Pollen biology and hormesis: Pollen germination and pollen tube elongation

This paper evaluated the occurrence of hormetic dose responses in pollen reported over the past eight decades. Hormetic doses responses were induced by a wide range of chemical and physical agents in 34 plant species for pollen germination and pollen tube growth/elongation. Agents inducing such hormetic dose/concentration responses in pollen included nutrients, growth-promoting agents, plant and animal hormones, toxic substances, including heavy metals such as cadmium, gaseous pollutants such as ozone, as well as ionizing and non-ionizing radiation. This paper provides further evidence for the broad generality of the hormesis dose response, supporting substantial prior findings that the hormetic response is independent of biological model, inducing agent, and endpoints measured. Given the widespread potential of inducing hormetic dose responses in pollen, these findings indicate the need to explore their emerging biological, ecological, agricultural, economic and public health implications.

Gamete fusion is facilitated by two sperm cell-expressed DUF679 membrane proteins

Successful double fertilization in flowering plants relies on two coordinated gamete fusion events, but the underlying molecular processes are not well understood. We show that two sperm-specific DOMAIN OF UNKNOWN FUNCTION 679 membrane proteins (DMP8 and DMP9) facilitate gamete fusion, with a greater effect on sperm-egg fusion than on sperm-central cell fusion. We also show that sperm adhesion and sperm cell separation depend on egg cell-secreted EGG CELL 1 proteins.

Advances in the study of egg activation of higher plants

Fertilization in higher plants induces many structural and physiological changes in the fertilized egg, and represents the transition from the haploid female gamete to the diploid zygote, the first cell of a sporophyte. Some changes are induced extremely rapidly following fusion with sperm cells and are the preclusions of egg activation. This review focuses on the early changes that occur in the egg after fusion with sperm cells, but before nuclear fusion. Reported changes include cell shrinkage, cell wall formation, polarity change, oscillation in Ca2+ concentration, and DNA synthesis. In addition, the current understanding of egg activation is summarized and the possible functional relevance of the changes is explored.

Fertilization Mechanisms in Flowering Plants

Compared with the animal kingdom, fertilization is particularly complex in flowering plants (angiosperms). Sperm cells of angiosperms have lost their motility and require transportation as a passive cargo by the pollen tube cell to the egg apparatus (egg cell and accessory synergid cells). Sperm cell release from the pollen tube occurs after intensive communication between the pollen tube cell and the receptive synergid, culminating in the lysis of both interaction partners. Following release of the two sperm cells, they interact and fuse with two dimorphic female gametes (the egg and the central cell) forming the major seed components embryo and endosperm, respectively. This process is known as double fertilization. Here, we review the current understanding of the processes of sperm cell reception, gamete interaction, their pre-fertilization activation and fusion, as well as the mechanisms plants use to prevent the fusion of egg cells with multiple sperm cells. The role of Ca(2+) is highlighted in these various processes and comparisons are drawn between fertilization mechanisms in flowering plants and other eukaryotes, including mammals.

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.

Brassinosteroids promote Arabidopsis pollen germination and growth

Pollen tubes are among the fastest tip-growing plant cells and represent an excellent experimental system for studying the dynamics and spatiotemporal control of polarized cell growth. However, investigating pollen tube tip growth in the model plant Arabidopsis remains difficult because in vitro pollen germination and pollen tube growth rates are highly variable and largely different from those observed in pistils, most likely due to growth-promoting properties of the female reproductive tract. We found that in vitro grown Arabidopsis pollen respond to brassinosteroid (BR) in a dose-dependent manner. Pollen germination and pollen tube growth increased nine- and fivefold, respectively, when media were supplemented with 10 µM epibrassinolide (epiBL), resulting in growth kinetics more similar to growth in vivo. Expression analyses show that the promoter of one of the key enzymes in BR biosynthesis, CYP90A1/CPD, is highly active in the cells of the reproductive tract that form the pathway for pollen tubes from the stigma to the ovules. Pollen tubes grew significantly shorter through the reproductive tract of a cyp90a1 mutant compared to the wild type, or to a BR perception mutant. Our results show that epiBL promotes pollen germination and tube growth in vitro and suggest that the cells of the reproductive tract provide BR compounds to stimulate pollen tube growth.

Imaging sexual reproduction in Arabidopsis using fluorescent markers

Sexual reproduction in higher plants is a stealth process as most events occur within tissues protected by multiple surrounding cell layers. Female gametes are produced inside the embryo sac surrounded by layers of ovule integument cells. Upon double fertilization, two male gametes are released at one end of the embryo sac and migrate towards their respective female partner to generate the embryo and its feeding tissue, the endosperm, within a seed. Since the early discovery of plant reproduction, advances in microscopy have contributed enormously to our understanding of this process (Faure and Dumas, Plant Physiol 125:102-104, 2001). Recently, live imaging of double fertilization has been possible using a set of fluorescent markers for gametes in Arabidopsis. The following chapter will detail protocols to study male and female gametogenesis and double fertilization in living tissues using fluorescent markers.

Downregulation of egg cell-secreted EC1 is accompanied with delayed gamete fusion and polytubey

One major player known to be essential for successful gamete interactions during double fertilization in Arabidopsis thaliana is the recently identified family of egg cell-secreted EC1 proteins. Both gamete fusion events are affected in EC1-deficient female gametophytes. Here, we show that the number of ovules with unfused sperm cells is considerably higher than the number of undeveloped seeds in the same ec1-RNAi knockdown lines. We found that some sperm cells are able to fuse with the female gametes even 2 to 3 days after pollination, as reflected by delayed embryo and endosperm development, and by polytubey. We propose that the egg cell secretes EC1 proteins upon sperm arrival to promote rapid sperm activation, thereby accelerating gamete fusion and preventing polytubey.

Downregulation of egg cell-secreted EC1 is accompanied with delayed gamete fusion and polytubey

One major player known to be essential for successful gamete interactions during double fertilization in Arabidopsis thaliana is the recently identified family of egg cell-secreted EC1 proteins. Both gamete fusion events are affected in EC1-deficient female gametophytes. Here, we show that the number of ovules with unfused sperm cells is considerably higher than the number of undeveloped seeds in the same ec1-RNAi knockdown lines. We found that some sperm cells are able to fuse with the female gametes even 2 to 3 days after pollination, as reflected by delayed embryo and endosperm development, and by polytubey. We propose that the egg cell secretes EC1 proteins upon sperm arrival to promote rapid sperm activation, thereby accelerating gamete fusion and preventing polytubey.

Egg cell-secreted EC1 triggers sperm cell activation during double fertilization

Double fertilization is the defining characteristic of flowering plants. However, the molecular mechanisms regulating the fusion of one sperm with the egg and the second sperm with the central cell are largely unknown. We show that gamete interactions in Arabidopsis depend on small cysteine-rich EC1 (EGG CELL 1) proteins accumulating in storage vesicles of the egg cell. Upon sperm arrival, EC1-containing vesicles are exocytosed. The sperm endomembrane system responds to exogenously applied EC1 peptides by redistributing the potential gamete fusogen HAP2/GCS1 (HAPLESS 2/GENERATIVE CELL SPECIFIC 1) to the cell surface. Furthermore, fertilization studies with ec1 quintuple mutants show that successful male-female gamete interactions are necessary to prevent multiple-sperm cell delivery. Our findings provide evidence that mutual gamete activation, regulated exocytosis, and sperm plasma membrane modifications govern flowering plant gamete interactions.

Pathfinding in angiosperm reproduction: pollen tube guidance by pistils ensures successful double fertilization

Sexual reproduction in flowering plants is unique in multiple ways. Distinct multicellular gametophytes contain either a pair of immotile, haploid male gametes (sperm cells) or a pair of female gametes (haploid egg cell and homodiploid central cell). After pollination, the pollen tube, a cellular extension of the male gametophyte, transports both male gametes at its growing tip and delivers them to the female gametes to affect double fertilization. The pollen tube travels a long path and sustains its growth over a considerable amount of time in the female reproductive organ (pistil) before it reaches the ovule, which houses the female gametophyte. The pistil facilitates the pollen tube's journey by providing multiple, stage-specific, nutritional, and guidance cues along its path. The pollen tube interacts with seven different pistil cell types prior to completing its journey. Consequently, the pollen tube has a dynamic gene expression program allowing it to continuously reset and be receptive to multiple pistil signals as it migrates through the pistil. Here, we review the studies, including several significant recent advances, that led to a better understanding of the multitude of cues generated by the pistil tissues to assist the pollen tube in delivering the sperm cells to the female gametophyte. We also highlight the outstanding questions, draw attention to opportunities created by recent advances and point to approaches that could be undertaken to unravel the molecular mechanisms underlying pollen tube-pistil interactions.

Identification of genes expressed in the angiosperm female gametophyte

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

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.

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

BACKGROUND

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

SCOPE

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

Family life at close quarters: communication and constraint in angiosperm seed development

The formation of viable angiosperm seeds involves the co-ordinated growth and development of three genetically distinct organisms, the maternally derived seed coat and the zygotic embryo and endosperm. The physical relationships of these tissues are initially established during the specification and differentiation of the female gametophyte within the tissues of the developing ovule. The molecular programmes implicated in both ovule and seed development involve elements of globally important pathways (such as auxin signalling), as well as ovule- and seed-specific pathways. Recurrent themes, such as the precisely controlled death of specific cell types and the regulation of cell-cell communication and nutrition by the selective establishment of symplastic and apoplastic barriers, appear to play key roles in both pre- and post-fertilization seed development. Much of post-fertilization seed growth occurs during a key developmental window shortly after fertilization and involves the dramatic expansion of the young endosperm, constrained by surrounding maternal tissues. The complex tissue-specific regulation of carbohydrate metabolism in specific seed compartments has been shown to provide a driving force for this early seed expansion. The embryo, which is arguably the most important component of the seed, appears to be only minimally involved in early seed development. Given the evolutionary and agronomic importance of angiosperm seeds, the complex combination of communication pathways which co-ordinate their growth and development remains remarkably poorly understood.