基于S-核酸酶的自交不亲和性的分子机制

自交不亲和性是一种广泛存在于显花植物中的种内生殖障碍,可以抑制近亲繁殖而促进异交。其中,以茄科、玄参科和蔷薇科为代表的配子体自交不亲和性是最常见的类型。这类自交不亲和性是由单一的多态性S-位点所控制。目前的研究发现这一位点至少包含两个自交不亲和反应特异性决定因子：花柱中的S-核酸酶和花粉中的SLF（S-Locus F-box）蛋白。该文将主要介绍并讨论基于S-核酸酶的自交不亲和性分子机制的研究进展。     

基于S- 核酸酶的自交不亲和性的分子机制

自交不亲和性是一种广泛存在于显花植物中的种内生殖障碍, 可以抑制近亲繁殖而促进异交。其中, 以茄科、玄参科和蔷薇科为代表的配子体自交不亲和性是最常见的类型。这类自交不亲和性是由单一的多态性S-位点所控制。目前的研究发现这一位点至少包含两个自交不亲和反应特异性决定因子: 花柱中的S-核酸酶和花粉中的SLF(S-Locus F-box)蛋白。该文将主要介绍并讨论基于S-核酸酶的自交不亲和性分子机制的研究进展。     

Gametophytic self-incompatibility: contrasting mechanisms for Nicotiana and Papaver

Since Darwin first noted that not all plants produce self-seed, several mechanisms that regulate the acceptance or rejection of pollen during fertilization have been recognized, of which self-incompatibility (SI) is the most widespread. Over the past few years much progress has been made in understanding the molecular and cellular processes involved in SI. Here we review recent studies of the SI systems of Nicotiana alata and Papaver rhoeas. The SI systems are both determined by a single, multi-allelic gametophytically controlled S-gene, but involve quite different mechanisms.     

The different mechanisms of sporophytic self-incompatibility

Flowering plants have evolved a multitude of mechanisms to avoid self-fertilization and promote outbreeding. Self-incompatibility (SI) is by far the most common of these, and is found in ca. 60% of flowering plants. SI is a genetically controlled pollen-pistil recognition system that provides a barrier to fertilization by self and self-related pollen in hermaphrodite (usually co-sexual) flowering plants. Two genetically distinct forms of SI can be recognized: gametophytic SI (GSI) and sporophytic SI (SSI), distinguished by how the incompatibility phenotype of the pollen is determined. GSI appears to be the most common mode of SI and can operate through at least three different mechanisms, two of which have been characterized extensively at a molecular level in the Solanaceae and Papaveraceae. Because molecular studies of SSI have been largely confined to species from the Brassicaceae, predominantly Brassica species, it is not yet known whether SSI, like GSI, can operate through different molecular mechanisms. Molecular studies of SSI are now being carried out on Ipomoea trifida (Convolvulaceae) and Senecio squalidus (Asteraceae) and are providing important preliminary data suggesting that SSI in these two families does not share the same molecular mechanism as that of the Brassicaceae. Here, what is currently known about the molecular regulation of SSI in the Brassicaceae is briefly reviewed, and the emerging data on SSI in I. trifida, and more especially in S. squalidus, are discussed.     

Self-incompatibility in Papaver: signalling to trigger PCD in incompatible pollen

Sexual reproduction in higher plants uses pollination, involving interactions between pollen and pistil. Self-incompatibility (SI) prevents self-fertilization, providing an important mechanism to promote outbreeding. SI is controlled by the S-locus; discrimination occurs between incompatible pollen, which is rejected, while compatible pollen can achieve fertilization. In Papaver rhoeas, S proteins encoded by the pistil part of the S-locus interact with incompatible pollen to effect rapid inhibition of tip growth. This self-incompatible interaction triggers a Ca(2+)-dependent signalling cascade. SI-specific events triggered in incompatible pollen include rapid depolymerization of the actin cytoskeleton; phosphorylation of soluble inorganic pyrophosphatases, and activation of a MAPK. It has recently been shown that programmed cell death (PCD) is triggered by SI. This provides a precise mechanism for the specific destruction of 'self' pollen. Recent data providing evidence for SI-induced caspase-3-like protease activity, and the involvement of actin depolymerization and MAPK activation in SI-mediated PCD will be discussed. These studies not only significantly advance our understanding of the mechanisms involved in SI, but also contribute to our understanding of functional links between signalling components and initiation of PCD in a plant cell. Recent data demonstrating SI-mediated modification of soluble inorganic pyrophosphatases are also described.     

显花植物自体和异体花粉识别的分子机理研究(英文)

显花植物的受精涉及许多识别过程;其中第一个是雌性生殖组织心皮对花粉的识别。自交不亲和性（Ｓｅｌｆ－ｉｎｃｏｍｐａｔｉｂｉｌｉｔｙ,ＳＩ）是一种广泛分布于显花植物的种内生殖障碍。在多数自交不亲和的植物中,ＳＩ的遗传控制比较简单,受控于一个由复等位基因构成的单一位点,称为Ｓ位点。在以茄科、玄参科和蔷薇科为代表的配子体自交不亲和植物中,Ｓ位点编码一类核酸酶,即Ｓ核酸酶（Ｆｉｇ．１）,控制ＳＩ在花柱中的表达,但是与花粉自交不亲和性的表达无关。后者可能由与Ｓ核酸酶不同的基因控制,这种基因常被称为花粉Ｓ基因。它是目前了解显花植物花粉识别生化和分子机理的关键。近来;通过对影响花粉ＳＩ表达突变体的分子遗传分析提出了一个花粉Ｓ基因产物如何与Ｓ核酸酶相互作用完成自体和异体花粉识别过程的模型（Ｆｉｇ．２）。另外,描述了两个在金鱼草中克隆花粉Ｓ基因的方法,即Ｓ位点选择性的转座子标记和图位克隆。     

The Skp1‐like protein SSK1 is required for cross‐pollen compatibility in S‐RNase‐based self‐incompatibility

The self‐incompatibility (SI) response occurs widely in flowering plants as a means of preventing self‐fertilization. In these self/non‐self discrimination systems, plant pistils reject self or genetically related pollen. In the Solanaceae, Plantaginaceae and Rosaceae, pistil‐secreted S‐RNases enter the pollen tube and function as cytotoxins to specifically arrest self‐pollen tube growth. Recent studies have revealed that the S‐locus F‐box (SLF) protein controls the pollen expression of SI in these families. However, the precise role of SLF remains largely unknown. Here we report that PhSSK1 (Petunia hybrida SLF‐interacting Skp1‐like1), an equivalent of AhSSK1 of Antirrhinum hispanicum, is expressed specifically in pollen and acts as an adaptor in an SCF(Skp1‐Cullin1‐F‐box)^SLF complex, indicating that this pollen‐specific SSK1‐SLF interaction occurs in both Petunia and Antirrhinum, two species from the Solanaceae and Plantaginaceae, respectively. Substantial reduction of PhSSK1 in pollen reduced cross‐pollen compatibility (CPC) in the S‐RNase‐based SI response, suggesting that the pollen S determinant contributes to inhibiting rather than protecting the S‐RNase activity, at least in solanaceous plants. Furthermore, our results provide an example that a specific Skp1‐like protein other than the known conserved ones can be recruited into a canonical SCF complex as an adaptor.     

Self-incompatibility systems: barriers to self-fertilization in flowering plants

Flowering plants (angiosperms) are the most prevalent and evolutionarily advanced group of plants. Success of these plants is owed to several unique evolutionary adaptations that aid in reproduction: the flower, the closed carpel, double fertilization, and the ultimate products of fertilization, seeds enclosed in the fruit. Angiosperms exhibit a vast array of reproductive strategies, including both asexual and sexual, the latter of which includes both self-fertilization and cross-fertilization. Asexual reproduction and self-fertilization are important reproductive strategies in a variety of situations, such as when mates are scarce or when the environment remains relatively stable. However, reproductive strategies promoting cross-fertilization are critical to angiosperm success, since they contribute to the creation of genetically diverse populations, which increase the probability that at least one individual in a population will survive given changing environmental conditions. The evolution of several physical and genetic barriers to self-fertilization or fertilization among closely related individuals is thus widespread in angiosperms. A major genetic barrier to self-fertilization is self-incompatibility (SI), which allows female reproductive cells to discriminate between "self" and "non-self" pollen, and specifically reject self pollen. Evidence for the importance of SI in angiosperm evolution lies in the highly diverse set of mechanisms used by various angiosperm families for recognition of self pollen tube development and preventing self-fertilization.     

S-RNase complexes and pollen rejection

Biochemical interactions between the pollen and the pistil allow plants fine control over fertilization. S-RNase-based pollen rejection is among the most widespread and best understood of these interactions. At least three plant families have S-RNase-based self-incompatibility (SI) systems, and S-RNases have also been implicated in interspecific pollen rejection. Although S-RNases determine the specificity of SI, other genes are required for the pollen rejection system to function. Progress is being made toward identifying these non-S-RNase factors. HT-protein, first identified as a non-S-RNase factor that was required for SI in Nicotiana alata, has now been implicated in other species as well. In addition, several pistil proteins bind to S-RNase in vitro. One hypothesis is that S-RNase forms a complex with these proteins in vivo that is the active form of S-RNase in pollen rejection.     

Gametophytic self-incompatibility inhibits pollen tube growth using different mechanisms

Self-incompatibility (SI) is one of the most important mechanisms used by plants to prevent self-pollination and consequently inbreeding. It is genetically controlled by the S-locus, which allows the recognition and rejection of ‘self’ (S-phenotypically identical) pollen. Gametophytically controlled SI (GSI) is the most widespread SI system. To date, only two forms have been elucidated in detail at the molecular level, revealing two different stigmatic S-genes. Here we summarize the evidence for the use of two different mechanisms to inhibit incompatible pollen tube growth. Because the limited data suggest the independent evolution of these two GSI systems, it would be interesting to explore other GSI systems to determine the extent of the mechanistic diversity.     

Patterns of variation within self-incompatibility loci

Diverse self-incompatibility (SI) mechanisms permit flowering plants to inhibit fertilization by pollen that express specificities in common with the pistil. Characteristic of at least two model systems is greatly reduced recombination across large genomic tracts surrounding the S-locus, which regulates SI. In three angiosperm families, including the Solanaceae, the gene that controls the expression of gametophytic SI in the pistil encodes a ribonuclease (S-RNase). The gene that controls pollen SI expression is currently unknown, although several candidates have recently been proposed. Although each candidate shows a high level of polymorphism and complete allelic disequilibrium with the S-RNase gene, such properties may merely reflect tight linkage to the S-locus, irrespective of any functional role in SI. We analyzed the magnitude and nature of nucleotide variation, with the objective of distinguishing likely candidates for regulators of SI from other genes embedded in the S-locus region. We studied the S-RNase gene of the Solanaceae and 48A, a candidate for the pollen gene in this system, and we also conducted a parallel analysis of the regulators of sporophytic SI in Brassica, a system in which both the pistil and pollen genes are known. Although the pattern of variation shown by the pollen gene of the Brassica system is consistent with its role as a determinant of pollen specificity, that of 48A departs from expectation. Our analysis further suggests that recombination between 48A and S-RNase may have occurred during the interval spanned by the gene genealogy, another indication that 48A may not regulate SI expression in pollen.     

The Arabidopsis CrRLK1L protein kinases BUPS1 and BUPS2 are required for normal growth of pollen tubes in the pistil

In flowering plants, the interaction of pollen tubes with female tissues is important for the accomplishment of double fertilization. Little information is known about the mechanisms that underlie signalling between pollen tubes and female tissues. In this study, two Arabidopsis pollen tube‐expressed CrRLK1L protein kinases, Buddha's Paper Seal 1 (BUPS1) and BUPS2, were identified as being required for normal tip growth of pollen tubes in the pistil. They are expressed prolifically in pollen and pollen tubes and are localized on the plasma membrane of the pollen tube tip region. Mutations in BUPS1 drastically reduced seed set. Most of the bups1 mutant pollen tubes growing in the pistil exhibited a swollen pollen tube tip, leading to failure of fertilization. The bups2 pollen tubes had a slightly abnormal morphology but could still accomplish double fertilization. The bups1 bups2 double mutant exhibited a slightly enhanced phenotype compared to the single bups1 mutants. The BUPS1 proteins could form homomers and heteromers with BUPS2, whereas BUPS2 could only form heteromers with BUPS1. The BUPS proteins could interact with the Arabidopsis pollen‐expressed RopGEFs in the yeast two‐hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays. The results indicated that the BUPSs may mediate normal polar growth of pollen tubes in the pistil.     

Two Arabidopsis AGC kinases are critical for the polarized growth of pollen tubes

Reproduction of flowering plants requires the growth of pollen tubes to deliver immotile sperm for fertilization. Pollen tube growth resembles that of polarized metazoan cells, in that some molecular mechanisms underlying cell polarization and growth are evolutionarily conserved, including the functions of Rho GTPases and the dynamics of the actin cytoskeleton. However, a role for AGC kinases, crucial signaling mediators in polarized metazoan cells, has yet to be shown in pollen tubes. Here we demonstrate that two Arabidopsis AGC kinases are critical for polarized growth of pollen tubes. AGC1.5 and AGC1.7 are pollen-specific genes expressed during late developmental stages. Pollen tubes of single mutants had no detectable phenotypes during in vitro or in vivo germination, whereas those of double mutants were wider and twisted, due to frequent changes of growth trajectory in vitro . Pollen tubes of the double mutant also had reduced growth and were probably compromised in response to guidance cues in vivo . In the agc1.5 background, downregulation of AGC1.7 using an antisense construct phenocopied the growth defect of double mutant pollen tubes, providing additional support for a redundant function of AGC1.5/1.7 in pollen tube growth. Using the actin marker mouse Talin, we show that pollen tubes of double mutants had relatively unaffected longitudinal actin cables but had ectopic filamentous actin, indicating disturbed control of polarity. Our results demonstrate that AGC1.5 and AGC1.7 are critical components of the internal machinery of the pollen tube leading to polarized growth of pollen tubes.     

Intra- and interspecific reproductive barriers inCrocus (Iridaceae)

The reproductive system was studied inCrocus genus (Iridaceae) following intra- and interspecific pollination, by using light and scanning electron microscopy. The results suggest that the stigma-style tract of theCrocus pistil is a mere promoter of pollen tube growth, while intra- and interspecific discrimination of compatible and incompatible pollen occurs in the ovarian tract. Here, the transmitting tissue consists of special epidermal cells, whose granular or floccular secretions provide the selective medium for the growth of pollen tubes. The ovarian self-incompatibility (SI) is widespread within the genus, resulting in a partial or complete suppression of self-fertilization. Moreover, postzygotic SI mechanisms, as well as postzygotic mechanisms of unknown nature, seem to be recurrent and both are responsible for seed abortion. The interspecific ovarian incompatibility concerns only unrelated crosses; crosses between related fertile species succeed both in fertilization and seed-set.     

Incompatibility in angiosperms

 Since Darwinian times considerable knowledge has accumulated on the distribution, physiology and genetics of self-incompatibility (SI) in higher plants. In the second half of this century the first attempts were made to identify the biochemical bases of SI. These included thediscovery that cutinase enables pollen tube penetration at the surface* of the stigma in Cruciferae, sorting of segregation pollen S-phenotypes by serological techniques, a lock-and-key model of the SI reaction, the first detection and characterisation of SI proteins and the discovery of the role of the tapetum in the determination of pollen phenotypes in homomorphic sporophytic SI. This pioneering work was followed by a worldwide effort to identify and understand the cellular and molecular processes which lead to the recognition and rejection of SI pollen. The present review article summarizes briefly the current state of knowledge in areas essential for the understanding and exploitation of SI and outlines new information that has become available during recent years. Received: 14 March 1997 / Revision accepted: 10 June 1997     

Molecular evolution of the s locus controlling mating in the brassicaceae

Flowering plants possess self-incompatibility (SI) mechanisms that promote outbreeding and thereby increase their genetic diversity. In the self-incompatible Brassicaceae, recognition and rejection of self-pollen is based on a receptor-ligand interaction between male and female SI determinants. A transmembrane receptor kinase (S locus Receptor Kinase, SRK) determines the SI specificity in stigmatic cells, whereas a pollen coat-localized ligand (S locus Cysteine-Rich, SCR) determines the SI specificity in pollen. During recent years, major advances have been made in the understanding of the molecular basis of self-pollen recognition by stigmatic cells. In this review, we will focus on evolutionary aspects of the SI system in Brassicaceae. We will describe how the study of the molecular aspect of SI, not only in the historical Brassica model but also in Arabidopsis species, has contributed to highlight certain aspects of evolution of SI in the Brassicaceae.     

Growth and development of conifer pollen tubes

Conifer pollen tubes are an important but underused experimental system in plant biology. They represent a major evolutionary step in male gametophyte development as an intermediate form between the haustorial pollen tubes of cycads and Ginkgo and the structurally reduced and faster growing pollen tubes of flowering plants. Conifer pollen grains are available in large quantities, most can be stored for several years, and they grow very well in culture. The study of pollen tube growth and development furthers our understanding of conifer reproduction and contributes towards our ability to improve on their productivity. This review covers taxonomy and morphology to cell, developmental, and molecular biology. It explores recent advances in research on conifer pollen and pollen tubes in vivo, focusing on pollen wall structure, male gametophyte development within the pollen wall, pollination mechanisms, pollen tube growth and development, and programmed cell death. It also explores recent research in vitro, including the cellular mechanisms underlying pollen tube elongation, in vitro fertilization, genetic transformation and gene expression, and pine pollen tube proteomics. With the ongoing sequencing of the Pinus taeda genome in several labs, we expect the use of conifer pollen tubes as an experimental system to increase in the next decade.     

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.     

Evidence for ovarian self-incompatibility as a cause of self-sterility in the relictual woody angiosperm,Pseudowintera axillaris (Winteraceae)

Species within the genus Pseudowintera exhibit high rates of self-sterility. Self-sterility in the genus has been previously posited-but not confirmed-to be the result of late-acting ovarian self-incompatibility (OSI) functioning within nucellar tissue of the ovule to prevent self pollen tubes from entering the embryo sac. Structural and functional aspects of pollen-carpel interactions and early seed development following cross- and self-pollination were investigated in P. axillaris to determine the site, timing and possible mechanisms of self-sterility. No significant differences were observed between pollen tube growth, ovule penetration and double fertilization following cross- and self-pollination. Pollen tubes exhibited phasic growth in an extracellular matrix composed of proteins and carbohydrates, as well as arabinogalactans/arabinogalactan proteins. A uniform failure in embryo sac development prior to division of the zygote was apparent within 15 d following double fertilization by self gametes. Results indicate that SI mechanisms in P. axillaris do not prevent double fertilization from occurring. Instead, mechanisms of self-sterility affect post-zygotic development of the embryo sac. Although self-sterility may be attributed to inbreeding depression, given the post-zygotic nature of failure in embryo sac development, the possibility of late-acting OSI is discussed.     

Imaging fertilization in flowering plants, not so abominable after all

Although the discovery of double fertilization in flowering plants took place at the end of the nineteenth century little progress had been made in understanding the cellular and molecular mechanisms involved until the end of the twentieth century. After attempts to study fertilization with isolated male and female gametes, researchers turned to Arabidopsis thaliana as a model for genetic analysis and in vivo imaging. The development of confocal imaging and fluorescent proteins, coupled with new molecular insights into cell fate specification of plant gametes, allowed the development of robust markers for cells participating in double fertilization. These markers enabled the imaging of double fertilization in vivo in Arabidopsis. These studies have been coupled with the identification and molecular characterization of genes controlling fertilization in Arabidopsis. Live imaging has already provided new insights on sperm cell delivery, the equivalence of the fate of the sperm cells, gamete fusion, and re-initiation of the zygotic life. This review covers these topics and outlines many important aspects of double fertilization that remain unknown.