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。     

Molecular analysis of the stylar-expressed Solanum chacoense small asparagine-rich protein family related to the HT modifier of gametophytic self-incompatibility in Nicotiana

Gametophytic self-incompatibility (GSI) systems involving the expression of stylar ribonucleases have been described and extensively studied in many plant families including the Solanaceae, Rosaceae and Scrophulariaceae. Pollen recognition and rejection is governed in the style by specific ribonucleases called S-RNases, but in many self-incompatibility (SI) systems, modifier loci that can modulate the SI response have been described at the genetic level. Here, we present at the molecular level, the isolation and characterization of two Solanum chacoense homologues of the Nicotiana HT modifier that had been previously shown to be necessary for the SI reaction to occur in N. alata (McClure et al., 1999). HT homologues from other solanaceous species have also been isolated and a phylogenetic analysis reveals that the HT genes fall into two groups. In S. chacoense, these small proteins named ScHT-A and ScHT-B are expressed in the style and are developmentally regulated during anthesis identically to the S-RNases as well as following compatible and incompatible pollination. To elucidate the precise role of each HT isoform, antisense ScHT-A and RNAi ScHT-B lines were generated. Conversion from SI to self-compatibility (SC) was only observed in RNAi ScHT-B lines with reduced levels of ScHT-B mRNA. These results confirm the role of the HT modifier in solanaceous SI and indicate that only the HT-B isoform is directly involved in SI.     

Genomic organization of the Papaver rhoeas self-incompatibility S(1) locus

The self-incompatibility (SI) response in Papaver rhoeas depends upon the cognate interaction between a pollen-expressed receptor and a stigmatically expressed ligand. The genes encoding these components are situated within the S-locus. In order for SI to be maintained, the genes encoded by the S-locus must be co-inherited with no recombination between them. Several hypotheses, including sequence heterogeneity and chromosomal position, have been put forward to explain the maintenance of the S-locus in the SI systems of the Brassicaceae and the Solanaceae. A region of the Papaver rhoeas genome encompassing part of the self-incompatibility S(1) locus has been cloned and sequenced. The clone contains the gene encoding the stigmatic component of the response, but does not contain a putative pollen S-gene. The sequence surrounding the S(1) gene contains several diverse repetitive DNA elements. As such, the P. rhoeas S-locus bears similarities to the S-loci of other SI systems. An attempt to localize the P. rhoeas S-locus using fluorescence in situ hybridization (FISH) has also been made. The potential relevance of the findings to mechanisms of recombination suppression is discussed.     

Involvement of extracellular calcium influx in the self-incompatibility response of Papaver rhoeas

We have previously demonstrated that increases in cytosolic free Ca2+ are triggered by the self-incompatibility (SI) response in incompatible Papaver rhoeas (the field poppy) pollen. However, one key question that has not been answered is whether extracellular Ca2+ may be involved. To address this question, we have used an ion-selective vibrating probe to measure changes in extracellular Ca2+ fluxes around poppy pollen tubes. Our data reveal several findings. First, we confirm that there is an oscillating Ca2+ influx directed at the apex of the pollen tube; we also provide evidence that Ca2+ influx also occurs at the shanks of pollen tubes. Second, upon challenge with self-incompatibility (S) proteins, there is a stimulation of Ca2+ influx along the shank of incompatible pollen tubes, approximately 50 microm behind the pollen tube tip. This demonstration of SI-induced Ca2+ influx suggests a role for influx of extracellular Ca2+ in the SI response.     

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.     

Detection of Proteins Possibly Involved in Self-Incompatibility Response in Distylous Buckwheat

Buckwheat (Fagopyrum esculentum Moench) is a heterostylous plant displaying heteromorphic sporophytic self-incompatibility (SI). In order to detect proteins involved in SI, pistils from both long and short styles were isolated and then selfed or cross-pollinated. One-dimensional gel electrophoresis revealed that short pistils 2 h after selfing contained an unique 50 kDa protein. In the two-dimensional electrophoresis two distinct groups of proteins possibly involved in SI response were detected in the short, and one in the long pistils. This revised version was published online in July 2006 with corrections to the Cover Date.     

A cascade signal pathway occurs in self-incompatibility of Pyrus pyrifolia

Pear (Pyrus pyrifolia L.) possesses an S-RNase-based gametophytic self-incompatibility (GSI) system and S-RNase, the self-incompatibility (SI) determinant in the pistil, has also been implicated in the rejection of self-pollen and genetically identical pollen. We have demonstrated that S-RNase depolymerises actin cytoskeleton, triggers mitochondrial alteration and DNA degradation in the incompatible pollen tube, which indicates programmed cell death (PCD) may occur in SI response of Pyrus pyrifolia. Recently, we have identified that S-RNase specifically disrupted tip-localized reactive oxygen species (ROS) of incompatible pollen tube via arrest of ROS formation in mitochondria and cell walls in Pyrus pyrifolia. Furthermore, tip-localized ROS disruption not only decreased the Ca²⁺ current and depolymerised the actin cytoskeleton, but it also induced nuclear DNA degradation in the pollen tube. The results mentioned above indicate that a cascade signal pathway may occur in SI of Pyrus pyrifolia and PCD is used to terminate the incompatible pollen tubes growth. In this addendum, we review the cascade signal pathway of Pyrus pyrifolia SI.Key words: S-RNase, programmed cell death, reactive oxygen species, actin cytoskeleton, Ca²⁺ current, nuclear DNA     

Alterations in the actin cytoskeleton of pollen tubes are induced by the self-incompatibility reaction in Papaver rhoeas

Self-incompatibility (SI) is a genetically controlled process used to prevent self-pollination. In Papaver rhoeas, the induction of SI is triggered by a Ca(2)+-dependent signaling pathway that results in the rapid and S allele-specific inhibition of pollen tube tip growth. Tip growth of cells is dependent on a functioning actin cytoskeleton. We have investigated the effect of self-incompatibility (S) proteins on the actin cytoskeleton in poppy pollen tubes. Here, we report that the actin cytoskeleton of incompatible pollen tubes is rapidly and dramatically rearranged during the SI response, not only in our in vitro SI system but also in vivo. We demonstrate that nonspecific inhibition of growth does not result in similar actin rearrangements. Because the SI-induced alterations are not observed if growth stops, this clearly demonstrates that these alterations are triggered by the SI signaling cascade rather than merely resulting from the consequent inhibition of growth. We establish a detailed time course of events and discuss the mechanisms that might be involved. Our data strongly implicate a role for the actin cytoskeleton as a target for signaling pathways involved in the SI response of P. rhoeas.     

Variation in the self-incompatibility response within and among populations of the tropical shrub Witheringia solanacea (Solanaceae)

Breakdown of genetically enforced self-incompatibility (SI), an extremely common and important evolutionary transition in plants, has conventionally been conceived as a qualitative rather than a quantitative change. We evaluated qualitative and quantitative variation in SI for four populations of Witheringia solanacea in Costa Rica, examining growth of self-pollen tubes in pollinations of buds and mature flowers. We also measured levels of RNase production in styles to determine whether enzyme production was correlated with differences in self-rejection. The two small populations contained both self-compatible (SC) individuals and obligate outcrossers (female or SI). Plants in the two large populations were uniformly SI as revealed by pollen tube growth, although several of these individuals sporadically set seed autogamously. Stylar RNase activity did not differ significantly between bud and mature flowers, but self-pollen tube growth did differ, suggesting that a gene product in addition to S-RNase is responsible for developmental onset of SI. Population-level differences in RNase activity were consistent with differences in the strength of the rejection response in bud pollinations, suggesting that a threshold level of S-RNase, in combination with other factors, is necessary for SI. Our results support a growing body of evidence that not only qualitative variation in SI, but also quantitative variation may be functionally significant.     

Identification of a ubiquitin-binding structure in the S-locus F-box protein controlling S-RNase-based self-incompatibility

In flowering plants,self-incompatibility(SI) serves as an important intraspecific reproductive barrier to promote outbreeding.In species from the Solanaceae,Plantaginaceae and Rosaceae,S-RNase and SLF(S-locus F-box) proteins have been shown to control the female and male specificity of SI,respectively.However,little is known about structure features of the SLF protein apart from its conserved F-box domain.Here we show that the SLF C-terminal region possesses a novel ubiquitin-binding domain(UBD) structure conserved among the SLF protein family.By using an ex vivo system of Nicotiana benthamiana,we found that the UBD mediates the SLF protein turnover by the ubiquitin—proteasome pathway.Furthermore,we detected that the SLF protein was directly involved in S-RNase degradation.Taken together,our results provide a novel insight into the SLF structure and highlight a potential role of SLF protein stability and degradation in S-RNase-based self-incompatibility.     

The origin and function of self-incompatibility in flowering plants

Summary The evolutionary significance of self-incompatibility (SI) traditionally has been linked to reduced inbreeding through enforced outcrossing. This view is founded on the premise that outcrossing reduces inbreeding. It is important, when considering the evolutionary significance of any genetic system, to try to distinguish those factors related to the evolution of, from those related to the maintenance of, the system in question. Three factors are considered important for the maintenance of SI: (1) phylogenetic constraint in species descended from SI ancestors, (2) reduced inbreeding in populations, and (3) fitness benefits to individuals resulting from the avoidance of selfing. I suggest that the first two factors should be rejected when considering the origin of SI (whether one or more origins are hypothesized) and that the increase in individual fitness resulting from the avoidance of self-fertilization among individuals that are heterozygous for deleterious alleles may be sufficient to account for the origin of SI. Self-fertilization in plants (except in species that predominantly self-fertilize) generally results in a reduction in fitness of some individuals due to the increased expression of deleterious or lethal recessive alleles, regardless of the degree of inbreeding in the population or the frequency of the allele in question. Inbreeding is a consequence of population structure in many outcrossing plant species. Complex (multi-locus and multi-allelic) systems of SI exist that reduce inbreeding. However, it is argued that these are derived either from simpler systems of SI that may have very little or no effect on inbreeding, in which case any effect on level of inbreeding is secondary, or are not true self-incompatibility systems and are part of a regulatory system that serves to balance the level of inbreeding and outbreeding. Multi-locus and multi-allelic systems of SI and heteromorphic systems of SI are discussed in terms of derived versus ancestral characteristics. A reassessment of the role of breeding systems in the development of a population structure promoting inbreeding is suggested, which may have been of crucial importance in the success and diversification of angiosperms.     

How far are we from unravelling self-incompatibility in grasses?

The genetic and physiological mechanisms involved in limiting self-fertilization in angiosperms, referred to as self-incompatibility (SI), have significant effects on population structure and have potential diversification and evolutionary consequences. Up to now, details of the underlying genetic control and physiological basis of SI have been elucidated in two different gametophytic SI (GSI) systems, the S-RNase SI and the Papaver SI systems, and the sporophytic SI (SSI) system (Brassica). In the grass family (Poaceae), which contains all the cereal and major forage crops, SI has been known for half a century to be controlled gametophytically by two multiallelic and independent loci, S and Z. But still none of the gene products for S and Z is known and only limited information on related biochemical responses is available. Here we compare current knowledge of grass SI with that of other well-characterized SI systems and speculate about the relationship between SSI and grass SI. Additionally, we discuss comparative mapping as a tool for the further investigation of grass SI.     

Increased Phosphorylation of a 26-kD Pollen Protein Is Induced by the Self-Incompatibility Response in Papaver rhoeas

We have investigated whether specific protein phosphorylation events are induced in Papaver rhoeas pollen as a consequence of the self-incompatibility (SI) response. Pollen grown in vitro in the presence of 32P-orthophosphate was challenged with biologically active recombinant S proteins, and pollen proteins were extracted and analyzed. The results provide strong evidence that the increased phosphorylation of a 26-kD protein of pl 6.2, p26, is specifically induced by the SI response. This phosphorylation event occurs in living pollen tubes and was observed specifically when pollen was challenged with S proteins that are incompatible with the S alleles carried by the pollen and not when pollen was challenged with compatible or incompatible heat-denatured S proteins. Further characterization demonstrated that p26 comprises two phosphoproteins, p26.1 and p26.2, that are found in soluble and microsomal fractions, respectively. Increased phosphorylation of p26.1 is implicated in the SI response and appears to be Ca2+ and calmodulin dependent. These data argue for the involvement of a Ca2+-dependent protein kinase requiring calmodulin-like domains, whose activation comprises an intracellular signal mediating the SI response in P. rhoeas pollen.     

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.     

Signal-mediated depolymerization of actin in pollen during the self-incompatibility response

Signal perception and the integration of signals into networks that effect cellular changes is essential for all cells. The self-incompatibility (SI) response in field poppy pollen triggers a Ca(2+)-dependent signaling cascade that results in the inhibition of incompatible pollen. SI also stimulates dramatic alterations in the actin cytoskeleton. By measuring the amount of filamentous (F-) actin in pollen before and during the SI response, we demonstrate that SI stimulates a rapid and large reduction in F-actin level that is sustained for at least 1 h. This represents quantitative evidence for stimulus-mediated depolymerization of F-actin in plant cells by a defined biological stimulus. Surprisingly, there are remarkably few examples of sustained reductions in F-actin levels stimulated by a biologically relevant ligand. Actin depolymerization also was achieved in pollen by treatments that increase cytosolic free Ca(2+) artificially, providing evidence that actin is a target for the Ca(2+) signals triggered by the SI response. By determining the cellular concentrations and binding constants for native profilin from poppy pollen, we show that profilin has Ca(2+)-dependent monomeric actin-sequestering activity. Although profilin is likely to contribute to stimulus-mediated actin depolymerization, our data suggest a role for additional actin binding proteins. We propose that Ca(2+)-mediated depolymerization of F-actin may be a mechanism whereby SI-induced tip growth inhibition is achieved.     

Modes and rates of selfing and associated inbreeding depression in the self-incompatible plant Senecio squalidus (Asteraceae): a successful colonizing species in the British Isles

The strength of the self-incompatibility (SI) response in Senecio squalidus was measured across its British range. Geographic variation in SI was investigated and the extent and inheritance of pseudo-self-compatibility (PSC) and inbreeding depression were determined. Mean self-fruit-set per capitulum was calculated for individuals and sample populations. The heritability of PSC and the magnitude of inbreeding depression were assessed by comparing selfing rates and fitness trait values between SI and PSC parent-progeny lines. SI was found to be strongly expressed in S. squalidus throughout its British range, with only 3.1% of the individuals sampled showing PSC. This PSC had relatively low heritability with stronger expression of SI in selfed progeny relative to PSC parents. Inbreeding depression was shown to be great in S. squalidus, with mean life history stage values ranging from 0.18 to 0.25. The strength of SI in S. squalidus appears not to have weakened in response to its rapid colonization of Britain. The avoidance of inbreeding depression is likely to be the primary factor maintaining strong SI in this successful colonizing species.