Arabinogalactan proteins: rising attention from plant biologists

Functional analysis of fasciclin-like arabinogalactan in carotenoid synthesis during tomato fruit ripening

Carotenoids are important pigmented nutrients synthesized by tomato fruits during ripening. To reveal the molecular mechanism underlying carotenoid synthesis during tomato fruit ripening, we analyzed carotenoid metabolites and transcriptomes in six development stages of tomato fruits. A total of thirty different carotenoids were detected and quantified in tomato fruits from 10 to 60 DPA. Based on differential gene expression profiles and WGCNA, we explored several genes that were highly significant and negatively correlated with lycopene, all of which encode fasciclin-like arabinogalactan proteins (FLAs). The FLAs are involved in plant signal transduction, however the functional role of these proteins has not been studied in tomato. Genome-wide analysis revealed that cultivated and wild tomato species contained 18 to 22 FLA family members, clustered into four groups, and mainly evolved by means of segmental duplication. The functional characterization of FLAs showed that silencing of SlFLA1, 5, and 13 were found to contribute to the early coloration of tomato fruits, and the expression of carotenoid synthesis-related genes was up-regulated in fruits that changed phenotypically, especially in SlFLA13-silenced plants. Furthermore, the content of multiple carotenoids (including (E/Z)-phytoene, lycopene, γ-carotene, and α-carotene) was significantly increased in SlFLA13-silenced fruits, suggesting that SlFLA13 has a potential inhibitory function in regulating carotenoid synthesis in tomato fruits. The results of the present study broaden the idea of analyzing the biological functions of tomato FLAs and preliminary evidence for the inhibitory role of SlFLA13 in carotenoid synthesis in fruit, providing the theoretical basis and a candidate for improving tomato fruit quality.

The different response of Brassica napus genotypes to microspore embryogenesis induced by heat shock and trichostatin A is not determined by changes in cell wall structure and composition but by different stress tolerance

During microspore embryogenesis, microspores are induced to develop into haploid embryos. In Brassica napus, microspore embryogenesis is induced by a heat shock (HS), which initially produces embryogenic structures with different cell wall architectures and compositions, and with different potentials to develop into embryos. The B. napus DH4079 and DH12075 genotypes have high and very low embryo yields, respectively. In DH12075, embryo yield is greatly increased by combining HS and the histone deacetylase (HDAC) inhibitor trichostatin A (TSA). However, we show that HS + TSA inhibits embryogenesis in the highly embryogenic DH4079 line. To ascertain why TSA has such different effects in these lines, we treated DH4079 and DH12075 microspore cultures with TSA and compared the cell wall structure and composition of the different embryogenic structures in both lines, specifically the in situ levels and distribution of callose, cellulose, arabinogalactan proteins and high and low methyl-esterified pectin. For both lines, HS + TSA led to the formation of cell walls unfavorable for embryogenesis progression, with reduced levels of arabinogalactan proteins, reduced cell adhesion of inner walls and altered pectin composition. Thus, TSA effects on cell walls cannot explain their different embryogenic response to TSA. We also applied TSA to DH4079 cultures at different times and concentrations before HS application, with no negative effects on embryogenic induction. These results indicate that DH4079 microspores are hypersensitive to combined TSA and HS treatments, and open up new hypotheses about the causes of such hypersensitivity.

The extracellular matrix of green algae

Green algae display a wide range of extracellular matrix (ECM) components that include various types of cell walls (CW), scales, crystalline glycoprotein coverings, hydrophobic compounds, and complex gels or mucilage. Recently, new information derived from genomic/transcriptomic screening, advanced biochemical analyses, immunocytochemical studies, and ecophysiology has significantly enhanced and refined our understanding of the green algal ECM. In the later diverging charophyte group of green algae, the CW and other ECM components provide insight into the evolution of plants and the ways the ECM modulates during environmental stress. Chlorophytes produce diverse ECM components, many of which have been exploited for various uses in medicine, food, and biofuel production. This review highlights major advances in ECM studies of green algae.

Characterization of the FLA Gene Family in Tomato (Solanum lycopersicum L.) and the Expression Analysis of SlFLAs in Response to Hormone and Abiotic Stresses

Fasciclin-like arabinogalactan proteins (FLAs), a subclass of arabinogalactan proteins (AGPs), participate in mediating plant growth, development, and response to abiotic stress. However, the characterization and function of FLAs in tomato are currently unknown. In this study, members of the tomato FLA family are characterized and analyzed in relation to their response to phytohormonal and abiotic stresses. The results show that a total of 24 FLA members were characterized in tomato. The structural domain analysis showed that these members have a high protein similarity. The expression profiles of different tissues indicated that the genes of most members of the tomato FLA gene family are highly expressed in roots, but to a lower extent in fruits. qRT-PCR analysis revealed that all 24 tomato FLA genes are responsive to ABA and MeJA. SlFLAs showed a positive response to salt and cold stress. SlFLA1, SlFLA12, and SlFLA14 are significantly induced under darkness. SlFLA1 and SlFLA3 are significantly induced under drought stress. This study provides a basis for a further understanding of the role of tomato FLA homologous genes in plant response to abiotic stress and lays the foundation for further research on the function of FLAs in tomato.

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.

Glycosylphosphatidylinositol-anchoring is required for the proper transport and extensive glycosylation of a classical arabinogalactan protein precursor in tobacco BY-2 cells

Many precursors of plant arabinogalactan proteins (AGPs) contain a C-terminal glycosylphosphatidylinositol (GPI)-anchoring signal. Using NtAGP1, a classical tobacco AGP, as a model, and green fluorescent protein (GFP) and sweet potato sporamin (SPO) as tags, we analyzed the localization and modification of AGP and its mutant without GPI-anchoring signal (AGPΔC) in tobacco BY-2 cells. The NtAGP1 fusion proteins migrated as large smear on SDS-polyacrylamide gel, and these proteins also localized preferentially to the plasma membrane. In contrast, fusions of AGPΔC with GFP and SPO yielded several forms: The largest were secreted, whereas others were recovered in the endomembrane organelles, including vacuoles. Comparison of the glycan structures of the microsomal SPO-AGP and the secreted SPO-AGPΔC using antibodies against the glycan epitopes of AGP indicated that the glycan structures of these proteins are different. These observations indicate that GPI-anchoring is required for the proper transport and glycosylation of the AGP precursor.

Type II arabinogalactans initiated by hydroxyproline-O-galactosyltransferases play important roles in pollen-pistil interactions

Arabinogalactan-proteins (AGPs) are hydroxyproline-rich glycoproteins containing a high sugar content and are widely distributed in the plant kingdom. AGPs have long been suggested to play important roles in sexual plant reproduction. The synthesis of their complex carbohydrates is initiated by a family of hydroxyproline galactosyltransferase (Hyp-GALT) enzymes which add the first galactose to Hyp residues in the protein backbone. Eight Hyp-GALT enzymes have been identified so far, and in the present work a mutant affecting five of these enzymes (galt2galt5galt7galt8galt9) was analyzed regarding the reproductive process. The galt25789 mutant presented a low seed set, and reciprocal crosses indicated a significant female gametophytic contribution to this mutant phenotype. Mutant ovules revealed abnormal callose accumulation inside the embryo sac and integument defects at the micropylar region culminating in defects in pollen tube reception. In addition, immunolocalization and biochemical analyses allowed the detection of a reduction in the amount of glucuronic acid in mutant ovary AGPs. Dramatically low amounts of high-molecular-weight Hyp-O-glycosides obtained following size exclusion chromatography of base-hydrolyzed mutant AGPs compared to the wild type indicated the presence of underglycosylated AGPs in the galt25789 mutant, while the monosaccharide composition of these Hyp-O-glycosides displayed no significant changes compared to the wild-type Hyp-O-glycosides. The present work demonstrates the functional importance of the carbohydrate moieties of AGPs in ovule development and pollen-pistil interactions.

Genome-Wide Comparative Analysis of the Fasciclin-like Arabinogalactan Proteins (FLAs) in Salicacea and Identification of Secondary Tissue Development-Related Genes

Fasciclin-like arabinogalactan proteins (FLAs) are a subclass of arabinogalactan proteins (AGPs) containing both AGP-like glycated domains and fasciclin (FAS) domains, which are involved in plant growth and development and synthesis of the cell wall. However, these proteins have not been identified or analyzed in willow, Salix, the sister genus of Populus. In this study, we performed a whole genome study of the FLA gene family of Salix suchowensis and compared it with the FLA gene family of Populus deltoides. The results showed the presence of 40 and 46 FLA genes in P. deltoides and S. suchowensis, distributed on 17 and 16 chromosomes, respectively. Four pairs of tandem repeat genes were found in willow, while poplar had no tandem repeat genes. Twelve and thirteen pairs of duplicated gene fragments were identified in poplar and willow, respectively. The multispecies phylogenetic tree showed that the FLA gene family could be divided into four groups (I-IV), with Group 1 showing significant expansion in woody plants. A gene expression analysis showed that PdeFLA19/27 in Group I of poplar was highly expressed, specifically during the secondary growth period of the stem and the rapid elongation of seed hairs. In the Group I genes of S. suchowensis, SsuFLA25/26/28 was also highly expressed during the secondary growth period, whereas increased expression of SsuFLA35 was associated with seed hair tissue. These results provide important clues about the differences in the FLA gene family during the evolution of herbs and woody plants, and suggest that the FLA gene family may play an essential role in regulating the secondary growth of woody plants. It also provides a reference for further studies on the regulation of secondary growth and seed hair development by FLA genes in poplar and willow.

Comprehensive Transcriptome Profiling Uncovers Molecular Mechanisms and Potential Candidate Genes Associated with Heat Stress Response in Chickpea

Chickpea (Cicer arietinum L.) production is highly susceptible to heat stress (day/night temperatures above 32/20 °C). Identifying the molecular mechanisms and potential candidate genes underlying heat stress response is important for increasing chickpea productivity. Here, we used an RNA-seq approach to investigate the transcriptome dynamics of 48 samples which include the leaf and root tissues of six contrasting heat stress responsive chickpea genotypes at the vegetative and reproductive stages of plant development. A total of 14,544 unique, differentially expressed genes (DEGs) were identified across different combinations studied. These DEGs were mainly involved in metabolic processes, cell wall remodeling, calcium signaling, and photosynthesis. Pathway analysis revealed the enrichment of metabolic pathways, biosynthesis of secondary metabolites, and plant hormone signal transduction, under heat stress conditions. Furthermore, heat-responsive genes encoding bHLH, ERF, WRKY, and MYB transcription factors were differentially regulated in response to heat stress, and candidate genes underlying the quantitative trait loci (QTLs) for heat tolerance component traits, which showed differential gene expression across tolerant and sensitive genotypes, were identified. Our study provides an important resource for dissecting the role of candidate genes associated with heat stress response and also paves the way for developing climate-resilient chickpea varieties for the future.

New insights on the expression patterns of specific Arabinogalactan proteins in reproductive tissues of Arabidopsis thaliana

Arabinogalactan proteins (AGPs) are hydroxyproline-rich glycoproteins containing a high proportion of carbohydrates, widely distributed in the plant kingdom and ubiquitously present in land plants. AGPs have long been suggested to play important roles in plant reproduction and there is already evidence that specific glycoproteins are essential for male and female gametophyte development, pollen tube growth and guidance, and successful fertilization. However, the functions of many of these proteins have yet to be uncovered, mainly due to the difficulty to study individual AGPs. In this work, we generated molecular tools to analyze the expression patterns of a subgroup of individual AGPs in different Arabidopsis tissues, focusing on reproductive processes. This study focused on six AGPs: four classical AGPs (AGP7, AGP25, AGP26, AGP27), one AG peptide (AGP24) and one chimeric AGP (AGP31). These AGPs were first selected based on their predicted expression patterns along the reproductive tissues from available RNA-seq data. Promoter analysis using β-glucuronidase fusions and qPCR in different Arabidopsis tissues allowed to confirm these predictions. AGP7 was mainly expressed in female reproductive tissues, more precisely in the style, funiculus, and integuments near the micropyle region. AGP25 was found to be expressed in the style, septum and ovules with higher expression in the chalaza and funiculus tissues. AGP26 was present in the ovules and pistil valves. AGP27 was expressed in the transmitting tissue, septum and funiculus during seed development. AGP24 was expressed in pollen grains, in mature embryo sacs, with highest expression at the chalazal pole and in the micropyle. AGP31 was expressed in the mature embryo sac with highest expression at the chalaza and, occasionally, in the micropyle. For all these AGPs a co-expression analysis was performed providing new hints on its possible functions. This work confirmed the detection in Arabidopsis male and female tissues of six AGPs never studied before regarding the reproductive process. These results provide novel evidence on the possible involvement of specific AGPs in plant reproduction, as strong candidates to participate in pollen-pistil interactions in an active way, which is significant for this field of study.

DEAP1 encodes a fasciclin-like arabinogalactan protein required for male fertility in rice

Arabinogalactan proteins (AGPs) are widely distributed in plant cells. Fasciclin-like AGPs (FLAs) belong to a subclass of AGPs that play important roles in plant growth and development. However, little is known about the biological functions of rice FLA. Herein, we report the identification of a male-sterile mutant of DEFECTIVE EXINE AND APERTURE PATTERNING1 (DEAP1) in rice. The deap1 mutant anthers produced aberrant pollen grains with defective exine formation and a flattened aperture annulus and exhibited slightly delayed tapetum degradation. DEAP1 encodes a plasma membrane-associated member of group III plant FLAs and is specifically and temporally expressed in reproductive cells and the tapetum layer during male development. Gene expression studies revealed reduced transcript accumulation of genes related to exine formation, aperture patterning, and tapetum development in deap1 mutants. Moreover, DEAP1 may interact with two rice D6 PROTEIN KINASE-LIKE3s (OsD6PKL3s), homologs of a known Arabidopsis aperture protein, to affect rice pollen aperture development. Our findings suggested that DEAP1 is involved in male reproductive development and may affect exine formation and aperture patterning, thereby providing new insights into the molecular functions of plant FLAs in male fertility.

Arabinogalactan Proteins: Focus on the Role in Cellulose Synthesis and Deposition during Plant Cell Wall Biogenesis

Arabinogalactan proteins (AGPs) belong to a family of glycoproteins that are widely present in plants. AGPs are mostly composed of a protein backbone decorated with complex carbohydrate side chains and are usually anchored to the plasma membrane or secreted extracellularly. A trickle of compelling biochemical and genetic evidence has demonstrated that AGPs make exciting candidates for a multitude of vital activities related to plant growth and development. However, because of the diversity of AGPs, functional redundancy of AGP family members, and blunt-force research tools, the precise functions of AGPs and their mechanisms of action remain elusive. In this review, we put together the current knowledge about the characteristics, classification, and identification of AGPs and make a summary of the biological functions of AGPs in multiple phases of plant reproduction and developmental processes. In addition, we especially discuss deeply the potential mechanisms for AGP action in different biological processes via their impacts on cellulose synthesis and deposition based on previous studies. Particularly, five hypothetical models that may explain the AGP involvement in cellulose synthesis and deposition during plant cell wall biogenesis are proposed. AGPs open a new avenue for understanding cellulose synthesis and deposition in plants.

Comprehensive Analysis of Arabinogalactan Protein-Encoding Genes Reveals the Involvement of Three BrFLA Genes in Pollen Germination in Brassica rapa

Arabinogalactan proteins (AGPs) are a superfamily of hydroxyproline-rich glycoproteins that are massively glycosylated, widely implicated in plant growth and development. No comprehensive analysis of the AGP gene family has been performed in Chinese cabbage (Brassica rapa ssp. chinensis). Here, we identified a total of 293 putative AGP-encoding genes in B. rapa, including 25 classical AGPs, three lysine-rich AGPs, 30 AG-peptides, 36 fasciclin-like AGPs (FLAs), 59 phytocyanin-like AGPs, 33 xylogen-like AGPs, 102 other chimeric AGPs, two non-classical AGPs and three AGP/extensin hybrids. Their protein structures, phylogenetic relationships, chromosomal location and gene duplication status were comprehensively analyzed. Based on RNA sequencing data, we found that 73 AGP genes were differentially expressed in the floral buds of the sterile and fertile plants at least at one developmental stage in B. rapa, suggesting a potential role of AGPs in male reproductive development. We further characterized BrFLA2, BrFLA28 and BrFLA32, three FLA members especially expressed in anthers, pollen grains and pollen tubes. BrFLA2, BrFLA28 and BrFLA32 are indispensable for the proper timing of pollen germination under high relative humidity. Our study greatly extends the repertoire of AGPs in B. rapa and reveals a role for three members of the FLA subfamily in pollen germination.

Mapping Pectic-Polysaccharide Epitopes in Cell Walls of Forage Chicory (Cichorium intybus) Leaves

The cell walls of forage chicory (Cichorium intybus) leaves are known to contain high proportions of pectic polysaccharides. However, little is known about the distribution of pectic polysaacharides among walls of different cell types/tissues and within walls. In this study, immunolabelling with four monoclonal antibodies was used to map the distribution of pectic polysaccharides in the cell walls of the laminae and midribs of these leaves. The antibodies JIM5 and JIM7 are specific for partially methyl-esterified homogalacturonans; LM5 and LM6 are specific for (1→4)-β-galactan and (1→5)-α-arabinan side chains, respectively, of rhamnogalacturonan I. All four antibodies labelled the walls of the epidermal cells with different intensities. JIM5 and JIM7, but not LM5 or LM6, labelled the middle lamella, tricellular junctions, and the corners of intercellular spaces of ground, xylem and phloem parenchyma. LM5, but not LM6, strongly labelled the walls of the few sclerenchyma fibres in the phloem of the midrib and lamina vascular bundles. The LM5 epitope was absent from some phloem parenchyma cells. LM6, but not LM5, strongly labelled the walls of the stomatal guard cells. The differential distribution of pectic epitopes among walls of different cell types and within walls may reflect the deposition and modification of these polysaccharides which are involved in cell wall properties and cell development.

The Chirality of Aggregated Yariv Reagents Correlates with Their AGP-Binding Ability*

Yariv reagents are glycosylated triphenylazo dyes that bind to arabinogalactan proteins (AGPs), proteoglycans found in plant cell walls that are integral for plant growth and development. Yariv reagents are widely utilized as imaging, purification, and quantification tools for AGPs and represent the only small molecule probe for interrogating AGP function. The ability of Yariv reagents to bind to AGPs is dependent on the structure of the terminal glycoside on the dye. The reason for this selectivity has not been understood until the present work. Using circular dichroism spectroscopy, we show that the Yariv reagents form supramolecular aggregates with helical chirality. More significantly, the ability of the Yariv reagent to bind AGPs is correlated with this helical chirality. This finding paves the way towards developing a more detailed understanding of the nature of the Yariv-AGP complex, and the design of AGP-binding reagents with higher affinities and selectivities.

Cell Wall Composition and Structure Define the Developmental Fate of Embryogenic Microspores in Brassica napus

Microspore cultures generate a heterogeneous population of embryogenic structures that can be grouped into highly embryogenic structures [exine-enclosed (EE) and loose bicellular structures (LBS)] and barely embryogenic structures [compact callus (CC) and loose callus (LC) structures]. Little is known about the factors behind these different responses. In this study we performed a comparative analysis of the composition and architecture of the cell walls of each structure by confocal and quantitative electron microscopy. Each structure presented specific cell wall characteristics that defined their developmental fate. EE and LBS structures, which are responsible for most of the viable embryos, showed a specific profile with thin walls rich in arabinogalactan proteins (AGPs), highly and low methyl-esterified pectin and callose, and a callose-rich subintinal layer not necessarily thick, but with a remarkably high callose concentration. The different profiles of EE and LBS walls support the development as suspensorless and suspensor-bearing embryos, respectively. Conversely, less viable embryogenic structures (LC) presented the thickest walls and the lowest values for almost all of the studied cell wall components. These cell wall properties would be the less favorable for cell proliferation and embryo progression. High levels of highly methyl-esterified pectin are necessary for wall flexibility and growth of highly embryogenic structures. AGPs seem to play a role in cell wall stiffness, possibly due to their putative role as calcium capacitors, explaining the positive relationship between embryogenic potential and calcium levels.

The Arabinogalactan Protein Family of Centaurium erythraea Rafn

Centaurium erythraea (centaury) is a medicinal plant with exceptional developmental plasticity in vitro and vigorous, often spontaneous, regeneration via shoot organogenesis and somatic embryogenesis, during which arabinogalactan proteins (AGPs) play an important role. AGPs are highly glycosylated proteins belonging to the super family of O-glycosylated plant cell surface hydroxyproline-rich glycoproteins (HRGPs). HRGPs/AGPs are intrinsically disordered and not well conserved, making their homology-based mining ineffective. We have applied a recently developed pipeline for HRGP/AGP mining, ragp, which is based on machine learning prediction of proline hydroxylation, to identify HRGP sequences in centaury transcriptome and to classify them into motif and amino acid bias (MAAB) classes. AGP sequences with low AG glycomotif representation were also identified. Six members of each of the three AGP subclasses, fasciclin-like AGPs, receptor kinase-like AGPs and AG peptides, were selected for phylogenetic and expression analyses. The expression of these 18 genes was recorded over 48 h following leaf mechanical wounding, as well as in 16 tissue samples representing plants from nature, plants cultivated in vitro, and developmental stages during shoot organogenesis and somatic embryogenesis. None of the selected genes were upregulated during both wounding recovery and regeneration. Possible functions of AGPs with the most interesting expression profiles are discussed.

Links between Regulatory Systems of ROS and Carbohydrates in Reproductive Development

To thrive on the earth, highly sophisticated systems to finely control reproductive development have been evolved in plants. In addition, deciphering the mechanisms underlying the reproductive development has been considered as a main research avenue because it leads to the improvement of the crop yields to fulfill the huge demand of foods for the growing world population. Numerous studies revealed the significance of ROS regulatory systems and carbohydrate transports and metabolisms in the regulation of various processes of reproductive development. However, it is poorly understood how these mechanisms function together in reproductive tissues. In this review, we discuss mode of coordination and integration between ROS regulatory systems and carbohydrate transports and metabolisms underlying reproductive development based on the hitherto findings. We then propose three mechanisms as key players that integrate ROS and carbohydrate regulatory systems. These include ROS-dependent programmed cell death (PCD), mitochondrial and respiratory metabolisms as sources of ROS and energy, and functions of arabinogalactan proteins (AGPs). It is likely that these key mechanisms govern the various signals involved in the sequential events required for proper seed production.

FLA14 is required for pollen development and preventing premature pollen germination under high humidity in Arabidopsis

BACKGROUND

As an important subfamily of arabinogalactan proteins (AGPs), fasciclin-like AGPs (FLAs) contribute to various aspects of growth, development and adaptation, yet their function remains largely elusive. Despite the diversity of FLAs, only two members, Arabidopsis FLA3 and rice MTR1, are reported to be involved in sexual reproduction. In this study, another Arabidopsis FLA-encoding gene, FLA14, was identified, and its role was investigated.

RESULTS

Arabidopsis FLA14 was found to be a pollen grain-specific gene. Expression results from fusion with green fluorescent protein showed that FLA14 was localized along the cell membrane and in Hechtian strands. A loss-of-function mutant of FLA14 showed no discernible defects during male gametogenesis, but precocious pollen germination occurred inside the mature anthers under high moisture conditions. Overexpression of FLA14 caused 39.2% abnormal pollen grains with a shrunken and withered appearance, leading to largely reduced fertility with short mature siliques and lower seed set. Cytological and ultramicroscopic observation showed that ectopic expression of FLA14 caused disruption at the uninucleate stage, resulting in either collapsed pollen with absent intine or pollen of normal appearance but with a thickened intine.

CONCLUSIONS

Taken together, our data suggest a role for FLA14 in pollen development and preventing premature pollen germination inside the anthers under high relative humidity in Arabidopsis.

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.

Glycosylphosphatidylinositol-anchored proteins mediate the interactions between pollen/pollen tube and pistil tissues

In flowering plants, pollen germination on the stigma and pollen tube growth in pistil tissues are critical for sexual plant reproduction, which are involved in the interactions between pollen/pollen tube and pistil tissues. GPI-anchored proteins (GPI-APs) are located on the external surface of the plasma membrane and function in various processes of sexual plant reproduction. The evidences suggest that GPI-APs participate in endosome machinery, Ca²⁺ oscillations, the development of the transmitting tract, the maintenance of the integrity of pollen tube, the enhancement of interactions of the receptor-like kinase (RLK) and ligand, and guidance of the growth of pollen tube, and so on. In this review, we will summarize the recent progress on the roles of GPI-APs in the interactions between pollen/pollen tube and pistil tissues during pollination, such as pollen germination on the stigma, pollen tube growth in the transmitting tract, pollen tube guidance to the ovule, and pollen tube reception in the embryo sac. We will also discuss the future outlook of GPI-APs in the interactions between pollen/pollen tube and pistil tissues.

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.

An Improved Protocol for the Synthesis and Purification of Yariv Reagents

Yariv reagents are glycoconjugate tris-azo dyes widely used in plant biology. These reagents are synthesized by diazo coupling between phloroglucinol and a para-diazophenyl glycoside. Despite their synthetic accessibility, well-defined protocols for obtaining pure Yariv reagents, and their complete compound characterization data, have not been reported. We report here optimized protocols used to synthesize, purify, and characterize a panel of six Yariv reagents and suggest approaches that could be valuable for the purification and characterization of other glycoconjugates as well.

Cell Wall Composition as a Marker of the Reprogramming of the Cell Fate on the Example of a Daucus carota (L.) Hypocotyl in Which Somatic Embryogenesis Was Induced

Changes in the composition of the cell walls are postulated to accompany changes in the cell's fate. We check whether there is a relationship between the presence of selected pectic, arabinogalactan proteins (AGPs), and extensins epitopes and changes in cell reprogramming in order to answer the question of whether they can be markers accompanying changes of cell fate. Selected antibodies were used for spatio-temporal immunolocalization of wall components during the induction of somatic embryogenesis. Based on the obtained results, it can be concluded that (1) the LM6 (pectic), LM2 (AGPs) epitopes are positive markers, but the LM5, LM19 (pectic), JIM8, JIM13 (AGPs) epitopes are negative markers of cells reprogramming to the meristematic/pluripotent state; (2) the LM8 (pectic), JIM8, JIM13, LM2 (AGPs) and JIM11 (extensin) epitopes are positive markers, but LM6 (pectic) epitope is negative marker of cells undergoing detachment; (3) JIM4 (AGPs) is a positive marker, but LM5 (pectic), JIM8, JIM13, LM2 (AGPs) are negative markers for pericycle cells on the xylem pole; (4) LM19, LM20 (pectic), JIM13, LM2 (AGPs) are constitutive wall components, but LM6, LM8 (pectic), JIM4, JIM8, JIM16 (AGPs), JIM11, JIM12 and JIM20 (extensins) are not constitutive wall components; (5) the extensins do not contribute to the cell reprogramming.

Genome-wide identification of fasciclin-like arabinogalactan proteins in jute and their expression pattern during fiber formation

Fasciclin-like arabinogalactan proteins (FLAs), a class of arabinogalactan proteins (AGPs) are involved in plant growth and development via cell communication and adhesion. FLAs were also associated with fiber and wood formation in plants but no information is available about the roles of FLA proteins during fibre development of jute. Here, we performed molecular characterization, evolutionary relationship and expression profiling of FLAs proteins in jute (Corchorus olitorius). In total, nineteen CoFLA genes have been identified in jute genome, which were divided into four classes like FLAs of other species based on protein structure and similarity. All CoFLAs have N-terminal signal peptide and one or two FAS domain while two FLAs lack well defined AGP region and eight FLAs were devoid of C-terminal glycosylphosphatidylinositol (GPI) anchor. Expression analysis of different regions of jute stem suggested their involvement in different fiber development stages. Four genes CoFLA 11, 12, 20, and 23 were highly or predominately expressed in fiber containing bark tissues while the expression levels of six CoFLA genes 02, 03, 04, 06, 14 and 19 were comparatively higher in stick. Higher transcripts levels of CoFLA 12 and 20 in the middle bark tissues suggest their involvement in fiber elongation. In contrast, the CoFLA 11 and 23 were more expressed in bottom bark tissues suggesting their potential involvement in secondary cell wall synthesis. Our study can serve as solid foundation for further functional exploration of FLAs and in future breeding program of jute aiming fiber improvement.

A cell surface arabinogalactan-peptide influences root hair cell fate

Root hairs (RHs) develop from specialized epidermal trichoblast cells, whereas epidermal cells that lack RHs are known as atrichoblasts. The mechanism controlling RH cell fate is only partially understood. RH cell fate is regulated by a transcription factor complex that promotes the expression of the homeodomain protein GLABRA 2 (GL2), which blocks RH development by inhibiting ROOT HAIR DEFECTIVE 6 (RHD6). Suppression of GL2 expression activates RHD6, a series of downstream TFs including ROOT HAIR DEFECTIVE 6 LIKE-4 (RSL4) and their target genes, and causes epidermal cells to develop into RHs. Brassinosteroids (BRs) influence RH cell fate. In the absence of BRs, phosphorylated BIN2 (a Type-II GSK3-like kinase) inhibits a protein complex that regulates GL2 expression. Perturbation of the arabinogalactan peptide (AGP21) in Arabidopsis thaliana triggers aberrant RH development, similar to that observed in plants with defective BR signaling. We reveal that an O-glycosylated AGP21 peptide, which is positively regulated by BZR1, a transcription factor activated by BR signaling, affects RH cell fate by altering GL2 expression in a BIN2-dependent manner. Changes in cell surface AGP disrupts BR responses and inhibits the downstream effect of BIN2 on the RH repressor GL2 in root epidermis.

Genetic and signalling pathways of dry fruit size: targets for genome editing-based crop improvement

Fruit is seed-bearing structures specific to angiosperm that form from the gynoecium after flowering. Fruit size is an important fitness character for plant evolution and an agronomical trait for crop domestication/improvement. Despite the functional and economic importance of fruit size, the underlying genes and mechanisms are poorly understood, especially for dry fruit types. Improving our understanding of the genomic basis for fruit size opens the potential to apply gene-editing technology such as CRISPR/Cas to modulate fruit size in a range of species. This review examines the genes involved in the regulation of fruit size and identifies their genetic/signalling pathways, including the phytohormones, transcription and elongation factors, ubiquitin-proteasome and microRNA pathways, G-protein and receptor kinases signalling, arabinogalactan and RNA-binding proteins. Interestingly, different plant taxa have conserved functions for various fruit size regulators, suggesting that common genome edits across species may have similar outcomes. Many fruit size regulators identified to date are pleiotropic and affect other organs such as seeds, flowers and leaves, indicating a coordinated regulation. The relationships between fruit size and fruit number/seed number per fruit/seed size, as well as future research questions, are also discussed.

The best CRISPR/Cas9 versus RNA interference approaches for Arabinogalactan proteins' study

Arabinogalactan Proteins (AGPs) are hydroxyproline-rich proteins containing a high proportion of carbohydrates, widely spread in the plant kingdom. AGPs have been suggested to play important roles in plant development processes, especially in sexual plant reproduction. Nevertheless, the functions of a large number of these molecules, remains to be discovered. In this review, we discuss two revolutionary genetic techniques that are able to decode the roles of these glycoproteins in an easy and efficient way. The RNA interference is a frequently technique used in plant biology that promotes genes silencing. The Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-associated protein 9 (CRISPR/Cas9), emerged a few years ago as a revolutionary genome-editing technique that has allowed null mutants to be obtained in a wide variety of organisms, including plants. The two techniques have some differences between them and depending on the research objective, these may work as advantage or disadvantage. In the present work, we propose the use of the two techniques to obtain AGP mutants easily and quickly, helping to unravel the role of AGPs, surely a great asset for the future.

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.

Composition of the Reconstituted Cell Wall in Protoplast-Derived Cells of Daucus is Affected by Phytosulfokine (PSK)

Phytosulfokine-α (PSK), a peptidyl plant growth factor, has been recognized as a promising intercellular signaling molecule involved in cellular proliferation and dedifferentiation. It was shown that PSK stimulated and enhanced cell divisions in protoplast cultures of several species leading to callus and proembryogenic mass formation. Since PSK had been shown to cause an increase in efficiency of somatic embryogenesis, it was reasonable to check the distribution of selected chemical components of the cell walls during the protoplast regeneration process. So far, especially for the carrot, a model species for in vitro cultures, it has not been specified what pectic, arabinogalactan protein (AGP) and extensin epitopes are involved in the reconstruction of the wall in protoplast-derived cells. Even less is known about the correlation between wall regeneration and the presence of PSK during the protoplast culture. Three Daucus taxa, including the cultivated carrot, were analyzed during protoplast regeneration. Several antibodies directed against wall components (anti-pectin: LM19, LM20, anti-AGP: JIM4, JIM8, JIM13 and anti-extensin: JIM12) were used. The obtained results indicate a diverse response of the used Daucus taxa to PSK in terms of protoplast-derived cell development, and diversity in the chemical composition of the cell walls in the control and the PSK-treated cultures.

Arabinogalactan proteins mediate intercellular crosstalk in the ovule of apple flowers

AGP-rich glycoproteins mediate pollen-ovule interactions and cell patterning in the embryo sac of apple before and after fertilization. Glycoproteins are significant players in the dialog that takes place between growing pollen tubes and the stigma and style in the angiosperms. Yet, information is scarce on their possible involvement in the ovule, a sporophytic organ that hosts the female gametophyte. Apple flowers have a prolonged lapse of time between pollination and fertilization, offering a great system to study the developmental basis of glycoprotein secretion and their putative role during the last stages of the progamic phase and early seed initiation. For this purpose, the sequential pollen tube elongation within the ovary was examined in relation to changes in arabinogalactan proteins (AGPs) in the tissues of the ovule before and after fertilization. To evaluate what of these changes are developmentally regulated, unpollinated and pollinated flowers were compared. AGPs paved the pollen tube pathway in the ovules along the micropylar canal, and the nucellus entrance toward the synergids, which also developmentally accumulated AGPs at the filiform apparatus. Glycoproteins vanished from all these tissues following pollen tube passage, strongly suggesting a role in pollen-ovule interaction. In addition, AGPs marked the primary cell walls of the haploid cells of the female gametophyte, and they further built up in the cell walls of the embryo sac and developing embryo, layering the interactive walls of the three generations hosted in the ovule, the maternal sporophytic tissues, the female gametophyte, and the developing embryo.

The peritrophic membrane as a target of proteins that play important roles in plant defense and microbial attack

The peritrophic membrane (or peritrophic matrix: PM) is a thin membranous structure that lies along the midgut epithelium in the midgut lumen and consists of chitin and proteins. PM exists between ingested food material and midgut epithelium cells and it is on the frontline of insect-plant and insect-microbe interactions. Therefore, proteins that play major roles in plant defense against herbivorous insects and in microbial attack on insects should penetrate, destroy or modify the PM to accomplish their roles. Recently, it has become clear that some proteins crucial to plant defense or microbial attack have the PM as their primary target. In addition, several plant defense proteins have been reported to affect the PM, although it is still unclear whether the PM is their primary target. This review introduces several of these proteins: fusolin and enhancin, two proteins produced by insect viruses that greatly enhance infection of the viruses by disrupting the PM; the MLX56 family proteins found in mulberry latex as defense proteins against insect herbivores, which modify the PM to a thick structure that inhibits digestive processes; Mir1-CP, a defense cysteine protease from maize that inhibits the growth of insects at very low concentrations and degrades the PM structures; and chitinases and lectins. The importance, necessary characteristics, and modes of action of PM-targeting proteins are then discussed from a strategic point of view, by spotlighting the importance of selective permeability of the PM. Finally, the review discusses the possibility of applying PM-targeting proteins for the control of pest insects.

HD-AGPs as Speciation Genes: Positive Selection on a Proline-Rich Domain in Non-Hybridizing Species of Petunia, Solanum, and Nicotiana

Transmitting tissue-specific proteins (TTS proteins) are abundant in the extracellular matrix of Nicotiana pistils, and vital for optimal pollen tube growth and seed set. We have identified orthologs from several species in the Solanaceae, including Petunia axillaris axillaris and Petunia integrifolia. We refer to TTS proteins and their orthologs as histidine domain-arabinogalactan proteins (HD-AGPs). HD-AGPs have distinctive domains, including a small histidine-rich region and a C-terminal PAC domain. Pairwise comparisons between HD-AGPs of 15 species belonging to Petunia, Nicotiana, and Solanum show that the his-domain and PAC domain are under purifying selection. In contrast, a proline-rich domain (HV2) is conserved among cross-hybridizing species, but variant in species-pairs that are reproductively isolated by post-pollination pre-fertilization reproductive barriers. In particular, variation in a tetrapeptide motif (XKPP) is systematically correlated with the presence of an interspecific reproductive barrier. Ka/Ks ratios are not informative at the infrageneric level, but the ratios reveal a clear signature of positive selection on two hypervariable domains (HV1 and HV2) when HD-AGPs from five solanaceous genera are compared. We propose that sequence divergence in the hypervariable domains of HD-AGPs reinforces sympatric speciation in incipient species that may have first diverged as a consequence of pollinator preferences or other ecological factors.

Insights into secrets along the pollen tube pathway in need to be discovered

The process of plant fertilization provides an outstanding example of refined control of gene expression. During this elegant process, subtle communication occurs between neighboring cells, based on chemical signals, that induces cellular mechanisms of patterning and growth. Having faced an immediate issue of self-incompatibility responses, the pathway to fertilization starts once the stigmatic cells recognize a compatible pollen grain, and it continues with numerous players interacting to affect pollen tube growth and the puzzling process of navigation along the transmitting tract. The pollen tube goes through a guidance process that begins with a preovular stage (i.e. prior to the influence of the target ovule), with interactions with factors from the transmitting tissue. In the subsequent ovular-guidance stage a specific relationship develops between the pollen tube and its target ovule. This stage is divided into the funicular and micropylar guidance steps, with numerous receptors working in signalling cascades. Finally, just after the pollen tube has passed beyond the synergids, fusion of the gametes occurs and the developing seed-the ultimate aim of the process-will start to mature. In this paper, we review the existing knowledge of the crucial biological processes involved in pollen-pistil interactions that give rise to the new seed.

Hydroxyproline-Rich Glycoproteins as Markers of Temperature Stress in the Leaves of Brachypodium distachyon

Plants frequently encounter diverse abiotic stresses, one of which is environmental thermal stress. To cope with these stresses, plants have developed a range of mechanisms, including altering the cell wall architecture, which is facilitated by the arabinogalactan proteins (AGP) and extensins (EXT). In order to characterise the localisation of the epitopes of the AGP and EXT, which are induced by the stress connected with a low (4 °C) or a high (40 °C) temperature, in the leaves of Brachypodium distachyon, we performed immunohistochemical analyses using the antibodies that bind to selected AGP (JIM8, JIM13, JIM16, LM2 and MAC207), pectin/AGP (LM6) as well as EXT (JIM11, JIM12 and JIM20). The analyses of the epitopes of the AGP indicated their presence in the phloem and in the inner bundle sheath (JIM8, JIM13, JIM16 and LM2). The JIM16 epitope was less abundant in the leaves from the low or high temperature compared to the control leaves. The LM2 epitope was more abundant in the leaves that had been subjected to the high temperatures. In the case of JIM13 and MAC207, no changes were observed at the different temperatures. The epitopes of the EXT were primarily observed in the mesophyll and xylem cells of the major vascular bundle (JIM11, JIM12 and JIM20) and no correlation was observed between the presence of the epitopes and the temperature stress. We also analysed changes in the level of transcript accumulation of some of the genes encoding EXT, EXT-like receptor kinases and AGP in the response to the temperature stress. In both cases, although we observed the upregulation of the genes encoding AGP in stressed plants, the changes were more pronounced at the high temperature. Similar changes were observed in the expression profiles of the EXT and EXT-like receptor kinase genes. Our findings may be relevant for genetic engineering of plants with increased resistance to the temperature stress.

Arabinogalactan Proteins and the Extracellular Matrix of Charophytes: A Sticky Business

Charophytes represent the group of green algae whose ancestors invaded land and ultimately gave rise to land plants 450 million years ago. While Zygnematophyceae are believed to be the direct sister lineage to embryophytes, different members of this group (Penium, Spirogyra, Zygnema) and the advanced thallus forming Coleochaete as well as the sarcinoid basal streptophyte Chlorokybus were investigated concerning their vegetative extracellular matrix (ECM) properties. Many taxa exhibit adhesion phenomena that are critical for affixing to a substrate or keeping cells together in a thallus, however, there is a great variety in possible reactions to e.g., wounding. In this study an analysis of adhesion mechanisms revealed that arabinogalactan proteins (AGPs) are most likely key adhesion molecules. Through use of monoclonal antibodies (JIM13) or the Yariv reagent, AGPs were located in cell surface sheaths and cell walls that were parts of the adhesion focal zones on substrates including wound induced rhizoid formation. JIM5, detecting highly methyl-esterfied homoglacturonan and JIM8, an antibody detecting AGP glycan and LM6 detecting arabinans were also tested and a colocalization was found in several examples (e.g., Zygnema) suggesting an interplay between these components. AGPs have been described in this study to perform both, cell to cell adhesion in algae forming thalli and cell to surface adhesion in the filamentous forms. These findings enable a broader evolutionary understanding of the function of AGPs in charophyte green algae.

Analysis of AGP contribution to the dynamic assembly and mechanical properties of cell wall during pollen tube growth

Arabinogalactan proteins as cell wall structural proteins are involved in fundamental processes during plant development and growth. The aim of this study was to evaluate AGP function in the distribution of pectin, cellulose and callose along Fragaria x ananassa pollen tube and to associate the cell wall structure with local mechanical properties. We used Yariv reagent which interacts with AGPs and allows the observation of the assembly of cell walls without AGPs performing their function. Cytochemical, immunofluorescence labelling and atomic force microscope have been used to characterize the changes in cell wall structure and stiffness. It was shown that disordering of the structure of AGP present in cell walls affects the localization of cellulose, pectins and the secretion of callose. Changes in cell wall assembly are relevant to pollen tube mechanical properties. The stiffness gradient lengthwise through the axis of the pollen tube has demonstrated a significantly higher Young's modulus of the shank region than the growth zone. It has been revealed that the apex of the pollen tube cultured in the presence of Yariv reagent is stiffer (1.68 MPa) than the corresponding region of the pollen tube grown under control conditions (0.13-0.27 MPa). AGP affects the structure of the cell wall by changing the distribution of other components and the modification of their localization, and hence it plays a significant role in the mechanical properties of the cell wall.

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

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

Dynamic Changes in Arabinogalactan-Protein, Pectin, Xyloglucan and Xylan Composition of the Cell Wall During Microspore Embryogenesis in Brassica napus

Microspore embryogenesis is a manifestation of plant cell totipotency whereby new cell walls are formed as a consequence of the embryogenic switch. In particular, the callose-rich subintinal layer created immediately upon induction of embryogenesis was recently related to protection against stress. However, little is currently known about the functional significance of other compositional changes undergone by the walls of embryogenic microspores. We characterized these changes in Brassica napus at different stages during induction of embryogenic microspores and development of microspore-derived embryos (MDEs) by using a series of monoclonal antibodies specific for cell wall components, including arabinogalactan-proteins (AGPs), pectins, xyloglucan and xylan. We used JIM13, JIM8, JIM14 and JIM16 for AGPs, CCRC-M13, LM5, LM6, JIM7, JIM5 and LM7 for pectins, CCRC-M1 and LM15 for xyloglucan, and LM11 for xylan. By transmission electron microscopy and quantification of immunogold labeling on high-pressure frozen, freeze-substituted samples, we profiled the changes in cell wall ultrastructure and composition at the different stages of microspore embryogenesis. As a reference to compare with, we also studied in vivo microspores and maturing pollen grains. We showed that the cell wall of embryogenic microspores is a highly dynamic structure whose architecture, arrangement and composition changes dramatically as microspores undergo embryogenesis and then transform into MDEs. Upon induction, the composition of the preexisting microspore intine walls is remodeled, and unusual walls with a unique structure and composition are formed. Changes in AGP composition were related to developmental fate. In particular, AGPs containing the JIM13 epitope were massively excreted into the cell apoplast, and appeared associated to cell totipotency. According to the ultrastructure and the pectin and xyloglucan composition of these walls, we deduced that commitment to embryogenesis induces the formation of fragile, plastic and deformable cell walls, which allow for cell expansion and microspore growth. We also showed that these special walls are transient, since cell wall composition in microspore-derived embryos was completely different. Thus, once adopted the embryogenic developmental pathway and far from the effects of heat shock exposure, cell wall biosynthesis would approach the structure, composition and properties of conventional cell walls.

Substrate specificity of novel GH16 endo-β-(1→3)-galactanases acting on linear and branched β-(1→3)-galactooligosaccharides

Arabinogalactan proteins are proteoglycans located in the plant cell wall. Most arabinogalactan proteins are composed of carbohydrate moieties of β-(1→3)-galactan main chains with β-(1→6)-galactan side chains terminated by other glycans. In this study, three novel endo-β-(1→3)-galactanases were identified and the substrate specificity was further studied using well-defined galactan oligomers. Linear and branched β-(1→3)-linked galactans, which resemble the carbohydrate core of the arabinogalactan protein, were used for the characterization of endo-β-(1→3)-galactanases. The identified enzymes required at least three consecutive galactose residues for activity. Non-substituted regions were preferred, but substituents in the -2 and +2 and in some cases also -1 and +1 subsites were tolerated to some extent, depending on the branching pattern, however at a significantly lower rate/frequency.

Immunolocalization of AGPs and Pectins in Quercus suber Gametophytic Structures

The arabinogalactan proteins (AGPs) are highly glycosylated proteins, ubiquitous in plants that have been linked to numerous aspects of sexual reproduction in several plant species, including the monoecious tree species Quercus suber. AGPs are found in cell membranes and cell walls of all types of tissues, including reproductive cells and organs. Pectins are cell wall components that also have been shown to change in composition and quantity during the maturations of the male and female gametophyte in cork oak. These findings were only possible to reveal, due to the histological study of AGP and pectins epitopes by immunolabeling. The immunofluorescence microscopy technique uses antibodies linked to fluorophores and relies on the specificity of the antibody binding to its antigen, labeling the epitope with a fluorescent dye.In the method presented here, we explore the immunolocalization technique performed in male and female flowers of Quercus suber, using London Resin (LR-White) as the embedding medium, after vacuum fixation with formaldehyde/glutaraldehyde. An extensive description of all the aspects of this technique is provided, from the plant material developmental stages selection to the critical analysis of results performed, continuously supported by troubleshooting recommendations.

Arabinogalactan proteins and their sugar chains: functions in plant reproduction, research methods, and biosynthesis

The arabinogalactan protein (AGP) family is one of the most complex protein families and is ubiquitous in the plant kingdom. Moreover, it has been demonstrated to play various roles during plant reproduction. A typical AGP contains a hydroxyproline-rich core protein with high heterogeneity and varying numbers of polysaccharide side chains. However, the functions of the polysaccharide components (i.e. AG sugar chains) remain largely unknown due to the general difficulties associated with studying sugar chains in glycobiology. In recent years, methodological breakthroughs have resulted in substantial progress in AGP research. Here, we summarise the multiple roles of AGPs during plant gametophyte development and male-female communication, with a focus on recent advances. In addition, we discuss the analytical tools used in AGP research, and the biosynthesis and function of AG sugar chains. A comprehensive understanding of the AGP family will help clarify the mechanisms precisely controlling reproductive processes.

Abnormal swelling of the peritrophic membrane in Eri silkworm gut caused by MLX56 family defense proteins with chitin-binding and extensin domains

MLX56 family defense proteins, MLX56 and its close homolog LA-b, are chitin-binding defense proteins found in mulberry latex that show strong growth-inhibitions against caterpillars when fed at concentrations as low as 0.01%. MLX56 family proteins contain a unique structure with an extensin domain surrounded by two hevein-like chitin-binding domains, but their defensive modes of action remain unclear. Here, we analyzed the effects of MLX56 family proteins on the peritrophic membrane (PM), a thin and soft membrane consisting of chitin that lines the midgut lumen of insects. We observed an abnormally thick (>1/5 the diameter of midgut) hard gel-like membrane consisted of chitin and MLX56 family proteins, MLX56 and LA-b, in the midgut of the Eri silkworms, Samia ricini, fed a diet containing MLX56 family proteins, MLX56 and LA-b. When polyoxin AL, a chitin-synthesis-inhibitor, was added to the diet containing MLX56 family proteins, the toxicity of MLX56 family proteins disappeared and PM became thinner and fragmented. These results suggest that MLX56 family proteins, through their chitin-binding domains, bind to the chitin framework of PM, then through their extensin-domain (gum arabic-like structure), which functions as swelling agent, expands PM into an abnormally thick membrane that inhibits the growth of insects. This study shows that MLX56 family proteins are plant defense lectins with a totally unique mode of action, and reveals the functions of extensin domains and arabinogalactan proteins as swelling (gel-forming) agents of plants.

Arabinogalactan proteins: Immunolocalization in the developing ovary of a facultative apomict Fragaria x ananassa (Duch.)

BACKGROUND AND AIMS

Arabinogalactan proteins are present in the extracellular matrix and their occurrence is developmentally regulated. The studies were carried out to localise arabinogalactan proteins in ovary cells of Fragaria x ananassa Duchesne (strawberry) during megasporogenesis, megagametogenesis, and formation of the embryo.

METHODS

The research included studies of ovary histochemistry and immunofluorescence labelling of AGP epitopes was performed with antibodies JIM13, JIM15 and MAC207. The use of the immunogold labelling method allowed specific detection of AGP epitopes at the subcellular level.

KEY RESULTS

The localization of AGPs was studied in the cells of the ovary wall and elements building the developing ovule i.e. the integument, nucellus, archespore, megaspores, embryo sac, and embryo of a facultative apomict Fragaria x ananassa cv. 'Mount Everest'. For the first time the presence of AGP epitopes at the stage of a multicellular archespore was described. The occurrence of AGPs in the functional megaspore walls is related to selection of a megaspore continuing development; during later stages of development, AGPs are also evident markers of the female gametophyte. The intense fluorescence indicates the presence of AGPs in the embryo sac wall as well as in the cytoplasm compartment of the egg apparatus and around the secondary nucleus of the central cell. The localization of AGPs in the ovule of F. x ananassa resembles the distribution of these proteins in amphimictic plants.

CONCLUSIONS

Arabinogalactan proteins occur in similar parts of the ovule of amphimictic and apomictic plants. The results confirm the participation of AGPs in reproductive structures as a useful marker during development of female gametophyte.

Chemical signaling for pollen tube guidance at a glance

Pollen tube guidance is a unique navigating system that is required for the successful sexual reproduction of plants. As plant sperm cells are non-motile and egg cells are embedded deep inside the female tissues, a pollen tube delivers the two sperm cells that it contains by growing towards the ovule, in which the egg cell resides. Pollen tube growth towards the ovule is precisely controlled and divided into two stages, preovular and ovular guidance. In this Cell Science at a Glance article and accompanying poster, we provide a comprehensive overview of pollen tube guidance and highlight some of the attractant peptides used during ovular guidance. We further discuss the precise one-to-one guidance system that exists in multi-ovular plants. The pollen tube-blocking system, which is mediated by male–female crosstalk communication, to avoid attraction of multiple pollen tubes, is also reviewed.

Pollen tube growth and guidance: Occam's razor sharpened on a molecular arabinogalactan glycoprotein Rosetta Stone

Occam's Razor suggests a new model of pollen tube tip growth based on a novel Hechtian oscillator that integrates a periplasmic arabinogalactan glycoprotein-calcium (AGP-Ca²⁺ ) capacitor with tip-localized AGPs as the source of tip-focussed cytosolic Ca²⁺ oscillations: Hechtian adhesion between the plasma membrane and the cell wall of the growing tip acts as a piconewton force transducer that couples the internal stress of a rapidly growing wall to the plasma membrane. Such Hechtian transduction opens stretch-activated Ca²⁺ channels and activates H⁺ -ATPase proton pump efflux that dissociates periplasmic AGP-Ca²⁺ resulting in a Ca²⁺ influx that activates exocytosis of wall precursors. Thus, a highly simplified pectic primary cell wall regulates its own synthesis by a Hechtian growth oscillator that regulates overall tip growth. By analogy with the three cryptic inscriptions of the classical Rosetta Stone, the Hechtian Hypothesis translates classical AGP function as a Ca²⁺ capacitor, pollen tube guide and wall plasticizer into a simple but widely applicable model of tip growth. Even wider ramifications of the Hechtian oscillator may implicate AGPs in osmosensing or gravisensing and other tropisms, leading us yet further towards the Holy Grail of plant growth.

Pollen-pistil interaction in pawpaw (Asimina triloba), the northernmost species of the mainly tropical family Annonaceae

PREMISE OF THE STUDY

The pawpaw, Asimina triloba, is an underutilized fruit crop native to North America that belongs to the mainly tropical, early-divergent family Annonaceae. Asimina is the only genus within the Annonaceae with species adapted to cold climates. A thorough analysis of its reproductive biology, specifically pollen-pistil interaction during the progamic phase, is essential to understand both its adaptation to cold climates and how to optimize its fertilization and fruit set.

METHODS

We characterized pollen-pistil interaction in Asimina triloba, including the floral cycle and anatomy, stigmatic receptivity, and the pollen tube pathway. We used a combination of histological, cytological, and immunolocalization approaches.

KEY RESULTS

Asimina triloba has a gynoecium formed by plicate carpels with a short stylar canal. Unicellular papillae form a continuous tissue covered by a copious secretion from the stigma to the ovary, which is most prominent on the stigma surface where it forms an extragynoecial compitum. Compared to the stigmas of other species in the Annonaceae, the stigmas of A. triloba show a long stigmatic receptivity associated with a long flowering cycle. Stigmatic receptivity is concomitant with the secretion of cell-wall-related arabinogalactan proteins (AGPs).

CONCLUSIONS

A long female phase with a long period of stigmatic receptivity is unusual among protogynous flowers of the magnoliid clade, suggesting a derived condition of A. triloba within the Annonaceae. This phase further correlates with the presence of cell-wall-related arabinogalactan proteins in the secretion, which may indicate the conservation of these glycoproteins during stigmatic receptivity and pollen tube growth in angiosperms.

Identification of fasciclin-like arabinogalactan proteins in textile hemp (Cannabis sativa L.): in silico analyses and gene expression patterns in different tissues

BACKGROUND

The fasciclin-like arabinogalactan proteins (FLAs) belong to the arabinogalactan protein (AGP) superfamily and are known to play different physiological roles in plants. This class of proteins was shown to participate in plant growth, development, defense against abiotic stresses and, notably, cell wall biosynthesis. Although some studies are available on the characterization of FLA genes from different species, both woody and herbaceous, no detailed information is available on the FLA family of textile hemp (Cannabis sativa L.), an economically important fibre crop.

RESULTS

By searching the Cannabis genome and EST databases, 23 CsaFLAs have been here identified which are divided into four phylogenetic groups. A real-time qPCR analysis performed on stem tissues (isolated bast fibres and shivs sampled at three heights), hypocotyls (6-9-12-15-17-20 days-old), whole seedlings, roots, leaves and female/male flowers of the monoecious fibre variety Santhica 27, indicates that the identified FLA genes are differentially expressed. Interestingly, some hemp FLAs are expressed during early phases of fibre growth (elongation), while others are more expressed in the middle and base of the stem and thus potentially involved in secondary cell wall formation (fibre thickening). The bioinformatic analysis of the promoter regions shows that the FLAs upregulated in the younger regions of the stem share a conserved motif related to flowering control and regulation of photoperiod perception. The promoters of the FLA genes expressed at higher levels in the older stem regions, instead, share a motif putatively recognized by MYB3, a transcriptional repressor belonging to the MYB family subgroup S4.

CONCLUSIONS

These results point to the existence of a transcriptional network fine-tuning the expression of FLA genes in the older and younger regions of the stem, as well as in the bast fibres/shivs of textile hemp. In summary, our study paves the way for future analyses on the biological functions of FLAs in an industrially relevant fibre crop.

Polymorphism and structure of style-specific arabinogalactan proteins as determinants of pollen tube growth in Nicotiana

BACKGROUND

Pollen tube growth and fertilization are key processes in angiosperm sexual reproduction. The transmitting tract (TT) of Nicotiana tabacum controls pollen tube growth in part by secreting pistil extensin-like protein III (PELPIII), transmitting-tract-specific (TTS) protein and 120 kDa glycoprotein (120 K) into the stylar extracellular matrix. The three arabinogalactan proteins (AGP) are referred to as stylar AGPs and are the focus of this research. The transmitting tract regulates pollen tube growth, promoting fertilization or rejecting pollen tubes.

RESULTS

The N-terminal domain (NTD) of the stylar AGPs is proline rich and polymorphic among Nicotiana spp. The NTD was predicted to be mainly an intrinsically disordered region (IDR), making it a candidate for protein-protein interactions. The NTD is also the location for the majority of the predicted O-glycosylation sites that were variable among Nicotiana spp. The C-terminal domain (CTD) contains an Ole e 1-like domain, that was predicted to form beta-sheets that are similar in position and length among Nicotiana spp. and among stylar AGPs. The TTS protein had the greatest amino acid and predicted O-glycosylation conservation among Nicotiana spp. relative to the PELPIII and 120 K. The PELPIII, TTS and 120 K genes undergo negative selection, with dn/ds ratios of 0.59, 0.29 and 0.38 respectively. The dn/ds ratio for individual species ranged from 0.4 to 0.9 and from 0.1 to 0.8, for PELPIII and TTS genes, respectively. These data indicate that PELPIII and TTS genes are under different selective pressures. A newly discovered AGP gene, Nicotiana tabacum Proline Rich Protein (NtPRP), was found with a similar intron-exon configuration and protein structure resembling other stylar AGPs, particularly TTS.

CONCLUSIONS

Further studies of the NtPRP gene are necessary to elucidate its biological role. Due to its high similarity to the TTS gene, NtPRP may be involved in pollen tube guidance and growth. In contrast to TTS, both PELPIII and 120 K genes are more diverse indicating a possible role in speciation or mating preference of Nicotiana spp. We hypothesize that the stylar AGPs and NtPRP share a common origin from a single gene that duplicated and diversified into four distinct genes involved in pollen-style interactions.

Transcriptomic analysis links gene expression to unilateral pollen-pistil reproductive barriers

BACKGROUND

Unilateral incompatibility (UI) is an asymmetric reproductive barrier that unidirectionally prevents gene flow between species and/or populations. UI is characterized by a compatible interaction between partners in one direction, but in the reciprocal cross fertilization fails, generally due to pollen tube rejection by the pistil. Although UI has long been observed in crosses between different species, the underlying molecular mechanisms are only beginning to be characterized. The wild tomato relative Solanum habrochaites provides a unique study system to investigate the molecular basis of this reproductive barrier, as populations within the species exhibit both interspecific and interpopulation UI. Here we utilized a transcriptomic approach to identify genes in both pollen and pistil tissues that may be key players in UI.

RESULTS

We confirmed UI at the pollen-pistil level between a self-incompatible population and a self-compatible population of S. habrochaites. A comparison of gene expression between pollinated styles exhibiting the incompatibility response and unpollinated controls revealed only a small number of differentially expressed transcripts. Many more differences in transcript profiles were identified between UI-competent versus UI-compromised reproductive tissues. A number of intriguing candidate genes were highly differentially expressed, including a putative pollen arabinogalactan protein, a stylar Kunitz family protease inhibitor, and a stylar peptide hormone Rapid ALkalinization Factor. Our data also provide transcriptomic evidence that fundamental processes including reactive oxygen species (ROS) signaling are likely key in UI pollen-pistil interactions between both populations and species.

CONCLUSIONS

Gene expression analysis of reproductive tissues allowed us to better understand the molecular basis of interpopulation incompatibility at the level of pollen-pistil interactions. Our transcriptomic analysis highlighted specific genes, including those in ROS signaling pathways that warrant further study in investigations of UI. To our knowledge, this is the first report to identify candidate genes involved in unilateral barriers between populations within a species.