MYB98 positively regulates a battery of synergid-expressed genes encoding filiform apparatus localized proteins

Attraction of tip-growing pollen tubes by the female gametophyte

Pollen tube guidance is the mechanism whereby the direction of pollen tube growth is controlled by female cells of the pistil. Some key genes and molecules have recently been identified as being involved in pollen tube guidance. In this review article, we discuss the molecular basis of pollen tube guidance, especially in Arabidopsis thaliana, by summarizing recent progress in various plant species. Attractant molecules and receptors for gametophytic pollen tube guidance are the focus of this article.

Cell-cell communication and signalling pathways within the ovule: from its inception to fertilization

Cell-cell communication pervades every aspect of the life of a plant. It is particularly crucial for the development of the gametes and their subtle interaction leading to double fertilization. The ovule is composed of a funiculus, one or two integuments, and a gametophyte surrounded by nucellus tissue. Proper ovule and embryo sac development are critical to reproductive success. To allow fertilization, the correct relative positioning and differentiation of the embryo sac cells are essential. Integument development is also intimately linked with the normal development of the female gametophyte; the sporophyte and gametophyte are not fully independent tissues. Inside the gametophyte, numerous signs of cell-cell communication take place throughout development, including cell fate patterning, fertilization and the early stages of embryogenesis. This review highlights the current evidence of cell-cell communication and signalling elements based on structural and physiological observations as well as the description and characterization of mutants in structurally specific genes. By combining data from different species, models of cell-cell interactions have been built, particularly for the establishment of the germline, for the progression through megagametogenesis and for double fertilization.

AtPV42a and AtPV42b redundantly regulate reproductive development in Arabidopsis thaliana

Background

The conserved SNF1/AMPK/SnRK1 complexes are global regulators of metabolic responses in eukaryotes and play a key role in the control of energy balance. Although α-type subunits of the SnRK1 complex have been characterized in several plant species, the biological function of β-type and γ-type subunits remains largely unknown. Here, we characterized AtPV42a and AtPV42b, the two homologous genes in Arabidopsis, which encode cystathionine-β-synthase (CBS) domain-containing proteins that belong to the PV42 class of γ-type subunits of the plant SnRK1 complexes.

Methodology/Principal Findings

Real-time polymerase chain reaction was performed to examine the expression of AtPV42a and AtPV42b in various tissues. Transgenic plants that expressed artificial microRNAs targeting these two genes were created. Reproductive organ development and fertilization in these plants were examined by various approaches, including histological analysis, scanning electron microscopy, transmission electron microscopy, and phenotypic analyses of reciprocal crosses between wild-type and transgenic plants. We found that AtPV42a and AtPV42b were expressed in various tissues during different developmental stages. Transgenic plants where AtPV42a and AtPV42b were simultaneously silenced developed shorter siliques and reduced seed sets. Such low fertility phenotype resulted from deregulation of late stamen development and impairment of pollen tube attraction conferred by the female gametophyte.

Conclusions

Our results demonstrate that AtPV42a and AtPV42b play redundant roles in regulating male gametogenesis and pollen tube guidance, indicating that the Arabidopsis SnRK1 complexes might be involved in the control of reproductive development.

Whole genome analysis of gene expression reveals coordinated activation of signaling and metabolic pathways during pollen-pistil interactions in Arabidopsis

Plant reproduction depends on the concerted activation of many genes to ensure correct communication between pollen and pistil. Here, we queried the whole transcriptome of Arabidopsis (Arabidopsis thaliana) in order to identify genes with specific reproductive functions. We used the Affymetrix ATH1 whole genome array to profile wild-type unpollinated pistils and unfertilized ovules. By comparing the expression profile of pistils at 0.5, 3.5, and 8.0 h after pollination and applying a number of statistical and bioinformatics criteria, we found 1,373 genes differentially regulated during pollen-pistil interactions. Robust clustering analysis grouped these genes in 16 time-course clusters representing distinct patterns of regulation. Coregulation within each cluster suggests the presence of distinct genetic pathways, which might be under the control of specific transcriptional regulators. A total of 78% of the regulated genes were expressed initially in unpollinated pistil and/or ovules, 15% were initially detected in the pollen data sets as enriched or preferentially expressed, and 7% were induced upon pollination. Among those, we found a particular enrichment for unknown transcripts predicted to encode secreted proteins or representing signaling and cell wall-related proteins, which may function by remodeling the extracellular matrix or as extracellular signaling molecules. A strict regulatory control in various metabolic pathways suggests that fine-tuning of the biochemical and physiological cellular environment is crucial for reproductive success. Our study provides a unique and detailed temporal and spatial gene expression profile of in vivo pollen-pistil interactions, providing a framework to better understand the basis of the molecular mechanisms operating during the reproductive process in higher plants.

Identification of genes expressed in the angiosperm female gametophyte

Until recently, identification of gene regulatory networks controlling the development of the angiosperm female gametophyte has presented a significant challenge to the plant biology community. The angiosperm female gametophyte is fairly inaccessible because it is a highly reduced structure relative to the sporophyte and is embedded within multiple layers of the sporophytic tissue of the ovule. Moreover, although mutations affecting the female gametophyte can be readily isolated, their analysis can be difficult because most affect genes involved in basic cellular processes that are also required in the diploid sporophyte. In recent years, expression-based approaches in multiple species have begun to uncover gene sets expressed in specific female gametophyte cells as a means of identifying regulatory networks controlling cell differentiation in the female gametophyte. Here, recent efforts to identify and analyse gene expression programmes in the Arabidopsis female gametophyte are reviewed.

Cysteine-rich peptides (CRPs) mediate diverse aspects of cell-cell communication in plant reproduction and development

Cell-cell communication in plants is essential for the correct co-ordination of reproduction, growth, and development. Studies to dissect this mode of communication have previously focussed primarily on the action of plant hormones as mediators of intercellular signalling. In animals, peptide signalling is a well-documented intercellular communication system, however, relatively little is known about this system in plants. In recent years, numerous reports have emerged about small, secreted peptides controlling different aspects of plant reproduction. Interestingly, most of these peptides are cysteine-rich, and there is convincing evidence suggesting multiple roles for related cysteine-rich peptides (CRPs) as signalling factors in developmental patterning as well as during plant pathogen responses and symbiosis. In this review, we discuss how CRPs are emerging as key signalling factors in regulating multiple aspects of vegetative growth and reproductive development in plants.

Nuclear behavior, cell polarity, and cell specification in the female gametophyte

In flowering plants, the haploid gamete-forming generation comprises only a few cells and develops within the reproductive organs of the flower. The female gametophyte has become an attractive model system to study the genetic and molecular mechanisms involved in pattern formation and gamete specification. It originates from a single haploid spore through three free nuclear division cycles, giving rise to four different cell types. Research over recent years has allowed to catch a glimpse of the mechanisms that establish the distinct cell identities and suggests dynamic cell-cell communication to orchestrate not only development among the cells of the female gametophyte but also the interaction between male and female gametophytes. Additionally, cytological observations and mutant studies have highlighted the importance of nuclei migration- and positioning for patterning the female gametophyte. Here we review current knowledge on the mechanisms of cell specification in the female gametophyte, emphasizing the importance of positional cues for the establishment of distinct molecular profiles.

Achievement of genetics in plant reproduction research: the past decade for the coming decade

In the last decade, a variety of innovations of emerging technologies in science have been accomplished. Advanced research environment in plant science has made it possible to obtain whole genome sequence in plant species. But now we recognize this by itself is not sufficient to understand the overall biological significance. Since Gregor Mendel established a principle of genetics, known as Mendel's Laws of Inheritance, genetics plays a prominent role in life science, and this aspect is indispensable even in modern plant biology. In this review, we focus on achievements of genetics on plant sexual reproduction research in the last decade and discuss the role of genetics for the coming decade. It is our hope that this will shed light on the importance of genetics in plant biology and provide valuable information to plant biologists.

The R2R3 MYB transcription factor DUO1 activates a male germline-specific regulon essential for sperm cell differentiation in Arabidopsis

The male germline in flowering plants arises through asymmetric division of a haploid microspore. The resulting germ cell undergoes mitotic division and specialization to produce the two sperm cells required for double fertilization. The male germline-specific R2R3 MYB transcription factor DUO1 POLLEN1 (DUO1) plays an essential role in sperm cell specification by activating a germline-specific differentiation program. Here, we show that ectopic expression of DUO1 upregulates a significant number (~63) of germline-specific or enriched genes, including those required for fertilization. We validated 14 previously unknown DUO1 target genes by demonstrating DUO1-dependent promoter activity in the male germline. DUO1 is shown to directly regulate its target promoters through binding to canonical MYB sites, suggesting that the DUO1 target genes validated thus far are likely to be direct targets. This work advances knowledge of the DUO1 regulon that encompasses genes with a range of cellular functions, including transcription, protein fate, signaling, and transport. Thus, the DUO1 regulon has a major role in shaping the germline transcriptome and functions to commit progenitor germ cells to sperm cell differentiation.

Distinct gene expression profiles in egg and synergid cells of rice as revealed by cell type-specific microarrays

Double fertilization in flowering plants refers to a process in which two sperm cells, carried by the pollen tube, fertilize both the egg and the central cell after their release into a synergid cell of the female gametophyte. The molecular processes by which the female gametophytic cells express their unique functions during fertilization are not well understood. Genes expressed in egg and synergid cells might be important for multiple stages of the plant reproductive process. Here, we profiled genome-wide gene expression in egg and synergid cells in rice (Oryza sativa), a model monocot, using a nonenzymatic cell isolation technique. We found that the expression profiles of the egg and synergid cells were already specified at the micropylar end of the female gametophyte during the short developmental period that comprises the three consecutive mitotic nuclear divisions after megaspore generation. In addition, we identified a large number of genes expressed in the rice egg and synergid cells and characterized these genes using Gene Ontology analysis. The analysis suggested that epigenetic and posttranscriptional regulatory mechanisms are involved in the specification and/or maintenance of these cells. Comparisons between the rice profiles and reported Arabidopsis (Arabidopsis thaliana) profiles revealed that genes enriched in the egg/synergid cell of rice were distinct from those in Arabidopsis.

Green love talks; cell-cell communication during double fertilization in flowering plants

BACKGROUND

Flowering plant seeds originate from a unique double-fertilization event, which involves two sperm cells and two female gametes, the egg cell and the central cell. For many years our knowledge of mechanisms involved in angiosperm fertilization remained minimal. It was obvious that several signals were required to explain how the male gametes are delivered inside the maternal reproductive tissues to the two female gametes but their molecular nature remained unknown. The difficulties in imaging the double-fertilization process prevented the identification of the mode of sperm cell delivery. It was believed that the two sperm cells were not functionally equivalent.

SCOPE

We review recent studies that have significantly improved our understanding of the early steps of double fertilization. The attractants of the pollen tube have been identified as small proteins produced by the synergid cells that surround the egg cell. Genetic studies have identified the signalling pathways required for the release of male gametes from the pollen tube. High-resolution imaging of the trajectory of the two male gametes showed that their transport does not involve the synergid cells directly and that isomorphic male gametes are functionally equivalent. We also outline major outstanding issues in the field concerned with the barrier against polyspermy, gamete recognition and mechanisms that prevent interspecies crosses.

Annexins

Annexins are multifunctional lipid-binding proteins. Plant annexins are expressed throughout the life cycle and are under environmental control. Their association or insertion into membranes may be governed by a range of local conditions (Ca(2+), pH, voltage or lipid identity) and nonclassical sorting motifs. Protein functions include exocytosis, actin binding, peroxidase activity, callose synthase regulation and ion transport. As such, annexins appear capable of linking Ca(2+), redox and lipid signalling to coordinate development with responses to the biotic and abiotic environment. Significant advances in plant annexin research have been made in the past 2 yr. Here, we review the basis of annexin multifunctionality and suggest how these proteins may operate in the life and death of a plant cell.

The female gametophyte

The angiosperm female gametophyte is critical for plant reproduction. It contains the egg cell and central cell that become fertilized and give rise to the embryo and endosperm of the seed, respectively. Female gametophyte development begins early in ovule development with the formation of a diploid megaspore mother cell that undergoes meiosis. One resulting haploid megaspore then develops into the female gametophyte. Genetic and epigenetic processes mediate specification of megaspore mother cell identity and limit megaspore mother cell formation to a single cell per ovule. Auxin gradients influence female gametophyte polarity and a battery of transcription factors mediate female gametophyte cell specification and differentiation. The mature female gametophyte secretes peptides that guide the pollen tube to the embryo sac and contains protein complexes that prevent seed development before fertilization. Post-fertilization, the female gametophyte influences seed development through maternal-effect genes and by regulating parental contributions. Female gametophytes can form by an asexual process called gametophytic apomixis, which involves formation of a diploid female gametophyte and fertilization-independent development of the egg into the embryo. These functions collectively underscore the important role of the female gametophyte in seed and food production.

Receptor-like kinases as surface regulators for RAC/ROP-mediated pollen tube growth and interaction with the pistil

BACKGROUND

RAC/ROPs are RHO-type GTPases and are known to play diverse signalling roles in plants. Cytoplasmic RAC/ROPs are recruited to the cell membrane and activated in response to extracellular signals perceived and mediated by cell surface-located signalling assemblies, transducing the signals to regulate cellular processes. More than any other cell types in plants, pollen tubes depend on continuous interactions with an extracellular environment produced by their surrounding tissues as they grow within the female organ pistil to deliver sperm to the female gametophyte for fertilization.

SCOPE

We review studies on pollen tube growth that provide compelling evidence indicating that RAC/ROPs are crucial for regulating the cellular processes that underlie the polarized cell growth process. Efforts to identify cell surface regulators that mediate extracellular signals also point to RAC/ROPs being the molecular switches targeted by growth-regulating female factors for modulation to mediate pollination and fertilization. We discuss a large volume of work spanning more than two decades on a family of pollen-specific receptor kinases and some recent studies on members of the FERONIA family of receptor-like kinases (RLKs).

SIGNIFICANCE

The research described shows the crucial roles that two RLK families play in transducing signals from growth regulatory factors to the RAC/ROP switch at the pollen tube apex to mediate and target pollen tube growth to the female gametophyte and signal its disintegration to achieve fertilization once inside the female chamber.

Development of flowering plant gametophytes

Plant reproduction occurs through the production of gametes by a haploid generation, the gametophyte. Flowering plants have highly reduced male and female gametophytes, called pollen grains and embryo sacs, respectively, consisting of only a few cells. Gametophytes are critical for sexual reproduction, but detailed understanding of their development remains poor as compared to the diploid sporophyte. This article reviews recent progress in understanding the mechanisms underlying gametophytic development and function in flowering plants. The focus is on genes and molecules involved in the processes of initiation, growth, cell specification, and fertilization of the male and female gametophytes derived primarily from studies in model systems.

RALFs: peptide regulators of plant growth

Peptide signaling regulates a variety of developmental processes and environmental responses in plants. For example, the peptide systemin induces the systemic defense response in tomato and defensins are small cysteine-rich proteins that are involved in the innate immune system of plants. The CLAVATA3 peptide regulates meristem size and the SCR peptide is the pollen self-incompatibility recognition factor in the Brassicaceae. LURE peptides produced by synergid cells attract pollen tubes to the embryo sac. RALFs are a recently discovered family of plant peptides that play a role in plant cell growth.

Progress and Promise in using Arabidopsis to Study Adaptation, Divergence, and Speciation

Fundamental questions remain to be answered on how lineages split and new species form. The Arabidopsis genus, with several increasingly well characterized species closely related to the model system A. thaliana, provides a rare opportunity to address key questions in speciation research. Arabidopsis species, and in some cases populations within a species, vary considerably in their habitat preferences, adaptations to local environments, mating system, life history strategy, genome structure and chromosome number. These differences provide numerous open doors for understanding the role these factors play in population divergence and how they may cause barriers to arise among nascent species. Molecular tools available in A. thaliana are widely applicable to its relatives, and together with modern comparative genomic approaches they will provide new and increasingly mechanistic insights into the processes underpinning lineage divergence and speciation. We will discuss recent progress in understanding the molecular basis of local adaptation, reproductive isolation and genetic incompatibility, focusing on work utilizing the Arabidopsis genus, and will highlight several areas in which additional research will provide meaningful insights into adaptation and speciation processes in this genus.

Pollen tube guidance by attractant molecules: LUREs

Sexual reproduction in flowering plants requires pollen-tube guidance, which is thought to be mediated by chemoattractants derived from target ovules. To date, however, no convincing evidence has been reported of a particular molecule being the true attractant. Emerging data indicate that two synergid cells, which are on either side of the egg cell, emit a diffusible, species-specific signal to attract the pollen tube at the last step of pollen-tube guidance. Recently, it was demonstrated that LUREs (LURE1 and LURE2), cysteine-rich polypeptides secreted from the synergid cell, are the key molecules in pollen-tube guidance. In this review, we summarize the mechanism of pollen-tube guidance, with special focus on gametophytic guidance and the attractants.

Identification of transcription-factor genes expressed in the Arabidopsis female gametophyte

BACKGROUND

In flowering plants, the female gametophyte is typically a seven-celled structure with four cell types: the egg cell, the central cell, the synergid cells, and the antipodal cells. These cells perform essential functions required for double fertilization and early seed development. Differentiation of these distinct cell types likely involves coordinated changes in gene expression regulated by transcription factors. Therefore, understanding female gametophyte cell differentiation and function will require dissection of the gene regulatory networks operating in each of the cell types. These efforts have been hampered because few transcription factor genes expressed in the female gametophyte have been identified. To identify such genes, we undertook a large-scale differential expression screen followed by promoter-fusion analysis to detect transcription-factor genes transcribed in the Arabidopsis female gametophyte.

RESULTS

Using quantitative reverse-transcriptase PCR, we analyzed 1,482 Arabidopsis transcription-factor genes and identified 26 genes exhibiting reduced mRNA levels in determinate infertile 1 mutant ovaries, which lack female gametophytes, relative to ovaries containing female gametophytes. Spatial patterns of gene transcription within the mature female gametophyte were identified for 17 transcription-factor genes using promoter-fusion analysis. Of these, ten genes were predominantly expressed in a single cell type of the female gametophyte including the egg cell, central cell and the antipodal cells whereas the remaining seven genes were expressed in two or more cell types. After fertilization, 12 genes were transcriptionally active in the developing embryo and/or endosperm.

CONCLUSIONS

We have shown that our quantitative reverse-transcriptase PCR differential-expression screen is sufficiently sensitive to detect transcription-factor genes transcribed in the female gametophyte. Most of the genes identified in this study have not been reported previously as being expressed in the female gametophyte. Therefore, they might represent novel regulators and provide entry points for reverse genetic and molecular approaches to uncover the gene regulatory networks underlying female gametophyte development.

Defensin-like ZmES4 mediates pollen tube burst in maize via opening of the potassium channel KZM1

Species-preferential osmotic pollen tube burst and sperm discharge in maize involve induced opening of the pollen tube-expressed potassium channel KZM1 by the egg apparatus-derived defensin-like protein ZmES4.

In contrast to animals and lower plant species, sperm cells of flowering plants are non-motile and are transported to the female gametes via the pollen tube, i.e. the male gametophyte. Upon arrival at the female gametophyte two sperm cells are discharged into the receptive synergid cell to execute double fertilization. The first players involved in inter-gametophyte signaling to attract pollen tubes and to arrest their growth have been recently identified. In contrast the physiological mechanisms leading to pollen tube burst and thus sperm discharge remained elusive. Here, we describe the role of polymorphic defensin-like cysteine-rich proteins ZmES1-4 (Zea mays embryo sac) from maize, leading to pollen tube growth arrest, burst, and explosive sperm release. ZmES1-4 genes are exclusively expressed in the cells of the female gametophyte. ZmES4-GFP fusion proteins accumulate in vesicles at the secretory zone of mature synergid cells and are released during the fertilization process. Using RNAi knock-down and synthetic ZmES4 proteins, we found that ZmES4 induces pollen tube burst in a species-preferential manner. Pollen tube plasma membrane depolarization, which occurs immediately after ZmES4 application, as well as channel blocker experiments point to a role of K⁺-influx in the pollen tube rupture mechanism. Finally, we discovered the intrinsic rectifying K⁺ channel KZM1 as a direct target of ZmES4. Following ZmES4 application, KZM1 opens at physiological membrane potentials and closes after wash-out. In conclusion, we suggest that vesicles containing ZmES4 are released from the synergid cells upon male-female gametophyte signaling. Subsequent interaction between ZmES4 and KZM1 results in channel opening and K⁺ influx. We further suggest that K⁺ influx leads to water uptake and culminates in osmotic tube burst. The species-preferential activity of polymorphic ZmES4 indicates that the mechanism described represents a pre-zygotic hybridization barrier and may be a component of reproductive isolation in plants.

Sperm cells of animals and lower plants are mobile and can swim to the oocyte or egg cell. In contrast, flowering plants generate immobile sperm encased in a pollen coat to protect them from drying out and are transported via the pollen tube cell towards the egg apparatus to achieve double fertilization. Upon arrival the pollen tube tip bursts to deliver two sperm cells, one fusing with the egg cell to generate the embryo and the other fusing with the central cell to generate the endosperm. Here, we report the mechanisms leading to pollen tube burst and sperm discharge in maize. We found that before fertilization the defensin-like protein ZmES1-4 is stored in the secretory zone of the egg apparatus cells and that pollen tubes cannot discharge sperm in ZmES1-4 knock-down plants. Application of chemically synthesized ZmES4 leads to pollen tube burst within seconds in maize, but not in other plant species, suggesting this mechanism may be species specific. Finally, we identified the pollen tube-expressed potassium channel KZM1 as a target of ZmES4, which opens after ZmES4 treatment and probably leads to K⁺ influx and sperm release after osmotic burst.

A pollen-specific RALF from tomato that regulates pollen tube elongation

Rapid Alkalinization Factors (RALFs) are plant peptides that rapidly increase the pH of plant suspension cell culture medium and inhibit root growth. A pollen-specific tomato (Solanum lycopersicum) RALF (SlPRALF) has been identified. The SlPRALF gene encodes a preproprotein that appears to be processed and released from the pollen tube as an active peptide. A synthetic SlPRALF peptide based on the putative active peptide did not affect pollen hydration or viability but inhibited the elongation of normal pollen tubes in an in vitro growth system. Inhibitory effects of SlPRALF were detectable at concentrations as low as 10 nm, and complete inhibition was observed at 1 mum peptide. At least 10-fold higher levels of alkSlPRALF, which lacks disulfide bonds, were required to see similar effects. A greater effect of peptide was observed in low-pH-buffered medium. Inhibition of pollen tube elongation was reversible if peptide was removed within 15 min of exposure. Addition of 100 nm SlPRALF to actively growing pollen tubes inhibited further elongation until tubes were 40 to 60 mum in length, after which pollen tubes became resistant to the peptide. The onset of resistance correlated with the timing of the exit of the male germ unit from the pollen grain into the tube. Thus, exogenous SlPRALF acts as a negative regulator of pollen tube elongation within a specific developmental window.

VERDANDI is a direct target of the MADS domain ovule identity complex and affects embryo sac differentiation in Arabidopsis

In Arabidopsis thaliana, the three MADS box genes SEEDSTICK (STK), SHATTERPROOF1 (SHP1), and SHP2 redundantly regulate ovule development. Protein interaction studies have shown that a multimeric complex composed of the ovule identity proteins together with the SEPALLATA MADS domain proteins is necessary to determine ovule identity. Despite the extensive knowledge that has become available about these MADS domain transcription factors, little is known regarding the genes that they regulate. Here, we show that STK, SHP1, and SHP2 redundantly regulate VERDANDI (VDD), a putative transcription factor that belongs to the plant-specific B3 superfamily. The vdd mutant shows defects during the fertilization process resulting in semisterility. Analysis of the vdd mutant female gametophytes indicates that antipodal and synergid cell identity and/or differentiation are affected. Our results provide insights into the pathways regulated by the ovule identity factors and the role of the downstream target gene VDD in female gametophyte development.

A role for LORELEI, a putative glycosylphosphatidylinositol-anchored protein, in Arabidopsis thaliana double fertilization and early seed development

In plants, double fertilization requires successful sperm cell delivery into the female gametophyte followed by migration, recognition and fusion of the two sperm cells with two female gametes. We isolated a null allele (lre-5) of LORELEI, which encodes a putative glycosylphosphatidylinositol (GPI)-anchored protein implicated in reception of the pollen tube by the female gametophyte. Although most lre-5 female gametophytes do not allow pollen tube reception, in those that do, early seed development is delayed. A fraction of lre-5/lre-5 seeds underwent abortion due to defect(s) in the female gametophyte. The aborted seeds contained endosperm but no zygote/embryo, reminiscent of autonomous endosperm development in the pollen tube reception mutants scylla and sirene. However, unpollinated lre-5/lre-5 ovules did not initiate autonomous endosperm development and endosperm development in aborted seeds began after central cell fertilization. Thus, the egg cell probably remained unfertilized in aborted lre-5/lre-5 seeds. The lre-5/lre-5 ovules that remain undeveloped due to defective pollen tube reception did not induce synergid degeneration and repulsion of supernumerary pollen tubes. In ovules, LORELEI is expressed during pollen tube reception, double fertilization and early seed development. Null mutants of LORELEI-like-GPI-anchored protein 1 (LLG1), the closest relative of LORELEI among three Arabidopsis LLG genes, are fully fertile and did not enhance reproductive defects in lre-5/lre-5 pistils, suggesting that LLG1 function is not redundant with that of LORELEI in the female gametophyte. Our results show that, besides pollen tube reception, LORELEI also functions during double fertilization and early seed development.

Pollen-pistil interactions regulating successful fertilization in the Brassicaceae

In the Brassicaceae, the acceptance of compatible pollen and the rejection of self-incompatible pollen by the pistil involves complex molecular communication systems between the pollen grain and the female reproductive structures. Preference towards species related-pollen combined with self-recognition systems, function to select the most desirable pollen; and thus, increase the plant's chances for the maximum number of successful fertilizations and vigorous offspring. The Brassicaceae is an ideal group for studying pollen-pistil interactions as this family includes a diverse group of agriculturally relevant crops as well as several excellent model organisms for studying both compatible and self-incompatible pollinations. This review will describe the cellular systems in the pistil that guide the post-pollination events, from pollen capture on the stigmatic papillae to pollen tube guidance to the ovule, with the final release of the sperm cells to effect fertilization. The interplay of other recognition systems, such as the self-incompatibility response and interspecific interactions, on regulating post-pollination events and selecting for compatible pollen-pistil interactions will also be explored.

Mechanistic insights from a quantitative analysis of pollen tube guidance

BACKGROUND

Plant biologists have long speculated about the mechanisms that guide pollen tubes to ovules. Although there is now evidence that ovules emit a diffusible attractant, little is known about how this attractant mediates interactions between the pollen tube and the ovules.

RESULTS

We employ a semi-in vitro assay, in which ovules dissected from Arabidopsis thaliana are arranged around a cut style on artificial medium, to elucidate how ovules release the attractant and how pollen tubes respond to it. Analysis of microscopy images of the semi-in vitro system shows that pollen tubes are more attracted to ovules that are incubated on the medium for longer times before pollen tubes emerge from the cut style. The responses of tubes are consistent with their sensing a gradient of an attractant at 100-150 mum, farther than previously reported. Our microscopy images also show that pollen tubes slow their growth near the micropyles of functional ovules with a spatial range that depends on ovule incubation time.

CONCLUSIONS

We propose a stochastic model that captures these dynamics. In the model, a pollen tube senses a difference in the fraction of receptors bound to an attractant and changes its direction of growth in response; the attractant is continuously released from ovules and spreads isotropically on the medium. The model suggests that the observed slowing greatly enhances the ability of pollen tubes to successfully target ovules. The relation of the results to guidance in vivo is discussed.

Peptide signaling in pollen-pistil interactions

Plant fertilization is achieved through the involvement of various pollen-pistil interactions. Self-/non-self-recognition in pollination is important to avoid inbreeding, and directional and sustainable control of pollen tube growth is critical for the pollen tube to deliver male germ cells. Recently, various secreted peptides (polypeptides) have been reported to be involved in cell-cell communication of pollen-pistil interactions. These include determinants of self-incompatibility, factors for pollen germination and tube growth, and pollen tube attractants. Interestingly, many of them are cysteine-rich peptides/polypeptides (CRPs). In this review, I focus on the peptides involved in pollen-pistil interactions and discuss properties of peptide signaling in each step from pollination to fertilization.

Pattern formation in miniature: the female gametophyte of flowering plants

Plant reproduction involves gamete production by a haploid generation, the gametophyte. For flowering plants, a defining characteristic in the evolution from the ‘naked-seed’ plants, or gymnosperms, is a reduced female gametophyte, comprising just seven cells of four different types – a microcosm of pattern formation and gamete specification about which only little is known. However, several genes involved in the differentiation, fertilization and post-fertilization functions of the female gametophyte have been identified and, recently, the morphogenic activity of the plant hormone auxin has been found to mediate patterning and egg cell specification. This article reviews recent progress in understanding the pattern formation, maternal effects and evolution of this essential unit of plant reproduction.

Female gametophyte development in flowering plants

The multicellular female gametophyte, a unique feature of higher plants, provides us with an excellent experimental system to address fundamental questions in biology. During the past few years, we have gained significant insight into the mechanisms that control embryo sac polarity, gametophytic cell specification, and recognition between male and female gametophytic cells. An auxin gradient has been shown for the first time to function in the female gametophyte to regulate gametic cell fate, and key genes that control gametic cell fate have also been identified. This review provides an overview of these exciting discoveries with a focus on molecular and genetic data.

Analyzing female gametophyte development and function: There is more than one way to crack an egg

In flowering plants, gametes are formed in specialized haploid structures, termed gametophytes. The female gametophyte is a few-celled structure that integrates such diverse functions as pollen tube attraction, sperm cell release, gamete fusion and seed initiation. These processes are realized by distinct cell types, which ensure reproductive success in a coordinated manner. In the past decade, much progress has been made concerning the molecular nature of the functions carried out by the different cell types. Here, we review recent work that has shed light on female gametophyte development and function with a particular focus on approaches that have led to the isolation of genes involved in these processes.

Nectar and pollination drops: how different are they?

BACKGROUND

Pollination drops and nectars (floral nectars) are secretions related to plant reproduction. The pollination drop is the landing site for the majority of gymnosperm pollen, whereas nectar of angiosperm flowers represents a common nutritional resource for a large variety of pollinators. Extrafloral nectars also are known from all vascular plants, although among the gymnosperms they are restricted to the Gnetales. Extrafloral nectars are not generally involved in reproduction but serve as 'reward' for ants defending plants against herbivores (indirect defence).

SCOPE

Although very different in their task, nectars and pollination drops share some features, e.g. basic chemical composition and eventual consumption by animals. This has led some authors to call these secretions collectively nectar. Modern techniques that permit chemical analysis and protein characterization have very recently added important information about these sugary secretions that appear to be much more than a 'reward' for pollinating (floral nectar) and defending animals (extrafloral nectar) or a landing site for pollen (pollination drop).

CONCLUSIONS

Nectar and pollination drops contain sugars as the main components, but the total concentration and the relative proportions are different. They also contain amino acids, of which proline is frequently the most abundant. Proteomic studies have revealed the presence of common functional classes of proteins such as invertases and defence-related proteins in nectar (floral and extrafloral) and pollination drops. Invertases allow for dynamic rearrangement of sugar composition following secretion. Defence-related proteins provide protection from invasion by fungi and bacteria. Currently, only few species have been studied in any depth. The chemical composition of the pollination drop must be investigated in a larger number of species if eventual phylogenetic relationships are to be revealed. Much more information can be provided from further proteomic studies of both nectar and pollination drop that will contribute to the study of plant reproduction and evolution.

Defensin-like polypeptide LUREs are pollen tube attractants secreted from synergid cells

For more than 140 years, pollen tube guidance in flowering plants has been thought to be mediated by chemoattractants derived from target ovules. However, there has been no convincing evidence of any particular molecule being the true attractant that actually controls the navigation of pollen tubes towards ovules. Emerging data indicate that two synergid cells on the side of the egg cell emit a diffusible, species-specific signal to attract the pollen tube at the last step of pollen tube guidance. Here we report that secreted, cysteine-rich polypeptides (CRPs) in a subgroup of defensin-like proteins are attractants derived from the synergid cells. We isolated synergid cells of Torenia fournieri, a unique plant with a protruding embryo sac, to identify transcripts encoding secreted proteins as candidate molecules for the chemoattractant(s). We found two CRPs, abundantly and predominantly expressed in the synergid cell, which are secreted to the surface of the egg apparatus. Moreover, they showed activity in vitro to attract competent pollen tubes of their own species and were named as LUREs. Injection of morpholino antisense oligomers against the LUREs impaired pollen tube attraction, supporting the finding that LUREs are the attractants derived from the synergid cells of T. fournieri.

Truncation of a protein disulfide isomerase, PDIL2-1, delays embryo sac maturation and disrupts pollen tube guidance in Arabidopsis thaliana

Pollen tubes must navigate through different female tissues to deliver sperm to the embryo sac for fertilization. Protein disulfide isomerases play important roles in the maturation of secreted or plasma membrane proteins. Here, we show that certain T-DNA insertions in Arabidopsis thaliana PDIL2-1, a protein disulfide isomerase (PDI), have reduced seed set, due to delays in embryo sac maturation. Reciprocal crosses indicate that these mutations acted sporophytically, and aniline blue staining and scanning electron microscopy showed that funicular and micropylar pollen tube guidance were disrupted. A PDIL2-1-yellow fluorescent protein fusion was mainly localized in the endoplasmic reticulum and was expressed in all tissues examined. In ovules, expression in integument tissues was much higher in the micropylar region in later developmental stages, but there was no expression in embryo sacs. We show that reduced seed set occurred when another copy of full-length PDIL2-1 or when enzymatically active truncated versions were expressed, but not when an enzymatically inactive version was expressed, indicating that these T-DNA insertion lines are gain-of-function mutants. Our results suggest that these truncated versions of PDIL2-1 function in sporophytic tissues to affect ovule structure and impede embryo sac development, thereby disrupting pollen tube guidance.

AthaMap, integrating transcriptional and post-transcriptional data

The AthaMap database generates a map of predicted transcription factor binding sites (TFBS) for the whole Arabidopsis thaliana genome. AthaMap has now been extended to include data on post-transcriptional regulation. A total of 403 173 genomic positions of small RNAs have been mapped in the A. thaliana genome. These identify 5772 putative post-transcriptionally regulated target genes. AthaMap tools have been modified to improve the identification of common TFBS in co-regulated genes by subtracting post-transcriptionally regulated genes from such analyses. Furthermore, AthaMap was updated to the TAIR7 genome annotation, a graphic display of gene analysis results was implemented, and the TFBS data content was increased. AthaMap is freely available at http://www.athamap.de/.

Double fertilization - caught in the act

In flowering plants, fertilization is unique because it involves two pairs of male and female gametes, a process known as double fertilization. Here, we provide an overview of the field and a detailed review of the outstanding recent advances, including in vivo imaging of double fertilization and the identification of a signaling pathway controlling the release of the male gametes and of a protein involved in gamete membrane fusion. These recent results are stepping stones for further research; our knowledge of double fertilization is expanding as newly discovered molecular pathways are explored and new mutants are characterized. Controlling plant fertilization is essential for seed production, and molecular understanding of double fertilization will provide the tools to improve crops and breeding programs.

The MYB98 subcircuit of the synergid gene regulatory network includes genes directly and indirectly regulated by MYB98

The female gametophyte contains two synergid cells that play a role in many steps of the angiosperm reproductive process, including pollen tube guidance. At their micropylar poles, the synergid cells have a thickened and elaborated cell wall: the filiform apparatus that is thought to play a role in the secretion of the pollen tube attractant(s). MYB98 regulates an important subcircuit of the synergid gene regulatory network (GRN) that functions to activate the expression of genes required for pollen tube guidance and filiform apparatus formation. The MYB98 subcircuit comprises at least 83 downstream genes, including 48 genes within four gene families (CRP810, CRP3700, CRP3730 and CRP3740) that encode Cys-rich proteins. We show that the 11 CRP3700 genes, which include DD11 and DD18, are regulated by a common cis-element, GTAACNT, and that a multimer of this sequence confers MYB98-dependent synergid expression. The GTAACNT element contains the MYB98-binding site identified in vitro, suggesting that the 11 CRP3700 genes are direct targets of MYB98. We also show that five of the CRP810 genes, which include DD2, lack a functional GTAACNT element, suggesting that they are not directly regulated by MYB98. In addition, we show that the five CRP810 genes are regulated by the cis-element AACGT, and that a multimer of this sequence confers synergid expression. Together, these results suggest that the MYB98 branch of the synergid GRN is multi-tiered and, therefore, contains at least one additional downstream transcription factor.

Programmed cell death in plants: new insights into redox regulation and the role of hydrogen peroxide

Programmed cell death (PCD), the highly regulated dismantling of cells, is essential for plant growth and survival. PCD plays key roles in embryo development, formation and maturation of many cell types and tissues, and plant reaction/adaptation to environmental conditions. Reactive oxygen species (ROS) are not only toxic by products of aerobic metabolism with strictly controlled cellular levels, but they also function as signaling agents regulating many biological processes and producing pleiotropic effects. Over the last decade, ROS have become recognized as important modulators of plant PCD. Molecular genetic approaches using plant mutants and transcriptome studies related to ROS-mediated PCD have revealed a wide array of plant-specific cell death regulators and have contributed to unraveling the elaborate redox signaling network. This review summarizes the biological processes, in which plant PCD participates and discusses the signaling functions of ROS with emphasis on hydrogen peroxide.

The central cell plays a critical role in pollen tube guidance in Arabidopsis

The sperm cell of flowering plants cannot migrate unaided and must be transported by the pollen tube cell of the male gametophyte to achieve successful fertilization. Long-distance pollen tube guidance is controlled by the seven-celled female gametophyte, the embryo sac. Previous reports showed that the synergid cell of the embryo sac is essential for pollen tube guidance. Here, we report the identification of a central cell guidance (ccg) mutant, which is defective in micropylar pollen tube guidance. CCG encodes a nuclear protein with an N-terminal conserved zinc beta-ribbon domain that is functionally interchangeable with that of TFIIB in yeast. This suggests that CCG might act as a transcription regulator for pollen tube guidance. CCG is expressed in the central cell of the female gametophyte. Expression of CCG in the central cell alone is sufficient to restore the normal pollen tube guidance phenotype, demonstrating that the central cell plays a critical role in pollen tube guidance.