The transcript composition of egg cells changes significantly following fertilization in wheat (Triticum aestivum L.)

From the notch signaling pathway to ribosome biogenesis

Nearly 240 WD repeat proteins have been identified from the Arabidopsis genome. Among these, some well characterized WDR proteins were shown to regulate various developmental processes in plants.1 We have recently isolated in Solanum chacoense a homolog of the Drosophila NOTCHLESS gene. In Drosophila, NOTCHLESS regulates the activity of the Notch signaling pathway through a direct interaction with the intracellular domain of the Notch receptor. Although the Notch signaling pathway does not exist in yease and plants, the NLE gene is conserved in animals, plants and yeast. Furthermore, functional conservation was suggested by expression of the plant NLE gene in Drosophila. In plants, underexpression of the plant NLE gene altered numerous developmental processes including seed development, and resulted in reduced aerial organ size and organ numbers, in delayed flowering, and in an increased stomatal index. Surprisingly, the link between these pleiotropic phenotypes is the recently discovered of the involvement of NLE in ribosome biogenesis, emphasizing its role in proper cellular growth and proliferation during plant development.

Heterosis associated gene expression in maize embryos 6 days after fertilization exhibits additive, dominant and overdominant pattern

Heterosis is important for conventional plant breeding and is intensively used to increase the productivity of crop plants. Genetic processes shortly after fertilization might be of particular importance with respect to heterosis, because coordination of the diverse genomes establishes a basis for future performance of the sporophyte. Here we demonstrate a strong crossbreeding advantage of hybrid maize embryos as early as 6 days after fertilization in a modern maize hybrid and provide the first embryo specific analysis of associated gene expression pattern at this early stage of development. We identified differentially expressed genes between hybrid embryos and the parental genotypes by a combined approach of suppression subtractive hybridization and differential screening by microarray hybridizations. Association of heterosis in embryos with genes related to signal transduction and other regulatory processes was implied by the enrichment of these functional classes among the identified gene set. Quantitative RT-PCR analysis validated the expression pattern of 7 of 12 genes analysed and revealed predominantly additive, but also dominant and overdominant expression patterns in hybrid embryos. These patterns indicate that gene regulatory interactions among parental alleles act at this early developmental stage and the genes identified provide entry points for the exploration of gene regulatory networks associated with the specification of the phenomenon heterosis in the plant life cycle.

Asymmetric Cell Division in Plant Development

Plant embryogenesis creates a seedling with a basic body plan. Post-embryonically the seedling elaborates with a lifelong ability to develop new tissues and organs. As a result asymmetric cell divisions serve essential roles during embryonic and postembryonic development to generate cell diversity. This review highlights selective cases of asymmetric division in the model plant Arabidopsis thaliana and describes the current knowledge on fate determinants and mechanisms involved. Common themes that emerge are: 1. role of the plant hormone auxin and its polar transport machinery; 2. a MAP kinase signaling cascade and; 3. asymmetric segregating transcription factors that are involved in several asymmetric cell divisions.

EST generation and analyses towards identifying female gametophyte-specific genes in Zea mays L

The embryo sac (female gametophyte) plays an important role in double fertilization. The female gametophyte is composed of four specific cell types: the synergids that attract pollen tubes, the egg cell and central cell which are fusion partners for the two sperm cells, and the antipodal cells whose function is unknown. As a resource for gene discovery and to help identify genes exhibiting cell-specific expression patterns, we constructed cDNA libraries from female gametophytes and from egg cells of maize and sequenced more than 8,500 ESTs. These libraries represent diverse transcripts, potentially corresponding to 3,850 genes (contigs and singletons) from the female gametophyte and 963 genes (contigs and singletons) from the egg cell. In each collection, 16% of the contigs/singletons have no matches in databases and 3-5% encode hypothetical proteins; novel hypothetical proteins (not found within the female gametophyte contigs) were identified among the egg cell contigs. We examined 65 contigs by RT-PCR and 19 genes that were potentially female gametophyte-specific were identified. We used in situ hybridization to determine expression specificity for seven genes: one transcript was expressed both in the egg cell and in the central cell, one was expressed in the egg cell and synergids, two were expressed in the central cell, two were expressed in the synergids, and one was expressed in the central cell and the synergids. Four of these encode small, potentially secreted peptides that are dissimilar except for a conserved triple cysteine motif near their C-terminus. These EST resources should prove useful for identifying female gametophyte or cell-specific genes.

AGL80 is required for central cell and endosperm development in Arabidopsis

During plant reproduction, the central cell of the female gametophyte becomes fertilized to produce the endosperm, a storage tissue that nourishes the developing embryo within the seed. The molecular mechanisms controlling the specification and differentiation of the central cell are poorly understood. We identified a female gametophyte mutant in Arabidopsis thaliana, fem111, that is affected in central cell development. In fem111 female gametophytes, the central cell's nucleolus and vacuole fail to mature properly. In addition, endosperm development is not initiated after fertilization of fem111 female gametophytes. fem111 contains a T-DNA insertion in AGAMOUS-LIKE80 (AGL80). FEM111/AGL80 is a member of the MADS box family of genes that likely encode transcription factors. An AGL80-green fluorescent protein fusion protein is localized to the nucleus. Within the ovule and seed, FEM111/AGL80 is expressed exclusively in the central cell and uncellularized endosperm. FEM111/AGL80 expression is also detected in roots, leaves, floral stems, anthers, and young flowers by real-time RT-PCR. FEM111/AGL80 is required for the expression of two central cell-expressed genes, DEMETER and DD46, but not for a third central cell-expressed gene, FERTILIZATION-INDEPENDENT SEED2. Together, these data suggest that FEM111/AGL80 functions as a transcription factor within the central cell gene regulatory network and controls the expression of downstream genes required for central cell development and function.

Differential gene expression in egg cells and zygotes suggests that the transcriptome is restructed before the first zygotic division in tobacco

We applied suppression subtractive hybridization and mirror orientation selection to compare gene expression profiles of isolated Nicotiana tabacum cv SR1 zygotes and egg cells. Our results revealed that many differentially expressed genes in zygotes were transcribed de novo after fertilization. Some of these genes are critical to zygote polarity and pattern formation during early embryogenesis. This suggests that the transcriptome is restructed in zygote and that the maternal-to-zygotic transition happens before the first zygotic division, which is much earlier in higher plants than in animals. The expressed sequence tags used in this study provide a valuable resource for future research on fertilization and early embryogenesis.

Cell-cell communication during double fertilization

Double fertilization in flowering seed plants requires intercellular signaling events between many interacting partners. The four cell types of the seven-celled female gametophyte communicate with each other to establish and maintain their identity. They secrete signaling molecules to guide the male gametophyte and to mediate sperm cell discharge and transport towards the two female gametes (the egg and central cell). After fusion of the gametes, guidance signals have to be removed to prevent polyspermy, embryo and endosperm development is induced generating daughter cells or nuclear regions of a different fate, and cell death is induced in the surrounding ovular cells. Until recently, little was known about the molecular nature of the signaling molecules that are involved in these processes. Now, small secreted proteins and peptides have been identified as prime candidates mediating several of these communication events.

The fertilization-induced DNA replication factor MCM6 of maize shuttles between cytoplasm and nucleus, and is essential for plant growth and development

The eukaryotic genome is duplicated exactly once per cell division cycle. A strategy that limits every replication origin to a single initiation event is tightly regulated by a multiprotein complex, which involves at least 20 protein factors. A key player in this regulation is the evolutionary conserved hexameric MCM2-7 complex. From maize (Zea mays) zygotes, we have cloned MCM6 and characterized this essential gene in more detail. Shortly after fertilization, expression of ZmMCM6 is strongly induced. During progression of zygote and proembryo development, ZmMCM6 transcript amounts decrease and are low in vegetative tissues, where expression is restricted to tissues containing proliferating cells. The highest protein amounts are detectable about 6 to 20 d after fertilization in developing kernels. Subcellular localization studies revealed that MCM6 protein shuttles between cytoplasm and nucleoplasm in a cell cycle-dependent manner. ZmMCM6 is taken up by the nucleus during G1 phase and the highest protein levels were observed during late G1/S phase. ZmMCM6 is excluded from the nucleus during late S, G2, and mitosis. Transgenic maize was generated to overexpress and down-regulate ZmMCM6. Plants displaying minor antisense transcript amounts were reduced in size and did not develop cobs to maturity. Down-regulation of ZmMCM6 gene activity seems also to affect pollen development because antisense transgenes could not be propagated via pollen to wild-type plants. In summary, the transgenic data indicate that MCM6 is essential for both vegetative as well as reproductive growth and development in plants.

Construction and screening of subtracted cDNA libraries from limited populations of plant cells: a comparative analysis of gene expression between maize egg cells and central cells

The analysis of cell type-specific gene expression is an essential step in understanding certain biological processes during plant development, such as differentiation. Although methods for isolating specific cell types have been established, the application of cDNA subtraction to small populations of isolated cell types for direct identification of specific or differentially expressed transcripts has not yet been reported. As a first step in the identification of genes expressed differentially between maize egg cells and central cells, we have manually isolated these types of cell, and applied a suppression-subtractive hybridization (SSH) strategy. After microarray screening of 1030 cDNAs obtained from the subtracted libraries, we identified 340 differentially expressed clones. Of these, 142 were sequenced, which resulted in the identification of 62 individual cDNAs. The expression patterns of 20 cDNAs were validated by quantitative RT-PCR, through which we identified five transcripts with cell type-specific expression. The specific localization of some of these transcripts was also confirmed by in situ hybridization on embryo sac sections. Taken together, our data demonstrate the effectiveness of our approach in identifying differentially expressed and cell type-specific transcripts of relatively low abundance. This was also confirmed by the identification of previously reported egg cell- and central cell-specific genes in our screen. Importantly, from our analysis we identified a significant number of novel sequences not present in other embryo sac or, indeed, in other plant expressed sequence tag (EST) databases. Thus, in combination with standard EST sequencing and microarray hybridization strategies, our approach of differentially screening subtracted cDNAs will add substantially to the expression information in spatially highly resolved transcriptome analyses.

A 6374 unigene set corresponding to low abundance transcripts expressed following fertilization in Solanum chacoense Bitt, and characterization of 30 receptor-like kinases

In order to characterize regulatory genes that are expressed in ovule tissues after fertilization we have undertaken an EST sequencing project in Solanum chacoense, a self-incompatible wild potato species. Two cDNA libraries made from ovule tissues covering embryo development from zygote to late torpedo-stage were constructed and plated at high density on nylon membranes. To decrease EST redundancy and enrich for transcripts corresponding to weakly expressed genes a self-probe subtraction method was used to select the colonies harboring the genes to be sequenced. 7741 good sequences were obtained and, from these, 6374 unigenes were isolated. Thus, the self-probe subtraction resulted in a strong enrichment in singletons, a decrease in the number of clones per contigs, and concomitantly, an enrichment in the total number of unigenes obtained (82%). To gain insights into signal transduction events occurring during embryo development all the receptor-like kinases (or protein receptor kinases) were analyzed by quantitative real-time RT-PCR. Interestingly, 28 out of the 30 RLK isolated were predominantly expressed in ovary tissues or young developing fruits, and 23 were transcriptionaly induced following fertilization. Thus, the self-probe subtraction did not select for genes weakly expressed in the target tissue while being highly expressed elsewhere in the plant. Of the receptor-like kinases (RLK) genes isolated, the leucine-rich repeat (LRR) family of RLK was by far the most represented with 25 members covering 11 LRR classes.

Is there more than one way to attract a pollen tube?

ZmEA1 (Zea mays egg apparatus 1) is expressed only in the egg and synergid cells. Embryo sacs with presumed reduced expression of ZmEA1 fail to attract pollen tubes. Together with data from Arabidopsis mutants and from elegant laser ablation experiments in Torenia fournieri, these results indicate that embryo sacs send signals to the incoming pollen tubes. We need to decipher how such signals are perceived and determine if the signals are species-specific.