Arabinogalactan-proteins: a class of extracellular matrix proteoglycans involved in plant growth and development

Identification and expression profiles of xylogen-like arabinogalactan protein (XYLP) gene family in Phyllostachys edulis in different developmental tissues and under various abiotic stresses

Xylogen-like arabinogalactan protein (XYLP) is an atypical lipid transport protein. In this study, 23 Phyllostachys edulis XYLPs were identified, and their proteins contain characteristic structures of AGP and nsLTP domain. All PeXYLPs can be divided into four clades, and their genes were unevenly distributed on 11 chromosome scaffolds. Collinear analysis revealed that segmental duplication was the main driver for PeXYLP family expansion. The cis-acting elements presented in the promoter are involved in various regulations of PeXYLPs expression. G.O. annotation revealed that PeXYLPs are mainly interested in lipid transport and synthesis and primarily function at the plasma membrane. Transcriptome analysis revealed that PeXYLPs were spatiotemporally expressed and displayed significant variability during various tissue development. Besides that, some PeXYLPs also respond to multiple phytohormones and abiotic stresses. By semi-quantitative RT-PCR, the response of some PeXYLPs to MeJA was confirmed, and the proteins were shown to localize to the plasma membrane mainly. WGCNA in defined regions of fast-growing bamboo shoots revealed that 5 PeXYLPs in 4 gene co-expression modules showed a positive module-trait relationship with three fast-growing regions. This systematic analysis of the PeXYLP family will provide a foundation for further insight into the functions of individual PeXYLP in a specific tissue or organ development, phytohormone perception, and stress responses in the future.

Review: structure and modifications of arabinogalactan proteins (AGPs)

The aim of this report is to provide general information on the molecular structure and synthesis of arabinogalactan proteins (AGPs) in association to their physiological significance. Assessment of genetic modifications of the activity of enzymes involved in the AGP biosynthesis is an efficient tool to study AGP functions. Thus, P4H (prolyl 4 hydroxylase) mutants, GLCAT (β-glucuronosyltransferase) mutants, and GH43 (glycoside hydrolase family 43) mutants have been described. We focused on the overview of AGPs modifications observed at the molecular, cellular, and organ levels. Inhibition of the hydroxylation process results in an increase in the intensity of cell divisions and thus, has an impact on root system length and leaf area. In turn, overexpression of P4H genes stimulates the density of root hairs. A mutation in GLCAT genes responsible for the transfer of glucuronic acid to the AGP molecule revealed that the reduction of GlcA in AGP disrupts the substantial assembly of the primary cell wall. Furthermore, silencing of genes encoding GH43, which has the ability to hydrolyze the AGP glycan by removing incorrectly synthesized β-1,3-galactans, induces changes in the abundance of other cell wall constituents, which finally leads to root growth defects. This information provides insight into AGPs as a crucial players in the structural interactions present in the plant extracellular matrix.

Functional characterization of hydroxyproline-O-galactosyltransferases for Arabidopsis arabinogalactan-protein synthesis

BACKGROUND

Arabinogalactan-proteins (AGPs) are structurally complex hydroxyproline-rich cell wall glycoproteins ubiquitous in the plant kingdom. AGPs biosynthesis involves a series of post-translational modifications including the addition of type II arabinogalactans to non-contiguous Hyp residues. To date, eight Hyp-galactosyltransferases (Hyp-GALTs; GALT2-GALT9) belonging to CAZy GT31, are known to catalyze the addition of the first galactose residues to AGP protein backbones and enable subsequent AGP glycosylation. The extent of genetic redundancy, however, remains to be elucidated for the Hyp-GALT gene family.

RESULTS

To examine their gene redundancy and functions, we generated various multiple gene knock-outs, including a triple mutant (galt5 galt8 galt9), two quadruple mutants (galt2 galt5 galt7 galt8, galt2 galt5 galt7 galt9), and one quintuple mutant (galt2 galt5 galt7 galt8 galt9), and comprehensively examined their biochemical and physiological phenotypes. The key findings include: AGP precipitations with β-Yariv reagent showed that GALT2, GALT5, GALT7, GALT8 and GALT9 act redundantly with respect to AGP glycosylation in cauline and rosette leaves, while the activity of GALT7, GALT8 and GALT9 dominate in the stem, silique and flowers. Monosaccharide composition analysis showed that galactose was decreased in the silique and root AGPs of the Hyp-GALT mutants. TEM analysis of 25789 quintuple mutant stems indicated cell wall defects coincident with the observed developmental and growth impairment in these Hyp-GALT mutants. Correlated with expression patterns, galt2, galt5, galt7, galt8, and galt9 display equal additive effects on insensitivity to β-Yariv-induced growth inhibition, silique length, plant height, and pollen viability. Interestingly, galt7, galt8, and galt9 contributed more to primary root growth and root tip swelling under salt stress, whereas galt2 and galt5 played more important roles in seed morphology, germination defects and seed set. Pollen defects likely contributed to the reduced seed set in these mutants.

CONCLUSION

Additive and pleiotropic effects of GALT2, GALT5, GALT7, GALT8 and GALT9 on vegetative and reproductive growth phenotypes were teased apart via generation of different combinations of Hyp-GALT knock-out mutants. Taken together, the generation of higher order Hyp-GALT mutants demonstrate the functional importance of AG polysaccharides decorating the AGPs with respect to various aspects of plant growth and development.

Structural study of a pectic polysaccharide fraction isolated from "mountain tea" (Sideritis scardica Griseb.)

The present study was conducted to investigate the structural characteristics of an acid-extracted polysaccharide fraction from mountain tea. The monosaccharide composition revealed that uronic acids (72.4 mol%) considerably predominated in the fraction, followed by smaller amounts of galactose (14.5 mol%) and glucose (6.2 mol%). The fraction contained mostly a highly methyl-esterified homogalacturonan (HG) - 71 mol%. The pectin had a high molecular weight population (∼60-100 kDa). Enzymatic fingerprinting was employed with a combination of HG degrading enzymes and LC-HILIC-MS, HPAEC, HPSEC to examine the structure in greater detail. Unsaturated oligomers released indicated the presence of large blocks of highly methyl-esterified GalA residues. Furthermore, the presence of blocks of non-esterified GalA residues and partly methyl-esterified and acetylated GalA residues in HG domain was demonstrated. The research findings provide a basis for further investigations regarding biological activity and commercial exploitation of mountain tea.

Three Decades of Advances in Arabinogalactan-Protein Biosynthesis

Arabinogalactan-proteins (AGPs) are a large, complex, and highly diverse class of heavily glycosylated proteins that belong to the family of cell wall hydroxyproline-rich glycoproteins. Approximately 90% of the molecules consist of arabinogalactan polysaccharides, which are composed of arabinose and galactose as major sugars and minor sugars such as glucuronic acid, fucose, and rhamnose. About half of the AGP family members contain a glycosylphosphatidylinositol (GPI) lipid anchor, which allows for an association with the outer leaflet of the plasma membrane. The mysterious AGP family has captivated the attention of plant biologists for several decades. This diverse family of glycoproteins is widely distributed in the plant kingdom, including many algae, where they play fundamental roles in growth and development processes. The journey of AGP biosynthesis begins with the assembly of amino acids into peptide chains of proteins. An N-terminal signal peptide directs AGPs toward the endoplasmic reticulum, where proline hydroxylation occurs and a GPI anchor may be added. GPI-anchored AGPs, as well as unanchored AGPs, are then transferred to the Golgi apparatus, where extensive glycosylation occurs by the action of a variety glycosyltransferase enzymes. Following glycosylation, AGPs are transported by secretory vesicles to the cell wall or to the extracellular face of the plasma membrane (in the case of GPI-anchored AGPs). GPI-anchored proteins can be released from the plasma membrane into the cell wall by phospholipases. In this review, we present an overview of the accumulated knowledge on AGP biosynthesis over the past three decades. Particular emphasis is placed on the glycosylation of AGPs as the sugar moiety is essential to their function. Recent genetics and genomics approaches have significantly contributed to a broader knowledge of AGP biosynthesis. However, many questions remain to be elucidated in the decades ahead.

Multi-omics Approaches To Decipher the Impact of Diet and Host Physiology on the Mammalian Gut Microbiome

In recent years, various studies have demonstrated that the gut microbiota influences host metabolism. However, these studies were focused primarily on a single or a limited range of host species, thus preventing a full exploration of possible taxonomic and functional adaptations by gut microbiota members as a result of host-microbe coevolution events. In the current study, the microbial taxonomic profiles of 250 fecal samples, corresponding to 77 host species that cover the mammalian branch of the tree of life, were reconstructed by 16S rRNA gene-based sequence analysis. Moreover, shotgun metagenomics was employed to investigate the metabolic potential of the fecal microbiomes of 24 mammals, and subsequent statistical analyses were performed to assess the impact of host diet and corresponding physiology of the digestive system on gut microbiota composition and functionality. Functional data were confirmed and extended through metatranscriptome assessment of gut microbial populations of eight animals, thus providing insights into the transcriptional response of gut microbiota to specific dietary lifestyles. Therefore, the analyses performed in this study support the notion that the metabolic features of the mammalian gut microbiota have adapted to maximize energy extraction from the host's diet.IMPORTANCE Diet and host physiology have been recognized as main factors affecting both taxonomic composition and functional features of the mammalian gut microbiota. However, very few studies have investigated the bacterial biodiversity of mammals by using large sample numbers that correspond to multiple mammalian species, thus resulting in an incomplete understanding of the functional aspects of their microbiome. Therefore, we investigated the bacterial taxonomic composition of 250 fecal samples belonging to 77 host species distributed along the tree of life in order to assess how diet and host physiology impact the intestinal microbial community by selecting specific microbial players. Conversely, the application of shotgun metagenomics and metatranscriptomics approaches to a group of selected fecal samples allowed us to shed light on both metabolic features and transcriptional responses of the intestinal bacterial community based on different diets.

Golgi-localized exo-β1,3-galactosidases involved in cell expansion and root growth in Arabidopsis

Plant arabinogalactan proteins (AGPs) are a diverse group of cell surface- and wall-associated glycoproteins. Functionally important AGP glycans are synthesized in the Golgi apparatus, but the relationships among their glycosylation levels, processing, and functionalities are poorly understood. Here, we report the identification and functional characterization of two Golgi-localized exo-β-1,3-galactosidases from the glycosyl hydrolase 43 (GH43) family in Arabidopsis thaliana GH43 loss-of-function mutants exhibited root cell expansion defects in sugar-containing growth media. This root phenotype was associated with an increase in the extent of AGP cell wall association, as demonstrated by Yariv phenylglycoside dye quantification and comprehensive microarray polymer profiling of sequentially extracted cell walls. Characterization of recombinant GH43 variants revealed that the exo-β-1,3-galactosidase activity of GH43 enzymes is hindered by β-1,6 branches on β-1,3-galactans. In line with this steric hindrance, the recombinant GH43 variants did not release galactose from cell wall-extracted glycoproteins or AGP-rich gum arabic. These results indicate that the lack of exo-β-1,3-galactosidase activity alters cell wall extensibility in roots, a phenotype that could be explained by the involvement of galactosidases in AGP glycan biosynthesis.

Arabinogalactan proteins: Distribution during the development of male and female gametophytes

Arabinogalactan proteins (AGPs), i.e. a subfamily of hydroxyproline-rich proteins (HRGPs), are widely distributed in the plant kingdom. For many years, AGPs have been connected with the multiple phases of plant reproduction and developmental processes. Currently, extensive knowledge is available about their various functions, i.e. involvement in pollen grain formation, initiation of pollen grain germination, pollen tube guidance in the transmission tissue of pistil and ovule nucellus, and function as a signaling molecule during cell-cell communication. Although many studies have been performed, the mechanism of action, the heterogeneous molecule structure, and the connection with other extracellular matrix components have not been sufficiently explained. The aim of this work was to gather and describe the most important information on the distribution of AGPs in gametophyte development. The present review provides a summary of the first reports about AGPs and the most recent knowledge about their functions during male and female gametophyte formation.

Back to the future with the AGP-Ca2+ flux capacitor

BACKGROUND

Arabinogalactan proteins (AGPs) are ubiquitous in green plants. AGPs comprise a widely varied group of hydroxyproline (Hyp)-rich cell surface glycoproteins (HRGPs). However, the more narrowly defined classical AGPs massively predominate and cover the plasma membrane. Extensive glycosylation by pendant polysaccharides O-linked to numerous Hyp residues like beads of a necklace creates a unique ionic compartment essential to a wide range of physiological processes including germination, cell extension and fertilization. The vital clue to a precise molecular function remained elusive until the recent isolation of small Hyp-arabinogalactan polysaccharide subunits; their structural elucidation by nuclear magentic resonance imaging, molecular simulations and direct experiment identified a 15-residue consensus subunit as a β-1,3-linked galactose trisaccharide with two short branched sidechains each with a single glucuronic acid residue that binds Ca(2+) when paired with its adjacent sidechain.

SCOPE

AGPs bind Ca(2+) (Kd ∼ 6 μm) at the plasma membrane (PM) at pH ∼5·5 but release it when auxin-dependent PM H(+)-ATPase generates a low periplasmic pH that dissociates AGP-Ca(2+) carboxylates (pka ∼3); the consequential large increase in free Ca(2+) drives entry into the cytosol via Ca(2+) channels that may be voltage gated. AGPs are thus arguably the primary source of cytosolic oscillatory Ca(2+) waves. This differs markedly from animals, in which cytosolic Ca(2+) originates mostly from internal stores such as the sarcoplasmic reticulum. In contrast, we propose that external dynamic Ca(2+) storage by a periplasmic AGP capacitor co-ordinates plant growth, typically involving exocytosis of AGPs and recycled Ca(2+), hence an AGP-Ca(2+) oscillator.

CONCLUSIONS

The novel concept of dynamic Ca(2+) recycling by an AGP-Ca(2+) oscillator solves the long-standing problem of a molecular-level function for classical AGPs and thus integrates three fields: AGPs, Ca(2+) signalling and auxin. This accounts for the involvement of AGPs in plant morphogenesis, including tropic and nastic movements.

Identification of Novel Peptidyl Serine α-Galactosyltransferase Gene Family in Plants

In plants, serine residues in extensin, a cell wall protein, are glycosylated with O-linked galactose. However, the enzyme that is involved in the galactosylation of serine had not yet been identified. To identify the peptidyl serine O-α-galactosyltransferase (SGT), we chose Chlamydomonas reinhardtii as a model. We established an assay system for SGT activity using C. reinhardtii and Arabidopsis thaliana cell extracts. SGT protein was partially purified from cell extracts of C. reinhardtii and analyzed by tandem mass spectrometry to determine its amino acid sequence. The sequence matched the open reading frame XP_001696927 in the C. reinhardtii proteome database, and a corresponding DNA fragment encoding 748 amino acids (BAL63043) was cloned from a C. reinhardtii cDNA library. The 748-amino acid protein (CrSGT1) was produced using a yeast expression system, and the SGT activity was examined. Hydroxylation of proline residues adjacent to a serine in acceptor peptides was required for SGT activity. Genes for proteins containing conserved domains were found in various plant genomes, including A. thaliana and Nicotiana tabacum. The AtSGT1 and NtSGT1 proteins also showed SGT activity when expressed in yeast. In addition, knock-out lines of AtSGT1 and knockdown lines of NtSGT1 showed no or reduced SGT activity. The SGT1 sequence, which contains a conserved DXD motif and a C-terminal membrane spanning region, is the first example of a glycosyltransferase with type I membrane protein topology, and it showed no homology with known glycosyltransferases, indicating that SGT1 belongs to a novel glycosyltransferase gene family existing only in the plant kingdom.

Proteoglycans from Boswellia serrata Roxb. and B. carteri Birdw. and identification of a proteolytic plant basic secretory protein

Water-soluble high molecular weight compounds were isolated in yields of 21-22% from the oleogum of Boswellia serrata and B. carteri. Using anion exchange chromatography and gel permeation chromatography, different proteoglycans were purified and characterized, leading to four principally different groups: (i) Hyp-/Ser-rich extensins with O-glycosidic attached arabinan side chains; (ii) Modified extensins, with arabinogalactosylated side chains containing GlA and 4-O-Me-GlcA; (iii) Glycoproteins with N-glycosidic side chains containing higher amounts of Fuc, Man and GluNH(2,) featuring a 200 kD metalloproteinase that has been de novo sequenced and is described for the first time; (iv) Type II arabinogalactans-proteins. Significant differences between the gums from the two species were observed in the protein content (6% vs 22%), offering the possibility of a quick differentiation of gums from both species for analytical quality control. The data also offer an insight into the plant response towards wound-closing by the formation of extensin and AGP-containing gum.

Arabinogalactan proteins in root and pollen-tube cells: distribution and functional aspects

BACKGROUND

Arabinogalactan proteins (AGPs) are complex proteoglycans of the cell wall found in the entire plant kingdom and in almost all plant organs. AGPs encompass a large group of heavily glycosylated cell-wall proteins which share common features, including the presence of glycan chains especially enriched in arabinose and galactose and a protein backbone particularly rich in hydroxyproline residues. However, AGPs also exhibit strong heterogeneities among their members in various plant species. AGP ubiquity in plants suggests these proteoglycans are fundamental players for plant survival and development.

SCOPE

In this review, we first present an overview of current knowledge and specific features of AGPs. A section devoted to major tools used to study AGPs is also presented. We then discuss the distribution of AGPs as well as various aspects of their functional properties in root tissues and pollen tubes. This review also suggests novel directions of research on the role of AGPs in the biology of roots and pollen tubes.

GhAGP31, a cotton non-classical arabinogalactan protein, is involved in response to cold stress during early seedling development

Arabinogalactan proteins (AGPs), a superfamily of highly glycosylated hydroxyproline-rich glycoproteins, are widely implicated in plant growth and development. A gene (including its cDNA), designated GhAGP31, encoding a non-classical AGP protein was isolated from cotton (Gossypium hirsutum). The deduced GhAGP31 protein contains the conserved features of non-classical AGPs: a putative signal peptide, N-terminal histidine-rich stretch, middle repetitive proline-rich domain and a cysteine-containing 'PAC' domain. GFP fluorescence assay demonstrated that GhAGP31 protein was localised on cell walls. GhAGP31 transcripts were mainly detected in roots, hypocotyls and ovules, but little or almost none were detected in other tissues. In particular, expression of GhAGP31 was developmentally regulated in roots. Further study demonstrated that GhAGP31 expression in cotton roots was remarkably up-regulated by cold stress. Expression of the GUS gene driven by the GhAGP31 promoter was also dramatically enhanced in roots of transgenic Arabidopsis seedlings under cold treatment. Additionally, overexpression of GhAGP31 in yeast and Arabidopsis significantly improved the freezing tolerance of yeast cells and cold tolerance of Arabidopsis seedlings. These data imply that GhAGP31 protein may be involved in the response to cold stress during early root development of cotton.

The fasciclin-like arabinogalactan protein gene, FLA3, is involved in microspore development of Arabidopsis

Arabinogalactan proteins are widely distributed in plant tissues and cells, and may function in the growth and development of higher plants. To our knowledge, there is currently no direct evidence concerning the involvement of fasciclin-like arabinogalactan proteins (FLA) in sexual reproduction in Arabidopsis. In this study, Arabidopsis FLA3 was found to be specifically expressed in pollen grains and tubes. Subcellular localization showed that FLA3 anchors tightly to the plasma membrane, and its glycosylphosphatidylinositol anchor may affect its localization. FLA3-RNA interference transgenic plants had approximately 50% abnormal pollen grains (including shrunken and wrinkled phenotypes) which lacked viability. Cytological observations revealed that pollen abortion occurred during the transition from uninucleate microspores to bicellular pollens, with abnormal cellulose distribution seen by calcofluor white staining. Transmission electron microscopy showed that the basic structure of the exine layer in aberrant pollen was normal, but the intine layer appeared to have some abnormalities. Taken together, these results suggest that FLA3 is involved in microspore development and may affect pollen intine formation, possibly by participating in cellulose deposition. In FLA3-overexpressing transgenic plants, defective elongation of the stamen filament and reduced female fertility led to short siliques with low seed set, which suggested that ectopic expression of FLA3 in tissues may reduce or disrupt cell growth and then result in defects throughout the plant.

Identification of novel genes potentially involved in somatic embryogenesis in chicory (Cichorium intybus L.)

BACKGROUND

In our laboratory we use cultured chicory (Cichorium intybus) explants as a model to investigate cell reactivation and somatic embryogenesis and have produced 2 chicory genotypes (K59, C15) sharing a similar genetic background. K59 is a responsive genotype (embryogenic) capable of undergoing complete cell reactivation i.e. cell de- and re-differentiation leading to somatic embryogenesis (SE), whereas C15 is a non-responsive genotype (non-embryogenic) and is unable to undergo SE. Previous studies 1 showed that the use of the beta-D-glucosyl Yariv reagent (beta-GlcY) that specifically binds arabinogalactan-proteins (AGPs) blocked somatic embryo production in chicory root explants. This observation indicates that beta-GlcY is a useful tool for investigating somatic embryogenesis (SE) in chicory. In addition, a putative AGP (DT212818) encoding gene was previously found to be significantly up-regulated in the embryogenic K59 chicory genotype as compared to the non-embryogenic C15 genotype suggesting that this AGP could be involved in chicory re-differentiation 2. In order to improve our understanding of the molecular and cellular regulation underlying SE in chicory, we undertook a detailed cytological study of cell reactivation events in K59 and C15 genotypes, and used microarray profiling to compare gene expression in these 2 genotypes. In addition we also used beta-GlcY to block SE in order to identify genes potentially involved in this process.

RESULTS

Microscopy confirmed that only the K59, but not the C15 genotype underwent complete cell reactivation leading to SE formation. beta-GlcY-treatment of explants blocked in vitro SE induction, but not cell reactivation, and induced cell wall modifications. Microarray analyses revealed that 78 genes were differentially expressed between induced K59 and C15 genotypes. The expression profiles of 19 genes were modified by beta-GlcY-treatment. Eight genes were both differentially expressed between K59 and C15 genotypes during SE induction and transcriptionally affected by beta-GlcY-treatment: AGP (DT212818), 26 S proteasome AAA ATPase subunit 6 (RPT6), remorin (REM), metallothionein-1 (MT1), two non-specific lipid transfer proteins genes (SDI-9 and DEA1), 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase), and snakin 2 (SN2). These results suggest that the 8 genes, including the previously-identified AGP gene (DT212818), could be involved in cell fate determination events leading to SE commitment in chicory.

CONCLUSION

The use of two different chicory genotypes differing in their responsiveness to SE induction, together with beta-GlcY-treatment represented an efficient tool to discriminate cell reactivation from the SE morphogenetic pathway. Such an approach, together with microarray analyses, permitted us to identify several putative key genes related to the SE morphogenetic pathway in chicory.

Genome-wide identification, classification, and expression analysis of the arabinogalactan protein gene family in rice (Oryza sativa L.)

Arabinogalactan proteins (AGPs) comprise a family of hydroxyproline-rich glycoproteins that are implicated in plant growth and development. In this study, 69 AGPs are identified from the rice genome, including 13 classical AGPs, 15 arabinogalactan (AG) peptides, three non-classical AGPs, three early nodulin-like AGPs (eNod-like AGPs), eight non-specific lipid transfer protein-like AGPs (nsLTP-like AGPs), and 27 fasciclin-like AGPs (FLAs). The results from expressed sequence tags, microarrays, and massively parallel signature sequencing tags are used to analyse the expression of AGP-encoding genes, which is confirmed by real-time PCR. The results reveal that several rice AGP-encoding genes are predominantly expressed in anthers and display differential expression patterns in response to abscisic acid, gibberellic acid, and abiotic stresses. Based on the results obtained from this analysis, an attempt has been made to link the protein structures and expression patterns of rice AGP-encoding genes to their functions. Taken together, the genome-wide identification and expression analysis of the rice AGP gene family might facilitate further functional studies of rice AGPs.

Expression and localization of GhH6L, a putative classical arabinogalactan protein in cotton (Gossypium hirsutum)

Arabinogalactan proteins (AGPs) are a large family of highly glycosylated of hydroxyproline-rich glycoproteins that play important roles in plant growth, development, and signal transduction. A cDNA encoding a putative classical AGP named GhH6L was isolated from cotton fiber cDNA libraries, and the deduced protein contains 17 copies of repetitive motif of X-Y-proline-proline-proline (where X is serine or alanine and Y is threonine or serine). Northern blotting analysis and quantitative RT-PCR results showed that it was preferentially expressed in 10 days post-anthesis (dpa) fibers and was also developmentally regulated. A promoter fragment was isolated from cotton (Gossypium hirsutum) by genome walking PCR. Expression of beta-glucuronidase (GUS) gene under the GhH6L promoter was examined in the transgenic Arabidopsis plants; only petiole and pedicel were stained, no staining was detected in other tissues. Subcellular localization indicated that GhH6L was localized to the plasma membrane and in the cytoplasm. These data further our understanding of GhH6L as well as shed light on functional insight to GhH6L in cotton.

Localization of an arabinogalactan protein epitope and the effects of Yariv phenylglycoside during zygotic embryo development of Arabidopsis thaliana

Arabinogalactan proteins (AGPs) are a class of highly glycosylated proteins widely distributed in higher plants and thought to be involved in plant growth and development. In the present paper, Western blotting with the monoclonal antibodies JIM4, JIM13, and LM2 showed that JIM13 reacted best with total protein extracts from flowers and siliques of Arabidopsis thaliana. This monoclonal antibody was therefore used as a probe to localize the AGP epitope in zygotic embryos at different developmental stages. Immunofluorescent labeling with JIM13 showed that AGPs were mainly distributed in the embryo proper and the top 1 to 2 cells and basal part of suspensors. The results of immunogold labeling confirmed the JIM13 epitope distribution in the different cells of the suspensor. AGP immunofluorescence was also observed at the shoot apex meristem during transition from the globular to the heart embryo stage, but this gradually disappeared after the torpedo stage. After (beta-D-Glc)(3) Yariv phenylglycoside (betaGlcY), a synthetic reagent that specifically binds to AGPs, was added to A. thaliana ovule culture medium, the survival rate and frequency of development of ovules at the zygote stage decreased in a concentration-dependent manner, with complete inhibition at 100 microM. The frequency of embryo differentiation from the globular stage to heart or later stages also decreased sharply. When betaGlcY was removed 24 h after inoculation, the inhibitory effects were reversible in a concentration-dependent and time-dependent manner. The results show that betaGlcY can inhibit embryo development and differentiation in A. thaliana, and the inhibitory effects are concentration dependent and reversible, indicating that AGPs are involved in embryo differentiation and shoot meristem formation. The possible roles of AGPs in A. thaliana zygotic embryo development are also discussed.

Arabinogalactan proteins: involvement in plant growth and morphogenesis

Arabinogalactan proteins (AGPs) are highly glycosylated hydroxyproline-containing variously located proteoglycans dynamically regulated in the course of plant ontogenesis. Special functions of AGPs are still unclear, but their involvement in vegetative growth and reproduction of plants is well established. This review considers data on the structure, biosynthesis, and metabolism of AGPs. Special attention is given to involvement of AGPs in growth and morphogenesis, and possible mechanisms of their regulatory action are considered. AGPs are also compared with animal proteoglycans.

An exo-beta-1,3-galactanase having a novel beta-1,3-galactan-binding module from Phanerochaete chrysosporium

An exo-beta-1,3-galactanase gene from Phanerochaete chrysosporium has been cloned, sequenced, and expressed in Pichia pastoris. The complete amino acid sequence of the exo-beta-1,3-galactanase indicated that the enzyme consists of an N-terminal catalytic module with similarity to glycoside hydrolase family 43 and an additional unknown functional domain similar to carbohydrate-binding module family 6 (CBM6) in the C-terminal region. The molecular mass of the recombinant enzyme was estimated as 55 kDa based on SDS-PAGE. The enzyme showed reactivity only toward beta-1,3-linked galactosyl oligosaccharides and polysaccharide as substrates but did not hydrolyze beta-1,4-linked galacto-oligosaccharides, beta-1,6-linked galacto-oligosaccharides, pectic galactan, larch arabinogalactan, arabinan, gum arabic, debranched arabinan, laminarin, soluble birchwood xylan, or soluble oat spelled xylan. The enzyme also did not hydrolyze beta-1,3-galactosyl galactosaminide, beta-1,3-galactosyl glucosaminide, or beta-1,3-galactosyl arabinofuranoside, suggesting that it specifically cleaves the internal beta-1,3-linkage of two galactosyl residues. High performance liquid chromatographic analysis of the hydrolysis products showed that the enzyme produced galactose from beta-1,3-galactan in an exo-acting manner. However, no activity toward p-nitrophenyl beta-galactopyranoside was detected. When incubated with arabinogalactan proteins, the enzyme produced oligosaccharides together with galactose, suggesting that it is able to bypass beta-1,6-linked galactosyl side chains. The C-terminal CBM6 did not show any affinity for known substrates of CBM6 such as xylan, cellulose, and beta-1,3-glucan, although it bound beta-1,3-galactan when analyzed by affinity electrophoresis. Frontal affinity chromatography for the CBM6 moiety using several kinds of terminal galactose-containing oligosaccharides as the analytes clearly indicated that the CBM6 specifically interacted with oligosaccharides containing a beta-1,3-galactobiose moiety. When the degree of polymerization of galactose oligomers was increased, the binding affinity of the CBM6 showed no marked change.

Post-translational modifications of arabinogalactan-peptides of Arabidopsis thaliana. Endoplasmic reticulum and glycosylphosphatidylinositol-anchor signal cleavage sites and hydroxylation of proline

We have developed a method for separating the deglycosylated protein/peptide backbones of the small arabinogalactan (AG)-peptides from the larger classical arabinogalactan-proteins (AGPs). AGPs are an important class of plant proteoglycans implicated in plant growth and development. Separation of AG-peptides enabled us to identify eight of 12 AG-peptides from Arabidopsis thaliana predicted from genomic sequences. Of the remaining four, two have low abundance based on expressed sequence tag databases and the other two are only present in pollen (At3g20865) or flowers (At3g57690) and therefore would not be detected in our analysis. Characterization of AG-peptides was performed using matrix-assisted laser desorption ionization-time of flight mass spectrometry and tandem mass spectrometry protein sequencing. These data provide (i) experimental evidence that AG-peptides are processed in vivo for the addition of a glycosylphosphatidylinositol (GPI) anchor, (ii) cleavage site information for both the endoplasmic reticulum secretion signal and the GPI-anchor signal for eight of the 12 AG-peptides, and (iii) experimental evidence that the Gly-Pro motif is hydroxylated in vivo. Furthermore, we show that AtAGP16 is GPI-anchored despite its unusually long hydrophobic C-terminal GPI-signal sequence. Prior to this work, the GPI-anchor cleavage site for only two plant proteins, NaAGP1 from Nicotiana alata and PcAGP1 from Pyrus communis, had been determined experimentally. Characterization of the post-translational modifications of AG-peptides contributes toward obtaining the complete primary structure of this class of biologically important plant proteoglycans and provides a greater understanding of post-translational modifications of plant proteins.

Characterization of the Arabidopsis lysine-rich arabinogalactan-protein AtAGP17 mutant (rat1) that results in a decreased efficiency of agrobacterium transformation

Arabinogalactan-proteins (AGPs) are a family of complex proteoglycans widely distributed in plants. The Arabidopsis rat1 mutant, previously characterized as resistant to Agrobacterium tumefaciens root transformation, is due to a mutation in the gene for the Lys-rich AGP, AtAGP17. We show that the phenotype of rat1 correlates with down-regulation of AGP17 in the root as a result of a T-DNA insertion into the promoter of AGP17. Complementation of rat1 plants by a floral dip method with either the wild-type AGP17 gene or cDNA can restore the plant to a wild-type phenotype in several independent transformants. Based on changes in PR1 gene expression and a decrease in free salicylic acid levels upon Agrobacterium infection, we suggest mechanisms by which AGP17 allows Agrobacterium rapidly to reduce the systemic acquired resistance response during the infection process.

Structure of a hydroxyproline (Hyp)-arabinogalactan polysaccharide from repetitive Ala-Hyp expressed in transgenic Nicotiana tabacum

A synthetic gene encoding the fusion protein (Ala-Hyp)(51)-enhanced green fluorescent protein expressed in Nicotiana tabacum cells produced a fusion glycoprotein with all proline residues hydroxylated and substituted with an arabinogalactan polysaccharide. Alkaline hydrolysis of the fusion glycoprotein yielded a population of hydroxyproline (Hyp)-arabinogalactan polysaccharides ranging in size from 13 to 26 saccharide residues/Hyp, with a median size of 15-17 residues. We isolated a 15-residue Hyp-arabinogalactan for structure determination by sugar analyses and one- and two-dimensional nuclear magnetic resonance techniques that provided the assignment of proton and carbon signals of a small polysaccharide O-linked to the hydroxyl group of Hyp. The polysaccharide consisted of a 1,3-linked beta-D-Galp backbone with a single 1,6-linked beta-D-Galp "kink." The backbone had two side chains of Galp substituted at position 3 with an arabinose di- or trisaccharide and at position 6 with glucuronic acid or rhamnosyl glucuronic acid. Energy-minimized space-filling molecular models showed hydrogen bonding within polysaccharides attached to repetitive Ala-Hyp and also between polysaccharides and the peptide backbone. Polysaccharides distorted the peptide Ramachandran angles consistent with the circular dichroic spectra of isolated (Ala-Hyp)(51) and its reversion to a polyproline II-like helix after deglycosylation. This first complete structure of a Hyp-arabinogalactan polysaccharide shows that computer-based molecular modeling of Hyp-rich glycoproteins is now feasible and supports the suggestion that small repetitive subunits comprise larger arabinogalactan polysaccharides.

Using genomic resources to guide research directions. The arabinogalactan protein gene family as a test case

Arabinogalactan proteins (AGPs) are extracellular hydroxyproline-rich proteoglycans implicated in plant growth and development. The protein backbones of AGPs are rich in proline/hydroxyproline, serine, alanine, and threonine. Most family members have less than 40% similarity; therefore, finding family members using Basic Local Alignment Search Tool searches is difficult. As part of our systematic analysis of AGP function in Arabidopsis, we wanted to make sure that we had identified most of the members of the gene family. We used the biased amino acid composition of AGPs to identify AGPs and arabinogalactan (AG) peptides in the Arabidopsis genome. Different criteria were used to identify the fasciclin-like AGPs. In total, we have identified 13 classical AGPs, 10 AG-peptides, three basic AGPs that include a short lysine-rich region, and 21 fasciclin-like AGPs. To streamline the analysis of genomic resources to assist in the planning of targeted experimental approaches, we have adopted a flow chart to maximize the information that can be obtained about each gene. One of the key steps is the reformatting of the Arabidopsis Functional Genomics Consortium microarray data. This customized software program makes it possible to view the ratio data for all Arabidopsis Functional Genomics Consortium experiments and as many genes as desired in a single spreadsheet. The results for reciprocal experiments are grouped to simplify analysis and candidate AGPs involved in development or biotic and abiotic stress responses are readily identified. The microarray data support the suggestion that different AGPs have different functions.

Tomato LeAGP-1 arabinogalactan-protein purified from transgenic tobacco corroborates the Hyp contiguity hypothesis

Functional analysis of the hyperglycosylated arabinogalactan-proteins (AGPs) attempts to relate biological roles to the molecular properties that result largely from O-Hyp glycosylation putatively coded by the primary sequence. The Hyp contiguity hypothesis predicts contiguous Hyp residues as attachment sites for arabino-oligosaccharides (arabinosides) and clustered, non-contiguous Hyp residues as arabinogalactan polysaccharide sites. Although earlier tests of naturally occurring hydroxyproline-rich glycoproteins (HRGPs) and HRGPs designed by synthetic genes were consistent with a sequence-driven code, the predictive value of the hypothesis starting from the DNA sequences of known AGPs remained untested due to difficulties in purifying a single AGP for analysis. However, expression in tobacco (Nicotiana tabacum) of the major tomato (Lycopersicon esculentum) AGP, LeAGP-1, as an enhanced green fluorescent protein fusion glycoprotein (EGFP)-LeAGP-1, increased its hydrophobicity sufficiently for chromatographic purification from other closely related endogenous AGPs. We also designed and purified two variants of LeAGP-1 for future functional analysis: one lacking the putative glycosylphosphatidylinositol (GPI)-anchor signal sequence; the other lacking a 12-residue internal lysine-rich region. Fluorescence microscopy of plasmolysed cells confirmed the location of LeAGP-1 at the plasma membrane outer surface and in Hechtian threads. Hyp glycoside profiles of the fusion glycoproteins gave ratios of Hyp-polysaccharides to Hyp-arabinosides plus non-glycosylated Hyp consistent with those predicted from DNA sequences by the Hyp contiguity hypothesis. These results demonstrate a route to the purification of AGPs and the use of the Hyp contiguity hypothesis for predicting the Hyp O-glycosylation profile of an HRGP from its DNA sequence.

An arabinogalactan protein(s) is a key component of a fraction that mediates local intercellular communication involved in tracheary element differentiation of zinnia mesophyll cells

Local intercellular communication is involved in tracheary element (TE) differentiation of zinnia (Zinnia elegans L.) mesophyll cells and mediated by a proteinous macromolecule, which was designated xylogen. To characterize and isolate xylogen, a bioassay system to monitor the activity of xylogen was developed, in which mesophyll cells were embedded in microbeads of agarose gel at a low (2.0-4.3x10(4) cells ml(-1)) or high density (8.0-9.0x10(4) cells ml(-1)) and microbeads of different cell densities were cultured together in a liquid medium to give a total density of 2.1-2.5x10(4) cells ml(-1). Without any additives, the frequency of TE differentiation was much smaller in the low-density microbeads than in the high-density microbeads. This low level of TE differentiation in the low-density microbeads was attributable to the shortage of xylogen. When cultures were supplemented with conditioned medium (CM) prepared from zinnia cell suspensions undergoing TE differentiation, the frequency of TE differentiation in the low-density microbeads increased remarkably, indicating the activity of xylogen in the CM. The xylogen activity in CM was sensitive to proteinase treatments. Xylogen was bound to galactose-specific lectins such as Ricinus communis agglutinin and peanut agglutinin, and precipitated by beta-glucosyl Yariv reagent. These results indicate that xylogen is a kind of arabinogalactan protein.

NaAGP4 is an arabinogalactan protein whose expression is suppressed by wounding and fungal infection in Nicotiana alata

Arabinogalactan proteins (AGPs) are proteoglycans secreted by plant cells that have been implicated in plant growth and development. Most AGPs cloned to date possess highly labile glycosylphosphatidylinositol (GPI) lipid anchors. These anchors transiently attach AGPs to the plasma membrane before they are released into the cell wall following GPI anchor hydrolysis. We have isolated and partially sequenced the protein core of an AGP purified from styles of Nicotiana alata. The protein sequence data were utilised to clone the AGP's gene, NaAGP4. This AGP shares about 78% sequence identity with the tomato AGP LeAGP-1. RNA gel blot analyses of different plant organs indicate that NaAGP4 is expressed in the same tissues and at similar levels as LeAGP-1. Furthermore, NaAGP4 like LeAGP-1 is rapidly suppressed by tissue wounding and by pathogen infection. We believe NaAGP4 and LeAGP-1 are the first described examples of orthologous AGPs from different plant species. In contrast, another AGP from N. alata, NaAGP1, is comparatively unaffected by wounding and pathogen infection, although this AGP is expressed in similar tissues and at similar levels as NaAGP4.

Structure of the glycosylphosphatidylinositol anchor of an arabinogalactan protein from Pyrus communis suspension-cultured cells

Arabinogalactan proteins (AGPs) are proteoglycans of higher plants, which are implicated in growth and development. We recently have shown that two AGPs, NaAGP1 (from Nicotiana alata styles) and PcAGP1 (from Pyrus communis cell suspension culture), are modified by the addition of a glycosylphosphatidylinositol (GPI) anchor. However, paradoxically, both AGPs were buffer soluble rather than membrane associated. We now show that pear suspension cultured cells also contain membrane-bound GPI-anchored AGPs. This GPI anchor has the minimal core oligosaccharide structure, D-Manalpha(1-2)-D-Manalpha(1-6)-D-Manalpha(1-4)-D-GlcN -inositol, which is consistent with those found in animals, protozoa, and yeast, but with a partial beta(1-4)-galactosyl substitution of the 6-linked Man residue, and has a phosphoceramide lipid composed primarily of phytosphingosine and tetracosanoic acid. The secreted form of PcAGP1 contains a truncated GPI lacking the phosphoceramide moiety, suggesting that it is released from the membrane by the action of a phospholipase D. The implications of these findings are discussed in relation to the potential mechanisms by which GPI-anchored AGPs may be involved in signal transduction pathways.

A small asparagine-rich protein required for S-allele-specific pollen rejection in Nicotiana

Although S-locus RNases (S-RNases) determine the specificity of pollen rejection in self-incompatible (SI) solanaceous plants, they alone are not sufficient to cause S-allele-specific pollen rejection. To identify non-S-RNase sequences that are required for pollen rejection, a Nicotiana alata cDNA library was screened by differential hybridization. One clone, designated HT, hybridized strongly to RNA from N. alata styles but not to RNA from Nicotiana plumbaginifolia, a species known to lack one or more factors necessary for S-allele-specific pollen rejection. Sequence analysis revealed a 101-residue ORF including a putative secretion signal and an asparagine-rich domain near the C terminus. RNA blot analysis showed that the HT-transcript accumulates in the stigma and style before anthesis. The timing of HT-expression lags slightly behind S(C10)-RNase in SI N. alata S(C10)S(C10) and is well correlated with the onset of S-allele-specific pollen rejection in the style. An antisense-HT construct was prepared to test for a role in pollen rejection. Transformed (N. plumbaginifolia x SI N. alata S(C10)S(C10)) hybrids with reduced levels of HT-protein continued to express S(C10)-RNase but failed to reject S(C10)-pollen. Control hybrids expressing both S(C10)-RNase and HT-protein showed a normal S-allele-specific pollen rejection response. We conclude that HT-protein is directly implicated in pollen rejection.

Presence of a glycosylphosphatidylinositol lipid anchor on rose arabinogalactan proteins

Arabinogalactan proteins constitute a class of plant cell surface proteoglycans with widespread occurrence and suggested functions in various aspects of plant growth and development, including cell proliferation, expansion, marking, and death. Previous investigations of subcellular fractions from suspension-cultured cells of "Paul's Scarlet" rose (Rosa sp.) have revealed extensive structural similarity between some soluble arabinogalactan proteins from the cell wall space and some plasma membrane-associated arabinogalactan proteins, thus inspiring the present investigation of the mechanism through which these inherently water-soluble molecules are held on the plasma membrane. Several lines of evidence gained through a combination of methods including reversed-phase chromatography, treatment with phosphatidylinositol-specific phospholipase C, and chemical structural analysis now show that some rose arabinogalactan proteins carry a ceramide class glycosylphosphatidylinositol lipid anchor. The predominant form of the ceramide is composed of tetracosanoic acid and 4-hydroxysphinganine. Plasma membrane vesicles readily shed arabinogalactan proteins by an inherent mechanism that appears to involve a phospholipase. This finding has significance toward understanding the biosynthesis, localization, and function of arabinogalactan proteins and toward stimulating other studies that may expand the currently very short list of higher plant proteins found to carry such membrane lipid anchors.

Novel genes induced during an arbuscular mycorrhizal (AM) symbiosis formed between Medicago truncatula and Glomus versiforme

Many terrestrial plant species are able to form symbiotic associations with arbuscular mycorrhizal fungi. Here we have identified three cDNA clones representing genes whose expression is induced during the arbuscular mycorrhizal symbiosis formed between Medicago truncatula and an arbuscular mycorrhizal fungus, Glomus versiforme. The three clones represent M. truncatula genes and encode novel proteins: a xyloglucan endotransglycosylase-related protein, a putative arabinogalactan protein (AGP), and a putative homologue of the mammalian p110 subunit of initiation factor 3 (eIF3). These genes show little or no expression in M. truncatula roots prior to formation of the symbiosis and are significantly induced following colonization by G. versiforme. The genes are not induced in roots in response to increases in phosphate. This suggests that induction of expression during the symbiosis is due to the interaction with the fungus and is not a secondary effect of improved phosphate nutrition. In situ hybridization revealed that the putative AGP is expressed specifically in cortical cells containing arbuscules. The identification of two mycorrhiza-induced genes encoding proteins predicted to be involved in cell wall structure is consistent with previous electron microscopy data that indicated major alterations in the extracellular matrix of the cortical cells following colonization by mycorrhizal fungi.

Immunogold localization of plant surface arabinogalactan-proteins using glycerol liquid substitution and scanning electron microscopy

We have studied the spatial distributions of arabinogalactan-protein (AGP) epitopes on the surface of maize embryogenic calli and roots using monoclonal antibodies JIM4 and MAC207. For this purpose, a new immunogold-scanning electron microscopy (SEM) method was employed which is based on liquid substitution of samples with glycerol. Using this method, we were able to show that the AGP epitopes are distributed along callus and root surfaces and they decorate filamentous structures. In callus cells, the JIM4 epitope was specifically enriched in the outer extracellular layers covering compact clusters of embryogenic meristematic callus cells. In roots, the MAC207 epitope was abundant on the root epidermal surface corresponding to the outer root pellicle, but was only occasionally found on the mucilage layer covering the root cap cells. Silver-enhanced gold particles, indicating AGP epitopes, were often linearly arranged suggesting that AGPs associate with filamentous structures both on the surface of embryogenic calli and root epidermal cells. These results indicate that AGPs are components of the outer extracellular layers and networks that cover the surface of roots and cells undergoing somatic embryogenesis.

Arabinogalactan-proteins from Nicotiana alata and Pyrus communis contain glycosylphosphatidylinositol membrane anchors

Arabinogalactan-proteins (AGPs) are a class of proteoglycans found in cell secretions and plasma membranes of plants. Attention is currently focused on their structure and their potential role in growth and development. We present evidence that two members of a major class of AGPs, the classical AGPs, AGPNa1 from styles of Nicotiana alata and AGPPc1 from cell suspension cultures of Pyrus communis, undergo C-terminal processing involving glycosylphosphatidylinositol membrane anchors. The evidence is that (i) the transmembrane helix at the C terminus predicted from the cDNA encoding these proteins is not present-the C-terminal amino acid is Asn87 and Ser97 for AGPNa1 and AGPPc1, respectively; (ii) both AGP protein backbones are substituted with ethanolamine at the C-terminal amino acid; and (iii) inositol, glucosamine, and mannose are present in the native AGPs. An examination of the deduced amino acid sequences of other classical AGP protein backbones shows that glycosylphosphatidylinositol-anchors may be a common feature of this class of AGPs.