Cell-cell communication during double fertilization

Polyphenol oxidases regulate pollen development through modulating flavonoids homeostasis in tobacco

Pollen development is critical in plant reproduction. Polyphenol oxidases (PPOs) genes encode defense-related enzymes, but the role of PPOs in pollen development remains largely unexplored. Here, we characterized NtPPO genes, and then investigated their function in pollen via creating NtPPO9/10 double knockout mutant (cas-1), overexpression 35S::NtPPO10 (cosp) line and RNAi lines against all NtPPOs in Nicotiana tabacum. NtPPOs were abundantly expressed in the anther and pollen (especially NtPPO9/10). The pollen germination, polarity ratio and fruit weights were significantly reduced in the NtPPO-RNAi and cosp lines, while they were normal in cas-1 likely due to compensation by other NtPPO isoforms. Comparisons of metabolites and transcripts between the pollen of WT and NtPPO-RNAi, or cosp showed that decreased enzymatic activity of NtPPOs led to hyper-accumulation of flavonoids. This accumulation might reduce the content of ROS. Ca²⁺ and actin levels also decreased in pollen of the transgenic lines.Thus, the NtPPOs regulate pollen germination through the flavonoid homeostasis and ROS signal pathway. This finding provides novel insights into the native physiological functions of PPOs in pollen during reproduction.

SWI2/SNF2 chromatin remodeling ATPases SPLAYED and BRAHMA control embryo development in rice

Chromatin remodeling ATPases OsSYD and OsBRM are involved in shoot establishment, and both affect OSH gene transcription. OsSYD protein interacts with RFL, but OsBRM does not. In plants, SPLAYED (SYD) and BRAHMA (BRM) encode chromatin remodeling ATPases that use the energy derived from ATP hydrolysis to restructure nucleosomes and render certain genomic regions available to transcription factors. However, the function of SYD and BRM on rice growth and development is unknown. Here, we constructed ossyd and osbrm mutants using CRISPR/Cas9 technology and analyzed the effects of mutations on rice embryo development. We discovered that the ossyd and osbrm mutants exhibited severe defects during embryonic development, whereas endosperm development was normal. These results indicated that the development of the embryo and endosperm is independent of each other. Consequently, the ossyd- and osbrm-null mutants did not germinate due to the abnormal embryos. Furthermore, we observed the embryos of ossyd- and osbrm-null mutants, and they indeed had distinct differentiation defects in shoot establishment, acquired during embryogenesis. To verify the function of OsSYD and OsBRM in embryogenesis, we measured the transcript levels of marker genes at different stages. Compared with wild type, the expression levels of multiple OSH genes were significantly reduced in the mutants, which was consistent with the defective shoot establishment phenotypes. The interaction between SYD and RICE FLORICAULA/LFY (RFL) was revealed using a yeast two-hybrid screening system, suggesting that the interaction between the LFY homolog and chromatin remodeling ATPases is ubiquitous in plants. Collectively, our findings provide the basis for elucidating the function of OsSYD and OsBRM during embryo development in rice.

Global Survey of Alternative Splicing in Rice by Direct RNA Sequencing During Reproductive Development: Landscape and Genetic Regulation

Alternative splicing is a widespread phenomenon, which generates multiple isoforms of the gene product. Reproductive development is the key process for crop production. Although numerous forms of alternative splicing have been identified in model plants, large-scale study of alternative splicing dynamics during reproductive development in rice has not been conducted. Here, we investigated alternative splicing of reproductive development of young panicles (YP), unfertilized florets (UF) and fertilized florets (F) in rice using direct RNA sequencing, small RNA sequencing, and degradome sequencing. We identified a total of 35,317 alternative splicing (AS) events, among which 67.2% splicing events were identified as novel alternative splicing events. Intron retention (IR) was the most abundant alternative splicing subtype. Splicing factors that differentially expressed and alternatively spliced could result in global alternative splicing. Global analysis of miRNAs-targets prediction revealed that alternative spliced transcripts affected miRNAs' targets during development. Degradome sequencing detected only 6.8% of the differentially alternative splicing transcripts, suggesting a productive transcripts generation during development. In addition, alternative splicing isoforms of Co-like, a transcription factor, interacted with Casein kinase 1-like protein HD1 (CKI) examined in luciferase assay, which could modulate normal male-floral organs development and flowering time. These results reveal that alternative splicing is intensely associated with developmental stages, and a high complexity of gene regulation.

The development of a hairless phenotype in barley roots treated with gold nanoparticles is accompanied by changes in the symplasmic communication

Uptake of water and nutrients by roots affects the ontogenesis of the whole plant. Nanoparticles, e.g. gold nanoparticles, have a broad range of applications in many fields which leads to the transfer of these materials into the environment. Thus, the understanding of their impact on the growth and development of the root system is an emerging issue. During our studies on the effect of positively charged gold nanoparticles on the barley roots, a hairless phenotype was found. We investigated whether this phenotype correlates with changes in symplasmic communication, which is an important factor that regulates, among others, differentiation of the rhizodermis into hair and non-hair cells. The results showed no restriction in symplasmic communication in the treated roots, in contrast to the control roots, in which the trichoblasts and atrichoblasts were symplasmically isolated during their differentiation. Moreover, differences concerning the root morphology, histology, ultrastructure and the cell wall composition were detected between the control and the treated roots. These findings suggest that the harmful effect of nanoparticles on plant growth may, among others, consist in disrupting the symplasmic communication/isolation, which leads to the development of a hairless root phenotype, thus limiting the functioning of the roots.

The Arabidopsis Receptor Kinase ZAR1 Is Required for Zygote Asymmetric Division and Its Daughter Cell Fate

Asymmetric division of zygote is critical for pattern formation during early embryogenesis in plants and animals. It requires integration of the intrinsic and extrinsic cues prior to and/or after fertilization. How these cues are translated into developmental signals is poorly understood. Here through genetic screen for mutations affecting early embryogenesis, we identified an Arabidopsis mutant, zygotic arrest 1 (zar1), in which zygote asymmetric division and the cell fate of its daughter cells were impaired. ZAR1 encodes a member of the RLK/Pelle kinase family. We demonstrated that ZAR1 physically interacts with Calmodulin and the heterotrimeric G protein Gβ, and ZAR1 kinase is activated by their binding as well. ZAR1 is specifically expressed micropylarly in the embryo sac at eight-nucleate stage and then in central cell, egg cell and synergids in the mature embryo sac. After fertilization, ZAR1 is accumulated in zygote and endosperm. The disruption of ZAR1 and AGB1 results in short basal cell and an apical cell with basal cell fate. These data suggest that ZAR1 functions as a membrane integrator for extrinsic cues, Ca2+ signal and G protein signaling to regulate the division of zygote and the cell fate of its daughter cells in Arabidopsis.

Exploration of rice pistil responses during early post-pollination through a combined proteomic and transcriptomic analysis

Pollen-stigma interaction is a multi-step and complex physiological process which contains different signaling and biochemical pathways. However, little is known about the molecular mechanism underlying this process in rice (Oryza sativa). In this study, the changes of gene expression were investigated through a combination study of transcriptome and proteome profiles in rice pistil during pollination. Totally, 1117 differentially expressed genes were identified, among which 962 and 167 were detected at transcriptional and protein level respectively. Functional categorization analysis showed that the genes involved in central metabolism were up-regulated, which can lead to the enhancement of these metabolisms. The reactive oxygen species (ROS) were over-accumulated in the stigma. In response to this, the proteins or transcripts involved in redox homeostasis regulation were differentially expressed. Furthermore, significant changes of protein ubiquitination and its related genes or proteins, especially some E3 ligases encoding genes, indicate that protein ubiquitination might play important roles in cell signal transduction during the pollination process. Our study sheds some lights on gene and protein expression profiles of rice pistil pollination process, and gives us a comprehensive understanding of the basic molecular mechanisms controlling pollination in rice.

BIOLOGICAL SIGNIFICANCE

Using RNA-seq, 2-DE and iTRAQ assays, we have generated the large-scale transcriptomic and proteomic data containing abundant information on genes involved in pollen and pistil interaction. Our results showed that ROS were significantly accumulated in stigma after pollination, and the abundance of genes involve in redox homeostasis system were changed variously. We also show that, changes of some E3 ligases encoding genes might indicate that protein ubiquitination play important roles in cell signal transduction during the pollination process. Data in this study might be helpful to deeply understand the pollination in rice.

Same same but different: sperm-activating EC1 and ECA1 gametogenesis-related family proteins

During double fertilization in Arabidopsis thaliana, the egg cell secretes small cysteine-rich EC1 (egg cell 1) proteins, which enable the arriving sperm pair to rapidly interact with the two female gametes. EC1 proteins are members of the large and unexplored group of ECA1 (early culture abundant 1) gametogenesis-related family proteins, characterized by a prolamin-like domain with six conserved cysteine residues that may form three pairs of disulfide bonds. The distinguishing marks of egg-cell-expressed EC1 proteins are, however, two short amino acid sequence motifs present in all EC1-like proteins. EC1 genes appear to encode the major CRPs (cysteine-rich proteins) expressed by the plant egg cell, and they are restricted to flowering plants, including the most basal extant flowering plant Amborella trichopoda. Many other ECA1 gametogenesis-related family genes are preferentially expressed in the synergid cell. Functional diversification among the ECA1 gametogenesis-related family is suggested by the different patterns of expression in the female gametophyte and the low primary sequence conservation.

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.

Embryo sac formation and early embryo development in Agave tequilana (Asparagaceae)

Agave tequilana is an angiosperm species that belongs to the family Asparagaceae (formerly Agavaceae). Even though there is information regarding to some aspects related to the megagametogenesis of A. tequilana, this is the first report describing the complete process of megasporogenesis, megagametogenesis, the early embryo and endosperm development process in detail. The objective of this work was to study and characterize all the above processes and the distinctive morphological changes of the micropylar and chalazal extremes after fertilization in this species. The agave plant material for the present study was collected from commercial plantations in the state of Jalisco, Mexico. Ovules and immature seeds, previously fixed in FAA and kept in ethanol 70%, were stained based on a tissue clarification technique by using a Mayer’s-Hematoxylin solution. The tissue clarification technique was successfully used for the characterization of the megasporogenesis, megagametogenesis, mature embryo sac formation, the early embryo and endosperm development processes by studying intact cells. The embryo sac of A. tequilana was confirmed to be of the monosporic Polygonum-type and an helobial endosperm formation. Also, the time-lapse of the developmental processes studied was recorded.

Increased symplasmic permeability in barley root epidermal cells correlates with defects in root hair development

It is well known that the process of plant cell differentiation depends on the symplasmic isolation of cells. Before starting the differentiation programme, the individual cell or group of cells should restrict symplasmic communication with neighbouring cells. We tested the symplasmic communication between epidermal cells in the different root zones of parental barley plants Hordeum vulgare L., cv. 'Karat' with normal root hair development, and two root hairless mutants (rhl1.a and rhl1.b). The results clearly show that symplasmic communication was limited during root hair differentiation in the parental variety, whereas in both root hairless mutants epidermal cells were still symplasmically connected in the corresponding root zone. This paper is the first report on the role of symplasmic isolation in barley root cell differentiation, and additionally shows that a disturbance in the restriction of symplasmic communication is present in root hairless mutants.

Asymmetric growth of root epidermal cells is related to the differentiation of root hair cells in Hordeum vulgare (L.)

The root epidermis of most vascular plants harbours two cell types, namely trichoblasts (capable of producing a root hair) and atrichoblasts. Here, in vivo analysis, confocal laser-scanning microscopy, transmission electron microscopy, histological analysis, and three-dimensional reconstruction were used to characterize the cell types present in the barley root epidermis and their distribution in the tissue. Both trichoblasts and atrichoblasts were present in the wild-type cultivars and could be distinguished from one another at an early stage. Trichoblast/atrichoblast differentiation depended on asymmetric cell expansion after a period of symmetrical cell division. After asymmetric growth, only the shorter epidermal cells could produce root hairs, whereas the longer cells became atrichoblasts. Moreover, the root epidermis did not develop root hairs at all if the epidermal cells did not differentiate into two asymmetric cell types. The root hairless phenotype of bald root barley (brb) and root hairless 1.b (rhl1.b) mutants was caused by a mutation in a gene related to the asymmetric expansion of the root epidermal cells. Additionally, the results showed that the mechanism of trichoblast/atrichoblast differentiation is not evolutionally conserved across the subfamilies of the Poaceae; in the Pooideae subfamily, both asymmetric division and asymmetric cell expansion have been observed.

Control of rice embryo development, shoot apical meristem maintenance, and grain yield by a novel cytochrome p450

Angiosperm seeds usually consist of two major parts: the embryo and the endosperm. However, the molecular mechanism(s) underlying embryo and endosperm development remains largely unknown, particularly in rice, the model cereal. Here, we report the identification and functional characterization of the rice GIANT EMBRYO (GE) gene. Mutation of GE resulted in a large embryo in the seed, which was caused by excessive expansion of scutellum cells. Post-embryonic growth of ge seedling was severely inhibited due to defective shoot apical meristem (SAM) maintenance. Map-based cloning revealed that GE encodes a CYP78A subfamily P450 monooxygenase that is localized to the endoplasmic reticulum. GE is expressed predominantly in the scutellar epithelium, the interface region between embryo and endosperm. Overexpression of GE promoted cell proliferation and enhanced rice plant growth and grain yield, but reduced embryo size, suggesting that GE is critical for coordinating rice embryo and endosperm development. Moreover, transgenic Arabidopsis plants overexpressing AtCYP78A10, a GE homolog, also produced bigger seeds, implying a conserved role for the CYP78A subfamily of P450s in regulating seed development. Taken together, our results indicate that GE plays critical roles in regulating embryo development and SAM maintenance.

Male-female crosstalk during pollen germination, tube growth and guidance, and double fertilization

Sperm cells of flowering plants are non-motile and thus require transportation to the egg apparatus via the pollen tube to execute double fertilization. During its journey, the pollen tube interacts with various sporophytic cell types that support its growth and guide it towards the surface of the ovule. The final steps of tube guidance and sperm delivery are controlled by the cells of the female gametophyte. During fertilization, cell-cell communication events take place to achieve and maximize reproductive success. Additional layers of crosstalk exist, including self-recognition and specialized processes to prevent self-fertilization and consequent inbreeding. In this review, we focus on intercellular communication between the pollen grain/pollen tube including the sperm cells with the various sporophytic maternal tissues and the cells of the female gametophyte. Polymorphic-secreted peptides and small proteins, especially those belonging to various subclasses of small cysteine-rich proteins (CRPs), reactive oxygen species (ROS)/NO signaling, and the second messenger Ca(2+), play center stage in most of these processes.

Actin cytoskeleton in the extra-ovular embryo sac of Utricularia nelumbifolia (Lentibulariaceae)

The actin cytoskeleton in the mature female gametophyte of angiosperms has been examined in only a few dicot and monocot species. The main purposes of this study were to identify how the actin cytoskeleton is arranged in the mature extra-ovular embryo sac in Utricularia nelumbifolia (Lentibulariaceae). We found that the extra-ovular part of the central cell has a well-developed actin cytoskeleton: actin microfilaments formed of long strands which run longitudinally or transversally to the long axis of the embryo sac. The exerted part of the central cell, which is exposed to the environment of the ovary chamber, is highly vacuolated and in the thin peripheral cytoplasm possesses a complicated network of actin microfilaments. The epidermal cells of the placenta that are in contact with the extra-ovular part of the embryo sac are crushed. The ultrastructure data of these cells are presented. We detected the accumulation of the actin cytoskeleton between the micropylar parts of the synergids and the extra-ovular part of central cell. This actin accumulation is unusual because in typical angiosperms the micropylar parts of the synergids form the apex of the female gametophyte.

LACHESIS-dependent egg-cell signaling regulates the development of female gametophytic cells

In contrast to animals, plant germ cells are formed along with accessory cells in specialized haploid generations, termed gametophytes. The female gametophyte of flowering plants consists of four different cell types, which exert distinct functions in the reproductive process. For successful fertilization, the development of the four cell types has to be tightly coordinated; however, the underlying mechanisms are not yet understood. We have previously isolated the lachesis (lis) mutant, which forms supernumerary gametes at the expense of adjacent accessory cells. LIS codes for the Arabidopsis homolog of the pre-mRNA splicing factor PRP4 and shows a dynamic expression pattern in the maturing female gametophyte. Here, we used LIS as a molecular tool to study cell-cell communication in the female gametophyte. We show that reducing LIS transcript amounts specifically in the egg cell, affects the development of all female gametophytic cells, indicating that cell differentiation in the female gametophyte is orchestrated by the egg cell. Among the defects observed is the failure of homotypic nuclei fusion in the central cell and, as a consequence, a block in endosperm formation. LIS-mediated egg cell signaling, thus, provides a safeguard mechanism that prevents the formation of nurturing tissue in the absence of a functional egg cell.

Using maize as a model to study pollen tube growth and guidance, cross-incompatibility and sperm delivery in grasses

BACKGROUND

In contrast to animals and lower plants such as mosses and ferns, sperm cells of flowering plants (angiosperms) are immobile and require transportation to the female gametes via the vegetative pollen tube cell to achieve double fertilization. The path of the pollen tube towards the female gametophyte (embryo sac) has been intensively studied in many intra- and interspecific crossing experiments with the aim of increasing the gene pool of crop plants for greater yield, improved biotic and abiotic stress resistance, and for introducing new agronomic traits. Many attempts to hybridize different species or genotypes failed due to the difficulty for the pollen tubes in reaching the female gametophyte. Detailed studies showed that these processes are controlled by various self-incompatible (intraspecific) and cross-incompatible (interspecific) hybridization mechanisms.

SCOPE

Understanding the molecular mechanisms of crossing barriers is therefore of great interest in plant reproduction, evolution and breeding research. In particular, pre-zygotic hybridization barriers related to pollen tube germination, growth, guidance and sperm delivery, which are considered the major hybridization controls in nature and thus also contribute to species isolation and speciation, have been intensively investigated. Despite this general interest, surprisingly little is known about these processes in the most important agronomic plant family, the Gramineae, Poaceae or grasses. Small polymorphic proteins and their receptors, degradation of sterility locus proteins and general compounds such as calcium, γ-aminobutyric acid or nitric oxide have been shown to be involved in progamic pollen germination, adhesion, tube growth and guidance, as well as sperm release. Most advances have been made in the Brassicaceae, Papaveraceae, Linderniaceae and Solanaceae families including their well-understood self-incompatibility (SI) systems. Grass species evolved similar mechanisms to control the penetration and growth of self-pollen to promote intraspecific outcrossing and to prevent fertilization by alien sperm cells. However, in the Poaceae, the underlying molecular mechanisms are still largely unknown.

CONCLUSIONS

We propose to develop maize (Zea mays) as a model to investigate the above-described processes to understand the associated intra- and interspecific crossing barriers in grasses. Many genetic, cellular and biotechnological tools including the completion of a reference genome (inbred line B73) have been established in the last decade and many more maize inbred genomes are expected to be available soon. Moreover, a cellular marker line database as well as large transposon insertion collections and improved Agrobacterium transformation protocols are now available. Additionally, the processes described above are well studied at the morphological level and a number of mutants have been described already, awaiting disclosure of the relevant genes. The identification of the first key players in pollen tube growth, guidance and burst show maize to be an excellent grass model to investigate these processes in more detail. Here we provide an overview of our current understanding of these processes in Poaceae with a focus on maize, and also include relevant discoveries in eudicot model species.

Identification of imprinted genes subject to parent-of-origin specific expression in Arabidopsis thaliana seeds

BACKGROUND

Epigenetic regulation of gene dosage by genomic imprinting of some autosomal genes facilitates normal reproductive development in both mammals and flowering plants. While many imprinted genes have been identified and intensively studied in mammals, smaller numbers have been characterized in flowering plants, mostly in Arabidopsis thaliana. Identification of additional imprinted loci in flowering plants by genome-wide screening for parent-of-origin specific uniparental expression in seed tissues will facilitate our understanding of the origins and functions of imprinted genes in flowering plants.

RESULTS

cDNA-AFLP can detect allele-specific expression that is parent-of-origin dependent for expressed genes in which restriction site polymorphisms exist in the transcripts derived from each allele. Using a genome-wide cDNA-AFLP screen surveying allele-specific expression of 4500 transcript-derived fragments, we report the identification of 52 maternally expressed genes (MEGs) displaying parent-of-origin dependent expression patterns in Arabidopsis siliques containing F1 hybrid seeds (3, 4 and 5 days after pollination). We identified these MEGs by developing a bioinformatics tool (GenFrag) which can directly determine the identities of transcript-derived fragments from (i) their size and (ii) which selective nucleotides were added to the primers used to generate them. Hence, GenFrag facilitates increased throughput for genome-wide cDNA-AFLP fragment analyses. The 52 MEGs we identified were further filtered for high expression levels in the endosperm relative to the seed coat to identify the candidate genes most likely representing novel imprinted genes expressed in the endosperm of Arabidopsis thaliana. Expression in seed tissues of the three top-ranked candidate genes, ATCDC48, PDE120 and MS5-like, was confirmed by Laser-Capture Microdissection and qRT-PCR analysis. Maternal-specific expression of these genes in Arabidopsis thaliana F1 seeds was confirmed via allele-specific transcript analysis across a range of different accessions. Differentially methylated regions were identified adjacent to ATCDC48 and PDE120, which may represent candidate imprinting control regions. Finally, we demonstrate that expression levels of these three genes in vegetative tissues are MET1-dependent, while their uniparental maternal expression in the seed is not dependent on MET1.

CONCLUSIONS

Using a cDNA-AFLP transcriptome profiling approach, we have identified three genes, ATCDC48, PDE120 and MS5-like which represent novel maternally expressed imprinted genes in the Arabidopsis thaliana seed. The extent of overlap between our cDNA-AFLP screen for maternally expressed imprinted genes, and other screens for imprinted and endosperm-expressed genes is discussed.

The walls have ears: the role of plant CrRLK1Ls in sensing and transducing extracellular signals

In plants, organ formation and cell elongation require the constant adjustment of the dynamic and adaptable cell wall in response to environmental cues as well as internal regulators, such as light, mechanical stresses, pathogen attacks, phytohormones, and other signaling molecules. The molecular mechanisms that perceive these cues and translate them into cellular responses to maintain integrity and remodelling of the carbohydrate-rich cell wall for the coordination of cell growth are still poorly understood. In the last 3 years, the function of six membrane-localized receptor-like kinases (RLKs) belonging to the CrRLK1L family has been linked to the control of cell elongation in vegetative and reproductive development. Moreover, the presence of putative carbohydrate-binding domains in the extracellular domains of these CrRLK1Ls makes this receptor family an excellent candidate for coordinating cell growth, cell-cell communication, and constant cell wall remodelling during the plant life cycle.

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.

Conserved molecular components for pollen tube reception and fungal invasion

During sexual reproduction in flowering plants such as Arabidopsis, a tip-growing pollen tube (PT) is guided to the synergid cells of the female gametophyte, where it bursts and releases the two sperm. Here we show that PT reception and powdery mildew (PM) infection, which involves communication between a tip-growing hypha and a plant epidermal cell, share molecular components. NORTIA (NTA), a member of the MLO family originally discovered in the context of PM resistance, and FERONIA (FER), a receptor-like kinase, both control PT reception in synergids. Homozygous fer mutants also display PM resistance, revealing a new function for FER and suggesting that conserved components, such as FER and distinct MLO proteins, are involved in both PT reception and PM infection.

ANK6, a mitochondrial ankyrin repeat protein, is required for male-female gamete recognition in Arabidopsis thaliana

Double fertilization in angiosperms involves several successive steps, including guidance and reception of the pollen tube and male-female gamete recognition. Each step entails extensive communication and interaction between two different reproductive cell or tissue types. Extensive research has focused on the pollen tube, namely, its interaction with the stigma and reception by maternal cells. Little is known, however, about the mechanism by which the gametes recognize each other and interact to form a zygote. We report that an ankyrin repeat protein (ANK6) is essential for fertilization, specifically for gamete recognition. ANK6 (At5g61230) was highly expressed in the male and female gametophytes before and during but not after fertilization. Genetic analysis of a T-DNA insertional mutant suggested that loss of function of ANK6 results in embryonic lethality. Moreover, male-female gamete recognition was found to be impaired only when an ank6 male gamete reached an ank6 female gamete, thereby preventing formation of homozygous zygotes. ANK6 was localized to the mitochondria, where it interacted with SIG5, a transcription initiation factor previously found to be essential for fertility. These results show that ANK6 plays a central role in male-female gamete recognition, possibly by regulating mitochondrial gene expression.

Gamete fusion site on the egg cell and autonomous establishment of cell polarity in the zygote

Gamete fusion activates the egg in animals and plants, and the gamete fusion site on the zygote might provide a possible cue for zygotic development and/or embryonic patterning. In angiosperms, a zygote generally divides into a two-celled proembryo consisting of an apical and a basal cell with different cell fates. This is a putative step in the formation of the apical-basal axis of the proembryo. We observed the positional relationship between the gamete fusion site and the division plane formed by zygotic cleavage using an in vitro fertilization system with rice gametes. There was no relationship between the gamete fusion site and the division plane leading to the two-celled proembryo. Thus, the gamete fusion site on the rice zygote does not appear to function as a determinant for positioning the zygote division plane, and the zygote apparently possesses autonomous potential to establish cell polarity along the apical-basal axis for its first cleavage.

An atlas of type I MADS box gene expression during female gametophyte and seed development in Arabidopsis

Members of the plant type I MADS domain subfamily have been reported to be involved in reproductive development in Arabidopsis (Arabidopsis thaliana). However, from the 61 type I genes in the Arabidopsis genome, only PHERES1, AGAMOUS-LIKE80 (AGL80), DIANA, AGL62, and AGL23 have been functionally characterized, which revealed important roles for these genes during female gametophyte and early seed development. The functions of the other genes are still unknown, despite the fact that the available single T-DNA insertion mutants have been largely investigated. The lack of mutant phenotypes is likely due to a considerable number of recent intrachromosomal duplications in the type I subfamily, resulting in nonfunctional genes in addition to a high level of redundancy. To enable a breakthrough in type I MADS box gene characterization, a framework needs to be established that allows the prediction of the functionality and redundancy of the type I genes. Here, we present a complete atlas of their expression patterns during female gametophyte and seed development in Arabidopsis, deduced from reporter lines containing translational fusions of the genes to green fluorescent protein and beta-glucuronidase. All the expressed genes were revealed to be active in the female gametophyte or developing seed, indicating that the entire type I subfamily is involved in reproductive development in Arabidopsis. Interestingly, expression was predominantly observed in the central cell, antipodal cells, and chalazal endosperm. The combination of our expression results with phylogenetic and protein interaction data allows a better identification of putative redundantly acting genes and provides a useful tool for the functional characterization of the type I MADS box genes in Arabidopsis.

Male gamete biology in flowering plants

Flowering plant reproduction is characterized by double fertilization, in which two diminutive brother sperm cells initiate embryo and endosperm. The role of the male gamete, although studied structurally for over a century at various levels, is still being explored on a molecular and cellular level. The potential of the male to influence development has been historically underestimated and the reasons for this are obvious: limitations provided by maternal imprinting, the much greater cellular volume of female gametes and the general paucity of paternal effects. However, as more is known about molecular expression of chromatin-modifying proteins, ubiquitin pathway proteins and transcription factors in sperm cells, as well as their ability to achieve effect by intaglio expression, passing transcripts directly into translation, the role of the male is likely to expand. Much of the expression in the male germline that appears to be distinct from patterns of pollen vegetative cell expression may be the result of chromosomal level regulation of transcription.

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.

Positional relationship between the gamete fusion site and the first division plane in the rice zygote

In angiosperms, a zygote generally divides into a two-celled proembryo consisting of an apical and a basal cell that possess different cell fates. This first division of the zygote is a putative step in the formation of the apical-basal axis of the proembryo. The gamete fusion activates the egg, and the gamete fusion site on the zygote has been reported to provide a possible cue for subsequent zygotic development and/or embryonic patterning in animals and plants. In this study, the gamete fusion site on the rice zygote was labelled by in vitro fertilization of a rice egg cell with a fluorescence-stained sperm cell. The positional relationship between the gamete fusion site and the division plane formed by zygotic cleavage was monitored using a fixed culture of the fusion site-labelled zygote until the two-celled proembryo stage. The results indicate that gamete fusion sites exist on two-celled proembryos with no relation to the position of the first division plane, and that the gamete fusion site on the rice zygote does not function as a determinant for positioning the zygote division plane.

The pollen tube journey in the pistil and imaging the in vivo process by two-photon microscopy

The process of pollen germination and tube growth in the pistil involves a series of cell-cell interactions, some facilitating fertilization while others prohibiting pollen tube access to the female gametophyte, either because of incompatibility or as a result of mechanisms to avert polyspermy and to ensure reproductive success. Understanding pollen tube growth and guidance to the female gametophyte has long been a pursuit among plant biologists, and observations indicate that diverse strategies may be adopted by different plant species. Recent studies in Arabidopsis, maize, and Torenia fournieri suggest that low molecular weight secretory molecules probably play major roles in the short-range attraction of pollen tubes to the female gametophyte. The process of pollen tube growth in the pistil occurs beneath several cell layers so much of the information that conveys the intimate partnership between penetrating pollen tubes and the female tissues has come from fixed samples and observations of in vitro pollen tube growth responses to female factors. A unique glimpse of the in vivo pollen germination and tube growth process is provided here by intra-vital two-photon excitation (TPE) microscopy of pollinated Arabidopsis pistils that remained on intact plants. Further discoveries of critical factors of male or female origins and how they control the pollen tube growth and fertilization process will broaden our understanding of the common themes and diverse strategies that plants have evolved to ensure reproductive success. The advancement of imaging technology to monitor pollination and fertilization and the development of probes to monitor various aspects of the pollen tube growth process, including pollen intracellular dynamics, will allow us to superimpose details obtained from studying pollen tube growth in culture conditions to interpret and understand the in vivo events.

Let's get physical: gamete interaction in flowering plants

Fertilization comprises a series of precisely orchestrated steps that culminate in the fusion of male and female gametes. The most intimate steps during fertilization encompass gamete recognition, adhesion and fusion. In animals, some binding-effector proteins and enzymes have been identified that act on the cell surfaces of the gametes to regulate gamete compatibility and fertilization success. In contrast, exploring plant gamete interaction during double fertilization, a characteristic trait of flowering plants, has been hampered for a long time because of the protected location of the female gametes and technical limitations. Over the last couple of years, however, the use of advanced methodologies, new imaging tools and new mutants has provided deeper insights into double fertilization, at both the cellular and the molecular level, especially for the model plant Arabidopsis thaliana. Most likely, one consequence of inventing double fertilization may be the co-evolution of special molecular mechanisms to govern each successful sperm delivery and efficient gamete recognition and fusion. In vivo imaging of double fertilization and the recent discovery of numerous female-gametophyte-specific expressed genes encoding small secreted proteins, some of whom were found to be essential for the fertilization process, support this hypothesis. Nevertheless, recent findings indicate that at least the membrane-merger step in plant gamete interaction may rely on an ancient and widely used gamete fusion system.

Disruption of the pollen-expressed FERONIA homologs ANXUR1 and ANXUR2 triggers pollen tube discharge

The precise delivery of male to female gametes during reproduction in eukaryotes requires complex signal exchanges and a flawless communication between male and female tissues. In angiosperms, molecular mechanisms have recently been revealed that are crucial for the dialog between male (pollen tube) and female gametophytes required for successful sperm delivery. When pollen tubes reach the female gametophyte, they arrest growth, burst and discharge their sperm cells. These processes are under the control of the female gametophyte via the receptor-like serine-threonine kinase (RLK) FERONIA (FER). However, the male signaling components that control the sperm delivery remain elusive. Here, we show that ANXUR1 and ANXUR2 (ANX1, ANX2), which encode the closest homologs of the FER-RLK in Arabidopsis, are preferentially expressed in pollen. Moreover, ANX1-YFP and ANX2-YFP fusion proteins display polar localization to the plasma membrane at the tip of the pollen tube. Finally, genetic analyses demonstrate that ANX1 and ANX2 function redundantly to control the timing of pollen tube discharge as anx1 anx2 double-mutant pollen tubes cease their growth and burst in vitro and fail to reach the female gametophytes in vivo. We propose that ANX-RLKs constitutively inhibit pollen tube rupture and sperm discharge at the tip of growing pollen tubes to sustain their growth within maternal tissues until they reach the female gametophytes. Upon arrival, the female FER-dependent signaling cascade is activated to mediate pollen tube reception and fertilization, while male ANX-dependent signaling is deactivated, enabling the pollen tube to rupture and deliver its sperm cells to effect fertilization.

ANXUR1 and 2, sister genes to FERONIA/SIRENE, are male factors for coordinated fertilization

In sexual reproduction, proper communication and cooperation between male and female organs and tissues are essential for male and female gametes to unite. In flowering plants, female sporophytic tissues and gametophytes direct a male pollen tube toward an egg apparatus, which consists of an egg cell and two synergid cells. The cell-cell communication between the pollen tube and the egg apparatus makes the tip of pollen tube rupture to release the sperm cell. To detect male factors involved in this communication, we screened mutants of receptor-like kinases expressed in pollen tubes and characterized ANXUR1 (ANX1) and ANXUR2 (ANX2) genes. Here we report that pollen tubes of anx1/anx2 mutants ruptured before arriving at the egg apparatus, suggesting that ANX1 and ANX2 are male factors controlling pollen tube behavior by directing rupture at proper timing. Furthermore, ANX1 and ANX2 were the most closely related paralogs of a female factor, FERONIA/SIRENE, controlling pollen tube behavior expressed in synergid cells. Our findings show that the coordinated behaviors of female and male reproductive apparatuses are regulated by these sister genes, whose duplication might play a role in the evolution of the fertilization system in flowering plants.

A collection of Ds insertional mutants associated with defects in male gametophyte development and function in Arabidopsis thaliana

Functional analyses of the Arabidopsis genome require analysis of the gametophytic generation, since approximately 10% of the genes are expressed in the male gametophyte and approximately 9% in the female gametophyte. Here we describe the genetic and molecular characterization of 67 Ds insertion lines that show reduced transmission through the male gametophyte. About half of these mutations are male gametophytic-specific mutations, while the others also affect female transmission. Genomic sequences flanking both sides of the Ds element were recovered for 39 lines; for 16 the Ds elements were inserted in or close to coding regions, while 7 were located in intergenic/unannotated regions of the genome. For the remaining 16 lines, chromosomal rearrangements such as translocations or deletions, ranging between 30 and 500 kb, were associated with the transposition event. The mutants were classified into five groups according to the developmental processes affected; these ranged from defects in early stages of gametogenesis to later defects affecting pollen germination, pollen tube growth, polarity or guidance, or pollen tube-embryo sac interactions or fertilization. The isolated mutants carry Ds insertions in genes with diverse biological functions and potentially specify new functions for several unannotated or unknown proteins.

GAMETOPHYTIC FACTOR 1, involved in pre-mRNA splicing, is essential for megagametogenesis and embryogenesis in Arabidopsis

RNA biogenesis is essential and vital for accurate expression of genes. It is obvious that cells cannot continue normal metabolism when RNA splicing is interfered with. sgt13018 is such a mutant, with partial loss of function of GAMETOPHYTIC FACTOR 1 (GFA1); a gene likely involved in RNA biogenesis in Arabidopsis. The mutant is featured in the phenotype of diminished female gametophyte development at stage FG5 and is associated with the arrest of early embryo development in Arabidopsis. Bioinformatics data showed that homologs of gene GFA1 in yeast and human encode putative U5 snRNP-specific proteins required for pre-mRNA splicing. Furthermore, the result of yeast two-hybrid assay indicated that GFA1 physically interacted with AtBrr2 and AtPrp8, the putative U5 snRNP components, of Arabidopsis. This investigation suggests that GFA1 is involved in mRNA biogenesis through interaction with AtBrr2 and AtPrp8 and functions in megagametogenesis and embryogenesis in plant.

The Saccharomyces cerevisiae PRM1 homolog in Neurospora crassa is involved in vegetative and sexual cell fusion events but also has postfertilization functions

Cell-cell fusion is essential for a variety of developmental steps in many eukaryotic organisms, during both fertilization and vegetative cell growth. Although the molecular mechanisms associated with intracellular membrane fusion are well characterized, the molecular mechanisms of plasma membrane merger between cells are poorly understood. In the filamentous fungus Neurospora crassa, cell fusion events occur during both vegetative and sexual stages of its life cycle, thus making it an attractive model for studying the molecular basis of cell fusion during vegetative growth vs. sexual reproduction. In the unicellular yeast Saccharomyces cerevisiae, one of the few proteins implicated in plasma membrane merger during mating is Prm1p; prm1Delta mutants show an approximately 50% reduction in mating cell fusion. Here we report on the role of the PRM1 homolog in N. crassa. N. crassa strains with deletions of a Prm1-like gene (Prm1) showed an approximately 50% reduction in both vegetative and sexual cell fusion events, suggesting that PRM1 is part of the general cell fusion machinery. However, unlike S. cerevisiae, N. crassa strains carrying a Prm1 deletion exhibited complete sterility as either a male or female mating partner, a phenotype that was not complemented in a heterokaryon with wild type (WT). Crosses with DeltaPrm1 strains were blocked early in sexual development, well before development of ascogenous hyphae. The DeltaPrm1 sexual defect in N. crassa was not suppressed by mutations in Sad-1, which is required for meiotic silencing of unpaired DNA (MSUD). However, mutations in Sad-1 increased the number of progeny obtained in crosses with a DeltaPrm1 (Prm1-gfp) complemented strain. These data indicate multiple roles for PRM1 during sexual development.

CLO/GFA1 and ATO are novel regulators of gametic cell fate in plants

The formation of gametes is a key step in the life cycle of any sexually reproducing organism. In flowering plants, gametes develop in haploid structures termed gametophytes that comprise a few cells. The female gametophyte forms gametic cells and flanking accessory cells. During a screen for regulators of egg-cell fate, we isolated three mutants, lachesis (lis), clotho (clo) and atropos (ato), that show deregulated expression of an egg-cell marker. We have previously shown that, in lis mutants, which are defective for the splicing factor PRP4, accessory cells can differentiate gametic cell fate. Here, we show that CLOTHO/GAMETOPHYTIC FACTOR 1 (CLO/GFA1) is necessary for the restricted expression of egg- and central-cell fate and hence reproductive success. Surprisingly, infertile gametophytes can be expelled from the maternal ovule tissue, thereby preventing the needless allocation of maternal resources to sterile tissue. CLO/GFA1 encodes the Arabidopsis homologue of Snu114, a protein that is considered to be an essential component of the spliceosome. In agreement with their proposed role in pre-mRNA splicing, CLO/GFA1 and LIS co-localize to nuclear speckles. Our data also suggest that CLO/GFA1 is necessary for the tissue-specific expression of LIS. Furthermore, we demonstrate that ATO encodes the Arabidopsis homologue of SF3a60, a protein that has been implicated in pre-spliceosome formation. Our results thus establish that the restriction of gametic cell fate is specifically coupled to the function of various core spliceosomal components.

Maternal control of male-gamete delivery in Arabidopsis involves a putative GPI-anchored protein encoded by the LORELEI gene

In Angiosperms, the male gametes are delivered to the female gametes through the maternal reproductive tissue by the pollen tube. Upon arrival, the pollen tube releases the two sperm cells, permitting double fertilization to take place. Although the critical role of the female gametophyte in pollen tube reception has been demonstrated, the underlying mechanisms remain poorly understood. Here, we describe lorelei, an Arabidopsis thaliana mutant impaired in sperm cell release, reminiscent of the feronia/sirène mutant. Pollen tubes reaching lorelei embryo sacs frequently do not rupture but continue to grow in the embryo sac. Furthermore, lorelei embryo sacs continue to attract additional pollen tubes after arrival of the initial pollen tube. The LORELEI gene is expressed in the synergid cells prior to fertilization and encodes a small plant-specific putative glucosylphosphatidylinositol-anchored protein (GAP). These results provide support for the concept of signaling mechanisms at the synergid cell membrane by which the female gametophyte recognizes the arrival of a compatible pollen tube and promotes sperm release. Although GAPs have previously been shown to play critical roles in initiation of fertilization in mammals, flowering plants appear to have independently evolved reproductive mechanisms that use the unique features of these proteins within a similar biological context.

Identification of small RNAs in late developmental stage of rice anthers

Small RNAs including microRNA (miRNA) and small interfering RNA (siRNA) are known as repressors of gene expression. There are many plant proteins involved in small RNA-mediated gene silencing, such as Dicer ribonucleases and RNA-dependent RNA polymerases. However, most of these proteins have been reported to be absent in the late developmental stage of the plant male gamete, pollen. In order to clarify the existence of the small RNAs during maturation of pollen, we cloned and sequenced small RNAs from rice anthers including tricellular pollen. From fifty six candidates of small RNAs, we identified two known miRNAs (miR166 and miR167), eight potential miRNAs, and ten putative heterochromatic siRNAs (hc-siRNAs). RNA gel blot analyses clearly showed that miR166 and miR167 were accumulated in the uninuclear pollen stage of anther development and remained until the tricellular pollen stage. Our cloning and RNA gel blot analyses of small RNAs led us to propose a possible function of small RNA-mediated gene regulation for the development of male gametes in rice.

Mutants with aberrant numbers of gametic cells shed new light on old questions

In contrast to animals, plant gametes form in distinct haploid generations, termed gametophytes. The female gametophyte of Arabidopsis consists of two gametic cells, the egg and central cell, which are flanked by accessory cells. The gametic cells differ with respect to morphology, molecular attributes and, importantly, their fate: whereas the egg cell, upon fertilisation, gives rise to the embryo, the central cell forms the endosperm. To ensure correct endosperm formation, not only the egg cell but also the central cell has to fuse with a sperm cell. The respective sperm cell pair is delivered by a single pollen tube. In some plant species, the two male gametes appear to express a different bias towards the female gametes. Such a preference consequently determines their respective contribution to either embryo or endosperm development. In Arabidopsis and many other species the sperm cells are indistinguishable and it has been discussed whether they possess an inherent preference for either of the female gametes. The recent isolation of mutants that form an aberrant number of either male or female gametes stimulates discussion, albeit with different results. Furthermore, some data indicate that the central cell is competent to initiate endosperm formation without a paternal contribution. These data support the theory that the endosperm is of gametophytic rather than sporophytic origin.

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.

The MADS domain protein DIANA acts together with AGAMOUS-LIKE80 to specify the central cell in Arabidopsis ovules

MADS box genes in plants consist of MIKC-type and type I genes. While MIKC-type genes have been studied extensively, the functions of type I genes are still poorly understood. Evidence suggests that type I MADS box genes are involved in embryo sac and seed development. We investigated two independent T-DNA insertion alleles of the Arabidopsis thaliana type I MADS box gene AGAMOUS-LIKE61 (AGL61) and showed that in agl61 mutant ovules, the polar nuclei do not fuse and central cell morphology is aberrant. Furthermore, the central cell begins to degenerate before fertilization takes place. Although pollen tubes are attracted and perceived by the mutant ovules, neither endosperm development nor zygote formation occurs. AGL61 is expressed in the central cell during the final stages of embryo sac development. An AGL61:green fluorescent protein-beta-glucoronidase fusion protein localizes exclusively to the polar nuclei and the secondary nucleus of the central cell. Yeast two-hybrid analysis showed that AGL61 can form a heterodimer with AGL80 and that the nuclear localization of AGL61 is lost in the agl80 mutant. Thus, AGL61 and AGL80 appear to function together to differentiate the central cell in Arabidopsis. We renamed AGL61 DIANA, after the virginal Roman goddess of the hunt.

A dialogue between the SIRENE pathway in synergids and the fertilization independent seed pathway in the central cell controls male gamete release during double fertilization in Arabidopsis

Angiosperms sexual reproduction involves interactions between the two female gametes in the embryo sac and the two male gametes released by the pollen tube. The two synergids of the embryo sac express the FERONIA/SIRENE receptor-like kinase, which controls the discharge of the two sperm cells from the pollen tube. FER/SRN may respond to a ligand from the pollen tube. Alternatively, the interaction between FER/SRN and a ligand from the embryo sac may lead to a state of competence of the synergids allowing pollen tube discharge. Here, we report the new mutant scylla (syl) impaired in the control of pollen tube discharge. This mutant also produces autonomous endosperm development in absence of fertilization-a trait associated with the FERTILIZATION INDEPENDENT SEED (FIS) mutant class. This led us to identify autonomous endosperm in srn mutants and to demonstrate synergistic interactions between srn and the fis mutants. In addition, the fis mutants display defects in pollen tube discharge as in srn and syl mutants, confirming the interaction between the two pathways. Our findings suggest that pollen tube discharge is controlled by an interaction between the synergids expressing SRN/FER and the central cell expressing FIS genes.

Diverse cell signalling pathways regulate pollen-stigma interactions: the search for consensus

Siphonogamy, the delivery of nonmotile sperm to the egg via a pollen tube, was a key innovation that allowed flowering plants (angiosperms) to carry out sexual reproduction on land without the need for water. This process begins with a pollen grain (male gametophyte) alighting on and adhering to the stigma of a flower. If conditions are right, the pollen grain germinates to produce a pollen tube. The pollen tube invades the stigma and grows through the style towards the ovary, where it enters an ovule, penetrates the embryo sac (female gametophyte) and releases two sperm cells, one of which fertilizes the egg, while the other fuses with the two polar nuclei of the central cell to form the triploid endosperm. The events before fertilization (pollen-pistil interactions) comprise a series of complex cellular interactions involving a continuous exchange of signals between the haploid pollen and the diploid maternal tissue of the pistil (sporophyte). In recent years, significant progress has been made in elucidating the molecular identity of these signals and the cellular interactions that they regulate. Here we review our current understanding of the cellular and molecular interactions that mediate the earliest of these interactions between the pollen and the pistil that occur on or within the stigma - the 'pollen-stigma interaction'.

Double fertilization in Arabidopsis thaliana involves a polyspermy block on the egg but not the central cell

In animal reproduction, thousands of sperm may compete to fertilize a single egg, but polyspermy blocks prevent multiple fertilization that would otherwise lead to death of the embryo. In flowering plants, successful seed development requires that only two sperm are delivered to the embryo sac, where each must fertilize a female gamete (egg or central cell) to produce the embryo and endosperm. Therefore, polyspermy must be avoided, not only to prevent abnormalities in offspring, but to ensure double fertilization. It is not understood how each sperm fertilizes only one female gamete, nor has the existence of polyspermy barriers been directly tested in vivo. Here, we sought evidence for polyspermy blocks in angiosperms using the polyspermic tetraspore (tes) mutant of Arabidopsis, which allows in-vivo challenge of egg and central cell with multiple male gametes. We show that tes mutant pollen tubes can transmit more than one sperm pair to an embryo sac, and that sperm from more than one pair can participate in fertilization. We detected endosperms but not embryos with ploidies that could only result from multiple fertilization. Our results therefore demonstrate an in-vivo polyspermy block on the egg, but not the central cell of a flowering plant.

Gamete recognition in higher plants: an abstruse but charming mystery

Although much effort has been made to uncover the mechanism underlying double fertilization, little knowledge has been acquired for understanding the molecular base of gamete recognition, mainly because of technical limitations. Still, progress has been made in terms of the mechanism, including the identification of candidate molecules that are involved in gamete recognition in angiosperms. New cues for gamete recognition have been found by the successful separation of the gametes and construction of gamete-specific cDNA libraries in several species, and the application of molecular approaches for studying this process by mutations. Thus, the topic is considered an abstruse but charming mystery.