Oxidative cross-linking of plasma membrane arabinogalactan proteins

An elastic proteinaceous envelope encapsulates the early Arabidopsis embryo

Plant external surfaces are often covered by barriers that control the exchange of molecules, protect from pathogens and offer mechanical integrity. A key question is when and how such surface barriers are generated. Post-embryonic surfaces have well-studied barriers, including the cuticle, and it has been previously shown that the late Arabidopsis thaliana embryo is protected by an endosperm-derived sheath deposited onto a primordial cuticle. Here, we show that both cuticle and sheath are preceded by another structure during the earliest stages of embryogenesis. This structure, which we named the embryonic envelope, is tightly wrapped around the embryonic surface but can be physically detached by cell wall digestion. We show that this structure is composed primarily of extensin and arabinogalactan O-glycoproteins and lipids, which appear to form a dense and elastic crosslinked embryonic envelope. The envelope forms in cuticle-deficient mutants and in a mutant that lacks endosperm. This embryo-derived envelope is therefore distinct from previously described cuticle and sheath structures. We propose that it acts as an expandable diffusion barrier, as well as a means to mechanically confine the embryo to maintain its tensegrity during early embryogenesis.

Maturation of demineralized arabinogalactan-proteins from Acacia seyal gum in dry state: Aggregation kinetics and structural properties of aggregates

The aggregation in dry state of mineral-loaded arabinogalactan-proteins (AGPs) from Acacia seyal gum (GA) generally occurs above 70 °C. This study focuses on the aggregation sensitivity of AGPs after their demineralization. The dry incubation in mild temperature (25 °C to 70 °C) of demineralized AGPs induced the formation of aggregates, not observed with GA. AGPs aggregated following a self-assembly mechanism for which temperature only modulated the aggregation rate. The activation energy was around 90-100 kJ·mol^-1 that could correspond to chemical condensation reactions induced by the AGPs surface dehydration. The aggregation kinetics were characterized by the formation of soluble aggregates during the first times of incubation, whose molar mass increased from 1 · 10⁶ g·mol^-1 to 6.7 · 10⁶ g·mol^-1 (SEC MALS) or 12 · 10⁶ g·mol^-1 (AF4 MALS) after 1.66 days of dry heating at 40 °C. These soluble aggregates revealed they adopted a similar conformation to that of not aggregated AGPs with a ν_h value around 0.45. Above 1.66 days at 40 °C, the soluble aggregates grew up to form microparticles with sizes ranging from 10 to around 200 μm. This study highlighted the protective role of cations from AGPs whose demineralization increased their sensibility to dry heating and their chemical reactivity for aggregation.

Exploring apoplast reorganization in the nodules of Lotus corniculatus L. growing on old Zn-Pb calamine wastes

Nodulation and symbiotic nitrogen fixation are important factors that determine legume growth. A pot experiment was carried out to determine the effects of Zn-Pb contamination on nodule apoplast (cell walls and intercellular spaces) of bird's foot trefoil (Lotus corniculatus L.) that spontaneously colonized old calamine wastes. The plants were grown in pots filled with sterile calamine substrate (M, metal treated) or expanded clay (NM, untreated) and inoculated with calamine-derived Lotus-nodulating Bradyrhizobium liaoningense. Apoplast reorganization in the nodules was examined using specific dyes for cellulose, pectin and lignin detection, and immuno-histochemical techniques based on monoclonal antibodies against xyloglucan (Lm25), pectins (Jim5 and Jim7), and structural proteins (arabinogalactan protein - Lm14 and extensin - Jim12). Microscopic analysis of metal-treated nodules revealed changes in the apoplast structure and composition of nodule cortex tissues and infected cells. Wall thickening was accompanied by intensified deposition of cellulose, xyloglucan, esterified pectin, arabinogalactan protein and extensin. The metal presence redirected also lignin and suberin deposition in the walls of the nodule cortex tissues. Our results showed reorganization of the apoplast of cortex tissues and infected cells of Lotus nodules under Zn-Pb presence. These changes in the apoplast structure and composition may have created actual barriers for the toxic ions. For this reason, they can be regarded as an element of legume defense strategy against metal stress that enables effective functioning of L. corniculatus-rhizobia symbiosis on Zn-Pb polluted calamine tailings.

Mutation of an Arabidopsis Golgi membrane protein ELMO1 reduces cell adhesion

Plant growth, morphogenesis and development involve cellular adhesion, a process dependent on the composition and structure of the extracellular matrix or cell wall. Pectin in the cell wall is thought to play an essential role in adhesion, and its modification and cleavage are suggested to be highly regulated so as to change adhesive properties. To increase our understanding of plant cell adhesion, a population of ethyl methanesulfonate-mutagenized Arabidopsis were screened for hypocotyl adhesion defects using the pectin binding dye Ruthenium Red that penetrates defective but not wild-type (WT) hypocotyl cell walls. Genomic sequencing was used to identify a mutant allele of ELMO1 which encodes a 20 kDa Golgi membrane protein that has no predicted enzymatic domains. ELMO1 colocalizes with several Golgi markers and elmo1-/- plants can be rescued by an ELMO1-GFP fusion. elmo1-/- exhibits reduced mannose content relative to WT but no other cell wall changes and can be rescued to WT phenotype by mutants in ESMERALDA1, which also suppresses other adhesion mutants. elmo1 describes a previously unidentified role for the ELMO1 protein in plant cell adhesion.

Spatial and Temporal Distribution of Arabinogalactan Proteins during Larix decidua Mill. Male Gametophyte and Ovule Interaction

The role of ArabinoGalactan Proteins (AGPs) in the sexual reproduction of gymnosperms is not as well documented as that of angiosperms. In earlier studies, we demonstrated that AGPs play important roles during ovule differentiation in Larix decidua Mill. The presented results encouraged us to carry out further studies focused on the functions of these unique glycoproteins during pollen/pollen tube and ovule interactions in Larix. We identified and analyzed the localization of AGPs epitopes by JIM4, JIM8, JIM13 and LM2 antibodies (Abs) in male gametophytes and ovule tissue during pollination, the progamic phase, and after fertilization and in vitro growing pollen tubes. Our results indicated that (1) AGPs recognized by JIM4 Abs play an essential role in the interaction of male gametophytes and ovules because their appearance in ovule cells is induced by physical contact between reproductive partners; (2) after pollination, AGPs are secreted from the pollen cytoplasm into the pollen wall and contact the extracellular matrix of stigmatic tip cells followed by micropylar canal cells; (3) AGPs synthesized in nucellus cells before pollen grain germination are secreted during pollen tube growth into the extracellular matrix, where they can directly interact with male gametophytes; (4) in vitro cultured pollen tube AGPs labeled with LM2 Abs participate in the germination of pollen grain, while AGPs recognized by JIM8 Abs are essential for pollen tube tip growth.

The FLA4-FEI Pathway: A Unique and Mysterious Signaling Module Related to Cell Wall Structure and Stress Signaling

Cell wall integrity control in plants involves multiple signaling modules that are mostly defined by genetic interactions. The putative co-receptors FEI1 and FEI2 and the extracellular glycoprotein FLA4 present the core components of a signaling pathway that acts in response to environmental conditions and insults to cell wall structure to modulate the balance of various growth regulators and, ultimately, to regulate the performance of the primary cell wall. Although the previously established genetic interactions are presently not matched by intermolecular binding studies, numerous receptor-like molecules that were identified in genome-wide interaction studies potentially contribute to the signaling machinery around the FLA4-FEI core. Apart from its function throughout the model plant Arabidopsis thaliana for the homeostasis of growth and stress responses, the FLA4-FEI pathway might support important agronomic traits in crop plants.

β-1,3-Glucanases and chitinases participate in the stress-related defence mechanisms that are possibly connected with modulation of arabinogalactan proteins (AGP) required for the androgenesis initiation in rye (Secale cereale L.)

This work presents the biochemical, cytochemical and molecular studies on two groups of PR proteins, β-1,3-glucanases and chitinases, and the arabinogalactan proteins (AGP) during the early stages of androgenesis induction in two breeding lines of rye (Secale cereale L.) with different androgenic potential. The process of androgenesis was initiated by tillers pre-treatments with low temperature, mannitol and/or reduced glutathione and resulted in microspores reprogramming and formation of androgenic structures what was associated with high activity of β-1,3-glucanases and chitinases. Some isoforms of β-1,3-glucanases, namely several acidic isoforms of about 26 kDa; appeared to be anther specific. Chitinases were well represented but were less variable. RT-qPCR revealed that the cold-responsive chitinase genes Chit1 and Chit2 were expressed at a lower level in the microspores and whole anthers while the cold-responsive Glu2 and Glu3 were not active. The stress pre-treatments modifications promoted the AGP accumulation. An apparent dominance of some AGP epitopes (LM2, JIM4 and JIM14) was detected in the androgenesis-responsive rye line. An abundant JIM13 epitopes in the vesicles and inner cell walls of the microspores and in the cell walls of the anther cell layers appeared to be the most specific for embryogenesis.

Arabinogalactan Proteins in Plant Roots - An Update on Possible Functions

Responsiveness to environmental conditions and developmental plasticity of root systems are crucial determinants of plant fitness. These processes are interconnected at a cellular level with cell wall properties and cell surface signaling, which involve arabinogalactan proteins (AGPs) as essential components. AGPs are cell-wall localized glycoproteins, often GPI-anchored, which participate in root functions at many levels. They are involved in cell expansion and differentiation, regulation of root growth, interactions with other organisms, and environmental response. Due to the complexity of cell wall functional and regulatory networks, and despite the large amount of experimental data, the exact molecular mechanisms of AGP-action are still largely unknown. This dynamically evolving field of root biology is summarized in the present review.

Fasciclin-Like Arabinogalactan-Protein 16 (FLA16) Is Required for Stem Development in Arabidopsis

The predominant Fascilin 1 (FAS1)-containing proteins in plants belong to the Fasciclin-Like Arabinogalactan-protein (FLA) family of extracellular glycoproteins. In addition to FAS1 domains, these multi-domain FLA proteins contain glycomotif regions predicted to direct addition of large arabinogalactan (AG) glycans and many contain signal sequences for addition of a glycosylphosphatidylinositol (GPI)-anchor to tether them to the plasma membrane. FLAs are proposed to play both structural and signaling functions by forming a range of interactions in the plant extracellular matrix, similar to FAS1-containing proteins in animals. FLA group B members contain two FAS1 domains and are not predicted to be GPI-anchored. None of the group B members have been functionally characterized or their sub-cellular location resolved, limiting understanding of their function. We investigated the group B FLA16 in Arabidopsis that is predominantly expressed in inflorescence tissues. FLA16 is the most highly expressed FLA in the stem after Group A members FLA11 and FLA12 that are stem specific. A FLA16-YFP fusion protein driven by the endogenous putative FLA16 promoter in wild type background showed expression in cells with secondary cell walls, and FLA16 displayed characteristics of cell wall glycoproteins with moderate glycosylation. Investigation of a fla16 mutant showed loss of FLA16 leads to reduced stem length and altered biomechanical properties, likely as a result of reduced levels of cellulose. Immuno-labeling indicated support for FLA16 location to the plasma-membrane and (apoplastic) cell wall of interfascicular stem fiber cells. Together these results indicate FLA16, a two-FAS1 domain FLAs, plays a role in plant secondary cell wall synthesis and function.

Structural Adaptation and Physiological Mechanisms in the Leaves of Anthyllis vulneraria L. from Metallicolous and Non-Metallicolous Populations

Calamine wastes highly contaminated with trace metals (TMs) are spontaneously inhabited by a legume plant Anthyllis vulneraria L. This study determined an adaptation strategy of metallicolous (M) A. vulneraria and compared it with that of the non-metallicolous (NM) ecotype. We hypothesized that TMs may lead to (i) leaf apoplast modifications and (ii) changes in the antioxidant machinery efficiency that facilitate plant growth under severe contamination. To verify our hypothesis, we implemented immunolabelling, transmission electron microscopy and biochemical measurements. NM leaves were larger and thicker compared to the M ecotype. Microscopic analysis of M leaves showed a lack of dysfunctions in mesophyll cells exposed to TMs. However, changes in apoplast composition and thickening of the mesophyll and epidermal cell walls in these plants were observed. Thick walls were abundant in xyloglucan, pectins, arabinan, arabinogalactan protein and extensin. The tested ecotypes differed also in their physiological responses. The metallicolous ecotype featured greater accumulation of photosynthetic pigments, enhanced activity of superoxide dismutase and increased content of specific phenol groups in comparison with the NM one. Despite this, radical scavenging activity at the level of 20% was similar in M and NM ecotypes, which may implicate effective reduction of oxidative stress in M plants. In summary, our results confirmed hypotheses and suggest that TMs induced cell wall modifications of leaves, which may play a role in metal stress avoidance in Anthyllis species. However, when TMs reach the protoplast, activation of antioxidant machinery may significantly strengthen the status of plants naturally growing in TM-polluted environment.

Properties of arabinogalactan-proteins in European pear (Pyrus communis L.) fruits

Arabinogalactans (AGs) and arabinogalactan-proteins (AGPs) were partially purified from an extract of fruits of the European pear (Pyrus communis L.) by DEAE-cellulose ion-exchange and Sepharose 6B gel-filtration chromatography. Among 7 AG(P)-containing fractions, a neutral AGP (SE-1) was confirmed to be highly purified (M_r 67,000) and rich in L-Ara and Gal; this fraction included a small amount (2.6%, w/w) of protein and showed the highest reactivity forming precipitate with β-Glc Yariv reagent among the 7 fractions, the intensity of which was comparable to that of gum arabic, a standard AGP. Another accompanying minor low-M_r neutral AGP (SE-2; M_r approx. 7200) still contained other polysaccharide (starch fragments) and did not show Yariv reactivity. The carbohydrate moieties of SE-1 consisted of consecutive (1 → 3)-linked β-galactosyl backbone chains substituted with side chains of (1 → 6)-linked β-galactosyl residues at O-6, to which mainly single α-l-arabinofuranosyl residues were attached through O-3. This structural feature was also observed for SE-2. Successive digestion of SE-1 with α-l-arabinofuranosidase and exo-β-(1 → 3)-galactanase with the aid of endo-β-(1 → 3)-galactanase released most (more than 98%, w/w) of the carbohydrate moieties as low-M_r fragments. These consisted of free L-Ara and Gal, and a series of β-(1 → 6)-galactooligosaccharides with degree of polymerization (dp) up to at least 17, indicative of attachment of (1 → 6)-linked β-galactosyl side chains of varying length along the (1 → 3)-linked β-galactosyl backbone chains.

Plant glycosylphosphatidylinositol anchored proteins at the plasma membrane-cell wall nexus

Approximately 1% of plant proteins are predicted to be post-translationally modified with a glycosylphosphatidylinositol (GPI) anchor that tethers the polypeptide to the outer leaflet of the plasma membrane. Whereas the synthesis and structure of GPI anchors is largely conserved across eukaryotes, the repertoire of functional domains present in the GPI-anchored proteome has diverged substantially. In plants, this includes a large fraction of the GPI-anchored proteome being further modified with plant-specific arabinogalactan (AG) O-glycans. The importance of the GPI-anchored proteome to plant development is underscored by the fact that GPI biosynthetic null mutants exhibit embryo lethality. Mutations in genes encoding specific GPI-anchored proteins (GAPs) further supports their contribution to diverse biological processes, occurring at the interface of the plasma membrane and cell wall, including signaling, cell wall metabolism, cell wall polymer cross-linking, and plasmodesmatal transport. Here, we review the literature concerning plant GPI-anchored proteins, in the context of their potential to act as molecular hubs that mediate interactions between the plasma membrane and the cell wall, and their potential to transduce the signal into the protoplast and, thereby, activate signal transduction pathways.

Extensin and Arabinogalactan-Protein Biosynthesis: Glycosyltransferases, Research Challenges, and Biosensors

Recent research, mostly in Arabidopsis thaliana, has led to the identification and characterization of the glycosyltransferases responsible for the biosynthesis of two of the most functionally important and abundant families of plant cell wall proteins, extensins, and arabinogalactan-proteins. Extensin glycosylation involves monogalactosylation of serine residues by O-α-serine galactosyltransferase and the addition of oligoarabinosides one to five arabinose units in length to contiguous hydroxyproline residues by a set of specific arabinosyltransferase enzymes, which includes hydroxyproline O-β-arabinosyltransferases, β-1,2-arabinosyltransferases, and at least one α-1,3-arabinosyltransferase. AGP glycosylation, however, is much more complex and involves the addition of large arabinogalactan polysaccharide chains to non-contiguous hydroxyproline residues. These arabinogalactan chains are composed of β-1,3-galactan backbones decorated with β-1,6-galactose side chains that are further modified with α-arabinose as well as other sugars, including β-(methyl)glucuronic acid, α-rhamnose, and α-fucose. Specific sets of hydroxyproline O-β-galactosyltransferases, β-1,3-galactosyltransferases, β-1,6-galactosyltransferases, α-arabinosyltransferases, β-glucuronosyltransferases, α-rhamnosyltransferases, and α-fucosyltransferases are responsible for the synthesis of these complex structures. This mini-review summarizes the EXT and AGP glycosyltransferases identified and characterized to date along with corresponding genetic mutant data, which addresses the functional importance of EXT and AGP glycosylation. In one case, genetic mutant data indicate that the carbohydrate moiety of arabinogalactan-proteins may serve as an extracellular biosensor or signal for normal cellular growth. Finally, future research challenges with respect to understanding the function of these enzymes more completely and discovering and characterizing additional glycosyltransferases responsible for extensin and arabinogalactan-protein biosynthesis are also discussed.

Identification of the Abundant Hydroxyproline-Rich Glycoproteins in the Root Walls of Wild-Type Arabidopsis, an ext3 Mutant Line, and Its Phenotypic Revertant

Extensins are members of the cell wall hydroxyproline-rich glycoprotein (HRGP) superfamily that form covalently cross-linked networks in primary cell walls. A knockout mutation in EXT3 (AT1G21310), the gene coding EXTENSIN 3 (EXT3) in Arabidopsis Landsberg erecta resulted in a lethal phenotype, although about 20% of the knockout plants have an apparently normal phenotype (ANP). In this study the root cell wall HRGP components of wild-type, ANP and the ext3 mutant seedlings were characterized by peptide fractionation of trypsin digested anhydrous hydrogen fluoride deglycosylated wall residues and by sequencing using LC-MS/MS. Several HRGPs, including EXT3, were identified in the wild-type root walls but not in walls of the ANP and lethal mutant. Indeed the ANP walls and walls of mutants displaying the lethal phenotype possessed HRGPs, but the profiles suggest that changes in the amount and perhaps type may account for the corresponding phenotypes.

Galactosyltransferases from Arabidopsis thaliana in the biosynthesis of type II arabinogalactan: molecular interaction enhances enzyme activity

BACKGROUND

Arabinogalactan proteins are abundant proteoglycans present on cell surfaces of plants and involved in many cellular processes, including somatic embryogenesis, cell-cell communication and cell elongation. Arabinogalactan proteins consist mainly of glycan, which is synthesized by post-translational modification of proteins in the secretory pathway. Importance of the variations in the glycan moiety of arabinogalactan proteins for their functions has been implicated, but its biosynthetic process is poorly understood.

RESULTS

We have identified a novel enzyme in the biosynthesis of the glycan moiety of arabinogalactan proteins. The At1g08280 (AtGALT29A) from Arabidopsis thaliana encodes a putative glycosyltransferase (GT), which belongs to the Carbohydrate Active Enzyme family GT29. AtGALT29A co-expresses with other arabinogalactan GTs, AtGALT31A and AtGLCAT14A. The recombinant AtGALT29A expressed in Nicotiana benthamiana demonstrated a galactosyltransferase activity, transferring galactose from UDP-galactose to a mixture of various oligosaccharides derived from arabinogalactan proteins. The galactose-incorporated products were analyzed using structure-specific hydrolases indicating that the recombinant AtGALT29A possesses β-1,6-galactosyltransferase activity, elongating β-1,6-galactan side chains and forming 6-Gal branches on the β-1,3-galactan main chain of arabinogalactan proteins. The fluorescence tagged AtGALT29A expressed in N. benthamiana was localized to Golgi stacks where it interacted with AtGALT31A as indicated by Förster resonance energy transfer. Biochemically, the enzyme complex containing AtGALT31A and AtGALT29A could be co-immunoprecipitated and the isolated protein complex exhibited increased level of β-1,6-galactosyltransferase activities compared to AtGALT29A alone.

CONCLUSIONS

AtGALT29A is a β-1,6-galactosyltransferase and can interact with AtGALT31A. The complex can work cooperatively to enhance the activities of adding galactose residues 6-linked to β-1,6-galactan and to β-1,3-galactan. The results provide new knowledge of the glycosylation process of arabinogalactan proteins and the functional significance of protein-protein interactions among O-glycosylation enzymes.

A galactosyltransferase acting on arabinogalactan protein glycans is essential for embryo development in Arabidopsis

Arabinogalactan proteins (AGPs) are a complex family of cell-wall proteoglycans that are thought to play major roles in plant growth and development. Genetic approaches to studying AGP function have met limited success so far, presumably due to redundancy within the large gene families encoding AGP backbones. Here we used an alternative approach for genetic dissection of the role of AGPs in development by modifying their glycan side chains. We have identified an Arabidopsis glycosyltransferase of CAZY family GT31 (AtGALT31A) that galactosylates AGP side chains. A mutation in the AtGALT31A gene caused the arrest of embryo development at the globular stage. The presence of the transcript in the suspensor of globular-stage embryos is consistent with a role for AtGALT31A in progression of embryo development beyond the globular stage. The first observable defect in the mutant is perturbation of the formative asymmetric division of the hypophysis, indicating an essential role for AGP proteoglycans in either specification of the hypophysis or orientation of the asymmetric division plane.

Arabinogalactan proteins in root-microbe interactions

Arabinogalactan proteins (AGPs) are among the most intriguing sets of macromolecules, specific to plants, structurally complex, and found abundantly in all plant organs including roots, as well as in root exudates. AGPs have been implicated in several fundamental plant processes such as development and reproduction. Recently, they have emerged as interesting actors of root-microbe interactions in the rhizosphere. Indeed, recent findings indicate that AGPs play key roles at various levels of interaction between roots and soil-borne microbes, either beneficial or pathogenic. Therefore, the focus of this review is the role of AGPs in the interactions between root cells and microbes. Understanding this facet of AGP function will undoubtedly improve plant health and crop protection.

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.

Expression of arabinogalactan proteins during tomato fruit ripening and in response to mechanical wounding, hypoxia and anoxia

Arabinogalactan proteins (AGPs) are highly glycosylated members of the superfamily of hydroxyproline-rich glycoproteins (HRGPs). Despite their implication in many aspects of plant growth and development little is known about their role in tomato fruit ripening (Solanum lycopersicum) and their response to abiotic stress in tomato fruits. A search of the currently available tomato genome database resulted in the identification of 34 genes encoding putative AGPs, with at least 20 of them being expressed in fruit. We monitored the abundance of AGPs bound by JIM8 and JIM13 monoclonal antibodies as well as the gene expression profiles of the Lys-rich LeAGP1 and two classical AGPs, SlAGP2 and SlAGP4. The JIM8- and JIM13-bound AGPs showed constitutive expression during fruit ripening and under hypoxic conditions, slight up-regulation to mechanical wounding in excised tomato fruit pericarp discs and up-regulation under anoxia indicating functional roles for these proteins in the developmental program of ripening and in response to abiotic stresses. Moreover, the SlAGP2 mRNA was significantly up-regulated during fruit ripening following the climacteric ethylene production, a pattern of expression similar to that of tomato fruit PG. The SlAGP4 and LeAGP1 mRNAs were up-regulated in response to mechanical wounding while under anoxia only the SlAGP4 transcript was induced. The protein and mRNA levels of these AGPs were induced under mechanical wounding while only JIM8-bound AGPs and SIAGP4 expression were induced under anoxic conditions. Our results indicate that selected tomato AGPs seem to play a role in fruit ripening as well as in response to mechanical wounding and anoxia.

Reactive oxygen species are involved in pollen tube initiation in kiwifruit

The role of reactive oxygen species (ROS) during pollen tube growth has been well established, but its involvement in the early germination stage is poorly understood. ROS production has been reported in germinating tobacco pollen, but evidence for a clear correlation between ROS and germination success remains elusive. Here, we show that ROS are involved in germination and pollen tube formation in kiwifruit. Using labelling with dihydrofluorescein diacetate (H(2) FDA) and nitroblue tetrazolium (NBT), endogenous ROS were detected immediately following pollen rehydration and during the lag phase preceding pollen tube emergence. Furthermore, extracellular H(2) O(2) was found to accumulate, beginning a few minutes after pollen suspension in liquid medium. ROS production was essential for kiwifruit pollen performance, since in the presence of compounds acting as superoxide dismutase/catalase mimic (Mn-5,10,15,20-tetrakis(1-methyl-4-pyridyl)21H,23H-porphin, Mn-TMPP) or as NADPH oxidase inhibitor (diphenyleneiodonium chloride, DPI), ROS levels were reduced and pollen tube emergence was severely or completely inhibited. Moreover, ROS production was substantially decreased in the absence of calcium, and by chromium and bisphenol A, which inhibit germination in kiwifruit. Peroxidase activity was cytochemically revealed after rehydration and during germination. In parallel, superoxide dismutase enzymes, particularly the Cu/Zn-dependent subtype - which function as superoxide radical scavengers - were detected by immunoblotting and by an in-gel activity assay in kiwifruit pollen, suggesting that ROS levels may be tightly regulated. Timing of ROS appearance, early localisation at the germination aperture and strict requirement for germination clearly suggest an important role for ROS in pollen grain activation and pollen tube initiation.

A bioinformatics approach to the identification, classification, and analysis of hydroxyproline-rich glycoproteins

Hydroxyproline-rich glycoproteins (HRGPs) are a superfamily of plant cell wall proteins that function in diverse aspects of plant growth and development. This superfamily consists of three members: hyperglycosylated arabinogalactan proteins (AGPs), moderately glycosylated extensins (EXTs), and lightly glycosylated proline-rich proteins (PRPs). Hybrid and chimeric versions of HRGP molecules also exist. In order to "mine" genomic databases for HRGPs and to facilitate and guide research in the field, the BIO OHIO software program was developed that identifies and classifies AGPs, EXTs, PRPs, hybrid HRGPs, and chimeric HRGPs from proteins predicted from DNA sequence data. This bioinformatics program is based on searching for biased amino acid compositions and for particular protein motifs associated with known HRGPs. HRGPs identified by the program are subsequently analyzed to elucidate the following: (1) repeating amino acid sequences, (2) signal peptide and glycosylphosphatidylinositol lipid anchor addition sequences, (3) similar HRGPs via Basic Local Alignment Search Tool, (4) expression patterns of their genes, (5) other HRGPs, glycosyl transferase, prolyl 4-hydroxylase, and peroxidase genes coexpressed with their genes, and (6) gene structure and whether genetic mutants exist in their genes. The program was used to identify and classify 166 HRGPs from Arabidopsis (Arabidopsis thaliana) as follows: 85 AGPs (including classical AGPs, lysine-rich AGPs, arabinogalactan peptides, fasciclin-like AGPs, plastocyanin AGPs, and other chimeric AGPs), 59 EXTs (including SP(5) EXTs, SP(5)/SP(4) EXTs, SP(4) EXTs, SP(4)/SP(3) EXTs, a SP(3) EXT, "short" EXTs, leucine-rich repeat-EXTs, proline-rich extensin-like receptor kinases, and other chimeric EXTs), 18 PRPs (including PRPs and chimeric PRPs), and AGP/EXT hybrid HRGPs.

Quantitative iTRAQ proteome and comparative transcriptome analysis of elicitor-induced Norway spruce (Picea abies) cells reveals elements of calcium signaling in the early conifer defense response

Long-lived conifer trees depend on both constitutive and induced defenses for resistance against a myriad of potential pathogens and herbivores. In species of spruce (Picea spp.), several of the late events of pathogen-, insect-, or elicitor-induced defense responses have previously been characterized at the anatomical, biochemical, transcriptome, and proteome levels in stems and needles. However, accurately measuring the early events of induced cellular responses in a conifer is technically challenging due to limitations in the precise timing of induction and tissue sampling from intact trees following insect or fungal treatment. In the present study, we used the advantages of Norway spruce (Picea abies) cell suspensions combined with chitosan elicitation to investigate the early proteome response in a conifer. A combination of iTRAQ labeling and a new design of iterative sample analysis employing data-dependent exclusion lists were used for proteome analysis. This approach improved the coverage of the spruce proteome beyond that achieved in any prior study in a conifer system. Comparison of elicitor-induced proteome and transcriptome responses in Norway spruce cells consistently identified features associated with calcium-mediated signaling and response to oxidative stress that have not previously been observed in the response of intact trees to fungal attack.

"Second extrinsic organizational mechanism" for orienting cellulose: modeling a role for the plasmalemmal reticulum

Oriented deposition of cellulose fibers by cellulose-synthesizing complexes typically occurs across the plasma membrane from microtubule bundles and is guided by them. However, aligned movement of the complexes can be shown even after applied oryzalin has depolymerized microtubules. Further, there is a claim that when (1) microtubules are depolymerized with oryzalin, (2) a microtubule-orienting stimulus is applied temporarily, and (3) oryzalin is washed out, the newly forming cellulose fibers are oriented with respect to the stimulus. With this in mind, the present paper gathers evidence from a diverse literature to suggest that the plasmalemmal reticulum, a major and structurally important form of cytoskeleton which connects cortical cytoplasm with wall, is a candidate to both independently and cooperatively participate in orienting microtubules and routing movements of cellulose-synthesizing complexes. Critical to this proposed function, the adhesion sites of the plasmalemmal reticulum have some morphological and molecular similarities to animal cell adhesion sites, known to play numerous integrative roles. The reticulum itself may be the morphological manifestation of the so-called lipid raft, previously known only on the basis of biochemical properties. According to the working model, the trusses interconnecting the adhesion sites shape the reticulum into apparently situation-dependent geometries. For example, in nongrowing or nonpolarized cells in which cellulose is deposited in brushy meshes, they form a nonpolar or weakly polar net; however, in elongating cells with oblique or otherwise polarized microtubules and newly forming cellulose fibers, there is suggestive evidence that net formation is dominated by trusses organized with correspondingly biased orientation. Consideration of such geometries and roles of the reticulum suggests several tests that could affirm, deny, or replace key aspects of this proposal to expand the theory of the peripheral cytoskeleton.

Defense-related signaling by interaction of arabinogalactan proteins and beta-glucosyl Yariv reagent inhibits gibberellin signaling in barley aleurone cells

Arabinogalactan proteins (AGPs) are hydroxyproline-rich glycoproteins present at the plasma membrane and in extracellular spaces. A synthetic chemical, beta-glucosyl Yariv reagent (beta-GlcY), binds specifically to AGPs. We previously reported that gibberellin signaling is specifically inhibited by beta-GlcY treatment in barley aleurone protoplasts. In the present study, we found that beta-GlcY also inhibited gibberellin-induced programmed cell death (PCD) in aleurone cells. We examined the universality and specificity of the inhibitory effect of beta-GlcY on gibberellin signaling using microarray analysis and found that beta-GlcY was largely effective in repressing gibberellin-induced gene expression. In addition, >100 genes were up-regulated by beta-GlcY in a gibberellin-independent manner, and many of these were categorized as defense-related genes. Defense signaling triggered by several defense system inducers such as jasmonic acid and a chitin elicitor could inhibit gibberellin-inducible events such as alpha-amylase secretion, PCD and expression of some gibberellin-inducible genes in aleurone cells. Furthermore, beta-GlcY repressed the gibberellin-inducible Ca2+-ATPase gene which is important for gibberellin-dependent gene expression, and induced known repressors of gibberellin signaling, two WRKY genes and a NAK kinase gene. These effects of beta-GlcY were also phenocopied by the chitin elicitor and/or jasmonic acid. These results indicate that gibberellin signaling is under the regulation of defense-related signaling in aleurone cells. It is also probable that AGPs are involved in the perception of stimuli causing defense responses.

Glutathione-S-Transferase is Detected During Somatic Embryogenesis in Chicory

Glutathione S-tranferases (GSTs) are a heterogeneous family of proteins, which perform diverse pivotal catalytic and non-enzymatic functions during plant development and in plant stress responses. Previous studies have shown that a GST activity (EC 2.5.1.18) is closely linked with the precocious phases of somatic embryogenesis in leaf tissues of an interspecific chicory hybrid (Cichorium intybus L. var. sativa x C. endivia L. var. latifolia). In order to learn more about the involvement of this enzyme in this process, in situ-hybridization as well as immunolocalization were performed in parallel. GST-mRNAs and proteins were colocalized in small veins, particularly in young protoxylem cell walls. During cell reactivation, the in situ and protein signals became less intense and were associated with chloroplasts. The GST-mRNAs and corresponding proteins were not always colocalized in the same tissues. While high amounts of transcripts could be detected in multicellular embryos, the proteins were not well labeled. Our results indicated that GSTs belong to a complex anti-oxidant mechanism within the cell, and also at the cell wall level. GSTs presence in reactivated cell and multicellular embryos is discussed in relation to redox cell status.

The biology of arabinogalactan proteins

Arabinogalactan proteins is an umbrella term applied to a highly diverse class of cell surface glycoproteins, many of which contain glycosylphosphatidylinositol lipid anchors. The structures of protein and glycan moieties of arabinogalactan proteins are overwhelmingly diverse while the "hydroxproline contiguity hypothesis" predicts arabinogalactan modification of members of many families of extracellular proteins. Descriptive studies using monoclonal antibodies reacting with carbohydrate epitopes on arabinogalactan proteins and experimental work using beta-Yariv reagent implicate arabinogalactan proteins in many biological processes of cell proliferation and survival, pattern formation and growth, and in plant microbe interaction. Advanced structural understanding of arabinogalactan proteins and an emerging molecular genetic definition of biological roles of individual arabinogalactan protein species, in conjunction with potentially analogous extracellular matrix components of animals, stimulate hypotheses about their mode of action. Arabinogalactan proteins might be soluble signals, or might act as modulators and coreceptors of apoplastic morphogens; their amphiphilic molecular nature makes them prime candidates of mediators between the cell wall, the plasma membrane, and the cytoplasm.

Reactive oxygen species produced by NADPH oxidase are involved in pollen tube growth

Tip-localized reactive oxygen species (ROS) were detected in growing pollen tubes by chloromethyl dichlorodihydrofluorescein diacetate oxidation, while tip-localized extracellular superoxide production was detected by nitroblue tetrazolium (NBT) reduction. To investigate the origin of the ROS we cloned a fragment of pollen specific tobacco NADPH oxidase (NOX) closely related to a pollen specific NOX from Arabidopsis. Transfection of tobacco pollen tubes with NOX-specific antisense oligodeoxynucleotides (ODNs) resulted in decreased amount of NtNOX mRNA, lower NOX activity and pollen tube growth inhibition. The ROS scavengers and the NOX inhibitor diphenylene iodonium chloride (DPI) inhibited growth and ROS formation in tobacco pollen tube cultures. Exogenous hydrogen peroxide (H2O2) rescued the growth inhibition caused by NOX antisense ODNs. Exogenous CaCl2 increased NBT reduction at the pollen tube tip, suggesting that Ca2+ increases the activity of pollen NOX in vivo. The results show that tip-localized ROS produced by a NOX enzyme is needed to sustain the normal rate of pollen tube growth and that this is likely to be a general mechanism in the control of tip growth of polarized plant cells.

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.

Analysis of the root-hair morphogenesis transcriptome reveals the molecular identity of six genes with roles in root-hair development in Arabidopsis

Root-hair morphogenesis is a model for studying the genetic regulation of plant cell development, and double-mutant analyses have revealed a complex genetic network underlying the development of this type of cell. Therefore, to increase knowledge of gene expression in root hairs and to identify new genes involved in root-hair morphogenesis, the transcriptomes of the root-hair differentiation zone of wild-type (WT) plants and a tip-growth defective root-hair mutant, rhd2-1, were compared using Affymetrix ATH1 GeneChips. A set of 606 genes with significantly greater expression in WT plants defines the 'root-hair morphogenesis transcriptome'. Compared with the whole genome, this set is highly enriched in genes known to be involved in root-hair morphogenesis. The additional gene families and functional groups enriched in the root-hair morphogenesis transcriptome are cell wall enzymes, hydroxyproline-rich glycoproteins (extensins) and arabinogalactan proteins, peroxidases, receptor-like kinases and proteins with predicted glycosylphosphatidylinositol (GPI) anchors. To discover new root-hair genes, 159 T-DNA insertion lines identified from the root-hair morphogenesis transcriptome were screened for defects in root-hair morphogenesis. This identified knockout mutations in six genes (RHM1-RHM6) that affected root-hair morphogenesis and that had not previously been identified at the molecular level: At2g03720 (similar to Escherichia coli universal stress protein); At3g54870 (armadillo-repeat containing kinesin-related protein); At4g18640 (leucine-rich repeat receptor-like kinase subfamily VI); At4g26690 (glycerophosphoryl diester phosphodiesterase-like GPI-anchored protein); At5g49270 (COBL9 GPI-anchored protein) and At5g65090 (inositol-1,4,5 triphosphate 5-phosphatase-like protein). The mutants were transcript null, their root-hair phenotypes were characterized and complementation testing with uncloned root-hair genes was performed. The results suggest a role for GPI-anchored proteins and lipid rafts in root-hair tip growth because two of these genes (At4g26690 and At5g49270) encode predicted GPI-anchored proteins likely to be associated with lipid rafts, and several other genes previously shown to be required for root-hair development also encode proteins associated with sterol-rich lipid rafts.

Regulation of cell length in the Arabidopsis thaliana root by the ethylene precursor 1-aminocyclopropane- 1-carboxylic acid: a matter of apoplastic reactions

Treatment of the Arabidopsis thaliana root with the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) immediately imposes a reduced maximal cell length beyond which further elongation is blocked. Here, we investigated possible apoplastic reactions involved in the inhibition of cell elongation. Five-day-old Arabidopsis seedlings were transferred to a growth medium supplemented with ACC and the effect on root cell length was recorded after 3 h of treatment. Altered characteristics in the apoplast of the nonelongating cells in the ACC-treated root, such as 'reactive oxygen species' (ROS) production and callose deposition, were detected using specific fluorochromes. The presence of functional hydroxyproline-rich glycoproteins (HRGPs) and the crosslinking of these cell-wall proteins are essential in limiting cell elongation. The ROS that drive the oxidative crosslinking of HRGPs, accumulate in the apoplast of cells in the zone where cell elongation stops. In the same cells, callose is deposited in the cell wall. The final cell length in the Arabidopsis root treated for a short period with ACC is determined in the zone of fast elongation. Both HRGPs crosslinking by ROS and callose deposition in the cell wall of this zone are suggested as causes for the reduced cell elongation.

Rhizobium colonization induced changes in membrane-bound and soluble hydroxyproline-rich glycoprotein composition in pea

Abundance and distribution of plant cell surface proteins of the hydroxyproline-rich glycoprotein (HRGP) class were studied in the pea-Rhizobium symbiosis using immunoblot analysis. The MAC 265-epitope was especially abundant in pea root nodules containing nitrogen-fixing Rhizobium bacteria. A 180-kDa MAC 265-HRGP dominated in pea shoot plasma membranes, while almost no MAC 265-HRGP was detected in root plasma membranes. We show here that a major difference between the plant-derived peribacteroid membrane of the symbiosomes and the root plasma membrane was the presence of a 100-kDa MAC 265-HRGP in the former. Arabinogalactan proteins (AGPs), as recognized by the monoclonal antibodies MAC 207 and JIM 8, were not detected in the peribacteroid membrane, while two isoforms (100 and 220 kDa) were detected in shoot and root plasma membranes. Specific MAC 265-HRGP isoforms were found in the peribacteroid space fraction of the symbiosomes and thus as soluble proteins in the interface between the symbionts. The abundance of the MAC 265-epitope was much reduced in non-nitrogen-fixing nodules when this phenotype resulted from a dicarboxylate transport mutation in Rhizobium. There was no reduction in the abundance of the MAC 265-epitope in non-fixing phenotypes resulting from a mutation in the plant. The results suggest that bacterial signals related to the bacterial ability to fix nitrogen, might be responsible for the regulation of HRGP expression in root nodules.

Arabinogalactan protein and wall-associated kinase in a plasmalemmal reticulum with specialized vertices

Arabinogalactan protein and wall-associated kinase (WAK) are suspected to be regulatory players at the interface between cytoplasm and cell wall. Both WAK(s) and arabinogalactan shown likely to represent arabinogalactan protein(s) have been visualized there with computational optical-sectioning microscopy. The arabinogalactan occurs in a polyhedral array at the external face of the cell membrane. WAK, and other proteins as yet unidentified, appear to fasten the membrane to the wall at vertices of the array. Evidence is presented that the array bears an important part of the mechanical stress experienced by the membrane, and it is speculated that the architectural organization of arabinogalactan protein, WAK, and other components of the array is critical for coordination of endomembrane activities, growth, and differentiation. The array has been named the plasmalemmal reticulum.

Membrane proteins and proteomics: un amour impossible?

Proteome analysis implies the ability to separate proteins as a first step prior to characterization. Thus, the overall performance of the analysis strongly depends on the performance of the separation tool, usually two-dimensional electrophoresis. This review shows how two-dimensional electrophoresis performs with membrane proteins from bacteria or animal or vegetable cells and tissues, the recent progress in this field, and it examines future prospects in this area.

If walls could talk

The plant cell wall is very complex, both in structure and function. The wall components and the mechanical properties of the wall have been implicated in conveying information that is important for morphogenesis. Proteoglycans, fragments of polysaccharides and the structural integrity of the wall may relay signals that influence cellular differentiation and growth control. Furthering our knowledge of cell wall structure and function is likely to have a profound impact on our understanding of how plant cells communicate with the extracellular environment.

Glycosylphosphatidylinositol-anchored cell-surface proteins from Arabidopsis

Remodeling of the plant cell surface occurs during the establishment of cell polarity, cellular differentiation, and organ development. This report demonstrates the existence of multiple glycosylphosphatidylinositol (GPI)-anchored proteins in the model plant Arabidopsis. Using two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), we also show that GPI-anchored proteins are a relatively abundant class of protein and that they are present at the plant plasma membrane. Furthermore, some of these proteins are released into the extracellular matrix. At least one of these is an arabinogalactan protein (AGP), a class of proteins known to be associated with cellular differentiation. Analysis of the amino acid sequences of two novel AGP-like proteins from Arabidopsis predicts that these proteins contain consensus signals for GPI-anchor addition. These findings support a model where GPI-anchored proteins are involved in the generation of specialized cell surfaces and extracellular signaling molecules.

Isolation, characterization and immunolocalization of a novel, modular tomato arabinogalactan-protein corresponding to the LeAGP-1 gene

Arabinogalactan-proteins (AGPs) are a family of hydroxy-proline-rich glycoproteins implicated to function in plant growth and development. This report focuses on a novel, modular AGP found in tomato, LeAGP-1, which was predicted by DNA cloning and herein verified at the protein level as a major AGP component. LeAGP-1 was isolated from tomato suspension-cultured cells and verified to be an AGP by precipitation with (beta-D-galactosyl)3 Yariv phenylglycoside and by amino acid composition analysis. Furthermore, LeAGP-1 was determined to correspond to LeAGP-1 clones based on three criteria: (1) amino acid composition identity, (2) amino acid sequence identity, and (3) specific immunoreactivity of glycosylated and deglycosylated LeAGP-1 with an antibody developed against the highly basic subdomain predicted from LeAGP-1 clones. The antibody was also used to immunolocalize LeAGP-1 in tomato to the cell surface of suspension-cultured cells, maturing metaxylem elements in young internodes and petioles, and stylar transmitting tissue cells. At the subcellular level, LeAGP-1 immunolocalized to the cell walls of these particular cells as well as to intercellular spaces between stylar transmitting tissue cells. LeAGP-1 now emerges as one of the most comprehensively studied AGPs in terms of (1) characterization at the genomic DNA, cDNA and protein levels, (2) known organ-specific and developmentally regulated mRNA expression patterns, (3) development of an antibody against a unique, peptide subdomain which specifically recognizes LeAGP-1 in its glycosylated and deglycosylated states, and (4) immunolocalization of a single, well-defined AGP molecule at the tissue and subcellular levels.

Cell walls: structures and signals

Cell walls harbor proteins and polysaccharides able to condition the development of a plant. In the past year, genes and enzymes modulating the composition and physical properties of walls have been characterized, and wall composition has been linked to the way a cell interacts with another cell, and to the way in which it differentiates. The sum of the signaling and physical activities of a cell wall may explain much about the control of development.