Arabinogalactan proteins: rising attention from plant biologists

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.

Pollen tube access to the ovule is mediated by glycoprotein secretion on the obturator of apple (Malus × domestica, Borkh)

BACKGROUND AND AIMS

Within the ovary, the obturator bridges the pathway of the pollen tube from the style to the ovule. Despite its widespread presence among flowering plants, its function has only been studied in a handful of species, and the molecules involved in pollen tube-obturator cross-talk have not been explored hitherto. This work evaluates the involvement of glucans and glycoproteins on pollen tube growth in the obturator of apple flowers ( Malus × domestica) .

METHODS

Pollen tube kinetics were sequentially examined in the pistil and related to changes occurring on the obturator using histochemistry and inmunocytochemistry. To discriminate between changes in the obturator induced by pollen tubes from those developmentally regulated, both pollinated and unpollinated pistils were examined.

KEY RESULTS

Pollen tube growth rates were slow in the stigma, faster in the style and slow again in the ovary. The arrival of pollen tubes at the obturator was concomitant with the secretion of proteins, saccharides and glycoprotein epitopes belonging to extensins and arabinogalactan proteins (AGPs). While some of these secretions - extensins and AGPs labelled by JIM13 - were developmentally regulated, others - AGPs labelled by JIM8 - were elicited by the presence of pollen tubes. Following pollen tube passage, all these glycoproteins were depleted.

CONCLUSIONS

The results show a timely secretion of glycoproteins on the obturator surface concomitant with pollen tube arrival at this structure. The fact that their secretion is depleted following pollen tube passage strongly suggests their role in regulating pollen tube access to the ovule. Remarkably, both the regulation of the secretion of the different glycoproteins, as well as their association with the performance of pollen tubes exhibit similarities with those observed in the stigma, in line with their common developmental origin.

Arabinogalactan Proteins as Interactors along the Crosstalk between the Pollen Tube and the Female Tissues

Arabinogalactan proteins (AGPs) have long been considered to be implicated in several steps of the reproductive process of flowering plants. Pollen tube growth along the pistil tissues requires a multiplicity of signaling pathways to be activated and turned off precisely, at crucial timepoints, to guarantee successful fertilization and seed production. In the recent years, an outstanding effort has been made by the plant reproduction scientific community in order to better understand this process. This resulted in the discovery of a fairly substantial number of new players essential for reproduction, as well as their modes of action and interactions. Besides all the indications of AGPs involvement in reproduction, there were no convincing evidences about it. Recently, several studies came out to prove what had long been suggested about this complex family of glycoproteins. AGPs consist of a large family of hydroxyproline-rich proteins, predicted to be anchored to the plasma membrane and extremely rich in sugars. These two last characteristics always made them perfect candidates to be involved in signaling mechanisms, in several plant developmental processes. New findings finally relate AGPs to concrete functions in plant reproduction. In this review, it is intended not only to describe how different molecules and signaling pathways are functioning to achieve fertilization, but also to integrate the recent discoveries about AGPs along this process.

Global DNA methylation variations after short-term heat shock treatment in cultured microspores of Brassica napus cv. Topas

Heat stress can induce the cultured microspores into embryogenesis. In this study, whole genome bisulphite sequencing was employed to study global DNA methylation variations after short-term heat shock (STHS) treatments in cultured microspores of Brassica napus cv. Topas. Our results indicated that treatment on cultured Topas microspores at 32 °C for 6 h triggered DNA hypomethylation, particularly in the CG and CHG contexts. And the total number of T32 (Topas 32 °C for 6 h) vs. T0 (Topas 0 h) differentially methylated region-related genes (DRGs) was approximately two-fold higher than that of T18 (Topas 18 °C for 6 h) vs. T0 DRGs, which suggested that 32 °C might be a more intense external stimulus than 18 °C resulting in more changes in the DNA methylation status of cultured microspores. Additionally, 32 °C treatment for 6 h led to increased CHG differential methylations of transposons (DMTs), which were mainly constituted by overlaps between the hypomethylated differentially methylated regions (hypo-DMRs) and transposon elements (TEs). Further analysis demonstrated that the DRGs and their paralogs exhibited differential methylated/demethylated patterns. To summarize, the present study is the first methylome analysis of cultured microspores in response to STHS and may provide valuable information on the roles of DNA methylation in heat response.

The end of temptation: the elimination of persistent synergid cell identity

In flowering plants, sexual reproduction culminates in double fertilization, which occurs after an ovule receives two sperm cells from a single pollen tube. Recent progress in pollen tube guidance, as well as analyses of fertilization-defective mutants, have highlighted a post-fertilization event that rapidly terminates pollen tube attraction. This event plays a crucial role in ensuring a one-to-one fertilization system between males and females. This phenomenon is controlled by the activity of persistent synergid cells, which secrete peptides that attract and thus guide the pollen tube. This review briefly introduces new findings on cell biology and signaling pathways that regulate the unique inactivation mechanism of persistent synergid cells.

Ultrastructural Immunolocalization of Arabinogalactan Protein, Pectin and Hemicellulose Epitopes Through Anther Development in Brassica napus

In this work, we performed an extensive and detailed analysis of the changes in cell wall composition during Brassica napus anther development. We used immunogold labeling to study the spatial and temporal patterns of the composition and distribution of different arabinogalactan protein (AGP), pectin, xyloglucan and xylan epitopes in high-pressure-frozen/freeze-substituted anthers, quantifying and comparing their relative levels in the different anther tissues and developmental stages. We used the following monoclonal antibodies: JIM13, JIM8, JIM14 and JIM16 for AGPs, LM5, LM6, JIM7, JIM5 and LM7 for pectins, CCRC-M1, CCRC-M89 and LM15 for xyloglucan, and LM11 for xylan. Each cell wall epitope showed a characteristic temporal and spatial labeling pattern. Microspore, pollen and tapetal cells showed similar patterns for each epitope, whereas the outermost anther layers (epidermis, endothecium and middle layers) presented remarkably different patterns. Our results suggested that AGPs, pectins, xyloglucan and xylan have specific roles during anther development. The AGP epitopes studied appeared to belong to AGPs specifically involved in microspore differentiation, and contributed first by the tapetum and then, upon tapetal dismantling, by the endothecium and middle layers. In contrast, the changes in pectin and hemicellulose epitopes suggested a specific role in anther dehiscence, facilitating anther wall weakening and rupture. The distribution of the different cell wall constituents is regulated in a tissue- and stage-specific manner, which seems directly related to the role of each tissue at each stage.

Maternal ENODLs Are Required for Pollen Tube Reception in Arabidopsis

During the angiosperm (flowering-plant) life cycle, double fertilization represents the hallmark between diploid and haploid generations [1]. The success of double fertilization largely depends on compatible communication between the male gametophyte (pollen tube) and the maternal tissues of the flower, culminating in precise pollen tube guidance to the female gametophyte (embryo sac) and its rupture to release sperm cells. Several important factors involved in the pollen tube reception have been identified recently [2-6], but the underlying signaling pathways are far from being understood. Here, we report that a group of female-specific small proteins, early nodulin-like proteins (ENODLs, or ENs), are required for pollen tube reception. ENs are featured with a plastocyanin-like (PCNL) domain, an arabinogalactan (AG) glycomodule, and a predicted glycosylphosphatidylinositol (GPI) anchor motif. We show that ENs are asymmetrically distributed at the plasma membrane of the synergid cells and accumulate at the filiform apparatus, where arriving pollen tubes communicate with the embryo sac. EN14 strongly and specifically interacts with the extracellular domain of the receptor-like kinase FERONIA, localized at the synergid cell surface and known to critically control pollen tube reception [6]. Wild-type pollen tubes failed to arrest growth and to rupture after entering the ovules of quintuple loss-of-function EN mutants, indicating a central role of ENs in male-female communication and pollen tube reception. Moreover, overexpression of EN15 by the endogenous promoter caused disturbed pollen tube guidance and reduced fertility. These data suggest that female-derived GPI-anchored ENODLs play an essential role in male-female communication and fertilization.

JAGGER, an AGP essential for persistent synergid degeneration and polytubey block in Arabidopsis

A precise control of sperm cells delivery and fusion to the egg cell and the central cell is fundamental for the accomplishment of successful double fertilization in flowering plants. This is mostly regulated by female gametophyte egg and central cells, which control the timing of synergids cell degeneration. We recently identified an arabinogalactan protein, AGP4, named JAGGER, that impairs the persistent synergid degeneration, and consequently leads to the attraction of more than one pollen tube into one embryo sac, a situation termed polytubey. jagger mutants revealed an increased rate of polytubey and persistent synergids that do not degenerate. This persistent synergid, is, as we suggested, the cell responsible for attracting an extra pollen tube into the embryo sacs.

Arabinogalactan proteins and pectin distribution during female gametogenesis in Quercus suber L

BACKGROUND AND AIMS

Quercus suber L. (cork oak) is one of the most important monoecious tree species in semi-arid regions of Southern Europe, with a high ecological value and economic potential. However, as a result of its long reproductive cycle, complex reproductive biology and recalcitrant seeds, conventional breeding is demanding. In its complex reproductive biology, little is known about the most important changes that occur during female gametogenesis. Arabinogalactan proteins (AGPs) and pectins are the main components of plant cell walls and have been reported to perform common functions in cell differentiation and organogenesis of reproductive plant structures. AGPs have been shown to serve as important molecules in several steps of the reproductive process in plants, working as signalling molecules, associated with the sporophyte-gametophyte transition, and pectins have been implicated in pollen-pistil interactions before double fertilization. In this study, the distribution of AGP and pectin epitopes was assessed during female gametogenesis.

METHODS

Immunofluorescence labelling of female flower cells was performed with a set of monoclonal antibodies (mAbs) directed to the carbohydrate moiety of AGPs (JIM8 and JIM13) and pectic homogalacturonans (HGs) (mAbs JIM5 and JIM7).

KEY RESULTS

The selective labelling obtained with AGP and pectin mAbs JIM8, JIM13, JIM5 and JIM7 during Q. suber female gametogenesis shows that AGPs and pectic HG can work as markers for mapping gametophytic cell differentiation in this species. Pectic HG showed different distribution patterns, depending on their levels of methyl esterification. Methyl-esterified HGs showed a uniform distribution in the overall female flower cells before fertilization and a more specific pattern after fertilization. A low methyl-ester pectin distribution pattern during the different developmental stages appears to be related to the pathway that pollen tubes follow to reach the embryo sac. AGPs showed a more sparse distribution in early stages of development, but specific labelling is shown in the synergids and their filiform apparatus.

CONCLUSIONS

The labelling obtained with anti-AGP and anti-pectin mAbs in Q. suber female flower cells showed a dynamic distribution of AGPs and pectic HGs, which may render these molecules useful molecular markers during female gametogenesis. Changes occurring during development will be determined in order to help describe cork oak ovule structural properties before and after fertilization, providing new insight to better understand Q. suber female gametogenesis.

The AMOR Arabinogalactan Sugar Chain Induces Pollen-Tube Competency to Respond to Ovular Guidance

Precise directional control of pollen-tube growth by pistil tissue is critical for successful fertilization of flowering plants [1-3]. Ovular attractant peptides, which are secreted from two synergid cells on the side of the egg cell, have been identified [4-6]. Emerging evidence suggests that the ovular directional cue is not sufficient for successful guidance but that competency control by the pistil is critical for the response of pollen tubes to the attraction signal [1, 3, 7]. However, the female molecule for this competency induction has not been reported. Here we report that ovular methyl-glucuronosyl arabinogalactan (AMOR) induces competency of the pollen tube to respond to ovular attractant LURE peptides in Torenia fournieri. We developed a method for assaying the response capability of a pollen tube by micromanipulating an ovule. Using this method, we showed that pollen tubes growing through a cut style acquired a response capability in the medium by receiving a sufficient amount of a factor derived from mature ovules of Torenia. This factor, named AMOR, was identified as an arabinogalactan polysaccharide, the terminal 4-O-methyl-glucuronosyl residue of which was necessary for its activity. Moreover, a chemically synthesized disaccharide, the β isomer of methyl-glucuronosyl galactose (4-Me-GlcA-β-(1→6)-Gal), showed AMOR activity. No specific sugar-chain structure of plant extracellular matrix has been identified as a bioactive molecule involved in intercellular communication. We suggest that the AMOR sugar chain in the ovary renders the pollen tube competent to the chemotropic response prior to final guidance by LURE peptides.

"Love Is Strong, and You're so Sweet": JAGGER Is Essential for Persistent Synergid Degeneration and Polytubey Block in Arabidopsis thaliana

Successful double fertilization and subsequent seed development in flowering plants requires the delivery of two sperm cells, transported by a pollen tube, into the embryo sac of an ovule. The embryo sac cells tightly control synergid cell death, and as a result the polyspermy block. Arabinogalactan proteins are highly glycosylated proteins thought to be involved in several steps of the reproductive process. We show that JAGGER, Arabinogalactan Protein 4, is an important molecule necessary to prevent the growth of multiple pollen tubes into one embryo sac in Arabidopsis thaliana. In jagger, an AGP4 knockout mutant, the pistils show impaired pollen tube blockage as a consequence of the survival of the persistent synergid. JAGGER seems to be involved in the signaling pathway that leads to a blockage of pollen tube attraction. Our results shed light on the mechanism responsible for preventing polyspermy in Arabidopsis and for safeguarding successful fertilization of all ovules in one pistil, ensuring seed set and the next generation.

The Diversity of the Pollen Tube Pathway in Plants: Toward an Increasing Control by the Sporophyte

Plants, unlike animals, alternate multicellular diploid, and haploid generations in their life cycle. While this is widespread all along the plant kingdom, the size and autonomy of the diploid sporophyte and the haploid gametophyte generations vary along evolution. Vascular plants show an evolutionary trend toward a reduction of the gametophyte, reflected both in size and lifespan, together with an increasing dependence from the sporophyte. This has resulted in an overlooking of the importance of the gametophytic phase in the evolution of higher plants. This reliance on the sporophyte is most notorious along the pollen tube journey, where the male gametophytes have to travel a long way inside the sporophyte to reach the female gametophyte. Along evolution, there is a change in the scenery of the pollen tube pathway that favors pollen competition and selection. This trend, toward apparently making complicated what could be simple, appears to be related to an increasing control of the sporophyte over the gametophyte with implications for understanding plant evolution.

A small multigene hydroxyproline-O-galactosyltransferase family functions in arabinogalactan-protein glycosylation, growth and development in Arabidopsis

BACKGROUND

Arabinogalactan-proteins (AGPs) are ubiquitous components of cell walls throughout the plant kingdom and are extensively post translationally modified by conversion of proline to hydroxyproline (Hyp) and by addition of arabinogalactan polysaccharides (AG) to Hyp residues. AGPs are implicated to function in various aspects of plant growth and development, but the functional contributions of AGP glycans remain to be elucidated. Hyp glycosylation is initiated by the action of a set of Hyp-O-galactosyltransferase (Hyp-O-GALT) enzymes that remain to be fully characterized.

RESULTS

Three members of the GT31 family (GALT3-At3g06440, GALT4-At1g27120, and GALT6-At5g62620) were identified as Hyp-O-GALT genes by heterologous expression in tobacco leaf epidermal cells and examined along with two previously characterized Hyp-O-GALT genes, GALT2 and GALT5. Transcript profiling by real-time PCR of these five Hyp-O-GALTs revealed overlapping but distinct expression patterns. Transiently expressed GALT3, GALT4 and GALT6 fluorescent protein fusions were localized within Golgi vesicles. Biochemical analysis of knock-out mutants for the five Hyp-O-GALT genes revealed significant reductions in both AGP-specific Hyp-O-GALT activity and β-Gal-Yariv precipitable AGPs. Further phenotypic analysis of these mutants demonstrated reduced root hair growth, reduced seed coat mucilage, reduced seed set, and accelerated leaf senescence. The mutants also displayed several conditional phenotypes, including impaired root growth, and defective anisotropic growth of root tips under salt stress, as well as less sensitivity to the growth inhibitory effects of β-Gal-Yariv reagent in roots and pollen tubes.

CONCLUSIONS

This study provides evidence that all five Hyp-O-GALT genes encode enzymes that catalyze the initial steps of AGP galactosylation and that AGP glycans play essential roles in both vegetative and reproductive plant growth.

Automated Glycan Assembly of Oligosaccharides Related to Arabinogalactan Proteins

Arabinogalactan proteins are heavily glycosylated proteoglycans in plants. Their glycan portion consists of type-II arabinogalactan polysaccharides whose heterogeneity hampers the assignment of the arabinogalactan protein function. Synthetic chemistry is key to the procurement of molecular probes for plant biologists. Described is the automated glycan assembly of 14 oligosaccharides from four monosaccharide building blocks. These linear and branched glycans represent key structural features of natural type-II arabinogalactans and will serve as tools for arabinogalactan biology.