-responsive changes observed in cabinet-grown plants are recapitulated in the field, we analyzed leaf transcript and metabolic profiles of Thellungiella growing in its native Yukon habitat during two years of contrasting meteorological conditions. We found 673 genes showing differential expression between field and unstressed, chamber-grown plants. There were comparatively few overlaps between genes expressed under field and cabinet treatment-specific conditions. Only 20 of 99 drought-responsive genes were expressed both in the field during a year of low precipitation and in plants subjected to drought treatments in cabinets. There was also a general pattern of lower abundance among metabolites found in field plants relative to control or stress-treated plants in growth cabinets. Nutrient availability may explain some of the observed differences. For example, proline accumulated to high levels in cold and salt-stressed cabinet-grown plants but proline content was, by comparison, negligible in plants at a saline Yukon field site. We show that proline accumulated in a stress-responsive manner in Thellungiella plants salinized in growth cabinets and in salt-stressed seedlings when nitrogen was provided at 1.0 mM. In seedlings grown on 0.1 mM nitrogen medium, the proline content was low while carbohydrates increased. The relatively higher content of sugar-like compounds in field plants and seedlings on low nitrogen media suggests that Thellungiella shows metabolic plasticity in response to environmental stress and that resource availability can influence the expression of stress tolerance traits under field conditions.

CONCLUSION

Comparisons between Thellungiella plants responding to stress in cabinets and in their natural habitats showed differences but also overlap between transcript and metabolite profiles. The traits in common offer potential targets for improving crops that must respond appropriately to multiple, concurrent stresses.

The basal level ethylene response is important to the wall and endomembrane structure in the hypocotyl cells of etiolated Arabidopsis seedlings

The sub-cellular events that occur during the ethylene-modulated cell elongation were characterized by examining the ultra-structure of etiolated Arabidopsis seedling hypocotyl cells. Preventing the basal level ethylene response facilitated cell elongation, and the cells exhibited wall loosening and separation phenotype. Nearby the wall separation sites were frequently associated with an increase in the cortical rough endoplasmic reticulum (rER) membranes, the presence of paramural bodies, and the circular Golgi formation. The cortical rER proliferation and circular Golgi phenotype were reverted by the protein biosynthesis inhibitor cycloheximide. The cortical rER membranes were longer when the ethylene response was prevented and shortened with elevated ethylene responses. Proteomic changes between wild type and the ethylene-insensitive mutant ethylene insensitive2 (ein2) seedling hypocotyls indicated that distinct subsets of proteins involving endomembrane trafficking, remodeling, and wall modifications were differentially expressed. FM4-64 staining supported the proteomic changes, which indicated reduced endocytosis activity with alleviation of the ethylene response. The basal level ethylene response has an important role in endomembrane trafficking, biological materials transport and maintenance of the endomembrane organization. It is possible that endomembrane alterations may partly associate with the wall modifications, though the biological significance of the alterations should be addressed in future studies.

Tomato GDSL1 is required for cutin deposition in the fruit cuticle

The plant cuticle consists of cutin, a polyester of glycerol, hydroxyl, and epoxy fatty acids, covered and filled by waxes. While the biosynthesis of cutin building blocks is well documented, the mechanisms underlining their extracellular deposition remain unknown. Among the proteins extracted from dewaxed tomato (Solanum lycopersicum) peels, we identified GDSL1, a member of the GDSL esterase/acylhydrolase family of plant proteins. GDSL1 is strongly expressed in the epidermis of growing fruit. In GDSL1-silenced tomato lines, we observed a significant reduction in fruit cuticle thickness and a decrease in cutin monomer content proportional to the level of GDSL1 silencing. A significant decrease of wax load was observed only for cuticles of the severely silenced transgenic line. Fourier transform infrared (FTIR) analysis of isolated cutins revealed a reduction in cutin density in silenced lines. Indeed, FTIR-attenuated total reflectance spectroscopy and atomic force microscopy imaging showed that drastic GDSL1 silencing leads to a reduction in ester bond cross-links and to the appearance of nanopores in tomato cutins. Furthermore, immunolabeling experiments attested that GDSL1 is essentially entrapped in the cuticle proper and cuticle layer. These results suggest that GDSL1 is specifically involved in the extracellular deposition of the cutin polyester in the tomato fruit cuticle.

Trans-α-xylosidase and trans-β-galactosidase activities, widespread in plants, modify and stabilize xyloglucan structures

Cell-wall components are hydrolysed by numerous plant glycosidase and glycanase activities. We investigated whether plant enzymes also modify xyloglucan structures by transglycosidase activities. Diverse angiosperm extracts exhibited transglycosidase activities that progressively transferred single sugar residues between xyloglucan heptasaccharide (XXXG or its reduced form, XXXGol) molecules, at 16 μM and above, creating octa- to decasaccharides plus smaller products. We measured remarkably high transglycosylation:hydrolysis ratios under optimized conditions. To identify the transferred monosaccharide(s), we devised a dual-labelling strategy in which a neutral radiolabelled oligosaccharide (donor substrate) reacted with an amino-labelled non-radioactive oligosaccharide (acceptor substrate), generating radioactive cationic products. For example, 37 μM [Xyl-³H]XXXG plus 1 mM XXLG-NH₂ generated ³H-labelled cations, demonstrating xylosyl transfer, which exceeded xylosyl hydrolysis 1.6- to 7.3-fold, implying the presence of enzymes that favour transglycosylation. The transferred xylose residues remained α-linked but were relatively resistant to hydrolysis by plant enzymes. Driselase digestion of the products released a trisaccharide (α-[³H]xylosyl-isoprimeverose), indicating that a new xyloglucan repeat unit had been formed. In similar assays, [Gal-³H]XXLG and [Gal-³H]XLLG (but not [Fuc-³H]XXFG) yielded radioactive cations. Thus plants exhibit trans-α-xylosidase and trans-β-galactosidase (but not trans-α-fucosidase) activities that graft sugar residues from one xyloglucan oligosaccharide to another. Reconstructing xyloglucan oligosaccharides in this way may alter oligosaccharin activities or increase their longevity in vivo. Trans-α-xylosidase activity also transferred xylose residues from xyloglucan oligosaccharides to long-chain hemicelluloses (xyloglucan, water-soluble cellulose acetate, mixed-linkage β-glucan, glucomannan and arabinoxylan). With xyloglucan as acceptor substrate, such an activity potentially affects the polysaccharide's suitability as a substrate for xyloglucan endotransglucosylase action and thereby modulates cell expansion. We conclude that certain proteins annotated as glycosidases can function as transglycosidases.

Trans-α-xylosidase, a widespread enzyme activity in plants, introduces (1→4)-α-d-xylobiose side-chains into xyloglucan structures

Angiosperms possess a retaining trans-α-xylosidase activity that catalyses the inter-molecular transfer of xylose residues between xyloglucan structures. To identify the linkage of the newly transferred α-xylose residue, we used [Xyl-(3)H]XXXG (xyloglucan heptasaccharide) as donor substrate and reductively-aminated xyloglucan oligosaccharides (XGO-NH(2)) as acceptor. Asparagus officinalis enzyme extracts generated cationic radioactive products ([(3)H]Xyl·XGO-NH(2)) that were Driselase-digestible to a neutral trisaccharide containing an α-[(3)H]xylose residue. After borohydride reduction, the trimer exhibited high molybdate-affinity, indicating xylobiosyl-(1→6)-glucitol rather than a di-xylosylated glucitol. Thus the trans-α-xylosidase had grafted an additional α-[(3)H]xylose residue onto the xylose of an isoprimeverose unit. The trisaccharide was rapidly acetolysed to an α-[(3)H]xylobiose, confirming the presence of an acetolysis-labile (1→6)-bond. The α-[(3)H]xylobiitol formed by reduction of this α-[(3)H]xylobiose had low molybdate-affinity, indicating a (1→2) or (1→4) linkage. In NaOH, the α-[(3)H]xylobiose underwent alkaline peeling at the moderate rate characteristic of a (1→4)-disaccharide. Finally, we synthesised eight non-radioactive xylobioses [α and β; (1↔1), (1→2), (1→3) and (1→4)] and found that the [(3)H]xylobiose co-chromatographed only with (1→4)-α-xylobiose. We conclude that Asparagus trans-α-xylosidase activity generates a novel xyloglucan building block, α-d-Xylp-(1→4)-α-d-Xylp-(1→6)-d-Glc (abbreviation: 'V'). Modifying xyloglucan structures in this way may alter oligosaccharin activities, or change their suitability as acceptor substrates for xyloglucan endotransglucosylase (XET) activity.

Cotton fiber: a powerful single-cell model for cell wall and cellulose research

Cotton fibers are single-celled extensions of the seed epidermis. They can be isolated in pure form as they undergo staged differentiation including primary cell wall synthesis during elongation and nearly pure cellulose synthesis during secondary wall thickening. This combination of features supports clear interpretation of data about cell walls and cellulose synthesis in the context of high throughput modern experimental technologies. Prior contributions of cotton fiber to building fundamental knowledge about cell walls will be summarized and the dynamic changes in cell wall polymers throughout cotton fiber differentiation will be described. Recent successes in using stable cotton transformation to alter cotton fiber cell wall properties as well as cotton fiber quality will be discussed. Futurec prospects to perform experiments more rapidly through altering cotton fiberwall properties via virus-induced gene silencing will be evaluated.

AtBGAL10 is the main xyloglucan β-galactosidase in Arabidopsis, and its absence results in unusual xyloglucan subunits and growth defects

In growing cells, xyloglucan is thought to connect cellulose microfibrils and regulate their separation during wall extension. In Arabidopsis (Arabidopsis thaliana), a significant proportion of xyloglucan side chains contain β-galactose linked to α-xylose at O2. In this work, we identified AtBGAL10 (At5g63810) as the gene responsible for the majority of β-galactosidase activity against xyloglucan. Xyloglucan from bgal10 insertional mutants was found to contain a large proportion of unusual subunits, such as GLG and GLLG. These subunits were not detected in a bgal10 xyl1 double mutant, deficient in both β-galactosidase and α-xylosidase. Xyloglucan from bgal10 xyl1 plants was enriched instead in XXLG/XLXG and XLLG subunits. In both cases, changes in xyloglucan composition were larger in the endoglucanase-accessible fraction. These results suggest that glycosidases acting on nonreducing ends digest large amounts of xyloglucan in wild-type plants, while plants deficient in any of these activities accumulate partly digested subunits. In both bgal10 and bgal10 xyl1, siliques and sepals were shorter, a phenotype that could be explained by an excess of nonreducing ends leading to a reinforced xyloglucan network. Additionally, AtBGAL10 expression was examined with a promoter-reporter construct. Expression was high in many cell types undergoing wall extension or remodeling, such as young stems, abscission zones, or developing vasculature, showing good correlation with α-xylosidase expression.

Genome-wide transcriptomic analysis of cotton under drought stress reveal significant down-regulation of genes and pathways involved in fibre elongation and up-regulation of defense responsive genes

Cotton is an important source of natural fibre used in the textile industry and the productivity of the crop is adversely affected by drought stress. High throughput transcriptomic analyses were used to identify genes involved in fibre development. However, not much information is available on cotton genome response in developing fibres under drought stress. In the present study a genome wide transcriptome analysis was carried out to identify differentially expressed genes at various stages of fibre growth under drought stress. Our study identified a number of genes differentially expressed during fibre elongation as compared to other stages. High level up-regulation of genes encoding for enzymes involved in pectin modification and cytoskeleton proteins was observed at fibre initiation stage. While a large number of genes encoding transcription factors (AP2-EREBP, WRKY, NAC and C2H2), osmoprotectants, ion transporters and heat shock proteins and pathways involved in hormone (ABA, ethylene and JA) biosynthesis and signal transduction were up-regulated and genes involved in phenylpropanoid and flavonoid biosynthesis, pentose and glucuronate interconversions and starch and sucrose metabolism pathways were down-regulated during fibre elongation. This study showed that drought has relatively less impact on fibre initiation but has profound effect on fibre elongation by down-regulating important genes involved in cell wall loosening and expansion process. The comprehensive transcriptome analysis under drought stress has provided valuable information on differentially expressed genes and pathways during fibre development that will be useful in developing drought tolerant cotton cultivars without compromising fibre quality.

We are good to grow: dynamic integration of cell wall architecture with the machinery of growth

Despite differences in cell wall composition between the type I cell walls of dicots and most monocots and the type II walls of commelinid monocots, all flowering plants respond to the same classes of growth regulators in the same tissue-specific way and exhibit the same growth physics. Substantial progress has been made in defining gene families and identifying mutants in cell wall-related genes, but our understanding of the biochemical basis of wall extensibility during growth is still rudimentary. In this review, we highlight insights into the physiological control of cell expansion emerging from genetic functional analyses, mostly in Arabidopsis and other dicots, and a few examples of genes of potential orthologous function in grass species. We discuss examples of cell wall architectural features that impact growth independent of composition, and progress in identifying proteins involved in transduction of growth signals and integrating their outputs in the molecular machinery of wall expansion.

A comprehensive overview of grain development in Brachypodium distachyon variety Bd21

A detailed and comprehensive understanding of seed reserve accumulation is of great importance for agriculture and crop improvement strategies. This work is part of a research programme aimed at using Brachypodium distachyon as a model plant for cereal grain development and filling. The focus was on the Bd21-3 accession, gathering morphological, cytological, and biochemical data, including protein, lipid, sugars, starch, and cell-wall analyses during grain development. This study highlighted the existence of three main developmental phases in Brachypodium caryopsis and provided an extensive description of Brachypodium grain development. In the first phase, namely morphogenesis, the embryo developed rapidly reaching its final morphology about 18 d after fertilization (DAF). Over the same period the endosperm enlarged, finally to occupy 80% of the grain volume. During the maturation phase, carbohydrates were continuously stored, mainly in the endosperm, switching from sucrose to starch accumulation. Large quantities of β-glucans accumulated in the endosperm with local variations in the deposition pattern. Interestingly, new β-glucans were found in Brachypodium compared with other cereals. Proteins (i.e. globulins and prolamins) were found in large quantities from 15 DAF onwards. These proteins were stored in two different sub-cellular structures which are also found in rice, but are unusual for the Pooideae. During the late stage of development, the grain desiccated while the dry matter remained fairly constant. Brachypodium exhibits some significant differences with domesticated cereals. Beta-glucan accumulates during grain development and this cell wall polysaccharide is the main storage carbohydrate at the expense of starch.

Distinguishing xyloglucanase activity in endo-β(1→4)glucanases

The ability of β-glucanases to cleave xyloglucans, a family of highly decorated β-glucans ubiquitous in plant biomass, has traditionally been overlooked in functional biochemical studies. An emerging body of data indicates, however, that a spectrum of xyloglucan specificity resides in diverse glycoside hydrolases from a range of carbohydrate-active enzyme families-including classic "cellulase" families. This chapter outlines a series of enzyme kinetic and product analysis methods to establish degrees of xyloglucan specificity and modes of action of glycosidases emerging from enzyme discovery projects.

Comparative structure and biomechanics of plant primary and secondary cell walls

Recent insights into the physical biology of plant cell walls are reviewed, summarizing the essential differences between primary and secondary cell walls and identifying crucial gaps in our knowledge of their structure and biomechanics. Unexpected parallels are identified between the mechanism of expansion of primary cell walls during growth and the mechanisms by which hydrated wood deforms under external tension. There is a particular need to revise current "cartoons" of plant cell walls to be more consistent with data from diverse approaches and to go beyond summarizing limited aspects of cell walls, serving instead as guides for future experiments and for the application of new techniques.

Organelle-nucleus cross-talk regulates plant intercellular communication via plasmodesmata

We use Arabidopsis thaliana embryogenesis as a model system for studying intercellular transport via plasmodesmata (PD). A forward genetic screen for altered PD transport identified increased size exclusion limit (ise) 1 and ise2 mutants with increased intercellular transport of fluorescent 10-kDa tracers. Both ise1 and ise2 exhibit increased formation of twinned and branched PD. ISE1 encodes a mitochondrial DEAD-box RNA helicase, whereas ISE2 encodes a DEVH-type RNA helicase. Here, we show that ISE2 foci are localized to the chloroplast stroma. Surprisingly, plastid development is defective in both ise1 and ise2 mutant embryos. In an effort to understand how RNA helicases that localize to different organelles have similar impacts on plastid and PD development/function, we performed whole-genome expression analyses. The most significantly affected class of transcripts in both mutants encode products that target to and enable plastid function. These results reinforce the importance of plastid-mitochondria-nucleus cross-talk, add PD as a critical player in the plant cell communication network, and thereby illuminate a previously undescribed signaling pathway dubbed organelle-nucleus-plasmodesmata signaling. Several genes with roles in cell wall synthesis and modification are also differentially expressed in both mutants, providing new targets for investigating PD development and function.

Differential transcript accumulation in chickpea during early phases of compatible interaction with a necrotrophic fungus Ascochyta rabiei

The initial phases of the disease establishment are very crucial for the compatible interactions. Pathogens must overcome the responses generated by the host for the onset of disease invasion. The compatible interaction is inadequately represented in plant-pathogen interaction studies. To gain broader insight into the early responses elicited by chickpea blight fungus Ascochyta rabiei during compatible interaction; we isolated early responsive genes of chickpea using PCR based suppression subtractive hybridization (SSH) strategy. We obtained ~250 unique genes after homology search and redundancy elimination. Based on their potential cellular functions, these genes were broadly classified into eleven different categories viz. stress, signaling, gene regulation, cellular metabolism and genes of unknown functions. Present study revealed few unexpected genes which have a possible role in induced immunity and disease progression. We employed macroarray, northern blot, real-time PCR and cluster analysis to develop transcript profiles. Most of the genes analyzed were early induced and were transcriptionally upregulated upon 24 h post inoculation. Our approach has rendered the isolation of early responsive genes involved in signaling and regulation of metabolic changes upon fungal infection. The information obtained will help to dissect the molecular mechanisms during compatible chickpea-Ascochyta interactions.

Phylogenetic variation in glycosidases and glycanases acting on plant cell wall polysaccharides, and the detection of transglycosidase and trans-β-xylanase activities

Wall polysaccharide chemistry varies phylogenetically, suggesting a need for variation in wall enzymes. Although plants possess the genes for numerous putative enzymes acting on wall carbohydrates, the activities of the encoded proteins often remain conjectural. To explore phylogenetic differences in demonstrable enzyme activities, we extracted proteins from 57 rapidly growing plant organs with three extractants, and assayed their ability to act on six oligosaccharides 'modelling' selected cell-wall polysaccharides. Based on reaction products, we successfully distinguished exo- and endo-hydrolases and found high taxonomic variation in all hydrolases screened: β-D-xylosidase, endo-(1→4)-β-D-xylanase, β-D-mannosidase, endo-(1→4)-β-D-mannanase, α-D-xylosidase, β-D-galactosidase, α-L-arabinosidase and α-L-fucosidase. The results, as GHATAbase, a searchable compendium in Excel format, also provide a compilation for selecting rich sources of enzymes acting on wall carbohydrates. Four of the hydrolases were accompanied, sometimes exceeded, by transglycosylase activities, generating products larger than the substrate. For example, during β-xylosidase assays on (1→4)-β-D-xylohexaose (Xyl₆), Marchantia, Selaginella and Equisetum extracts gave negligible free xylose but approximately equimolar Xyl₅ and Xyl₇, indicating trans-β-xylosidase activity, also found in onion, cereals, legumes and rape. The yield of Xyl₉ often exceeded that of Xyl₇₋₈, indicating that β-xylanase was accompanied by an endotransglycosylase activity, here called trans-β-xylanase, catalysing the reaction 2Xyl₆ → Xyl₃ + Xyl₉. Similar evidence also revealed trans-α-xylosidase, trans-α-arabinosidase and trans-α-arabinanase activities acting on xyloglucan oligosaccharides and (1→5)-α-L-arabino-oligosaccharides. In conclusion, diverse plants differ dramatically in extractable enzymes acting on wall carbohydrate, reflecting differences in wall polysaccharide composition. Besides glycosidase and glycanase activities, five new transglycosylase activities were detected. We propose that such activities function in the assembly and re-structuring of the wall matrix.

Constitutive expression of CaXTH3, a hot pepper xyloglucan endotransglucosylase/hydrolase, enhanced tolerance to salt and drought stresses without phenotypic defects in tomato plants (Solanum lycopersicum cv. Dotaerang)

The hot pepper xyloglucan endo-trans-gluco-sylase/hydrolase (CaXTH3) gene that was inducible by a broad spectrum of abiotic stresses in hot pepper has been reported to enhance tolerance to drought and high salinity in transgenic Arabidopsis. To assess whether CaXTH3 is a practically useful target gene for improving the stress tolerance of crop plants, we ectopically over-expressed the full-length CaXTH3 cDNA in tomato (Solanum lycopersicum cv. Dotaerang) and found that the 35S:CaXTH3 transgenic tomato plants exhibited a markedly increased tolerance to salt and drought stresses. Transgenic tomato plants exposed to a salt stress of 100 mM NaCl retained the chlorophyll in their leaves and showed normal root elongation. They also remained green and unwithered following exposure to 2 weeks of dehydration. A high proportion of stomatal closures in 35S:CaXTH3 was likely to be conferred by increased cell-wall remodeling activity of CaXTH3 in guard cell, which may reduce transpirational water loss in response to dehydration stress. Despite this increased stress tolerance, the transgenic tomato plants showed no detectable phenotype defects, such as abnormal morphology and growth retardation, under normal growth conditions. These results raise the possibility that CaXTH3 gene is appropriate for application in genetic engineering strategies aimed at improving abiotic stress tolerance in agriculturally and economically valuable crop plants.

Constitutive expression of CaXTH3, a hot pepper xyloglucan endotransglucosylase/hydrolase, enhanced tolerance to salt and drought stresses without phenotypic defects in tomato plants (Solanum lycopersicum cv. Dotaerang)

The hot pepper xyloglucan endo-trans-gluco-sylase/hydrolase (CaXTH3) gene that was inducible by a broad spectrum of abiotic stresses in hot pepper has been reported to enhance tolerance to drought and high salinity in transgenic Arabidopsis. To assess whether CaXTH3 is a practically useful target gene for improving the stress tolerance of crop plants, we ectopically over-expressed the full-length CaXTH3 cDNA in tomato (Solanum lycopersicum cv. Dotaerang) and found that the 35S:CaXTH3 transgenic tomato plants exhibited a markedly increased tolerance to salt and drought stresses. Transgenic tomato plants exposed to a salt stress of 100 mM NaCl retained the chlorophyll in their leaves and showed normal root elongation. They also remained green and unwithered following exposure to 2 weeks of dehydration. A high proportion of stomatal closures in 35S:CaXTH3 was likely to be conferred by increased cell-wall remodeling activity of CaXTH3 in guard cell, which may reduce transpirational water loss in response to dehydration stress. Despite this increased stress tolerance, the transgenic tomato plants showed no detectable phenotype defects, such as abnormal morphology and growth retardation, under normal growth conditions. These results raise the possibility that CaXTH3 gene is appropriate for application in genetic engineering strategies aimed at improving abiotic stress tolerance in agriculturally and economically valuable crop plants.

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.

Differences in enzymic properties of five recombinant xyloglucan endotransglucosylase/hydrolase (XTH) proteins of Arabidopsis thaliana

Xyloglucan endotransglucosylase/hydrolases (XTHs) are cell wall enzymes that are able to graft xyloglucan chains to oligosaccharides or to other available xyloglucan chains and/or to hydrolyse xyloglucan chains. As they are involved in the modification of the load-bearing cell-wall components, they are believed to be very important in the regulation of growth and development. Given the large number (33) of XTH genes in Arabidopsis and the overlapping expression patterns, specific enzymic properties may be expected. Five predominantly root-expressed Arabidopsis thaliana XTHs belonging to subgroup I/II were analysed here. These represent two sets of closely related genes: AtXTH12 and 13 on the one hand (trichoblast-enriched) and AtXTH17, 18, and 19 on the other (expressed in nearly all cell types in the root). They were all recombinantly produced in the yeast Pichia pastoris and partially purified by ammonium sulphate precipitation before they were subsequently all subjected to a series of identical in vitro tests. The kinetic properties of purified AtXTH13 were investigated in greater detail to rule out interference with the assays by contaminating yeast proteins. All five proteins were found to exhibit only the endotransglucosylase (XET; EC 2.4.1.207) activity towards xyloglucan and non-detectable endohydrolytic (XEH; EC 3.2.1.151) activity. Their endotransglucosylase activity was preferentially directed towards xyloglucan and, in some cases, water-soluble cellulose acetate, rather than to mixed-linkage β-glucan. Isoforms differed in optimum pH (5.0-7.5), in temperature dependence and in acceptor substrate preferences.

Lack of α-xylosidase activity in Arabidopsis alters xyloglucan composition and results in growth defects

Xyloglucan is the main hemicellulose in the primary cell walls of most seed plants and is thought to play a role in regulating the separation of cellulose microfibrils during growth. Xylose side chains block the degradation of the backbone, and α-xylosidase activity is necessary to remove them. Two Arabidopsis (Arabidopsis thaliana) mutant lines with insertions in the α-xylosidase gene AtXYL1 were characterized in this work. Both lines showed a reduction to undetectable levels of α-xylosidase activity against xyloglucan oligosaccharides. This reduction resulted in the accumulation of XXXG and XXLG in the liquid growth medium of Atxyl1 seedlings. The presence of XXLG suggests that it is a poor substrate for xyloglucan β-galactosidase. In addition, the polymeric xyloglucan of Atxyl1 lines was found to be enriched in XXLG subunits, with a concomitant decrease in XXFG and XLFG. This change can be explained by extensive exoglycosidase activity at the nonreducing ends of xyloglucan chains. These enzymes could thus have a larger role than previously thought in the metabolism of xyloglucan. Finally, Atxyl1 lines showed a reduced ability to control the anisotropic growth pattern of different organs, pointing to the importance of xyloglucan in this process. The promoter of AtXYL1 was shown to direct expression to many different organs and cell types undergoing cell wall modifications, including trichomes, vasculature, stomata, and elongating anther filaments.

Characterization of two divergent cDNAs encoding xyloglucan endotransglycosylase/hydrolase (XTH) expressed in Fragaria chiloensis fruit

Chilean strawberry (Fragaria chiloensis), the maternal progenitor of Fragaria×ananassa, has emerged as a new berry fruit with excellent organoleptic characteristics. The fast softening of strawberries is a limiting step for their commercialization. Fruit softening has been shown to be related to cell wall degradation. Several enzymatic activities related to this process have been isolated in strawberry fruit, however xyloglucan endotransglycosylase/hydrolase (XTH) enzymes have not been identified or characterized so far. Two XTH genes were identified in an EST database of F. chiloensis fruit with high homology to other plant XTHs. We isolated the full-length cDNAs associated to these ESTs in F. chiloensis (Fc-XTH1, Fc-XTH2). Phylogenetic analysis suggests that both F. chiloensis XTH genes belong to distant phylogenetic groups of XTHs. Moreover, DNA gel-blot analysis indicates different genomic organization between the two genes. By means of Real Time qPCR analysis, gene expression profiles show a transcriptional profile of Fc-XTH1 transcripts congruent with a probable role during strawberry ripening, while that exhibited by Fc-XTH2 could be related with vegetative processes like leaf growth. On the other hand, immunodetection and enzyme activity assays allow the detection of XTH-related proteins and high xyloglucan transglycosylating (XETA) and degrading (XDA) activities at the turning stage. The data presented confirms the existence of two divergent XTH genes, and XET and XEH activities, in F. chiloensis fruit.

Biological implications of the occurrence of 32 members of the XTH (xyloglucan endotransglucosylase/hydrolase) family of proteins in the bryophyte Physcomitrella patens

This comprehensive overview of the xyloglucan endotransglucosylase/hydrolase (XTH) family of genes and proteins in bryophytes, based on research using genomic resources that are newly available for the moss Physcomitrella patens, provides new insights into plant evolution. In angiosperms, the XTH genes are found in large multi-gene families, probably reflecting the diverse roles of individual XTHs in various cell types. As there are fewer cell types in P. patens than in angiosperms such as Arabidopsis and rice, it is tempting to deduce that there are fewer XTH family genes in bryophytes. However, the present study unexpectedly identified as many as 32 genes that potentially encode XTH family proteins in the genome of P. patens, constituting a fairly large multi-gene family that is comparable in size with those of Arabidopsis and rice. In situ localization of xyloglucan endotransglucosylase activity in this moss indicates that some P. patens XTH proteins exhibit biochemical functions similar to those found in angiosperms, and that their expression profiles are tissue-dependent. However, comparison of structural features of families of XTH genes between P. patens and angiosperms demonstrated the existence of several bryophyte-specific XTH genes with distinct structural and functional features that are not found in angiosperms. These bryophyte-specific XTH genes might have evolved to meet morphological and functional needs specific to the bryophyte. These findings raise interesting questions about the biological implications of the XTH family of proteins in non-seed plants.

Xyloglucan endotransglycosylase/hydrolase genes in cotton and their role in fiber elongation

Plant cell wall extensibility is mediated, in part, by xyloglucan endotransglycosylases/hydrolases (XTH) that are able to cleave and reattach xyloglucan polymers that make up the hemicelluloses matrix of type I cell walls. In Arabidopsis and other plants, XTHs are encoded by relatively large gene families that are regulated in specific spatial and temporal patterns. In silico screening of a cotton expressed sequence tag (EST) database identified 23 sequences with close sequence similarity to Arabidopsis XTH coding sequences. Analysis of full-length cotton cDNAs derived from these ESTs allow for the identification of three distinct GhXTH cDNAs (denoted GhXTH1, GhXTH2 and GhXTH3) based primarily on their 3' untranslated sequences. The three GhXTH genes were expressed differently with GhXTH1 predominantly expressed in elongating cotton fibers. The function of GhXTH1 in mediating cotton fiber elongation was analyzed in transgenic cotton plants that express a transgene consisting of the GhXTH1 coding sequence under transcriptional control of the CaMV 35S promoter. Plants that over-expressed GhXTH1 had increased XTH activity and produced mature cotton fibers that were between 15 and 20% longer than wild-type cotton plants under both greenhouse and field growth conditions. Segregation analysis showed that the 35S::GhXTH1 transgene acts as a dominant fiber length allele in transgenic cotton. These results confirm that GhXTH1 is the predominant XTH in elongating fibers and its expression limits cotton fiber elongation.