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

Applications of the Yariv reagent in polysaccharide analysis and plant physiology from theory to practice

Arabinogalactan (AG), a biologically active substance found abundantly in plants, is of significant interest in plant physiology due to its unique physicochemical properties. Yariv reagent, widely utilized in AG-II related applications, forms insoluble precipitates when bound to AG-II. This paper provides a comprehensive overview of the synthesis methods, physicochemical properties, and various dissociation methods of the Yariv reagent to enhance its utility in AG-II studies. Furthermore, the review explores the binding mechanisms and applications of the Yariv reagent, highlighting the advancements in studying the Yariv-AG complex in plant physiology. The aim of this review is to inspire new research ideas and foster novel applications of the Yariv reagent from synthesis to implementation.

A practical guide to in situ and ex situ characterisation of arabinogalactan proteins (AGPs) in fruits

BACKGROUND

Arabinogalactan proteins (AGPs) are plant cell components found in the extracellular matrix that play crucial roles in fruit growth and development. AGPs demonstrate structural diversity due to the presence of a protein domain and an expanded carbohydrate moiety. Considering their molecular structure, the modification of glycosylation is a primary factor contributing to the functional variety of AGPs.

MAIN BODY

Immunocytochemical methods are used for qualitative and quantitative analyses of AGPs in fruit tissues. These include in situ techniques such as immunofluorescence and immunogold labelling for visualising AGP distribution at different cellular levels and ex situ methods such as Western blotting and enzyme-linked immunoenzymatic assays (ELISA) for molecular characterisation and quantitative detection of isolated AGPs. The presented techniques were modified by considering the structure of AGPs and the changes that occur in fruit tissues during the development and ripening processes. These methods are based on antibodies that recognise carbohydrate chains, which are the only commercially available highly AGP-specific tools. These probes recognise AGP epitopes and identify structural modifications and changes in spatio-temporal distribution, shedding light on their functions in fruit.

CONCLUSION

This paper provides a concise overview of AGP research methods, emphasising their use in fruit tissue analysis and demonstrating the accessibility gaps in other tools used in such research (e.g. antibodies against protein moieties). It underscores fruit tissue as a valuable source of AGPs and emphasises the potential for future research to understand of AGP synthesis, degradation, and their roles in various physiological processes. Moreover, the application of advanced probes for AGP visualisation is a milestone in obtaining more detailed insights into the localisation and function of these proteins within fruit.

The translocase of the inner mitochondrial membrane 22-2 is required for mitochondrial membrane function during Arabidopsis seed development

The carrier translocase (also known as translocase of the inner membrane 22; TIM22 complex) is an important component of the mitochondrial protein import apparatus. However, the biological functions of AtTIM22-2 in Arabidopsis remain poorly defined. Here, we report studies on two tim22-2 mutants that exhibit defects in embryo and endosperm development, leading to seed abortion. AtTIM22-2, which was localized in mitochondria, was widely expressed in embryos and in various seedling organs. Loss of AtTIM22-2 function resulted in irregular mitochondrial cristae, decreased respiratory activity, and a lower membrane potential, together with changes in gene expression and enzyme activity related to reactive oxygen species (ROS) metabolism, leading to increased accumulation of ROS in the embryo. The levels of transcripts encoding mitochondrial protein import components were also altered in the tim22-2 mutants. Furthermore, mass spectrometry, bimolecular fluorescence complementation and co-immunoprecipitation assays revealed that AtTIM22-2 interacted with AtTIM23-2, AtB14.7 (a member of Arabidopsis OEP16 family encoded by At2G42210), and AT5G27395 (mitochondrial inner membrane translocase complex, subunit TIM44-related protein). Taken together, these results demonstrate that AtTIM22-2 is essential for maintaining mitochondrial membrane functions during seed development. These findings lay the foundations for a new model of the composition and functions of the TIM22 complex in higher plants.

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.

The Role of Arabinogalactan Type II Degradation in Plant-Microbe Interactions

Arabinogalactans (AGs) are structural polysaccharides of the plant cell wall. A small proportion of the AGs are associated with hemicellulose and pectin. Furthermore, AGs are associated with proteins forming the so-called arabinogalactan proteins (AGPs), which can be found in the plant cell wall or attached through a glycosylphosphatidylinositol (GPI) anchor to the plasma membrane. AGPs are a family of highly glycosylated proteins grouped with cell wall proteins rich in hydroxyproline. These glycoproteins have important and diverse functions in plants, such as growth, cellular differentiation, signaling, and microbe-plant interactions, and several reports suggest that carbohydrate components are crucial for AGP functions. In beneficial plant-microbe interactions, AGPs attract symbiotic species of fungi or bacteria, promote the development of infectious structures and the colonization of root tips, and furthermore, these interactions can activate plant defense mechanisms. On the other hand, plants secrete and accumulate AGPs at infection sites, creating cross-links with pectin. As part of the plant cell wall degradation machinery, beneficial and pathogenic fungi and bacteria can produce the enzymes necessary for the complete depolymerization of AGs including endo-β-(1,3), β-(1,4) and β-(1,6)-galactanases, β-(1,3/1,6) galactanases, α-L-arabinofuranosidases, β-L-arabinopyranosidases, and β-D-glucuronidases. These hydrolytic enzymes are secreted during plant-pathogen interactions and could have implications for the function of AGPs. It has been proposed that AGPs could prevent infection by pathogenic microorganisms because their degradation products generated by hydrolytic enzymes of pathogens function as damage-associated molecular patterns (DAMPs) eliciting the plant defense response. In this review, we describe the structure and function of AGs and AGPs as components of the plant cell wall. Additionally, we describe the set of enzymes secreted by microorganisms to degrade AGs from AGPs and its possible implication for plant-microbe interactions.

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.

In silico and expression analyses of fasciclin-like arabinogalactan proteins reveal functional conservation during embryo and seed development

KEY MESSAGE

The fasciclin-like arabinogalactan proteins organization into four groups is conserved and may be related to specific roles in developmental processes across angiosperms. Fasciclin-like arabinogalactan proteins (FLAs) are a subclass of arabinogalactan proteins (AGPs), which contain fasciclin-like domains in addition to typical AGP domains. FLAs are present across all embryophytes, and despite their low overall sequence similarity, conserved regions that define the fasciclin functional domain (FAS) have been identified, suggesting that the cell adhesion property is also conserved. FLAs in Arabidopsis have been organized into four subgroups according to the number and distribution of functional domains. Recent studies associated FLAs with cell wall-related processes where domain organization seemed to be related to functional roles. In Arabidopsis, FLAs containing a single FAS domain were found to be important for the integrity and elasticity of the plant cell wall matrix, and FLAs with two FAS domains and two AGP domains were found to be involved in maintaining proper cell expansion under salt stress conditions. The main purpose of the present work was to elucidate the expression pattern of selected FLA genes during embryo and seed development using RT-qPCR. AtFLA8 and AtFLA10, two Arabidopsis genes that stood out in previous microarray studies of embryo development, were further examined using promoter-driven gene reporter analyses. We also studied the expression of cork oak FLA genes and found that their expression partially parallels the expression patterns of the putative AtFLA orthologs. We propose that the functional organization of FLAs is conserved and may be related to fundamental aspects of embryogenesis and seed development across angiosperms. Phylogenetic studies were performed, and we show that the same basic four-subgroup organization described for Arabidopsis FLA gene classification is valid for most Arabidopsis FLA orthologs of several plant species, namely poplar, corn and cork oak.

Spatio-temporal localization of selected pectic and arabinogalactan protein epitopes and the ultrastructural characteristics of explant cells that accompany the changes in the cell fate during somatic embryogenesis in Arabidopsis thaliana

During somatic embryogenesis (SE), explant cells undergo changes in the direction of their differentiation, which lead to diverse cell phenotypes. Although the genetic bases of the SE have been extensively studied in Arabidopsis thaliana, little is known about the chemical characteristics of the wall of the explant cells, which undergo changes in the direction of differentiation. Thus, we examined the occurrence of selected pectic and AGP epitopes in explant cells that display different phenotypes during SE. Explants examinations have been supplemented with an analysis of the ultrastructure. The deposition of selected pectic and AGP epitopes in somatic embryos was determined. Compared to an explant at the initial stage, a/embryogenic/totipotent and meristematic/pluripotent cells were characterized by a decrease in the presence of AGP epitopes, b/the presence of AGP epitopes in differentiated cells was similar, and c/an increase of analyzed epitopes was detected in the callus cells. Totipotent cells could be distinguished from pluripotent cells by: 1/the presence of the LM2 epitope in the latest one, 2/the appearance of the JIM16 epitope in totipotent cells, and 3/the more abundant presence of the JIM7 epitope in the totipotent cells. The LM5 epitope characterized the wall of the cells that were localized within the mass of embryogenic domain. The JIM8, JIM13 and JIM16 AGP epitopes appeared to be the most specific for the callus cells. The results indicate a relationship between the developmental state of the explant cells and the chemical composition of the cell walls.

The distinct functions of two classical arabinogalactan proteins BcMF8 and BcMF18 during pollen wall development in Brassica campestris

Arabinogalactan proteins (AGPs) are extensively glycosylated hydroxyproline-rich glycoproteins ubiquitous in all plant tissues and cells. AtAGP6 and AtAGP11, the only two functionally known pollen-specific classical AGP encoding genes in Arabidopsis, are reported to have redundant functions in microspore development. BcMF18 and BcMF8 isolated from Brassica campestris are the orthologues of AtAGP6 and AtAGP11, respectively. In contrast to the functional redundancy of AtAGP6 and AtAGP11, single-gene disruption of BcMF8 led to deformed pollen grains with abnormal intine development and ectopic aperture formation in B. campestris. Here, we further explored the action of BcMF18 and its relationship with BcMF8. BcMF18 was specifically expressed in pollen during the late stages of microspore development. Antisense RNA transgenic lines with BcMF18 reduction resulted in aberrant pollen grains with abnormal cellulose distribution, lacking intine, cytoplasm and nuclei. Transgenic plants with repressive expression of both BcMF8 and BcMF18 showed a hybrid phenotype, expressing a mixture of the phenotypes of the single gene knockdown plant lines. In addition, we identified functional diversity between BcMF18/BcMF8 and AtAGP6/AtAGP11, mainly reflected by the specific contribution of BcMF18 and BcMF8 to pollen wall formation. These results suggest that, unlike the orthologous genes AtAGP6 and AtAGP11 in Arabidopsis, BcMF18 and BcMF8 are both integral to pollen biogenesis in B. campestris, acting through independent pathways during microspore development.

Pyridine 2,4-Dicarboxylic Acid Suppresses Tomato Seedling Growth

Pyridine 2,4-dicarboxylic acid is a structural analog of 2-oxoglutarate and is known to inhibit 2-oxoglutare-dependent dioxygenases. The effect of this inhibitor in tomato seedlings grown in MS media supplied with various concentrations of PDCA was investigated, resulting in shorter roots and hypocotyls in a dose-dependent manner. The partial inhibition of growth in roots was more drastic compared to hypocotyls and was attributed to a decrease in the elongation of root and hypocotyl cells. Concentrations of 100 and 250 μM of PDCA decreased hydroxyproline content in roots while only the 250 μM treatment reduced the hydroxyproline content in shoots. Seedlings treated with 100 μM PDCA exhibited enhanced growth of hypocotyl and cotyledon cells and higher hydroxyproline content resulting in cotyledons with greater surface area. However, no alterations in hypocotyl length were observed. Prolyl 4 hydroxylases (P4Hs) are involved in the O-glycosylation of AGPs and were also highly expressed during seedling growth. Moreover PDCA induced a decrease in the accumulation of HRGPs and particularly in AGPs-bound epitopes in a dose dependent-manner while more drastic reduction were observed in roots compared to shoots. In addition, bulged root epidermal cells were observed at the high concentration of 250 μM which is characteristic of root tissues with glycosylation defects. These results indicate that PDCA induced pleiotropic effects during seedling growth while further studies are required to better investigate the physiological significance of this 2-oxoglutarate analog. This pharmacological approach might be used as a tool to better understand the physiological significance of HRGPs and probably P4Hs in various growth and developmental programs in plants.

Synthesis and Application of Branched Type II Arabinogalactans

The synthesis of linear and (1 → 6)-branched β-(1 → 3)-d-galactans, structures found in plant arabinogalactan proteins (AGPs), is described. The synthetic strategy relies on iterative couplings of monosaccharide and disaccharide thioglycoside donors, followed by a late-stage glycosylation of heptagalactan backbone acceptors to introduce branching. A key finding from the synthetic study was the need to match protective groups in order to tune reactivity and ensure selectivity during the assembly. Carbohydrate microarrays were generated to enable the detailed epitope mapping of two monoclonal antibodies known to recognize AGPs: JIM16 and JIM133.

Gene Mining for Proline Based Signaling Proteins in Cell Wall of Arabidopsis thaliana

The cell wall (CW) as a first line of defense against biotic and abiotic stresses is of primary importance in plant biology. The proteins associated with cell walls play a significant role in determining a plant's sustainability to adverse environmental conditions. In this work, the genes encoding cell wall proteins (CWPs) in Arabidopsis were identified and functionally classified using geneMANIA and GENEVESTIGATOR with published microarrays data. This yielded 1605 genes, out of which 58 genes encoded proline-rich proteins (PRPs) and glycine-rich proteins (GRPs). Here, we have focused on the cellular compartmentalization, biological processes, and molecular functioning of proline-rich CWPs along with their expression at different plant developmental stages. The mined genes were categorized into five classes on the basis of the type of PRPs encoded in the cell wall of Arabidopsis thaliana. We review the domain structure and function of each class of protein, many with respect to the developmental stages of the plant. We have then used networks, hierarchical clustering and correlations to analyze co-expression, co-localization, genetic, and physical interactions and shared protein domains of these PRPs. This has given us further insight into these functionally important CWPs and identified a number of potentially new cell-wall related proteins in A. thaliana.

Identification, characterization, and transcription analysis of xylogen-like arabinogalactan proteins in rice (Oryza sativa L.)

BACKGROUND

Xylogen, a chimeric arabinogalactan protein containing a non-specific lipid transfer protein domain, can promote xylem cell differentiation. No comprehensive study has been carried out on the XYLP gene family in rice. As a first step in research on this gene family and as a useful strategy in general, a genome-wide analysis of the OsXYLP gene family is thus needed.

RESULTS

In this study, we identified 21 XYLP genes from the rice genome and comprehensively analyzed their protein structures, phylogenetic relationships, chromosomal locations, and gene duplication status. Our results indicate that gene duplication has played major roles in the expansion of the OsXYLP gene family. We used expressed sequence tag, microarray, massively parallel signature sequencing, and quantitative real-time PCR data to analyze OsXYLP gene expression during various developmental stages and under abiotic stress conditions. We found that many OsXYLP genes are abundantly expressed in vascular tissues and seeds, with some genes regulated under hormonal or abiotic stresses. In addition, we identified knockout mutants of OsXYLP7 and OsXYLP16 and discovered that the mutant xylp7 has a defect in stem height.

CONCLUSIONS

We analyzed expression profiles of 21 XYLP genes and characterized the structures and evolutionary relationships of their proteins. Our results demonstrate that the rice XYLP gene family may play roles in plant vascular system development and hormone signaling. Among the 21 detected OsXYLPs, 19 are newly identified genes encoding arabinogalactan proteins. Our results provide comprehensive insights that will assist future research on the biological functions of the rice XYLP gene family.

TRANSLOCASE OF THE INNER MEMBRANE9 and 10 are essential for maintaining mitochondrial function during early embryo cell and endosperm free nucleus divisions in Arabidopsis

In the life cycle of flowering plants, the sporophytic generation takes up most of the time and plays a dominant role in influencing plant growth and development. The embryo cell and endosperm free nucleus divisions establish the critical initiation phase of early sporophyte development, which forms mature seeds through a series of cell growth and differentiation events. Here, we report on the biological functions of two Arabidopsis (Arabidopsis thaliana) mitochondrial proteins, TRANSLOCASE OF THE INNER MEMBRANE9 (TIM9) and TIM10. We found that dysfunction of either AtTIM9 or AtTIM10 led to an early sporophyte-lethal phenotype; the embryo and endosperm both arrest division when the embryo proper developed to 16 to 32 cells. The abortion of tim9-1 and tim10 embryos at the 16/32-cell stage was caused by the loss of cell viability and the cessation of division in the embryo proper region, and this inactivation was due to the collapse of the mitochondrial structure and activity. Our characterization of tim9-1 and tim10 showed that mitochondrial membrane permeability increased and that cytochrome c was released from mitochondria into the cytoplasm in the 16/32-cell embryo proper, indicating that mitochondrial dysfunction occurred in the early sporophytic cells, and thus caused the initiation of a necrosis-like programmed cell death, which was further proved by the evidence of reactive oxygen species and DNA fragmentation tests. Consequently, we verified that AtTIM9 and AtTIM10 are nonredundantly essential for maintaining the mitochondrial function of early embryo proper cells and endosperm-free nuclei; these proteins play critically important roles during sporophyte initiation and development in Arabidopsis.

Arabinogalactan proteins and the extracellular matrix surface network during peach palm somatic embryogenesis

Somatic embryogenesis has been described in peach palm as a reliable method for its in vitro multiplication and conservation. In this study, we evaluated the possible role of arabinogalactan proteins (AGPs) during this morphogenetic pathway. The presence of Yariv reagent, a synthesized chemical antibody that specifically binds AGP molecules, affected somatic embryos and callus development rate, but no effect was observed on fresh weight increment. This substance also had profound effects on embryo morphology: somatic embryos presented loose cells in the protoderm and no signs of polarization could be observed. To better evaluate the role of AGPs, analyses of specific monoclonal antibodies (MAbs) against different AGP epitopes revealed a specific pattern of distribution for each epitope. MAb JIM13 had differential expression and showed intense signal on the embryogenic sector and some immediately adjacent layers. MAb JIM7 against pectin recognized cell walls and a specific layer over the developing somatic embryo, as well as over the shoot meristem region of mature somatic embryos. This corresponds to an extracellular matrix surface network (ECMSN) associated with the development of somatic embryos and closely related to the expression of MAb JIM13. Scanning electron microscopy confirmed the presence of an ECMSN covering a specific group of cells and ultra-structural analyses revealed that the ECMSN had lipophilic substances.

Cellular and molecular changes associated with somatic embryogenesis induction in Agave tequilana

In spite of the importance of somatic embryogenesis for basic research in plant embryology as well as for crop improvement and plant propagation, it is still unclear which mechanisms and cell signals are involved in acquiring embryogenic competence by a somatic cell. The aim of this work was to study cellular and molecular changes involved in the induction stage in calli of Agave tequilana Weber cultivar azul in order to gain more information on the initial stages of somatic embryogenesis in this species. Cytochemical and immunocytochemical techniques were used to identify differences between embryogenic and non-embryogenic cells from several genotypes. Presence of granular structures was detected after somatic embryogenesis induction in embryogenic cells; composition of these structures as well as changes in protein and polysaccharide distribution was studied using Coomassie brilliant blue and Periodic Acid-Schiff stains. Distribution of arabinogalactan proteins (AGPs) and pectins was investigated in embryogenic and non-embryogenic cells by immunolabelling using anti-AGP monoclonal antibodies (JIM4, JIM8 and JIM13) as well as an anti-methyl-esterified pectin-antibody (JIM7), in order to evaluate major modifications in cell wall composition in the initial stages of somatic embryogenesis. Our observations pointed out that induction of somatic embryogenesis produced accumulation of proteins and polysaccharides in embryogenic cells. Presence of JIM8, JIM13 and JIM7 epitopes were detected exclusively in embryogenic cells, which supports the idea that specific changes in cell wall are involved in the acquisition of embryogenic competence of A. tequilana.

The cytological changes of tobacco zygote and proembryo cells induced by beta-glucosyl Yariv reagent suggest the involvement of arabinogalactan proteins in cell division and cell plate formation

BACKGROUND

In dicotyledonous plant, the first asymmetric zygotic division and subsequent several cell divisions are crucial for proembryo pattern formation and later embryo development. Arabinogalactan proteins (AGPs) are a family of extensively glycosylated cell surface proteins that are thought to have important roles in various aspects of plant growth and development, including embryogenesis. Previous results from our laboratory show that AGPs are concerned with tobacco egg cell fertilization and zygotic division. However, how AGPs interact with other factors involved in zygotic division and proembryo development remains unknown.

RESULTS

In this study, we used the tobacco in vitro zygote culture system and series of meticulous cell biology techniques to investigate the roles of AGPs in zygote and proembryo cell division. For the first time, we examined tobacco proembryo division patterns detailed to every cell division. The bright-field images and statistical results both revealed that with the addition of an exogenous AGPs inhibitor, beta-glucosyl Yariv (beta-GlcY) reagent, the frequency of aberrant division increased remarkably in cultured tobacco zygotes and proembryos, and the cell plate specific locations of AGPs were greatly reduced after beta-GlcY treatment. In addition, the accumulations of new cell wall materials were also significantly affected by treating with beta-GlcY. Detection of cellulose components by Calcofluor white stain showed that strong fluorescence was located in the newly formed wall of daughter cells after the zygotic division of in vivo samples and the control samples from in vitro culture without beta-GlcY treatment; while there was only weak fluorescence in the newly formed cell walls with beta-GlcY treatment. Immunocytochemistry examination with JIM5 and JIM7 respectively against the low- and high-esterified pectins displayed that these two pectins located in opposite positions of zygotes and proembryos in vivo and the polarity was not affected by beta-GlcY. Furthermore, FM4-64 staining revealed that endosomes were distributed in the cell plates of proembryos, and the localization pattern was also affected by beta-GlcY treatment. These results were further confirmed by subsequent observation with transmission electron microscopy. Moreover, the changes to proembryo cell-organelles induced by beta-GlcY reagent were also observed using fluorescent dye staining technique.

CONCLUSIONS

These results imply that AGPs may not only relate to cell plate position decision, but also to the location of new cell wall components. Correlated with other factors, AGPs further influence the zygotic division and proembryo pattern establishment in tobacco.

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.

Distribution and change patterns of free IAA, ABP 1 and PM H⁺-ATPase during ovary and ovule development of Nicotiana tabacum L

Auxin plays key roles in flower induction, embryogenesis, seed formation and seedling development, but little is known about whether auxin regulates the development of ovaries and ovules before pollination. In the present report, we measured the content of free indole-3-acetic (IAA) in ovaries of Nicotiana tabacum L., and localized free IAA, auxin binding protein 1 (ABP1) and plasma membrane (PM) H⁺-ATPase in the ovaries and ovules. The level of free IAA in the developmental ovaries increased gradually from the stages of ovular primordium to the functional megaspore, but slightly decreased when the embryo sacs formed. Immunoenzyme labeling clearly showed that both IAA and ABP1 were distributed in the ovules, the edge of the placenta, vascular tissues and the ovary wall, while PM H⁺-ATPase was mainly localized in the ovules. By using immunogold labeling, the subcellular distributions of IAA, ABP1 and PM H⁺-ATPase in the ovules were also shown. The results suggest that IAA, ABP1 and PM H⁺-ATPase may play roles in the ovary and ovule initiation, formation and differentiation.

Roles of arabinogalactan proteins in cotyledon formation and cell wall deposition during embryo development of Arabidopsis

Arabinogalactan proteins (AGPs) are a class of highly glycosylated, widely distributed proteins in higher plants. In the previous study, we found that the green fluorescence from JIM13-labeled AGPs was mainly distributed in embryo proper and the basal part of suspensor but gradually disappeared after the torpedo-stage embryos in Arabidopsis. And (β-D-Glc)(3) Yariv phenylglycoside (βGlcY), a synthetic reagent that specifically binds to AGPs, could inhibit embryo development. In this study, as a continuous work, we investigated the AGP functions in embryo germination, cotyledon formation, and cell wall deposition in Arabidopsis embryos by using immunofluorescent, immunoenzyme, transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR) techniques. The results showed that 50 μM βGlcY caused inhibition of embryo germination, formation of abnormal cotyledon embryos, and disorder of cotyledon vasculature. Compared with the normal embryos in vitro and in vivo, the AGPs and pectin signals were quite weaker in the whole abnormal embryos, whereas the cellulose signal was stronger in the shoot apical meristem (SAM) of abnormal embryo by calcofluor white staining. The FTIR assay demonstrated that the cell wall of abnormal embryos was relatively poorer in pectins and richer in cellulose than those of normal embryos. By TEM observation, the SAM cells of the abnormal embryos had less cytoplasm, more plastid and starch grains, and larger vacuole than that of normal embryos. These results indicated that AGPs may play roles in embryo germination, cotyledon formation, cell wall cellulose and pectin deposition, and cell division potentiality during embryo development of Arabidopsis.

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.

Embryogenesis - the humble beginnings of plant life

Each plant starts life from the zygote formed by the fusion of an egg and a sperm cell. The zygote gives rise to a multicellular embryo that displays a basic plant body organization and is surrounded by nutritive endosperm and maternal tissue. How the body organization is generated had already been studied before the genome sequence of Arabidopsis thaliana was completed 10 years ago, but several regulatory mechanisms of embryo development have since been discovered or analysed in more detail. Although this progress did not strictly depend on the availability of the genome sequence itself, several advances were considerably facilitated. In this review, we mainly address early embryo development, highlighting general mechanisms and crucial regulators, including phytohormones, that are involved in patterning the embryo and were mainly analysed in the post-genome decade. We also highlight some unsolved problems, provide a brief outlook on the future of Arabidopsis embryo research, and discuss how the knowledge gained from Arabidopsis could be translated to crop species.

The alpha-N-acetyl-glucosaminidase gene is transcriptionally activated in male and female gametes prior to fertilization and is essential for seed development in Arabidopsis

Sugar residues in proteoglycan complexes carry important signalling and regulatory functions in biology. In humans, heparan sulphate is an example of such a complex polymer containing glucosamine and N-acetyl-glucosamine residues and is present in the extracellular matrix. Although heparan sulphate has not been found in plants, the At5g13690 gene encoding the alpha-N-acetyl-glucosaminidase (NAGLU), an enzyme involved in its catabolism, is present in the Arabidopsis genome. Among our collection of embryo-defective lines, a plant was identified in which the T-DNA had inserted into the AtNAGLU gene. The phenotype of atnaglu is an early arrest of seed development without apparent male or female gametophytic effects. These data demonstrated the essential function in Arabidopsis consistent with the contribution of NAGLU to the Sanfilippo syndrome in human. Expression of AtNAGLU in plants was shown to be prevalent during reproductive development. The presence of AtNAGLU mRNA was observed during early and late male gametogenesis and in each cell of the embryo sac at the time of fertilization. After fertilization, AtNAGLU was expressed in the embryo, suspensor, and endosperm until the cotyledonary stage embryo. This precise pattern of expression identifies the cells and tissues where a remodelling of the N-acetyl-glucosamine residues of proteoglycan complexes is occurring. This work provides original evidence of the important role of N-acetyl-glucosamines in plant reproductive development.

The cellular and molecular biology of conifer embryogenesis

Gymnosperms and angiosperms are thought to have evolved from a common ancestor c. 300 million yr ago. The manner in which gymnosperms and angiosperms form seeds has diverged and, although broad similarities are evident, the anatomy and cell and molecular biology of embryogenesis in gymnosperms, such as the coniferous trees pine, spruce and fir, differ significantly from those in the most widely studied model angiosperm Arabidopsis thaliana. Molecular analysis of signaling pathways and processes such as programmed cell death and embryo maturation indicates that many developmental pathways are conserved between angiosperms and gymnosperms. Recent genomics research reveals that almost 30% of mRNAs found in developing pine embryos are absent from other conifer expressed sequence tag (EST) collections. These data show that the conifer embryo differs markedly from other gymnosperm tissues studied to date in terms of the range of genes transcribed. Approximately 72% of conifer embryo-expressed genes are found in the Arabidopsis proteome and conifer embryos contain mRNAs of very similar sequence to key genes that regulate seed development in Arabidopsis. However, 1388 loblolly pine (Pinus taeda) embryo ESTs (11.4% of the collection) are novel and, to date, have been found in no other plant. The data imply that, in gymnosperm embryogenesis, differences in structure and development are achieved by subtle molecular interactions, control of spatial and temporal gene expression and the regulating agency of a few unique proteins.