Double fertilization in flowering plants: discovery, study methods and mechanisms

Reproduction Modes and Conservation Implications in Three Polyploid Sorbus Stenoendemics in Eastern Slovakia (Central Europe)

The remarkable species diversity of the genus Sorbus is a result of polyploidization and frequent hybridization between interacting species of different cytotypes. Moreover, hybridization is possible between several parental taxa. Gametophytic apomixis, which is common among polyploid Sorbus taxa, indicates the role of clonal reproduction in the evolutionary stabilization of hybridogeneous genotypes. The precise determination of the origin of seeds and their quantitative evaluation may elucidate inter-cytotype interactions, the potential role of mixed-cytotype populations in evolutionary success, and the long-term survival of some hybrid species. We investigated the reproduction modes of selected species of Sorbus in mixed-cytotype populations in eastern Slovakia, Central Europe. We determined the pollen quality, seed production rate, and the ploidy level of mature trees, as well as the origin of the embryo and endosperm in seeds of the stenoendemics S. amici-petri, S. dolomiticola, and S. hornadensis. The tetraploids S. amici-petri and S. hornadensis are characterized by regular and highly stainable pollen grains and reproduce predominantly via pseudogamous apomixis. In contrast, triploid S. dolomiticola usually has oval, heterogenous, and weakly stainable pollen grains, suggesting male meiotic irregularities. Although seeds originate via pseudogamous apomixis in S. dolomiticola as well, the ploidy level of sperm cells participating in the fertilization of central cells is usually determined by co-occurring species of different cytotypes. This suggests that maintaining mating partners is necessary for the long-term survival of a triploid species. We documented rare B_III hybrids and the residual sexuality in tetraploids. The distribution of seeds of meiotic and apomeiotic origins in S. amici-petri shows bimodal characteristics; however, genotypes with predominantly sexual seed types are rare. Reproduction modes documented in polyploid stenoendemics of Sorbus and inferred microevolutionary intercytotype relationships highlight the mixed-cytotype populations as the source of biodiversity in apomictic plant complexes. We suggest that conservation efforts should focus on maintaining the species and cytotypic diversity of Sorbus populations, especially when it comes to the conservation of triploid species.

Cross Inhibition of MPK10 and WRKY10 Participating in the Growth of Endosperm in Arabidopsis thaliana

The product of double fertilization produces seed, which contains three components: triploid endosperm, diploid embryo, and maternal seed coat. Amongst them, the endosperm plays a crucial role in coordinating seed growth. Mitogen-activated protein kinase (MAPK) cascades are conserved in eukaryotes and involved in signal transduction of plant development. MPK3, MPK6, and MPK10 form a small group of MPKs family in Arabidopsis thaliana. MPK3 and MPK6 are extensively studied and were found to be involved in diverse processes including plant reproduction. However, less is known about the function of MPK10. Here, we found WRKY10/MINI3, a member of HAIKU (IKU) pathway engaging in endosperm development, and MPK10 is high-specifically expressed in the early developmental endosperm but with opposite gradients. We further proved that MPK10 and WRKY10 cross-inhibit the expression of each other. The inhibition effect of MPK10 on gene expression of WRKY10 and the downstream targets is supported by the fact that MPK10 interacts with WRKY10 and suppresses the transcriptional activity of WRKY10. Constantly, mpk10 mutants produce big seeds while WRKY10/MINI3 positively regulate seed growth. Altogether, our data provides a model of WRKY10 and MPK10 regulating endosperm development with a unique cross inhibitory mechanism.

The Role of Anaphase-Promoting Complex/Cyclosome (APC/C) in Plant Reproduction

Most eukaryotic species propagate through sexual reproduction that requires male and female gametes. In flowering plants, it starts through a single round of DNA replication (S phase) and two consecutive chromosome segregation (meiosis I and II). Subsequently, haploid mitotic divisions occur, which results in a male gametophyte (pollen grain) and a female gametophyte (embryo sac) formation. In order to obtain viable gametophytes, accurate chromosome segregation is crucial to ensure ploidy stability. A precise gametogenesis progression is tightly regulated in plants and is controlled by multiple mechanisms to guarantee a correct evolution through meiotic cell division and sexual differentiation. In the past years, research in the field has shown an important role of the conserved E3-ubiquitin ligase complex, Anaphase-Promoting Complex/Cyclosome (APC/C), in this process. The APC/C is a multi-subunit complex that targets proteins for degradation via proteasome 26S. The functional characterization of APC/C subunits in Arabidopsis, which is one of the main E3 ubiquitin ligase that controls cell cycle, has revealed that all subunits investigated so far are essential for gametophytic development and/or embryogenesis.

Flow cytometric seed screen data are consistent with models of chromosome inheritance in asymmetrically compensating allopolyploids

Angiosperms have evolved a mechanism of double fertilization, which results in the production of a separate embryo (new individual) and endosperm (nutritive tissue). The flow cytometric seed screen (FCSS) was developed to infer plant reproduction modes based on endosperm-to-embryo DNA content ratio (P_ind ). A ratio of 1.5 indicates sexual reproduction, whereas higher values of ≥2.0 are consistent with apomixis. Although FCSS has been successfully applied to the study of sexual and asexual plants, the limits of FCSS and particularly its potential for determination of reproduction modes in hemisexual plants have not been explored. Here, we evaluated the application of FCSS to the study of reproduction modes in two asymmetrically compensating allopolyploids (ACAs), Onosma arenaria and Rosa canina. These two species are characterized by the presence of asexually inherited univalent-forming and sexually inherited bivalent-forming chromosome sets. They both use asymmetric meiosis, which eliminates univalent-forming chromosome sets from the male gamete and retains them in the female gamete. Different chromosomal behavior in male and female meiosis in these plants is reflected in different theoretically derived P_ind values, which deviate from a sexual 1.5 value. Here, we determined P_ind FCSS-based values in seeds of ACAs, and compared the results to sexual species. As expected, we determined that the mean P_ind is 1.51, 1.52, and 1.52 in the sexual plants, that is, Capsella bursa-pastoris, Crataegus monogyna, and O. pseudoarenaria, respectively. In the ACAs, different mean P_ind values were determined for O. arenaria (1.61) and R. canina (1.82). These values are consistent with the theoretical P_ind values determined based on models of chromosome inheritance. This study highlights the precision of flow cytometry in determining DNA content and it's utility in screening reproduction modes. Additionally, it advocates for more in-depth investigations into rapid screening of accessions where the P_ind ratio has deviated from the 1.5 value typical of sexual species, which may indicate meiotic irregularities.

Transcriptomic Analysis of Long Non-Coding RNAs and Coding Genes Uncovers a Complex Regulatory Network That Is Involved in Maize Seed Development

Long non-coding RNAs (lncRNAs) have been reported to be involved in the development of maize plant. However, few focused on seed development of maize. Here, we identified 753 lncRNA candidates in maize genome from six seed samples. Similar to the mRNAs, lncRNAs showed tissue developmental stage specific and differential expression, indicating their putative role in seed development. Increasing evidence shows that crosstalk among RNAs mediated by shared microRNAs (miRNAs) represents a novel layer of gene regulation, which plays important roles in plant development. Functional roles and regulatory mechanisms of lncRNAs as competing endogenous RNAs (ceRNA) in plants, particularly in maize seed development, are unclear. We combined analyses of consistently altered 17 lncRNAs, 840 mRNAs and known miRNA to genome-wide investigate potential lncRNA-mediated ceRNA based on “ceRNA hypothesis”. The results uncovered seven novel lncRNAs as potential functional ceRNAs. Functional analyses based on their competitive coding-gene partners by Gene Ontology (GO) and KEGG biological pathway demonstrated that combined effects of multiple ceRNAs can have major impacts on general developmental and metabolic processes in maize seed. These findings provided a useful platform for uncovering novel mechanisms of maize seed development and may provide opportunities for the functional characterization of individual lncRNA in future studies.

Proteomics Advances in the Understanding of Pollen-Pistil Interactions

The first key point to the successful pollination and fertilization in plants is the pollen-pistil interaction, referring to the cellular and molecular levels, which mainly involve the haploid pollen and the diploid pistil. The process is defined as “siphonogamy”, which starts from the capture of pollen by the epidermis of stigma and ends up with the fusion of sperm with egg. So far, the studies of the pollen-pistil interaction have been explicated around the self-compatibility and self-incompatibility (SI) process in different species from the molecular genetics and biochemistry to cellular and signal levels, especially the mechanism of SI system. Among them, numerous proteomics studies based on the advanced technologies from gel-system to gel-free system were conducted, focusing on the interaction, in order to uncover the mechanism of the process. The current review mainly focuses on the recent developments in proteomics of pollen-pistil interaction from two aspects: self-incompatible and compatible pollination. It might provide a comprehensive insight on the proteins that were involved in the regulation of pollen-pistil interaction.

Analysis of gamete membrane dynamics during double fertilization of Arabidopsis

Angiosperms have a unique sexual reproduction system called "double fertilization." One sperm cell fertilizes the egg and another sperm cell fertilizes the central cell. To date, plant gamete membrane dynamics during fertilization has been poorly understood. To analyze this unrevealed gamete subcellular behavior, live cell imaging analyses of Arabidopsis double fertilization were performed. We produced female gamete membrane marker lines in which fluorescent proteins conjugated with PIP2a finely visualized egg cell and central cell surfaces. Using those lines together with a sperm cell membrane marker line expressing GCS1-GFP, the double fertilization process was observed. As a result, after gamete fusion, putative sperm plasma membrane GFP signals were occasionally detected on the egg cell surface adjacent to the central cell. In addition, time-lapse imaging revealed that GCS1-GFP signals entered both the egg cell and the central cell in parallel with the sperm cell movement toward the female gametes during double fertilization. These findings suggested that the gamete fusion process based on membrane dynamics was composed of (1) plasma membrane fusion on male and female gamete surfaces, (2) entry of sperm internal membrane components into the female gametes, and (3) plasmogamy.

40S ribosome biogenesis co-factors are essential for gametophyte and embryo development

Ribosome biogenesis is well described in Saccharomyces cerevisiae. In contrast only very little information is available on this pathway in plants. This study presents the characterization of five putative protein co-factors of ribosome biogenesis in Arabidopsis thaliana, namely Rrp5, Pwp2, Nob1, Enp1 and Noc4. The characterization of the proteins in respect to localization, enzymatic activity and association with pre-ribosomal complexes is shown. Additionally, analyses of T-DNA insertion mutants aimed to reveal an involvement of the plant co-factors in ribosome biogenesis. The investigated proteins localize mainly to the nucleolus or the nucleus, and atEnp1 and atNob1 co-migrate with 40S pre-ribosomal complexes. The analysis of T-DNA insertion lines revealed that all proteins are essential in Arabidopsis thaliana and mutant plants show alterations of rRNA intermediate abundance already in the heterozygous state. The most significant alteration was observed in the NOB1 T-DNA insertion line where the P-A3 fragment, a 23S-like rRNA precursor, accumulated. The transmission of the T-DNA through the male and female gametophyte was strongly inhibited indicating a high importance of ribosome co-factor genes in the haploid stages of plant development. Additionally impaired embryogenesis was observed in some mutant plant lines. All results support an involvement of the analyzed proteins in ribosome biogenesis but differences in rRNA processing, gametophyte and embryo development suggested an alternative regulation in plants.

Emerging Roles for Non-Coding RNAs in Male Reproductive Development in Flowering Plants

Knowledge of sexual reproduction systems in flowering plants is essential to humankind, with crop fertility vitally important for food security. Here, we review rapidly emerging new evidence for the key importance of non-coding RNAs in male reproductive development in flowering plants. From the commitment of somatic cells to initiating reproductive development through to meiosis and the development of pollen—containing the male gametes (sperm cells)—in the anther, there is now overwhelming data for a diversity of non-coding RNAs and emerging evidence for crucial roles for them in regulating cellular events at these developmental stages. A particularly exciting development has been the association of one example of cytoplasmic male sterility, which has become an unparalleled breeding tool for producing new crop hybrids, with a non-coding RNA locus.

Calreticulin: conserved protein and diverse functions in plants

Calreticulin (CRT) is a key Ca2+-binding protein mainly resident in the endoplasmic reticulum (ER), which is highly conserved and extensively expressed in all eukaryotic organisms investigated. The protein plays important roles in a variety of cellular processes including Ca2+ signaling and protein folding. Although calreticulin has been well characterized in mammalian systems, increased investigations have demonstrated that plant CRTs have a number of specific properties different from their animal counterparts. Recent developments on plant CRTs have highlighted the significance of CRTs in plants growth and development as well as biotic and abiotic stress responses. There are at least two distinct groups of calreticulin isoforms in higher plants. Glycosylation of CRT was uniquely observed in plants. In this article, we will describe our current understanding of plant calreticulin gene family, protein structure, cellular localization, and diverse functions in plants. We also discuss the prospects of using this information for genetic improvements of crop plants.

Calreticulin: conserved protein and diverse functions in plants

Calreticulin (CRT) is a key Ca2+-binding protein mainly resident in the endoplasmic reticulum (ER), which is highly conserved and extensively expressed in all eukaryotic organisms investigated. The protein plays important roles in a variety of cellular processes including Ca2+ signaling and protein folding. Although calreticulin has been well characterized in mammalian systems, increased investigations have demonstrated that plant CRTs have a number of specific properties different from their animal counterparts. Recent developments on plant CRTs have highlighted the significance of CRTs in plants growth and development as well as biotic and abiotic stress responses. There are at least two distinct groups of calreticulin isoforms in higher plants. Glycosylation of CRT was uniquely observed in plants. In this article, we will describe our current understanding of plant calreticulin gene family, protein structure, cellular localization, and diverse functions in plants. We also discuss the prospects of using this information for genetic improvements of crop plants.

Sexual reproduction in higher plants I: fertilization and the initiation of zygotic program

Sexual plant reproduction is a critical developmental step in the life cycle of higher plants, to allow maternal and paternal genes to be transmitted in a highly regulated manner to the next generation. During evolution, a whole set of signal transduction machinery is developed by plants to ensure an error-free recognition between male and female gametes and initiation of zygotic program. In the past few years, the molecular machineries underlying this biological process have been elucidated, particularly on the importance of synergid cells in pollen tube guidance, the Ca(++) spike as the immediate response of fertilization and the epigenetic regulation of parental gene expressions in early zygotic embryogenesis. This review outlines the most recent development in this area.

Expressed sequence tag analysis of Lilium longiflorum generative cells

The generative cell, the male gametic cell progenitor in flowering plants, undergoes mitotic division to produce two sperm cells. We have examined the gene expression profile of the Lilium longiflorum (lily) generative cell by sequencing expressed sequence tags (ESTs). A total of 886 ESTs derived from the generative cell cDNA library were clustered into 637 unique ESTs comprising 123 cluster ESTs and 514 singleton ESTs. Thirty-nine percent of non-redundant ESTs showing similarity to Arabidopsis genes with known function were thus assigned putative functions. Genes related to the ubiquitin system were abundant, suggesting the key role of ubiquitin-dependent proteolysis in gametogenesis. A total of 168 and 129 non-redundant lily generative cell ESTs showed significant similarity to maize sperm cell ESTs and Arabidopsis male gametophyte-specific transcripts, respectively. Fifty-five ESTs appeared to have significant similarities to both maize sperm cell ESTs and Arabidopsis male gametophyte-specific genes, indicating conservation of male gamete-expressed genes across different plant genera. Thus our data provide a handle to identify Arabidopsis gamete-expressed genes and to investigate their function. Several of these genes are potential candidates for analyzing the molecular basis of fertilization and for investigating mechanisms of gamete-specific transcriptional regulation in Arabidopsis through bioinformatics-based approaches.

Plant gametogenesis: conservation and contrasts in development

Although the process of gamete formation in plants has many unique features, much has been learnt from the comparative analysis between plants and other eukaryotic systems. Plants have a number of factors that have made them desirable for the analysis of gamete development; these include late germline specification, the non-lethality of mutations affecting gamete development and the large size of their chromosomes. The availability of the fully annotatedArabidopsisgenome and comparative analysis using yeast, animal andE. colihas led to the identification and functional characterisation of many genes with roles in gamete development, principally those associated with meiosis, recombination and DNA repair. The advantages that plants give with the use of mutant screens to identify genes associated with gamete formation have also provided access to genes that are difficult to characterise by alternative routes. This has yielded novel information regarding the processes of gamete formation in higher plants. The times may now be changing with the advantages that plants provide serving to advance knowledge of gamete formation in other eukaryotic systems.

Some reflections on double fertilization, from its discovery to the present

The fusion of one sperm with the egg cell to form the embryo and of the other sperm with the polar fusion nucleus to give rise to the endosperm ('double fertilization') was discovered by Nawaschin in 1898 in the liliaceous plants, Lilium martagon and Fritillaria tenella. The occurrence of two fusion events analogous to double fertilization has recently been described in some gymnosperm species although the product of the second fusion is a transient embryo, rather than the endosperm as in angiosperms. Recent investigations in angiosperms describe the cell biology and nuclear cytology of double fertilization and the successful in vitro demonstration of the two fusion events using isolated egg cells, central cells, and sperm cells and the development of the fusion products into the embryo and endosperm. Molecular and genetic studies on the component elements of double fertilization have focused on the identification of mutants of Arabidopsis thaliana that display developmental patterns in the seed that result in autonomous endosperm development and even partial embryogenesis in the absence of fertilization. Characterization of the genes and their protein products has provided evidence for a predominant effect of maternal gametophytic genes and of silencing of paternal genes during double fertilization. Contents Summary 565 I. Introduction 566 II. Discovery of double fertilization 566 III. Seed development without double fertilization 568 IV. A case for double fertilization in gymnosperms 570 V. Structural and cytological perspectives on double fertilization 571 VI. In vitro double fertilization 575 VII. Genetic and molecular perspectives 576 VIII. Concluding comments 578 Acknowledgements 579 References 579.

The endosperm and the embryo of Arabidopsis thaliana are independently transformed through infiltration by Agrobacterium tumefaciens

Several experiments had indicated that in planta transformation of Arabidopsis thaliana by Agrobacterium involves the female germ line. In order to identify the precise stage at which transformation occurs we have monitored expression of a gusA reporter gene in the two products of the double fertilization of infiltrated plants. The plantlets and the remaining endosperm of seeds were separately tested after germination. It appeared that in the majority of cases only the plantlet or the endosperm were transformed. Based on transformation with two vectors borne by two different Agrobacterium strains, the minority of 'co-transformed' plantlets and endosperm can be explained by simultaneous but independent transformation events. These results indicate that mature female gametes could be the targets of T-DNA.