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.     

Early evolution of life cycles in embryophytes: A focus on the fossil evidence of gametophyte/sporophyte size and morphological complexity

Abstract Embryophytes (land plants) are distinguished from their green algal ancestors by diplobiontic life cycles, that is, alternation of multicellular gametophytic and sporophytic generations. The bryophyte sporophyte is small and matrotrophic on the dominant gametophyte; extant vascular plants have an independent, dominant sporophyte and a reduced gametophyte. The elaboration of the diplobiontic life cycle in embryophytes has been thoroughly discussed within the context of the Antithetic and the Homologous Theories. The Antithetic Theory proposes a green algal ancestor with a gametophyte‐dominant haplobiontic life cycle. The Homologous Theory suggests a green algal ancestor with alternation of isomorphic generations. The shifts that led from haplobiontic to diplobiontic life cycles and from gametophytic to sporophytic dominance are most probably related with terrestrial habitats. Cladistic studies strongly support the Antithetic Theory in repeatedly identifying charophycean green algae as the closest relatives of land plants. In recent years, exceptionally well‐preserved axial gametophytes have been described from the Rhynie chert (Lower Devonian, 410 Ma), and the complete life cycle of several Rhynie chert plants has been reconstructed. All show an alternation of more or less isomorphic generations, which is currently accepted as the plesiomorphic condition among all early polysporangiophytes, including basal tracheophytes. Here we review the existing evidence for early embryophyte gametophytes. We also discuss some recently discovered plants preserved as compression fossils and interpreted as gametophytes. All the fossil evidence supports the Antithetic Theory and indicates that the gametophytic generation/sporophytic generation size and complexity ratios show a gradual decrease along the land plant phylogenetic tree.     

Early evolution of life cycles in embryophytes:A focus on the fossil evidence of gametophyte/sporophyte size and morphological complexity

Embryophytes (land plants) are distinguished from their green algal ancestors by diplobiontic life cycles,that is,alternation of multicellular gametophytic and sporophytic generations.The bryophyte sporophyte is small and matrotrophic on the dominant gametophyte; extant vascular plants have an independent,dominant sporophyte and a reduced gametophyte.The elaboration of the diplobiontic life cycle in embryophytes has been thoroughly discussed within the context of the Antithetic and the Homologous Theories.The Antithetic Theory proposes a green algal ancestor with a gametophyte-dominant haplobiontic life cycle.The Homologous Theory suggests a green algal ancestor with alternation of isomorphic generations.The shifts that led from haplobiontic to diplobiontic life cycles and from gametophytic to sporophytic dominance are most probably related with terrestrial habitats.Cladistic studies strongly support the Antithetic Theory in repeatedly identifying charophycean green algae as the closest relatives of land plants.In recent years,exceptionally well-preserved axial gametophytes have been described from the Rhynie chert (Lower Devonian,410 Ma),and the complete life cycle of several Rhynie chert plants has been reconstructed.All show an alternation of more or less isomorphic generations,which is currently accepted as the plesiomorphic condition among all early polysporangiophytes,including basal tracheophytes.Here we review the existing evidence for early embryophyte gametophytes.We also discuss some recently discovered plants preserved as compression fossils and interpreted as gametophytes.All the fossil evidence supports the Antithetic Theory and indicates that the gametophytic generation/sporophytic generation size and complexity ratios show a gradual decrease along the land plant phylogenetic tree.     

Male gametophyte in maize: Influences of the gametophytic genotype

Summary Size and variation in pollen samples were investigated in several successively selfed generations. A significant decrease in mean diameter of pollen grains accompanied inbreeding; also decreased variation in pollen size from individual plants was observed. Since loss of developmental homeostasis in the sporophyte could affect variation in the gametophyte, sporophytic characters were observed as a control. The main conclusion reached in this study was that pollen diameter is influenced by the gametophytic genotype.     

雄配子体选择的遗传分化效应及其在植物改良中的应用

在植物世代交替的生活史中,配子体是产生配子和具有单倍数染色体的植物体,并且有自己的遗传表达信息.在配子产生的过程中以及配子结合之前,适者生存的法则对配子发挥着选择作用,只有最适应外界环境条件的配子才能通过竞争性受精并产生合子.雄配子体选择是影响植物遗传分化、演变和遗传多样性的重要因素,被认为是生物进化的有力动力.此外,由于植物基因组中约有2/3的基因表达交错发生在配子体阶段和孢子体阶段,因此雄配子体选择的结果会影响到下一代孢子体的表现型.在育种实践中,利用雄配子体选择对植物进行遗传改良,具有提高选择机率和缩短育种年限等优点.本文主要概述雄配子体选择与孢子体表型的关刎系、遗传分化效应及其在植物遗传改良中的应用,以期为雄配子选择相关研究提供参考     

Male gametophytic selection in maize

Summary There is evidence that male gametophyte selection is a widespread phenomenon in higher plants. The pollen tube growth rate is one of the main components of gametophyte selective value; genetic variability for this trait, due to the effect of single genes or to quantitative variation, has been described in maize. However, indication of gametophytic selection has been indirectly obtained; its effect was revealed by the positive relation observed between gametophyte competitive ability and sporophyte metrical traits.This paper considers the results of selection applied to gametophyte populations produced from single plants. The competitive ability of the lines was evaluated in comparison with that of a standard line by means of the pollen mixture technique. Sporophytic traits were measured in the hybrid progeny obtained by crossing selected S₃ and S₄ families with an unrelated single cross and an inbred line. Gametophyte selection produced inbred lines with high gametophyte competitive ability. In view of the selection procedure adopted, this result was interpreted as an indication of haploid expression of genes involved in the control of pollen tube growth. Moreover, this gametophytic trait was positively correlated with sporophytic traits (seedling weight, kernel weight and root tip growth in vitro), indicating that both groups of characters have a common genetic basis.     

Gene expression during the male gametophytic phase

Abstract

In recent years a number of experimental findings have indicated that in higher plants the gametophytic phase is able to express its own genetic information, a large part of which it shares with the sporophytic generation. Quantitative estimates of haploid and haplodiploid gene expression have been obtained by mRNA and isozyme analysis in several plant species: 60-70% of the genes are expressed in both pollen and plant, about 10% are pollen-specific, and 20% represent the sporophytic domain. Moreover, it has been demonstrated that stage-specific genes are expressed in the gametophytic generation: at least two sets of genes are activated during pollen development, others are expressed only in the postshedding period, during germination and tube growth. Studies have been made to ascertain the role played by gametophyte-expressed genes in pollen development; the in vivo and in vitro pollen tube growth rate has been revealed to be controlled by the gametophyte genome itself. Differential effects of specific chromosomal deficiencies on the development of maize pollen grains have indicated that components of normal microspore development are controlled by genes located in specific parts of the genome. For single gene analysis, gene transfer can be used; on the contrary, for traits with a multifactorial genetic control, direct proof of gene expression both in the gametophytic and the sporophytic generation can be obtained when selection is applied to the pollen population of a hybrid plant, and response to selection is observed in the resulting sporophytic progeny. Response to selection, applied at different stages of the gametophytic phase, has been described in the sporophytic progeny and this with regard to many adaptive traits; thus the phenomenon can have an important bearing on the genetic structure of natural populations and on higher plant evolution, it can also be used as a breeding tool to increase the efficiency of conventional selection methods.     

How was apical growth regulated in the ancestral land plant? Insights from the development of non-seed plants

Land plant life cycles are separated into distinct haploid gametophyte and diploid sporophyte stages. Indeterminate apical growth evolved independently in bryophyte (moss, liverwort, and hornwort) and fern gametophytes, and tracheophyte (vascular plant) sporophytes. The extent to which apical growth in tracheophytes co-opted conserved gametophytic gene networks, or exploited ancestral sporophytic networks, is a long-standing question in plant evolution. The recent phylogenetic confirmation of bryophytes and tracheophytes as sister groups has led to a reassessment of the nature of the ancestral land plant. Here, we review developmental genetic studies of apical regulators and speculate on their likely evolutionary history.

The combined results of recent developmental genetics and phylogenetics studies suggest that the ancestral sporophyte was more complex than previously thought.     

Cross talk between the sporophyte and the megagametophyte during ovule development

In seed plant ovules, the diploid maternal sporophytic generation embeds and sustains the haploid generation (the female gametophyte); thus, two independent generations coexist in a single organ. Many independent studies on Arabidopsis ovule mutants suggest that embryo sac development requires highly synchronized morphogenesis of the maternal sporophyte surrounding the gametophyte, since megagametogenesis is severely perturbed in most of the known sporophytic ovule development mutants. Which are the messenger molecules involved in the haploid–diploid dialogue? And furthermore, is this one way communication or is a feedback cross talk? In this review, we discuss genetic and molecular evidences supporting the presence of a cross talk between the two generations, starting from the first studies regarding ovule development and ending to the recently sporophytic identified genes whose expression is strictly controlled by the haploid gametophytic generation. We will mainly focus on Arabidopsis studies since it is the species more widely studied for this aspect. Furthermore, possible candidate molecules involved in the diploid–haploid generations dialogue will be presented and discussed.     

Intra- and extracellular lipid composition and associated gene expression patterns during pollen development in Brassica napus

Pollen development in angiosperms is regulated by the interaction of products contributed by both the gametophytic (haploid) and sporophytic (diploid) genomes. In entomophilous species, lipids are major products of both sporophytic and gametophytic metabolism during pollen development. Mature pollen grains of Brassica napus are shown to contain three major acyl lipid pools as follows: (i) the extracellular tryphine mainly consisting of medium-chain neutral esters; (ii) the intracellular membranes, particularly endoplasmic reticulum, mainly containing phospholipids; and (iii) the intracellular storage lipids, which are mostly triacylglycerols. This paper reports on the kinetics of accumulation of these lipid classes during pollen maturation and the expression patterns of several lipid biosynthetic genes and their protein products that are differentially regulated in developing microspores/ pollen grains (gametophyte) and tapetal cells (sporophyte) of B. napus. Detailed analysis of three members of the stearoyl-ACP desaturase (sad) gene family by Northern blotting, in situ hybridization and RT-PCR showed that the same individual genes were expressed both in gametophytic and sporophytic tissues, although under different temporal regulation. In the tapetum, maximal expression of two marker genes for lipid biosynthesis (sad and ear) occurred at a bud length of 2–3 mm, and the corresponding gene products SAD and EAR were detected by Western blotting in 3–4 mm buds, coinciding with the maximal rates of tapetal lipid accumulation. These lipids are released following tapetal cell disintegration and are relocated to form the major structural component of the extracellular tryphine layer that coats the mature pollen grain. In contrast, in developing microspores/pollen grains, maximal expression of the lipid marker genes sad, ear, acp and cyb5 was at the 3–5 mm bud stages, with the SAD and EAR gene products detected in 4–7 mm buds. This pattern of expression coincided with accumulation of the intracellular storage and membrane lipid components of pollen. These results suggest that, although the same genes may be expressed in the sporophytic tapetal cells and in gametophytic tissues, they are regulated differentially leading to the production of the various contrasting lipidic structures that are assembled together to give rise to a viable, fertile pollen grain.     

THE REGULATION OF ALTERNATION OF GENERATION IN FLOWERING PLANTS

The developmental changes involved in the alternation of generation represent the major gene-switching events in the life history of plants. While a large number of genes are common to both sporophyte and gametophyte, many thousand sequences are specifically expressed in each generation; indeed, certain key constituents (e.g. tubulin) are encoded by different genes in each generation, indicating that sporophyte and gametophyte are responding to different evolutionary pressures. Evidence is accumulating that major gene-switching events in plants, such as flowering, are regulated by complex control systems which ensures that development occurs only in the correct groups of cells at the appropriate time. A similar, or more sophisticated system might thus be expected to regulate alternation of generation. It is not possible to manipulate alternation of generation in a similar fashion to flowering, but study of apparent aberrations of development occurring in nature and in vitro suggests that alternation only occurs in cells which have become competent to receive particular developmental stimuli. Further, in certain cases, competent cells may be switched either into sporophytic or gametophytic developmental pathways depending upon the nature of the stimulus. Acquisition of competence seems to involve isolation of cells from the symplast, some cytoplasmic dedifferentiation, and perhaps cell cycle arrest or transition. The stimuli in vivo appear metabolic in nature, although embryogenesis may be activated by specific classes of glycoproteins. Interestingly, examination of agamospermic systems suggests that fertilization of the egg per se is not the signal which activates sporophytic development. Once competent cells have received the stimulus they start to develop, with no delay in a ‘determined’ state. Sporophytic and gametophytic development in vivo and in vitro both start with an asymmetric division, except for the female gametophyte which may arise via a range of developmental pathways, depending on the species.     

The Transformer Genes of the Fern Ceratopteris Simultaneously Promote Meristem and Archegonia Development and Repress Antheridia Development in the Developing Gametophyte

The sex of the haploid gametophyte of the fern Ceratopteris is determined by the presence or absence of the pheromone antheridiogen, which, when present, promotes male development and represses female development of the gametophyte. Several genes involved in sex determination in Ceratopteris have been identified by mutation. In this study, the epistatic interactions among new and previously described sex-determining mutants have been characterized. These results show that sex expression is regulated by two sets of genes defined by the FEM1 and TRA loci. Each promotes the expression of either male or female traits and simultaneously represses the expression of the other. A model describing how antheridiogen regulates the expression of these genes and the sex of the gametophyte is described. The observation that some gametophytic sex-determining mutants have phenotypic effects on the sporophyte plant indicates that sex determination in the Ceratopteris gametophyte is regulated by a mechanism that also regulates sporophyte development.     

The role of cytokinin in ovule development in Arabidopsis

The life cycle of higher plants alternates between the haploid gametophyte and diploid sporophyte. The female gametophyte (FG), surrounded by the sporophyte, develops within the ovule and orients along the chalazal/micropylar axis. This polarity is important in cell specification and development for both the ovule and FG. Previously, cytokinin was shown to act in the sporophytic tissue to regulate FG development.^1,2 In the highlighted study,³ we further showed that enriched cytokinin signaling in chalaza, the central domain of the ovule, is required for the specification of the functional megaspore, which usually occurs in the chalazal-most megaspore after meiosis. The restricted cytokinin signaling in the chalaza is achieved by localized cytokinin biosynthesis and perception. Here, we discuss the implications of this and other studies for the understanding of the role of two-component signaling in FG development and the genetic and cellular interactions between gametophytic and sporophytic cells. Further, we show that cytokinin-deficient mutants display distorted cell morphology in the inner integument and elevated mitotic activity in the maternal sporophyte. These results suggest that cytokinin negatively regulates cell proliferation in the sporophytic tissues surrounding the developing FG, consistent with previous results indicating that cytokinin deficiency causes an increase in the number of cells in the embryos and consequently an enlarged seed size.