Egg activation in flowering plants

Compared to animals and algae, egg activation in flowering plants is still poorly understood because of the inaccessibility and complexity of the fertilization process which is double and internal. However, the development of in vitro fertilization (IVF) systems in maize and a few other plants, despite some limitations, offers new possibilities for the study of early post- fusional events and signals leading to egg activation under defined conditions. This review reports recent data on calcium events induced by gamete fusion during maize IVF and presents perspectives on the role of calcium in egg activation and in early development. Received: 2 December 2000 / Accepted: 7 June 2001     

New lights in early steps of in vitro fertilization in plants

Studies using in vitro fertilization systems in animals and lower plants have led to a better understanding of the initial steps of fertilization and their underlying mechanisms. These mechanisms remain to be elucidated in flowering plants. Recent progress related to the development of in vitro fertilization systems using maize as a plant model is presented in this review. Their potential for leading to a better understanding of the process of gametic recognition and fusion and of the early events triggering egg activation and zygote formation are also discussed.     

The evolution of embryogeny in seed plants and the developmental origin and early history of endosperm

For almost a century, the formation of endosperm from a second and distinctive fertilization event has been viewed as a unique feature of flowering plants. However, until recently, the evolutionary origin of this unique embryo-nourishing entity remained a mystery. Based upon comparative developmental analysis of reproduction among basal angiosperms and their closest extant relatives, the Gnetales (Ephedra, Gnetum, and Welwitschia), it is possible to construct an explicit hypothesis of the events that led to the evolutionary establishment of double fertilization and endosperm. The formulation of this historical record is derived entirely from and dependent upon the determination of reproductive features that are likely to have characterized the common ancestors of angiosperms and Gnetales. Current evidence is most congruent with the concept that a process of double fertilization first evolved in a common ancestor of the Gnetales and angiosperms. Initially, however, the second fertilization product was diploid and yielded a supernumerary embryo. Subsequent to the divergence of the angiosperm lineage from its closest relatives (which include the Gnetales), modification of the development of the supernumerary embryo (derived from the second fertilization event) led to the establishment of an embryo-nourishing endosperm. Comparative analysis of patterns of embryogeny within Gnetales and angiosperms establishes that embryo development in the ancestors of flowering plants (with a rudimentary process of double fertilization) was ab initio cellular, and not free nuclear, as had previously been assumed. Thus, it is likely that the earliest flowering plants displayed an ab initio cellular pattern of endosperm development, whose expression was inherited from that of the supernumerary embryo of the ancestors of flowering plants.     

Fertilization recovery after defective sperm cell release in Arabidopsis

In animal fertilization, multiple sperms typically arrive at an egg cell to "win the race" for fertilization. However, in flowering plants, only one of many pollen tubes, conveying plant sperm cells, usually arrives at each ovule that harbors an egg cell. Plant fertilization has thus been thought to depend on the fertility of a single pollen tube. Here we report a fertilization recovery phenomenon in flowering plants that actively rescues the failure of fertilization of the first mutant pollen tube by attracting a second, functional pollen tube. Wild-type (WT) ovules of Arabidopsis thaliana frequently (～80%) accepted two pollen tubes when entered by mutant pollen defective in gamete fertility. In typical flowering plants, two synergid cells on the side of the egg cell attract pollen tubes, one of which degenerates upon pollen tube discharge. By semi-in vitro live-cell imaging we observed that fertilization was rescued when the second synergid cell accepted a WT pollen tube. Our results suggest that flowering plants precisely control the number of pollen tubes that arrive at each ovule and employ a fertilization recovery mechanism to maximize the likelihood of successful seed set.     

A one‐step rectification of sperm cell targeting ensures the success of double fertilization

Successful fertilization in animals depends on competition among millions of sperm cells, whereas double fertilization in flowering plants usually involves just one pollen tube releasing two immobile sperm cells. It is largely a mystery how the plant sperm cells fuse efficiently with their female targets within an embryo sac. We show that the initial positioning of sperm cells upon discharge from the pollen tube is usually inopportune for gamete fusions and that adjustment of sperm cell targeting occurs through release and re-adhesion of one sperm cell, while the other connected sperm cell remains in stagnation.This enables proper adhesion of each sperm cell to a female gamete and coordinates the gamete fusions. Our findings reveal inner embryo sac dynamics that ensure the reproductive success of flowering plants and suggest a requirement for sperm cell differentiation as the basis of double fertilization.     

Cellular programming of plant gene imprinting

Gene imprinting, the differential expression of maternal and paternal alleles, independently evolved in mammals and in flowering plants. A unique feature of flowering plants is a double-fertilization event in which the sperm fertilize not only the egg, which forms the embryo, but also the central cell, which develops into the endosperm (an embryo-supporting tissue). The distinctive mechanisms of gene imprinting in the endosperm, which involve DNA demethylation and histone methylation, begin in the central cell and sperm prior to fertilization. Flowering plants might have coevolved double fertilization and imprinting to prevent parthenogenetic development of the endosperm.     

Double fertilization in flowering plants: 1898-2008

A short review of the results of investigations in the field of plant embryology in vivo and in vitro which are directly connected with the discovery of double fertilization in flowering plants by S.G. Navashin is presented. These results have been obtained by using the methods of electron and fluorescence microscopy, cytophotometry, cultures of isolated ovules, sperms, eggs, and embryo sac central cells. The question on an origin of the female gametophyte of flowering plants, double fertilization, and endosperm are discussed. It is emphasized that the progress in this field is connected mostly with the study of molecular processes which control the development and functioning of a female gametophyte and sporophyte at the early stages of ontogenesis.     

施肥对金盏菊生长、开花及养分吸收的影响

研究了氮、磷、钾不同施肥处理对金盏菊（Calendula officinalisL.）生长、开花、干物质积累量及养分吸收的影响。结果表明：推荐施肥[N-P2O5-K2O：（0.4-0.2-0.3）g/kg]能有效促进金盏菊的生长及花朵开放,其中干物质积累量和总花朵数,分别比不施肥处理增加了264.3%和323.1%,比习惯施肥增加了16.0%和25.0%;不施肥、习惯施肥和推荐施肥三者干物质日积累量高峰均出现在移栽后29～35 d,分别为0.15、0.43、0.42 g/d;各处理氮、磷、钾吸收量之间均存在显著差异,推荐施肥处理N、P、K养分累积量分别为0.33、0.04、0.39 g/株,分别是不施肥处理的6.93倍、3.94倍和3.61倍,为习惯施肥的1.49倍、1.35倍和1.24倍,各处理金盏菊对N、P、K养分吸收比例变幅为1∶（0.13～0.24）∶（1.20～2.30）,平均比例为1∶0.17∶1.65。     

Fertilization in plants: Is calcium a key player?

For many years, the physiological significance of Ca(2+) oscillations has been a matter of debate, but the potential to encode and transduce information in the pattern of an oscillation is obvious. In this review, we only consider transients and oscillations observed during fertilization in plants with the major focus on flowering plants. After presenting data related to algae, fertilization mechanisms in flowering plants are defined as a multi-step phenomenon, starting with pollination during which calcium plays a key role, especially during pollen-stigma interactions (compatible and incompatible reactions). The pollen tube serves as a guide and a pathway for the sperm cells on their course towards their female target cells. For many years, the pollen tube has also been studied as an easily accessible in vitro model to elucidate the role of calcium on tip growth. Finally, in flowering plants, a unique double fertilization system is present. Interesting data obtained from an in vitro fertilization system in maize are presented and discussed. In addition, the new approaches made possible by Arabidopsis and Torenia and their potential limitations are covered.     

RLKs orchestrate the signaling in plant male-female interaction

Different from animals, sessile plants are equipped with a large receptor-like kinase (RLK) superfamily. RLKs are a family of single trans-membrane proteins with divergent N-terminal extracellular domains capped by a signal peptide and C-terminal intracellular kinase. Researches in the last two decades have uncovered an increasing number of RLKs that regulate plant development, stress response and sexual reproduction, highlighting a dominant role of RLK signaling in cell-to-cell communications. Sexual reproduction in flowering plants is featured by interactions between the male gametophyte and the female tissues to facilitate sperm delivery and fertilization. Emerging evidences suggest that RLKs regulate almost every aspect of plant reproductive process, especially during pollination. Therefore, in this review we will focus mainly on the function and signaling of RLKs in plant male-female interaction and discuss the future prospects on these topics.     

The epigenetic basis of gender in flowering plants and mammals

What makes a sperm male or an egg female, and how can we tell? A gamete's gender could be defined in many ways, such as the sex of the individual or organ that produced it, its cellular morphology, or its behaviour at fertilization. In flowering plants and mammals, however, there is an extra dimension to the gender of a gamete – due to parental imprinting, some of the genes it contributes to the next generation will have different expression patterns depending on whether they were maternally or paternally transmitted. The non-equivalence of gamete genomes, along with natural and experimental modification of imprinting, reveal a level of sexual identity that we describe as ‘epigender’. In this paper, we explore epigender in the life history of plants and animals, and its significance for reproduction and development.     

Further studies on seed transmission of broad bean stain virus and Echtes Ackerbohnenmosaik -Virus in field beans (Vicia faba)

Broad bean stain virus (BBSV) and Echtes Ackerbohnenmosaik-Virus (EAMV) were detected in the seed coat and embryo sac fluid of immature seeds from infected field beans (Viciafaba minor) by inoculation to Phaseolus vulgaris; BBSV was also detected in immature embryos. The proportion of seeds infected with either virus decreased during maturation. The viruses were transmitted to seedlings as often through fully ripened seeds from which the seed coats had been removed as through intact seeds. Both viruses were detected in pollen from infected plants, but in glasshouse tests only BBSV was transmitted through pollen to seeds. Delaying fertilization in plants infected with BBSV or EAMV seemed not to affect seed transmission of either virus. In glasshouse tests BBSV was transmitted more often through seeds from plants that were inoculated before flowering than during flowering, and was not transmitted through seeds from plants inoculated after flowering; EAMV was transmitted only through seeds from plants inoculated before flowering. In tests on seed from naturally infected plants BBSV was transmitted more often through seeds from plants that developed symptoms before flowering than during flowering. Both viruses were seed-borne in all cultivars tested and there was no marked difference in the frequency of transmission of either virus among the spring-sown cultivars most common in Britain. Both viruses persisted in seed for more than 4 yr.     

两种互逆资源梯度影响下刺叶锦鸡儿的种子选择性败育格局

种子的选择性败育在被子植物中普遍存在,开展结籽格局及其影响因素的研究有助于深入了解种子的形成机制及多样化的生殖对策。以刺叶锦鸡儿Caragana acanthophylla为材料,通过传粉过程、胚珠发育动态和资源分配状况的研究,以探讨种子的选择性败育格局及相关影响因素。结果显示：(1)刺叶锦鸡儿具高度自交不亲和性,为泛化的传粉系统,蜂类是主要的传粉者。自然状态的结实率为(86.00±4.96)%,不存在传粉限制,但受精具明显的时间效应。(2)刺叶锦鸡儿单花期4—5d,每朵花有(14.00±0.14)粒胚珠。胚珠的发育从荚果顶部开始,其中,开花后第3天荚果顶部胚珠开始膨大;第11天绝大多数胚珠出现了膨大,此时,在荚果基部的胚珠开始败育,随后荚果顶端受精后的胚珠也出现败育,最终仅荚果中部形成2—3粒种子。其结籽格局为成熟荚果中部胚珠,而败育荚果顶部和基部胚珠的选择性败育类型。(3)受微地形影响,居群内开花植株具斑块分布,对较少开花植株通过添加水肥进行资源调控后,结籽率显著提高,说明在种子形成过程中存在资源限制。综上所述,受精顺序和资源限制两个互逆的资源梯度决定了刺叶锦鸡儿的结籽格局。其中,...     

Kin selection and conflict in seed maturation

There are four genetically distinct components in the developing seeds of flowering plants: maternal sporophyte, gametophyte, endosperm, and embryo. Each component can potentially influence the quantity or quality of nutrients provided to the embryo of its seed, thereby reducing the amount available to embryos in other seeds of that plant. The theory of kin selection predicts that each component will be selected to favor its own embryo over the other embryos to the extent that it is more closely related to its own. Under this criterion, an embryo should be selected to try to acquire more nutrients than the endosperm should be selected to provide, the endosperm should try to supply more than the gametophyte should, and the gametophyte more than the parent sporophyte. Evidence for this conflict of interests is found in the higher frequency of endopolyploidy, nutrient-absorbing haustoria, and food storage tissues in the embryo and endosperm than in the gametophyte of maternal tissues.This theory also suggests how the gametophyte, which is the nurse tissue of gymnosperm seeds, was displaced from this role in the flowering plants by an endosperm initiated by a secondary fertilization. “Neoteny” in the pro-angiosperms created conditions in which (1) an endosperm initiated by double fertilization would be more closely related to the embryo than is the gametophyte and (2) the endosperm would be formed early enough to be of significant aid to the embryo.If this theory is correct it (1) requires a different approach to the study of seed morphology and physiology, (2) increases the plausibility of arguments that flowering plants are a polyphyletic group, (3) provides evidence that parents cannot always control the outcome of conflict with their offspring, and (4) forges a conceptual link in our understanding of the evolution of social interactions in plants and animals.     

Female control of male gamete delivery during fertilization in Arabidopsis thaliana

Fertilization in both animals and plants relies on the correct targeting of the male gametes to the female gametes. In flowering plants, the pollen tube carries two male gametes through the maternal reproductive tissues to the embryo sac, which contains two female gametes. The pollen tube then releases its two male gametes into a specialized receptor cell of the embryo sac, the synergid cell. The mechanisms controlling this critical step of gamete delivery are unknown. Here, data based on the new sirène (srn) mutant of Arabidopsis thaliana provide the first evidence for female control over male gamete delivery. Live imaging of fertilization shows that wild-type pollen tubes do not stop their growth and do not deliver their contents in srn embryo sacs.