Pollen-stigma interactions: Identification and characterization of surface components with recognition potential

In-Orchard Population Dynamics of Erwinia amylovora on Apple Flower Stigmas

Populations of the fire blight pathogen Erwinia amylovora Ea110 on apple flower stigmas were tracked over the course of apple bloom in field studies conducted between 2016 and 2019. In 18 of 23 experiments, flower stigmas inoculated on the first day of opening were found to harbor large (10⁶ to 10⁷ cells per flower) populations of E. amylovora when assessed 3 to 5 days postinoculation. However, populations inoculated on stigmas of flowers that were already open for 3 days did not reach 10⁶ cells per flower, and populations inoculated on stigmas of flowers that were already open for 5 days never exceeded 10⁴ cells per flower. During this study, ≥10-fold increases in E. amylovora stigma populations in a 24-h time period (termed population surges) were observed on 34.8, 20.0, and 4.0% of possible days on 1-, 3-, and 5-day-open flowers, respectively. Population surges occurred on days with average temperatures as high as 24.5 and as low as 6.1°C. Experiments incorporating more frequent sampling during days and overnight revealed that many population surges occurred between 10:00 p.m. and 2:00 a.m. A Pearson's correlation analysis of weather parameters occurring during surge events indicated that population surges were significantly associated with situations in which overnight temperatures increased or remained constant, in which wind speed decreased, and in which relative humidity increased. This study refines our knowledge of E. amylovora population dynamics and further indicates that E. amylovora is able to infect flowers during exposure to colder field temperatures than previously reported.

Complex reproductive secretions occur in all extant gymnosperm lineages: a proteomic survey of gymnosperm pollination drops

Complex protein-containing reproductive secretions are a conserved trait amongst all extant gymnosperms; the pollination drops of most groups include carbohydrate-modifying enzymes and defence proteins. Pollination drops are aqueous secretions that receive pollen and transport it to the ovule interior in gymnosperms (Coniferales, Cycadales, Ginkgoales, Gnetales). Proteins are well established as components of pollination drops in conifers (Coniferales) and Ephedra spp. (Gnetales), but it is unknown whether proteins are also present in the pollination drops of cycads (Cycadales), Ginkgo (Ginkgoales), Gnetum (Gnetales), or in the pollination drops produced by sterile ovules occurring on pollen plants in the Gnetales. We used liquid chromatography-tandem mass spectrometry followed by database-derived protein identification to conduct proteomic surveys of pollination drops collected from: Ceratozamia hildae, Zamia furfuracea and Cycas rumphii (Cycadales); Ginkgo biloba (Ginkgoales); Gnetum gnemon and Welwitschia mirabilis, including pollination drops from both microsporangiate and ovulate plants (Gnetales). We identified proteins in all samples: C. hildae (61), Z. furfuracea (40), C. rumphii (9), G. biloba (57), G. gnemon ovulate (17) and sterile ovules from microsporangiate plants (25) and W. mirabilis fertile ovules (1) and sterile ovules from microsporangiate plants (138). Proteins involved in defence and carbohydrate modification occurred in the drops of most groups, indicating conserved functions for proteins in pollination drops. Our study demonstrates that all extant gymnosperm groups produce complex reproductive secretions containing proteins, an ancient trait that likely contributed to the evolutionary success of seed plants.

The role of the integuments in pollen tube guidance in flowering plants

In angiosperms, pollen tube entry into the ovule generally takes place through the micropyle, but the exact role of the micropyle in pollen tube guidance remains unclear. A limited number of studies have examined eudicots with bitegmic micropyles, but information is lacking in ovules of basal/early-divergent angiosperms with unitegmic micropyles. We have evaluated the role of the micropyle in pollen tube guidance in an early-divergent angiosperm (Annona cherimola) and the evolutionarily derived Arabidopsis thaliana by studying γ-aminobutyric acid (GABA) and arabinogalactan proteins (AGPs) in wild-type plants and integument-defective mutants. A conserved inhibitory role of GABA in pollen tube growth was shown in A. cherimola, in which AGPs surround the egg apparatus. In Arabidopsis, the micropyle formed only by the outer integument in wuschel-7 mutants caused a partial defect in pollen tube guidance. Moreover, pollen tubes were not observed in the micropyle of an inner no outer (ino) mutant in Arabidopsis, but were observed in homologous ino mutants in Annona. The similar distribution of GABA and AGPs observed in the micropyle of Arabidopsis and Annona, together with the anomalies from specific integument mutants, support the role of the inner integument in preventing multiple tube entrance (polytubey) in these two phylogenetically distant genera.

The unique evolutionary pattern of the Hydroxyproline-rich glycoproteins superfamily in Chinese white pear (Pyrus bretschneideri)

BACKGROUND

The hydroxyproline-rich glycoprotein (HRGP) superfamily, comprising three families (arabinogalactan-proteins, AGPs; extensins, EXTs; proline-rich proteins, PRPs), is a class of proline-rich proteins that exhibit high diversity and are involved in many aspects of plant biology.

RESULTS

In this study, 838 HRGPs were identified from Chinese white pear (Pyrus bretschneideri) by searching for biased amino acid composition and conserved motifs. 405 HRGPs were derived from whole genome duplication (WGD) events which is suggested to be the major force of driving HRGPs expansion and the recent WGD event shared by apple and pear generated most duplicated HRGPs in pear. This duplication event drived the structural variation of the HRGPs encoding hydroxyproline (Hyp)-rich motifs. The rate of HRGPs evolution mainly impacted the Hyp-rich motifs even in chimeric HRGPs. During the evolution of 53 PRPs that are also typified by 7-deoxyloganetin glucosyltransferase-like genes, the duplication from PRP to non-PRP was indirectly modified by positive selection. These results suggested that the rate of HRGP evolution mainly influenced the Hyp-rich motifs even in chimeric HRGPs. The expression divergence of HRGPs was higher than that of other commonly duplicated genes. In pear pistil, 601 HRGPs exhibited expression, while in pear pollen, 285 HRGPs were expressed. The qPCR results revealed that Pbr036330.1 and Pbr010506.1 showed different expression profile in self-incompatibility of pear pistil.

CONCLUSIONS

The researches indicated that WGD events was the main duplication type during the evolution of HRGPs, and the highly variable Hyp-motifs might be accountable for the expansion, evolution and expression divergence of HRGPs and that this divergence may be responsible for the gain of new functions in plants.

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.

Nanospherical arabinogalactan proteins are a key component of the high-strength adhesive secreted by English ivy

Over 130 y have passed since Charles Darwin first discovered that the adventitious roots of English ivy (Hedera helix) exude a yellowish mucilage that promotes the capacity of this plant to climb vertical surfaces. Unfortunately, little progress has been made in elucidating the adhesion mechanisms underlying this high-strength adhesive. In the previous studies, spherical nanoparticles were observed in the viscous exudate. Here we show that these nanoparticles are predominantly composed of arabinogalactan proteins (AGPs), a superfamily of hydroxyproline-rich glycoproteins present in the extracellular spaces of plant cells. The spheroidal shape of the AGP-rich ivy nanoparticles results in a low viscosity of the ivy adhesive, and thus a favorable wetting behavior on the surface of substrates. Meanwhile, calcium-driven electrostatic interactions among carboxyl groups of the AGPs and the pectic acids give rise to the cross-linking of the exuded adhesive substances, favor subsequent curing (hardening) via formation of an adhesive film, and eventually promote the generation of mechanical interlocking between the adventitious roots of English ivy and the surface of substrates. Inspired by these molecular events, a reconstructed ivy-mimetic adhesive composite was developed by integrating purified AGP-rich ivy nanoparticles with pectic polysaccharides and calcium ions. Information gained from the subsequent tensile tests, in turn, substantiated the proposed adhesion mechanisms underlying the ivy-derived adhesive. Given that AGPs and pectic polysaccharides are also observed in bioadhesives exuded by other climbing plants, the adhesion mechanisms revealed by English ivy may forward the progress toward understanding the general principles underlying diverse botanic adhesives.

Arabinogalactan proteins: rising attention from plant biologists

Key message: AGP update: plant reproduction. Arabinogalactan proteins (AGPs) are a large family of hydroxyproline-rich proteins, heavily glycosylated, ubiquitous in land plants, including basal angiosperms and also in many algae. They have been shown to serve as important molecules in several steps of the reproductive process in plants. Due to their special characteristics, such as high sugar content and their means of association with the membrane, they are often perceived as likely candidates for many different aspects of the reproductive process such as signalling molecules, cell identity determinants, morphogens, nutrient sources and support for pollen tube growth, among others. Nevertheless, the study of these proteins pose many difficulties when it comes to studying them individually. Most of the work done involved the use of the β-glucosyl Yariv reagent and antibodies that recognize the carbohydrate epitopes only. Recently, new approaches have been used to study AGPs largely based in the remarkable growing volume of microarray data made available. Either using older techniques or the most recent ones, a clearer picture is emerging for the functions and mode of action of these molecules in the plant reproductive processes. Here, we present an overview about the most important studies made in this area, focusing on the latest advances and the possibilities for future studies in the field.

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

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

Role of proline in cell wall synthesis and plant development and its implications in plant ontogeny

Proline is a proteogenic amino acid and accumulates both under stress and non-stress conditions as a beneficial solute in plants. Recent discoveries point out that proline plays an important role in plant growth and differentiation across life cycle. It is a key determinant of many cell wall proteins that plays important roles in plant development. The role of extensins, arabinogalactan proteins and hydroxyproline- and proline-rich proteins as important components of cell wall proteins that play pivotal roles in cell wall signal transduction cascades, plant development and stress tolerance is discussed in this review. Molecular insights are also provided here into the plausible roles of proline transporters modulating key events in plant development. In addition, the roles of proline during seed developmental transitions including storage protein synthesis are discussed.

Differential expression patterns of arabinogalactan proteins in Arabidopsis thaliana reproductive tissues

Arabinogalactan proteins (AGPs) are heavily glycosylated proteins existing in all members of the plant kingdom and are differentially distributed through distinctive developmental stages. Here, we showed the individual distributions of specific Arabidopsis AGPs: AGP1, AGP9, AGP12, AGP15, and AGP23, throughout reproductive tissues and indicated their possible roles in several reproductive processes. AGP genes specifically expressed in female tissues were identified using available microarray data. This selection was confirmed by promoter analysis using multiple green fluorescent protein fusions to a nuclear localization signal, β-glucuronidase fusions, and in situ hybridization as approaches to confirm the expression patterns of the AGPs. Promoter analysis allowed the detection of a specific and differential presence of these proteins along the pathway followed by the pollen tube during its journey to reach the egg and the central cell inside the embryo sac. AGP1 was expressed in the stigma, style, transmitting tract, and the chalazal and funiculus tissues of the ovules. AGP9 was present along the vasculature of the reproductive tissues and AGP12 was expressed in the stigmatic cells, chalazal and funiculus cells of the ovules, and in the septum. AGP15 was expressed in all pistil tissues, except in the transmitting tract, while AGP23 was specific to the pollen grain and pollen tube. The expression pattern of these AGPs provides new evidence for the detection of a subset of specific AGPs involved in plant reproductive processes, being of significance for this field of study. AGPs are prominent candidates for male-female communication during reproduction.

Functional identification of a hydroxyproline-o-galactosyltransferase specific for arabinogalactan protein biosynthesis in Arabidopsis

Although plants contain substantial amounts of arabinogalactan proteins (AGPs), the enzymes responsible for AGP glycosylation are largely unknown. Bioinformatics indicated that AGP galactosyltransferases (GALTs) are members of the carbohydrate-active enzyme glycosyltransferase (GT) 31 family (CAZy GT31) involved in N- and O-glycosylation. Six Arabidopsis GT31 members were expressed in Pichia pastoris and tested for enzyme activity. The At4g21060 gene (named AtGALT2) was found to encode activity for adding galactose (Gal) to hydroxyproline (Hyp) in AGP protein backbones. AtGALT2 specifically catalyzed incorporation of [(14)C]Gal from UDP-[(14)C]Gal to Hyp of model substrate acceptors having AGP peptide sequences, consisting of non-contiguous Hyp residues, such as (Ala-Hyp) repetitive units exemplified by chemically synthesized (AO)7 and anhydrous hydrogen fluoride-deglycosylated d(AO)51. Microsomal preparations from Pichia cells expressing AtGALT2 incorporated [(14)C]Gal to (AO)7, and the resulting product co-eluted with (AO)7 by reverse-phase HPLC. Acid hydrolysis of the [(14)C]Gal-(AO)7 product released (14)C-radiolabel as Gal only. Base hydrolysis of the [(14)C]Gal-(AO)7 product released a (14)C-radiolabeled fragment that co-eluted with a Hyp-Gal standard after high performance anion-exchange chromatography fractionation. AtGALT2 is specific for AGPs because substrates lacking AGP peptide sequences did not act as acceptors. Moreover, AtGALT2 uses only UDP-Gal as the substrate donor and requires Mg(2+) or Mn(2+) for high activity. Additional support that AtGALT2 encodes an AGP GALT was provided by two allelic AtGALT2 knock-out mutants, which demonstrated lower GALT activities and reductions in β-Yariv-precipitated AGPs compared with wild type plants. Confocal microscopic analysis of fluorescently tagged AtGALT2 in tobacco epidermal cells indicated that AtGALT2 is probably localized in the endomembrane system consistent with its function.

Pollination in Nicotiana alata stimulates synthesis and transfer to the stigmatic surface of NaStEP, a vacuolar Kunitz proteinase inhibitor homologue

After landing on a wet stigma, pollen grains hydrate and germination generally occurs. However, there is no certainty of the pollen tube growth through the style to reach the ovary. The pistil is a gatekeeper that evolved in many species to recognize and reject the self-pollen, avoiding endogamy and encouraging cross-pollination. However, recognition is a complex process, and specific factors are needed. Here the isolation and characterization of a stigma-specific protein from N. alata, NaStEP (N. alata Stigma Expressed Protein), that is homologous to Kunitz-type proteinase inhibitors, are reported. Activity gel assays showed that NaStEP is not a functional serine proteinase inhibitor. Immunohistochemical and protein blot analyses revealed that NaStEP is detectable in stigmas of self-incompatible (SI) species N. alata, N. forgetiana, and N. bonariensis, but not in self-compatible (SC) species N. tabacum, N. plumbaginifolia, N. benthamiana, N. longiflora, and N. glauca. NaStEP contains the vacuolar targeting sequence NPIVL, and immunocytochemistry experiments showed vacuolar localization in unpollinated stigmas. After self-pollination or pollination with pollen from the SC species N. tabacum or N. plumbaginifolia, NaStEP was also found in the stigmatic exudate. The synthesis and presence in the stigmatic exudate of this protein was strongly induced in N. alata following incompatible pollination with N. tabacum pollen. The transfer of NaStEP to the stigmatic exudate was accompanied by perforation of the stigmatic cell wall, which appeared to release the vacuolar contents to the apoplastic space. The increase in NaStEP synthesis after pollination and its presence in the stigmatic exudates suggest that this protein may play a role in the early pollen-stigma interactions that regulate pollen tube growth in Nicotiana.

Characterization of a male sterile related gene BcMF15 from Brassica campestris ssp. chinensis

Data from cDNA-AFLP analysis based on the genome-wide transcriptional profiling on the flower buds of the male meiotic cytokinesis (mmc) mutant and its wild-type of Brassica campestris L. ssp. chinensis Makino, syn. B. rapa L. ssp. chinensis, indicated that mutation of the MMC gene resulted in changes in expression of a variety of genes. A transcript-derived fragment specifically accumulated in the wild-type flower buds was isolated, and the corresponding full-length cDNA and DNA was subsequently amplified. Bioinformatical analyses of this gene named BcMF15 (GenBank accession number EF600901) showed that it encoded a protein with 103 amino acids. The BcMF15 had a 88% nucleotide similarity to a lipid transfer protein-like gene. Moreover, sequence prediction indicated that BcMF15 might encode a membrane protein with a signal peptide at the N-terminus. Meanwhile, six domains were predicted in the deduced BcMF15 protein, such as the AAI domain existing in some crucial proteins of pollen development-preferential, signal peptide, transmembrane domain, vWF domain, ZnF_C4 domain, and Tryp_alpha_amyl domain. Spatial and temporal expression patterns analysis by RT-PCR indicated that BcMF15 was exclusively expressed in the fertile line, which indicated this gene is male sterile related. Phylogenetic analysis in Cruciferae revealed that the BcMF15 was relative conservative in evolution. We suppose BcMF15 may be a critical molecule in the transmembrane transportation and signal transduction during microspore development.

The biology of arabinogalactan proteins

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

Promoter activity of a putative pollen monosaccharide transporter in Petunia hybrida and characterisation of a transposon insertion mutant

For the growth of the male reproductive cells of plants, the pollen, the presence of sufficient sucrose or monosaccharides is of vital importance. From Petunia hybrida a pollen-specific putative monosaccharide transporter designated PMT1 (for petunia monosaccharide transporter) has been identified previously. The present work provides an in-depth analysis and characterisation of PMT1 in the context of pollen development with the GUS reporter gene and an insertion mutant. The promoter of the pollen-specific putative PMT1 gene has been isolated by inverse PCR and sequenced. Analysis of plants transformed with the promoter-GUS fusion confirmed the specificity of this gene, belonging to the late pollen-specific expressed genes. GUS activity was detected even after 24 h of in vitro pollen germination, at the pollen tube tip. To elucidate the importance of PMT1 for gametophyte development and fertilisation, we isolated a mutant plant containing a transposon insertion in the PMT1 gene by the dTph1 transposon-tagging PCR-based assay. The PMT1 mutant contained a dTph1 insertion in position 1474 bp of the transcribing part of the gene, before the last two transmembrane-spanning domains. Analysis of the progeny of the heterozygous mutant after selfing revealed no alterations in pollen viability and fertility. Mature pollen grains of a plant homozygous for the transposon insertion were able to germinate in vitro in a medium containing sucrose, glucose, or fructose, which indicates that PMT1 is not essential for pollen survival. Several explanations for these results are discussed in the present work.

Polar distribution dynamics of Con A binding sites in embryo sacs is temporally coupled by the fertilization process

The binding site distribution of concanavalin agglutinin (Con A) and wheat germ agglutinin (WGA) on embryo sacs at various developmental stages of Torenia fournieri L was studied by using a cooled Charge Coupled Device (CCD) and fluorescent Con A and WGA probes. The distribution patterns of Con A and WGA binding sites on embryo sacs changed during the fertilization process. The fluorescent signal indicating Con A binding sites was distributed evenly on the surface of the embryo sac wall before anthesis, was much denser on the micropylar end of the embryo sac wall and looked like a corona on the day of anthesis. After pollination, stronger fluorescence was present on the micropylar end of the embryo sac wall and the filiform apparatus (FA), showing an obvious polar distribution. When the pollen tube entered the embryo sac and reached a synergid, the fluorescence was still concentrated on the micropylar end and FA, and started to appear on the synergid. After fertilization, the polar distribution of the fluorescence gradually disappeared and an even distribution pattern was observed again on the embryo sac wall. These results revealed that the dynamic distribution of Con A binding sites was temporally coupled with the process of fertilization. WGA binding site distribution on the embryo sac was also investigated and showed a simple pattern but also regularly changed during the process of fertilization. The variation of these lectin binding sites during the fertilization process suggests that lectin binding site interactions may play a role in the process.

Restricted spatial expression of a high-affinity phosphate transporter in potato roots

Phosphorus deficiency limits plant growth, and high-affinity phosphate transporters, of the Pht1 family, facilitate phosphate uptake and translocation. The family is subdivided into root specific, phosphate deprivation induced members and those also expressed in leaves. An antibody to StPT2, a potato root specific transporter, detected two bands (52 kDa and 30 kDa) on western blots of root plasma membrane extracts that were most intense in whole extracts from the root tip and slightly increased throughout the root in response to phosphate depletion. RT-PCR, using StPT2 specific primers, confirmed these findings. Low power confocal immunofluorescent images showed StPT2 expression mainly in the elongation zone at the root tip. By contrast, a vacuolar pyrophosphatase and a plasma membrane ATPase antibody labelled the whole root. High power images showed, by comparison with alpha-tubulin, cell wall and plasma membrane ATPase labelling, that StPT2 was in the epidermal plasma membrane and restricted to the apical surface. This is the first evidence of polar plasma membrane localisation of a plant nutrient transporter and is consistent with a role for StPT2 in phosphate capture and uptake.

DcAGP1, a secreted arabinogalactan protein, is related to a family of basic proline-rich proteins

A cDNA corresponding to the core protein of an immunoaffinity-purified arabinogalactan protein (AGP) secreted aucus carota (carrot) cells in liquid culture was isolated. This cDNA, DcAGP1, encodes a new class of non-classical' AGP with strong similarity to a family of basic proline-rich proteins. The protein is rich in proline (17%), alanine (10%) and lysine (11%) and contains four distinct domains: a signal peptide, a proline-rich domain, a histidine-rich basic domain and a cysteine-containing 'PAC' domain that is found in a range of other cell wall proteins. The protein contains several sequence motifs found in otherwise unrelated cell wall proteins, but also displays some unique features. Northern blot analyses show that while the DcAGP1 transcript is abundant in the suspension-culture cells from which the AGP was obtained; in carrot seedlings the gene is only expressed at low levels in the roots and it is neither wound- nor stress-inducible. Furthermore, northern and western blot analyses demonstrate that the core polypeptide of DcAGP1 is differentially glycosylated in two different carrot suspension cultures. The unusual features of the protein sequence suggest that the DcAGP1 protein is a member of a family of basic proline-rich proteins defined by the C-terminal PAC domain, and the possible function(s) of the DcAGP1 protein is considered in the light of current views on AGP structure and function.

A pollen tube growth-promoting arabinogalactan protein from nicotiana alata is similar to the tobacco TTS protein

Upon germination on the stigma, pollen tubes elongate in the stylar transmitting tract, aided by female factors, with speed and directionality not mimicked in in vitro pollen tube growth cultures. We have shown that a stylar transmitting tissue arabinogalactan protein (AGP) from Nicotiana tabacum (tobacco), TTS protein, stimulates pollen tube growth in vivo and in vitro and attracts pollen tubes grown in a semi-in vivo culture system. It has been reported that the self-incompatible Nicotiana alata produced a stylar glycoprotein, GaRSGP, which had a backbone polypeptide that shared 97% identity with those of TTS proteins but some of its properties were different from those described for TTS proteins. We report here the characterization of a family of stylar transmitting tissue glycoproteins from N. alata that is virtually identical to tobacco TTS proteins and which we refer to as NaTTS proteins. Like their tobacco counterparts, NaTTS proteins are recognized by the traditional AGP-diagnostic reagent beta-glucosyl Yariv reagent, and they are also recognized by JIM13, a monoclonal antibody against AGP. NaTTS proteins also stimulate pollen tube elongation in vitro and attract pollen tubes in a semi-in vivo pollen tube culture system. Biochemical and immunological characterization of NaTTS proteins revealed that they have extraordinary variability in the extent of sugar modifications of their polypeptide backbones. The extent of sugar modifications on NaTTS proteins significantly affects their biochemical properties, influences how they interact with the transmitting tissue extracellular matrix, and affects their solubility from this matrix. Our results suggest that the strategy used to purify GaRSGP only recovered a less glycosylated, more tightly extracellular matrix-bound sub-population of the entire spectrum of N. alata TTS proteins.

Proteoglycans and related components in plant cells

After the context is set by a brief description of the plant cell surface, emphasis is placed on one class of cell surface components, the arabinogalactan proteins. An expansion of knowledge regarding the structure, expression, and function of these proteoglycans has been initiated and is being sustained through new experimental approaches, including the development of monoclonal antibody probes and the cloning of cDNAs corresponding to core polypeptides. An examination of the structure of both the polypeptide and carbohydrate components of arabinogalactan proteins is presented with emphasis placed on recently deduced core polypeptide sequences. Information about the biosynthesis and turnover of arabinogalactan proteins is incomplete, especially with regard to the carbohydrate component. Although functions of arabinogalactan proteins have not been clearly identified, regulated expression and several other lines of evidence point to involvement in plant reproductive development, pattern formation, and somatic embryogenesis, as well as in the underlying processes of cell division, cell expansion, and cell death. Arabinogalactan proteins are compared with animal proteoglycans and mucins, and the results of searches for plant analogues of other animal extracellular matrix components are examined.

Reproduction in seagrasses: pollination in Amphibolis antarctica

Pollination in the dioecious marine angiosperm Amphibolis antarctica has been studied in plants maintained in constant conditions. When the ripe filiform pollen is applied to the submerged stigmas, the grains adhere tenaciously to the receptive zones of the branches. Microsopical examination of the stigmas at this stage shows that a meniscus is developed at each point of pollen-stigma contact and that in every case this results from local coalescence of a proteinaceous substance coating the grain wall and a film of secretion product on the surface of the stigma. Once the grains are attached to the stigma, pollen germination begins. At the site of the future germinal aperture a small region of the pollen wall fabric is gelatinized, presumably by glycosidase activity. At the same time as this process, or shortly afterwards, a lens of polysaccharide is deposited beneath the incipient aperture. The lens hydrates and extrudes, pushing the lax region of wall outwards to form a surface papilla. Extension is limited and the apertural perforation originates when the degraded wall fabric in the distal region of the papilla finally disperses. The pollen tube then emerges through the aperture and rapidly grows towards the stigma, the tube tip eventually making contact with the secretion layer. Entry of the tube into the stigma requires enzymic erosion of the stigma cuticle, and esterases contained in the stigma secretion are probably implicated in the process, but penetration was not directly observed. None the less, cuticle lysis must occur because some hours after pollen attachment pollen tubes are seen in the epidermal tissue of the stigma branch and growing towards the ovary. The principal features of the seagrass pollination system are compared with their counterparts in the pollination system of the land-based flowering plants, and the comparisons reveal marked similarities as well as differences between them.

Characterization of cDNAs for stylar transmitting tissue-specific proline-rich proteins in tobacco

The pistil of flowers is a specialized organ which contains the female gametophytes and provides the structures necessary for pollination and fertilization. Pollen deposited on the stigmatic surface of a compatible plant germinates a pollen tube which penetrates the stigmatic papillae and grows intercellularly through the style towards the ovules in the ovary. Pollen tube growth is largely restricted to the transmitting tissue in the style. Therefore the stylar transmitting tissue is extremely important for the migration of the pollen cell towards the ovary. We have isolated two related cDNAs, transmitting tissue-specific (TTS)-1 and TTS-2, derived from two proline-rich protein (PRP)-encoding mRNAs that accumulate specifically in the transmitting tissue of tobacco. The deduced PRP sequences share similarities with proline-rich cell wall glycoproteins found in a variety of plants. TTS-1 and TTS-2 mRNAs are induced in very young floral buds, accumulate most abundantly during the later stages of flower development when style elongation is the most rapid, and remain at relatively high levels at anthesis. These mRNAs become undetectable in maturing green fruits. In situ hybridization shows that TTS-1 and TTS-2 mRNA accumulation is restricted to the transmitting tissue of the style. The possible roles that these transmitting tissue-specific PRPs may play in maintaining the structural integrity of the style or in the function of this organ is discussed.

Directed Movement of Latex Particles in the Gynoecia of Three Species of Flowering Plants

The secretory matrix of the stylar-transmitting tract of angiosperms has been characterized as a nutrient medium for the growth of pollen tubes, acting to guide tubes to the ovules. When nonliving particles (latex beads) were artificially introduced onto the transmitting tracts of styles of Hemerocallis flava, Raphanus raphanistrum, and Vicia faba, they were translocated to the ovary at rates similar to those of pollen tubes. Direct observations were made on the movement of individual beads along the secretory epidermis in the style and ovary of Vicia faba. The transmitting tract may play an active role in extending tube tips to their destination in the ovary.

Structural requirements for the binding of phenylglycosides to the surface of protoplasts

A variety of phenylglycosides have been synthesized and tested for binding to the surface of protoplasts from suspension-cultured cells of "Paul's Scarlet" rose (Rosa sp.). Multivalent phenylglycosides in the form of Yariv antigens (1,3,5,-tri-[p-glycosyloxyphenylazo]-2,4,6,-trihydroxybenzene) agglutinated the protoplasts. Fluorescence-labeled derivatives of other monovalent and polyvalent phenyl-beta-glycosides did not bind to the protoplast surface. Agglutination was induced by Yariv antigens only if these probes contained beta-anomeric, O-glycosidic linkages. Yariv antigens containing alpha-anomeric or thio-glycosidic linkages did not agglutinate protoplasts. These same structural features of Yariv antigens were also required for the precipitation of gum arabic-Yariv antigen complexes. The results suggest that plasma membranes of "Paul's Scarlet" rose protoplasts contain arabinogalactan-proteins that interact with phenyl-beta-glycosides. The results further show that binding at these plasma membrane sites is not solely dependent upon the carbohydrate portion of single phenylglycosides, but may also require specific spatial orientations of adjacent phenylglycosides.

Origins and genetic nonvariability of the proteins which diffuse from maize pollen

The major function of pollen is to deliver the sperm nuclei to the embryo sac. It does this by germinating and producing a pollen tube and thus provides a relatively simple developmental system for study. Mutants for many pollen functions are accessible, as it is a haploid cell. Mature pollen was fractionated into diffusible proteins, soluble proteins, and proteins insolubly associated with membrane or wall; these protein fractions have been quantified and cataloged by native and SDS polyacrylamide gel electrophoresis. Diffusible proteins are localized in the pollen grain wall whereas soluble proteins are cytoplasmic. The roles of haploid and diploid genomes in specifying these proteins is discussed. Pollen from maximally divergent maize lines was examined for quantitative and qualitative variation in the diffusible proteins. A surprising conservation was found for these proteins indicating some functional role which is, at present, unknown. Initial experiments on the incorporation of 35S-methionine into germinating pollen indicate that major representatives of the diffusible proteins are made within the pollen grain itself. They are presumably included in the pollen wall during development and diffuse out through the pore region. Studies with pollen mRNA and experiments on incorporation of 35S-methionine into developing anthers are underway and will identify the origin of these proteins. A knowledge of the basic developmental biology of maize pollen is a prerequisite to its judicious use as a monitor of environmental mutagens.

Antigenic determinants of a plant proteoglycan, the Gladiolus style arabinogalactan-protein

Antiserum has been raised to the arabinogalactan-protein of Gladiolus style mucilage. This macromolecule has been characterized and has a structure consistent with a 1 leads to 3-linked beta-galactan backbone with side branches of 1 leads to 6-linked beta-galactosyl residues, some of which carry terminal alpha-L-arabinofuranoside residues [Gleeson & Clarke (1979) Biochem. J. 181, 607-621]. The specificity of the antiserum has been investigated by immunoprecipitation with [3H]arabinogalactan-protein. THe 3H label was introduced into the arabinogalactan-protein by oxidation of the terminal galactose residues with galactose oxidase, followed by reduction with NaB3H4. The antigenic specificity of the antiserum was shown to be directed towards the carbohydrate component of the arabinogalactan-protein. D-galactose and L-arabinose were the most effective hapten inhibitors of the antiserum; other monosaccharides, N-acetyl-D-galactono-1,4-lactone, D-glucose, D-mannose, L-rhamnose. L-fucose and D-xylose, were all poor inhibitors. The antiserum showed preference for beta-galactosides over alpha-galactosides. Of the haptens examined, the disaccharide 6-O-beta-D-galactopyranosyl-D-galactopyranose was the most potent inhibitor. The antigenic features of the arabinogalactan-protein were investigated by examining the interaction of the antiserum with chemically and enzymically modified arabinogalactan-protein. Also, the cross-reactivity of structurally related polysaccharides and glycoproteins with the specific antiserum was assessed by a haemagglutination assay using erythrocytes coupled with specific antiserum. The results indicate that the dominant antigenic determinants of the arabinogalactan-protein are probably the side branches of 1 leads to 6 -linked beta-galactose residues bearing the terminal alpha-L-arabinose residues.

Pollen stigma interactions in Brassica oleracea

Recent studies on the mechanism of self-incompatibility in Brassica indicate the location, nature and mode of action of the molecules involved. Characteristics of the pollen surface and the stigma surface are described in detail, together with new information pertaining to the recognition molecules located therein. A sequence of events is outlined leading from pollination, through adhesion, hydration, germination, and tube growth to acceptance and ultimate compatibility. The characteristics of rejection of incompatible grains are described for each stage of the pollen-stigma interaction. It is proposed that recognition of proteins from the coating of self-pollen by the molecules in the pellicle results in the formation of a biologically-active complex which inhibits water supply to the incompatible grain, and that all other manifestations of incompatibility are a consequence of this initial response.