Purification, functional characterization, cloning, and identification of mutants of a seed-specific arabinan hydrolase in Arabidopsis

The In Silico Characterization of Monocotyledonous α-l-Arabinofuranosidases on the Example of Maize

Plant α-l-arabinofuranosidases remove terminal arabinose from arabinose-containing substrates such as plant cell wall polysaccharides, including arabinoxylans, arabinogalactans, and arabinans. In plants, de-arabinosylation of cell wall polysaccharides accompanies different physiological processes such as fruit ripening and elongation growth. In this report, we address the diversity of plant α-l-arabinofuranosidases of the glycoside hydrolase (GH) family 51 through their phylogenetic analysis as well as their structural features. The CBM4-like domain at N-terminus was found to exist only in GH51 family proteins and was detected in almost 90% of plant sequences. This domain is similar to bacterial CBM4, but due to substitutions of key amino acid residues, it does not appear to be able to bind carbohydrates. Despite isoenzymes of GH51 being abundant, in particular in cereals, almost half of the GH51 proteins in Poales have a mutation of the acid/base residue in the catalytic site, making them potentially inactive. Open-source data on the transcription and translation of GH51 isoforms in maize were analyzed to discuss possible functions of individual isoenzymes. The results of homology modeling and molecular docking showed that the substrate binding site can accurately accommodate terminal arabinofuranose and that arabinoxylan is a more favorable ligand for all maize GH51 enzymes than arabinan.

Endosperm weakening: The gateway to a seed's new life

Seed germination is a crucial stage in a plant's life cycle, during which the embryo, surrounded by several tissues, undergoes a transition from the quiescent to a highly active state. Endosperm weakening, a key step in this transition, plays an important role in radicle protrusion. Endosperm weakening is initiated upon water uptake, followed by multiple key molecular events occurring within and outside endosperm cells. Although available transcriptomes have provided information about pivotal genes involved in this process, a complete understanding of the signaling pathways are yet to be elucidated. Much remains to be learnt about the diverse intercellular signals, such as reactive oxygen species-mediated redox signals, phytohormone crosstalk, environmental cue-dependent oxidative phosphorylation, peroxisomal-mediated pectin degradation, and storage protein mobilization during endosperm cell wall loosening. This review discusses the evidences from recent researches into the mechanism of endosperm weakening. Further, given that the endosperm has great potential for manipulation by crop breeding and biotechnology, we offer several novel insights, which will be helpful in this research field and in its application to the improvement of crop production.

The disulfide bond as a key motif for the construction of multivalent glycoclusters

S-Glycosylated dendrons having a thioacetate group in their focal points led to multivalent glycoclusters by spontaneous O2-oxidation of sulfides.

A wheat bran inducible expression system for the efficient production of α-L-arabinofuranosidase in Bacillus subtilis

α-l-arabinofuranosidases (EC 3.2.1.55; AFs) cause the release of arabinosyl residues from hemicellulose polymers such as xylans, and are receiving increased levels of research attention as they could be applied in a range of processes that involve the enzymatic degradation of xylans. The secretory production of bacterial AFs has not been attempted previously. In this study, we designed a unique induction system for the production of a recombinant AF in Bacillus subtilis in order to exploit its enzymic degradation of wheat bran. We found that non-starch phytochemicals were more efficient than d-xylose when inducing the expression of T7 RNA polymerase and driving the transcription of AF by the T7 promoter. The host cell, B. subtilis (ATCC 6051a-derived strain 164T7P) was engineered to incorporate a DNA cassette that expressed T7 RNA polymerase under the control of a d-xylose inducible promoter (P_xylA). The T7 promoter engineered into 164T7P was initially tested and compared with P43 in terms of GFP expression; we found that the expression level of GFP by the T7 promoter was ten-fold higher than that achieved by P43. When cultured in a flask with gentle shaking, and with d-xylose as an inducer, the recombinant strain successfully expressed arbf, a family 51 (GH 51) glycoside hydrolase from Bacillus licheniformis, and secreted 141.4 ± 4.8 U/mL of enzyme, with a Km of 1.4 ± 0.1 mM and a kcat of 139.4 s^-1. However, the protein was devoid of a secretary signal peptide. When cultures were supplemented with wheat bran, the maximal yield of the secreted AF reached 194.8 ± 4.1 U/mL. The results provide a foundation for the high level production of heterologous proteins using wheat bran as the inducer in B. subtilis.

MADS-Box and bHLH Transcription Factors Coordinate Transmitting Tract Development in Arabidopsis thaliana

The MADS-domain transcription factor SEEDSTICK (STK) controls several aspects of plant reproduction. STK is co-expressed with CESTA (CES), a basic Helix-Loop-Helix (bHLH) transcription factor-encoding gene. CES was reported to control redundantly with the brassinosteroid positive signaling factors BRASSINOSTEROID ENHANCED EXPRESSION1 (BEE1) and BEE3 the development of the transmitting tract. Combining the stk ces-4 mutants led to a reduction in ovule fertilization due to a defect in carpel fusion which, caused the formation of holes at the center of the septum where the transmitting tract differentiates. Combining the stk mutant with the bee1 bee3 ces-4 triple mutant showed an increased number of unfertilized ovules and septum defects. The transcriptome profile of this quadruple mutant revealed a small subset of differentially expressed genes which are mainly involved in cell death, extracellular matrix and cell wall development. Our data evidence a regulatory gene network controlling transmitting tract development regulated directly or indirectly by a STK-CES containing complex and reveal new insights in the regulation of transmitting tract development by bHLH and MADS-domain transcription factors.

Metabolism of polysaccharides in dynamic middle lamellae during cotton fibre development

Evidence is presented that cotton fibre adhesion and middle lamella formation are preceded by cutin dilution and accompanied by rhamnogalacturonan-I metabolism. Cotton fibres are single cell structures that early in development adhere to one another via the cotton fibre middle lamella (CFML) to form a tissue-like structure. The CFML is disassembled around the time of initial secondary wall deposition, leading to fibre detachment. Observations of CFML in the light microscope have suggested that the development of the middle lamella is accompanied by substantial cell-wall metabolism, but it has remained an open question as to which processes mediate adherence and which lead to detachment. The mechanism of adherence and detachment were investigated here using glyco-microarrays probed with monoclonal antibodies, transcript profiling, and observations of fibre auto-digestion. The results suggest that adherence is brought about by cutin dilution, while the presence of relevant enzyme activities and the dynamics of rhamnogalacturonan-I side-chain accumulation and disappearance suggest that both attachment and detachment are accompanied by rhamnogalacturonan-I metabolism.

An Efficient Strategy Combining SSR Markers- and Advanced QTL-seq-driven QTL Mapping Unravels Candidate Genes Regulating Grain Weight in Rice

Development and use of genome-wide informative simple sequence repeat (SSR) markers and novel integrated genomic strategies are vital to drive genomics-assisted breeding applications and for efficient dissection of quantitative trait loci (QTLs) underlying complex traits in rice. The present study developed 6244 genome-wide informative SSR markers exhibiting in silico fragment length polymorphism based on repeat-unit variations among genomic sequences of 11 indica, japonica, aus, and wild rice accessions. These markers were mapped on diverse coding and non-coding sequence components of known cloned/candidate genes annotated from 12 chromosomes and revealed a much higher amplification (97%) and polymorphic potential (88%) along with wider genetic/functional diversity level (16-74% with a mean 53%) especially among accessions belonging to indica cultivar group, suggesting their utility in large-scale genomics-assisted breeding applications in rice. A high-density 3791 SSR markers-anchored genetic linkage map (IR 64 × Sonasal) spanning 2060 cM total map-length with an average inter-marker distance of 0.54 cM was generated. This reference genetic map identified six major genomic regions harboring robust QTLs (31% combined phenotypic variation explained with a 5.7-8.7 LOD) governing grain weight on six rice chromosomes. One strong grain weight major QTL region (OsqGW5.1) was narrowed-down by integrating traditional QTL mapping with high-resolution QTL region-specific integrated SSR and single nucleotide polymorphism markers-based QTL-seq analysis and differential expression profiling. This led us to delineate two natural allelic variants in two known cis-regulatory elements (RAV1AAT and CARGCW8GAT) of glycosyl hydrolase and serine carboxypeptidase genes exhibiting pronounced seed-specific differential regulation in low (Sonasal) and high (IR 64) grain weight mapping parental accessions. Our genome-wide SSR marker resource (polymorphic within/between diverse cultivar groups) and integrated genomic strategy can efficiently scan functionally relevant potential molecular tags (markers, candidate genes and alleles) regulating complex agronomic traits (grain weight) and expedite marker-assisted genetic enhancement in rice.

Identifying the ionically bound cell wall and intracellular glycoside hydrolases in late growth stage Arabidopsis stems: implications for the genetic engineering of bioenergy crops

Identifying the cell wall-ionically bound glycoside hydrolases (GHs) in Arabidopsis stems is important for understanding the regulation of cell wall integrity. For cell wall proteomics studies, the preparation of clean cell wall fractions is a challenge since cell walls constitute an open compartment, which is more likely to contain a mixture of intracellular and extracellular proteins due to cell leakage at the late growth stage. Here, we utilize a CaCl2-extraction procedure to isolate non-structural proteins from Arabidopsis whole stems, followed by the in-solution and in-gel digestion methods coupled with Nano-LC-MS/MS, bioinformatics and literature analyses. This has led to the identification of 75 proteins identified using the in-solution method and 236 proteins identified by the in-gel method, among which about 10% of proteins predicted to be secreted. Together, eight cell wall proteins, namely AT1G75040, AT5G26000, AT3G57260, AT4G21650, AT3G52960, AT3G49120, AT5G49360, and AT3G14067, were identified by the in-solution method; among them, three were the GHs (AT5G26000, myrosinase 1, GH1; AT3G57260, β-1,3-glucanase 2, GH17; AT5G49360, bifunctional XYL 1/α-L-arabinofuranosidase, GH3). Moreover, four more GHs: AT4G30270 (xyloglucan endotransferase, GH16), AT1G68560 (bifunctional α-l-arabinofuranosidase/XYL, GH31), AT1G12240 (invertase, GH32) and AT2G28470 (β-galactosidase 8, GH35), were identified by the in-gel solution method only. Notably, more than half of above identified GHs are xylan- or hemicellulose-modifying enzymes, and will likely have an impact on cellulose accessibility, which is a critical factor for downstream enzymatic hydrolysis of plant tissues for biofuels production. The implications of these cell wall proteins identified at the late growth stage for the genetic engineering of bioenergy crops are discussed.

The functions of the endosperm during seed germination

In angiosperms, a double fertilization event initiates the development of two distinct structures, the embryo and endosperm. The endosperm plays an important role in supporting embryonic growth by supplying nutrients, protecting the embryo and controlling embryo growth by acting as a mechanical barrier during seed development and germination. Its structure and function in the mature dry seed is divergent and specialized among different plant species. A subset of endospermic tissues are composed of living cells even after seed maturation, and play an active role in the regulation of seed germination. Transcriptome analysis has provided new insights into the regulatory functions of the endosperm during seed germination. It is well known that the embryo secretes signals to the endosperm to induce the degradation of the seed reserve and to promote endosperm weakening during germination. Recent advances in seed biology have shown that the endosperm is capable of sensing environmental signals, and can produce and secrete signals to regulate the growth of the embryo. Thus, germination is a systemic response that involves bidirectional interactions between the embryo and endosperm.

Heterologous expression of tomato glycoside hydrolase family 3 α-L-arabinofuranosidase/β-xylosidases in tobacco suspension cultured cells and synergic action of a family 51 isozyme under antisense suppression of the enzyme

Four cDNA clones (SlArf/Xyl1-4) encoding α-l-arabinofuranosidase/β-xylosidase belonging to glycoside hydrolase family 3 were obtained from tomato (Solanum lycopersicum) fruit. SlArf/Xyl1 was expressed in various organs. Its level was particularly high in flower and leaves but low in fruit. SlArf/Xyl3 was highly expressed in flower. On the contrary, SlArf/Xyl2 and 4 were expressed in early developmental stage in various organs. Comparison with SlArf/Xyl4, SlArf/Xyl2 expression was observed in earlier stages. The active recombinant proteins were obtained by using BY-2 tobacco (Nicotiana tabacum) suspension cultured cells. The SlArf/Xyl1 and 2 recombinant proteins showed a bi-functional activity of α-l-arabinofuranosidase/β-xylosidase while the SlArf/Xyl4 protein possessed a β-xylosidase activity predominantly. Neither enzyme activities were detected for the SlArf/Xyl3 protein under the same conditions. Although SlArf/Xyl2 possessed a bi-functional activity, it preferentially hydrolyzed arabinosyl residues from tomato hemicellulosic polysaccharides. Antisense suppression of SlArf/Xyl2 resulted in no apparent changes in the enzyme activities, monosaccharide composition or fruit phenotype. Increment of a family 51 α-l-arabinofuranosidase expression rather than that of family 3 resulted in a restoring the activity in SlArf/Xyl2-suppressed fruit. The ability of recombinant SlArf/Xyl2 to hydrolyze both arabinan and arabinoxylan is nearly identical to that of α-l-arabinofuranosidases belonging to family 51. Our results suggested that BY-2 cells are a useful expression system for obtaining active cell wall hydrolyzing enzymes. In addition, an α-l-arabinofuranosidase activity derived from SlArf/Xyl2 would be essential in young organ development and the action of the enzyme could be restored by the other enzyme belonging to a different family under a defective condition.

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.

Distinct cell wall architectures in seed endosperms in representatives of the Brassicaceae and Solanaceae

In some species, a crucial role has been demonstrated for the seed endosperm during germination. The endosperm has been shown to integrate environmental cues with hormonal networks that underpin dormancy and seed germination, a process that involves the action of cell wall remodeling enzymes (CWREs). Here, we examine the cell wall architectures of the endosperms of two related Brassicaceae, Arabidopsis (Arabidopsis thaliana) and the close relative Lepidium (Lepidium sativum), and that of the Solanaceous species, tobacco (Nicotiana tabacum). The Brassicaceae species have a similar cell wall architecture that is rich in pectic homogalacturonan, arabinan, and xyloglucan. Distinctive features of the tobacco endosperm that are absent in the Brassicaceae representatives are major tissue asymmetries in cell wall structural components that reflect the future site of radicle emergence and abundant heteromannan. Cell wall architecture of the micropylar endosperm of tobacco seeds has structural components similar to those seen in Arabidopsis and Lepidium endosperms. In situ and biomechanical analyses were used to study changes in endosperms during seed germination and suggest a role for mannan degradation in tobacco. In the case of the Brassicaceae representatives, the structurally homogeneous cell walls of the endosperm can be acted on by spatially regulated CWRE expression. Genetic manipulations of cell wall components present in the Arabidopsis seed endosperm demonstrate the impact of cell wall architectural changes on germination kinetics.

Tissue-specific transcriptome analysis reveals cell wall metabolism, flavonol biosynthesis and defense responses are activated in the endosperm of germinating Arabidopsis thaliana seeds

Seed germination is a result of the competition of embryonic growth potential and mechanical constraint by surrounding tissues such as the endosperm. To understand the processes occurring in the endosperm during germination, we analyzed tiling array expression data on dissected endosperm and embryo from 6 and 24 h-imbibed Arabidopsis seeds. The genes preferentially expressed in the endosperm of both 6 and 24 h-imbibed seeds were enriched for those related to cell wall biosynthesis/modifications, flavonol biosynthesis, defense responses and cellular transport. Loss of function of AtXTH31/XTR8, an endosperm-specific gene for a putative xyloglucan endotransglycosylase/hydrolase, led to faster germination. This suggests that AtXTH31/XTR8 is involved in the reinforcement of the cell wall of the endosperm during germination. In vivo flavonol staining by diphenyl boric acid aminoethyl ester (DPBA) showed flavonols accumulated in the endosperm of both dormant and non-dormant seeds, suggesting that this event is independent of germination. Notably, DPBA fluorescence was also intense in the embryo, but the fluorescent region was diminished around the radicle and lower half of the hypocotyl during germination. DPBA fluorescence was localized in the vacuoles during germination. Vacuolation was not seen in imbibed dormant seeds, suggesting that vacuolation is associated with germination. A gene for δVPE (vacuolar processing enzyme), a caspase-1-like cysteine proteinase involved in cell death, is expressed specifically in endosperms of 24 h-imbibed seeds. The δvpe mutant showed retardation of vacuolation, but this mutation did not affect the kinetics of germination. This suggests that vacuolation is a consequence, and not a trigger, of germination.

Major changes in the cell wall during silique development in Arabidopsis thaliana

Fruit development is a highly complex process, which involves major changes in plant metabolism leading to cell growth and differentiation. Changes in cell wall composition and structure play a major role in modulating cell growth. We investigated the changes in cell wall composition and the activities of associated enzymes during the dry fruit development of the model plant Arabidopsis thaliana. Silique development is characterized by several specific phases leading to fruit dehiscence and seed dispersal. We showed that early phases of silique growth were characterized by specific changes in non-cellulosic sugar content (rhamnose, arabinose, xylose, galactose and galacturonic acid). Xyloglucan oligosaccharide mass profiling further showed a strong increase in O-acetylated xyloglucans over the course of silique development, which could suggest a decreased capacity of xyloglucans to be associated with each other or to cellulose. The degree of methylesterification, mediated by the activity of pectin methylesterases (PMEs), decreased over the course of silique growth and dehiscence. The major changes in cell wall composition revealed by our analysis suggest that it could be major determinants in modulating cell wall rheology leading to growth or growth arrest.

Arabinan metabolism during seed development and germination in Arabidopsis

Arabinans are found in the pectic network of many cell walls, where, along with galactan, they are present as side chains of Rhamnogalacturonan l. Whilst arabinans have been reported to be abundant polymers in the cell walls of seeds from a range of plant species, their proposed role as a storage reserve has not been thoroughly investigated. In the cell walls of Arabidopsis seeds, arabinose accounts for approximately 40% of the monosaccharide composition of non-cellulosic polysaccharides of embryos. Arabinose levels decline to approximately 15% during seedling establishment, indicating that cell wall arabinans may be mobilized during germination. Immunolocalization of arabinan in embryos, seeds, and seedlings reveals that arabinans accumulate in developing and mature embryos, but disappear during germination and seedling establishment. Experiments using 14C-arabinose show that it is readily incorporated and metabolized in growing seedlings, indicating an active catabolic pathway for this sugar. We found that depleting arabinans in seeds using a fungal arabinanase causes delayed seedling growth, lending support to the hypothesis that these polymers may help fuel early seedling growth.

AtBXL1 encodes a bifunctional beta-D-xylosidase/alpha-L-arabinofuranosidase required for pectic arabinan modification in Arabidopsis mucilage secretory cells

Following pollination, the epidermal cells of the Arabidopsis (Arabidopsis thaliana) ovule undergo a complex differentiation process that includes the synthesis and polar secretion of pectinaceous mucilage followed by the production of a secondary cell wall. Wetting of mature seeds leads to the rapid bursting of these mucilage secretory cells to release a hydrophilic gel that surrounds the seed and is believed to aid in seed hydration and germination. A novel mutant is identified where mucilage release is both patchy and slow and whose seeds display delayed germination. While developmental analysis of mutant seeds reveals no change in mucilage secretory cell morphology, changes in monosaccharide quantities are detected, suggesting the mucilage release defect results from altered mucilage composition. Plasmid rescue and cloning of the mutant locus revealed a T-DNA insertion in AtBXL1, which encodes a putative bifunctional beta-d-xylosidase/alpha-l-arabinofuranosidase that has been implicated as a beta-d-xylosidase acting during vascular development. Chemical and immunological analyses of mucilage extracted from bxl1 mutant seeds and antibody staining of developing seed coats reveal an increase in (1-->5)-linked arabinans, suggesting that BXL1 is acting as an alpha-l-arabinofuranosidase in the seed coat. This implication is supported by the ability to rescue mucilage release through treatment of bxl1 seeds with exogenous alpha-l-arabinofuranosidases. Together, these results suggest that trimming of rhamnogalacturonan I arabinan side chains is required for correct mucilage release and reveal a new role for BXL1 as an alpha-l-arabinofuranosidase acting in seed coat development.

Purification, cloning and functional characterization of an endogenous beta-glucuronidase in Arabidopsis thaliana

Beta-glucuronidase (GUS) activities have been extensively characterized in bacteria, fungi, and animals, and the bacterial enzyme GUSA from Escherichia coli is commonly used as a reporter for gene expression studies in plants. Although endogenous GUS activity has been observed in plants, the nature and function of the enzymes involved remain elusive. Here we report on tissue-specific localization, partial purification and identification of AtGUS2, a GUS active under acidic conditions from Arabidopsis thaliana. This enzyme belongs to the GH79 family in the Carbohydrate-Active Enzymes database, which also includes mammalian heparanases that degrade the carbohydrate moieties of cell surface proteoglycans, and fungal enzymes active on arabinogalactan proteins (AGPs). We characterized a knockout insertion line (atgus2-1) and transgenic lines overexpressing AtGUS2 (Pro(35S):AtGUS2). Endogenous GUS activity assayed histochemically and biochemically was absent in atgus2-1 tissues and four times higher in Pro(35S):AtGUS2 lines. AGPs purified from atgus2-1 and Pro(35S):AtGUS2 seedlings showed higher and markedly lower glucuronic acid content, respectively. Our results suggest that endogenous GUS activity influences the sugar composition of the complex polysaccharide chains of AGPs. We also show that transgenics display hypocotyl and root growth defects compared to wild-type plants. Hypocotyl and root lengths are increased in Pro(35S):AtGUS2 seedlings, whereas hypocoyl length is reduced in atgus2-1 seedlings. These data are consistent with a role for the carbohydrate moieties of AGPs in cell growth.

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.

Physiological roles of plant glycoside hydrolases

The functions of plant glycoside hydrolases and transglycosidases have been studied using different biochemical and molecular genetic approaches. These enzymes are involved in the metabolism of various carbohydrates containing compounds present in the plant tissues. The structural and functional diversity of the carbohydrates implies a vast spectrum of enzymes involved in their metabolism. Complete genome sequence of Arabidopsis and rice has allowed the classification of glycoside hydrolases in different families based on amino acid sequence data. The genomes of these plants contain 29 families of glycoside hydrolases. This review summarizes the current research on plant glycoside hydrolases concerning their principal functional roles, which were attributed to different families. The majority of these plant glycoside hydrolases are involved in cell wall polysaccharide metabolism. Other functions include their participation in the biosynthesis and remodulation of glycans, mobilization of energy, defence, symbiosis, signalling, secondary plant metabolism and metabolism of glycolipids.

A naturally occurring mutation in an Arabidopsis accession affects a beta-D-galactosidase that increases the hydrophilic potential of rhamnogalacturonan I in seed mucilage

The Arabidopsis thaliana accession Shahdara was identified as a rare naturally occurring mutant that does not liberate seed mucilage on imbibition. The defective locus was found to be allelic to the mum2-1 and mum2-2 mutants. Map-based cloning showed that MUCILAGE-MODIFIED2 (MUM2) encodes the putative beta-D-galactosidase BGAL6. Activity assays demonstrated that one of four major beta-D-galactosidase activities present in developing siliques is absent in mum2 mutants. No difference was observed in seed coat epidermal cell structure between wild-type and mutant seed; however, weakening of the outer tangential cell wall by chemical treatment resulted in the release of mucilage from mum2 seed coat epidermal cells, and the mum2 mucilage only increased slightly in volume, relative to the wild type. Consistent with the absence of beta-D-galactosidase activity in the mutant, the inner layer of mucilage contained more Gal. The allocation of polysaccharides between the inner and outer mucilage layers was also modified in mum2. Mass spectrometry showed that rhamnogalacturonan I in mutant mucilage had more branching between rhamnose and hexose residues relative to the wild type. We conclude that the MUM2/BGAL6 beta-D-galactosidase is required for maturation of rhamnogalacturonan I in seed mucilage by the removal of galactose/galactan branches, resulting in increased swelling and extrusion of the mucilage on seed hydration.

A sub-proteome of Arabidopsis thaliana mature stems trapped on Concanavalin A is enriched in cell wall glycoside hydrolases

N-glycosylated proteins were isolated from Arabidopsis thaliana mature stems using affinity chromatography on Concanavalin A Sepharose, separated by 2D-electrophoresis and identified using nanoHPLC-MS/MS and MALDI-TOF MS. 102 glycoproteins were identified. 94% of these proteins were predicted by bioinformatics to be targeted to the secretory pathway and 87% of them were predicted to be localized in the cell wall or at the plasma membrane. 30% of these proteins belong to glycoside hydrolase (GH) families with some of them possibly involved in the hydrolysis of cell wall polysaccharides. The second major class of identified proteins comprises aspartyl and serine proteases. Other proteins are predicted to be oxido-reductases, contain interacting domains, are potentially involved in signalling or have an unknown function. This is, to our knowledge, the first survey of plant cell wall N-glycosylated proteins.

Thioimidoyl furanosides as first inhibitors of the α-l-arabinofuranosidase AbfD3

Two sets of five thioimidoyl alpha-L-arabino- and beta-D-galactofuranosides were designed, synthesized and subjected to docking studies to evaluate their ability to be recognized by the active site of the alpha-L-arabinofuranosidase AbfD3. Further in vitro assays showed that the targeted furanosides are the first potent inhibitors of this furanosyl hydrolase and that the most efficient one, the thiazolyl alpha-L-arabinofuranoside 1, is a competitive inhibitor having a KI of 1.4 microM.