Plant self-incompatibility systems: a molecular evolutionary perspective

Incompatibility recognition systems preventing self-fertilization have evolved several times in independent lineages of Angiosperm plants, and three main model systems are well characterized at the molecular level [the gametophytic self-incompatibility (SI) systems of Solanaceae, Rosaceae and Anthirrhinum, the very different system of poppy, and the system in Brassicaceae with sporophytic control of pollen SI reactions]. In two of these systems, the genes encoding both components of pollen-pistil recognition are now known, showing clearly that these two proteins are distinct, that is, SI is a lock-and-key mechanism. Here, we review recent findings in the three well-studied systems in the light of these results and analyse their implications for understanding polymorphism and coevolution of the two SI genes, in the context of a tightly linked genome region.     

The different mechanisms of gametophytic self-incompatibility

Self-incompatibility (SI) involves the recognition and rejection of self or genetically identical pollen. Gametophytic SI is probably the most widespread of the SI systems and, so far, two completely different SI mechanisms, which appear to have evolved separately, have been identified. One mechanism is the RNase system, which is found in the Solanaceae, Rosaceae and Scrophulariaceae. The other is a complex system, so far found only in the Papaveraceae, which involves the triggering of signal transduction cascade(s) that result in rapid pollen tube inhibition and cell death. Here, we present an overview of what is currently known about the mechanisms involved in controlling pollen tube inhibition in these two systems.     

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 flowering plants: the Brassica model.

Self-incompatibility (SI) is a widespread mechanism in flowering plants that prevents self-fertilization. Self-pollen recognition relies on the products of genes located at the S (self-incompatibility) locus. Significant progress towards understanding molecular interactions allowing stigmatic cells to recognize and reject self-pollen in Brassica has been made during the past two years. Thus, the male and female determinants responsible of the self-incompatibility (SI) response have been identified. The structural features of these molecules strongly suggest that SI response is triggered by a ligand-receptor interaction.     

Identification of differentially accumulating pistil proteins associated with self‐incompatibility of non‐heading Chinese cabbage

Non‐heading Chinese cabbage (Brassica campestris L. ssp. chinensis Makino), an important vegetable crop in China, exhibits a typical sporophytic self‐incompatibility (SI) system. To better understand the mechanism of SI response and identify potential candidate proteins involved in the SI system of this vegetable crop, the proteomic approach was taken to identify differential accumulating pistil proteins. Pistils were collected at 0 h and 2 h after self‐pollination at anthesis in self‐incompatible and compatible lines of non‐heading Chinese cabbage, and total proteins were extracted and separated by two‐dimensional gel electrophoresis (2‐DE). A total of 25 protein spots that displayed differential abundance were identified by matrix‐assisted laser desorption/ionisation‐time of flight mass spectrometry (MALDI–TOF/TOF MS) and peptide mass fingerprinting (PMF). Among them, 22 protein spots were confidently established. The mRNA levels of the corresponding genes were detected by quantitative RT‐PCR. The 22 identified protein spots are involved in energy metabolism (four), protein biosynthesis (three), photosynthesis (six), stress response and defence (five), and protein degradation (four). Among these potential candidate proteins, UDP‐sugar pyrophosphorylase could be involved in sucrose degradation to influence pollen germination and growth. Glutathione S–transferases could be involved in pollen maturation, and affect pollen fertility. Senescence‐associated cysteine protease, which is related to programmed cell death, could be mainly related to self pollen recognition of non‐heading Chinese cabbage. The study will contribute to further investigations of molecular mechanism of sporophytic SI in Brassicaceae.     

Petunia Germinating Pollen S/D3 Interacts with S-RNases in Petunia hybrida Vilm.

Self-Incompatibility （SI） Is a genetic mechanism of self/non-self pollen recognition to prevent self-fertilization In many flowering plants and, In most cases, this is controlled by a multl-allellc S-locus. S-RNase and Slocus F box （SLF） proteins have been shown to be the female and male determinants of gametophytlc selfIncompatibility （GSI）, respectively, In the Solanaceae, Scrophulariaceae and Rosaceae. Nevertheless, It is thought that additional factors are required for the SI response. Herein, we constructed a mature anther cDNA library from a self-Incompatible Petunia hybrida Vllm. line of the S3S3 haplotype. Using AhS2-RNase from Antirrhinum hispanicum as a bait for yeast two-hybrid screening, we found that petunia germinating pollen （PGP） S/D3 was capable of Interacting physically with the bait. However, the Interaction lacked haplotype specificity. The PGPS/D3 gene Is a single copy gene that Is expressed In tissues such as the style, ovary, pollen, and leaf. The PGPS/D3：：GFP （green fluorescence protein） construct was detected In both the membrane and cytoplasm. The Implications of these findings In the operation of S-RNase-based SI are discussed.     

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.     

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.     

Compatibility and incompatibility in S-RNase-based systems

Background: S-RNase-based self-incompatibility (SI) occurs in the Solanaceae, Rosaceae and Plantaginaceae. In all three families, compatibility is controlled by a polymorphic S-locus encoding at least two genes. S-RNases determine the specificity of pollen rejection in the pistil, and S-locus F-box proteins fulfill this function in pollen. S-RNases are thought to function as S-specific cytotoxins as well as recognition proteins. Thus, incompatibility results from the cytotoxic activity of S-RNase, while compatible pollen tubes evade S-RNase cytotoxicity. SCOPE: The S-specificity determinants are known, but many questions remain. In this review, the genetics of SI are introduced and the characteristics of S-RNases and pollen F-box proteins are briefly described. A variety of modifier genes also required for SI are also reviewed. Mutations affecting compatibility in pollen are especially important for defining models of compatibility and incompatibility. In Solanaceae, pollen-side mutations causing breakdown in SI have been attributed to the heteroallelic pollen effect, but a mutation in Solanum chacoense may be an exception. This has been interpreted to mean that pollen incompatibility is the default condition unless the S-locus F-box protein confers resistance to S-RNase. In Prunus, however, S-locus F-box protein gene mutations clearly cause compatibility. CONCLUSIONS: Two alternative mechanisms have been proposed to explain compatibility and incompatibility: compatibility is explained either as a result of either degradation of non-self S-RNase or by its compartmentalization so that it does not have access to the pollen tube cytoplasm. These models are not necessarily mutually exclusive, but each makes different predictions about whether pollen compatibility or incompatibility is the default. As more factors required for SI are identified and characterized, it will be possible to determine the role each process plays in S-RNase-based SI.     

Progress towards elucidating the mechanisms of self-incompatibility in the grasses: further insights from studies in Lolium

BACKGROUND AND SCOPE: Self-incompatibility (SI) in flowering plants ensures the maintenance of genetic diversity by ensuring outbreeding. Different genetic and mechanistic systems of SI among flowering plants suggest either multiple origins of SI or considerable evolutionary diversification. In the grasses, SI is based on two loci, S and Z, which are both polyallelic: an incompatible reaction occurs only if both S and Z alleles are matched in individual pollen with alleles of the pistil on which they alight. Such incompatibility is referred to as gametophytic SI (GSI). The mechanics of grass GSI is poorly understood relative to the well-characterized S-RNase-based single-locus GSI systems (Solanaceae, Rosaceae, Plantaginaceae), or the Papaver recognition system that triggers a calcium-dependent signalling network culminating in programmed cell death. There is every reason to suggest that the grass SI system represents yet another mechanism of SI. S and Z loci have been mapped using isozymes to linkage groups C1 and C2 of the Triticeae consensus maps in Secale, Phalaris and Lolium. Recently, in Lolium perenne, in order to finely map and identify S and Z, more closely spaced markers have been developed based on cDNA and repeat DNA sequences, in part from genomic regions syntenic between the grasses. Several genes tightly linked to the S and Z loci were identified, but so far no convincing candidate has emerged. RESEARCH AND PROGRESS: From subtracted Lolium immature stigma cDNA libraries derived from S and Z genotyped individuals enriched for SI potential component genes, kinase enzyme domains, a calmodulin-dependent kinase and a peptide with several calcium (Ca(2+)) binding domains were identified. Preliminary findings suggest that Ca(2+) signalling and phosphorylation may be involved in Lolium GSI. This is supported by the inhibition of Lolium SI by Ca(2+) channel blockers lanthanum (La(3+)) and verapamil, and by findings of increased phosphorylation activity during an SI response.     

Self/non-self discrimination in angiosperm self-incompatibility

Self-incompatibility (SI) in angiosperms prevents inbreeding and promotes outcrossing to generate genetic diversity. In many angiosperms, self/non-self recognition in SI is accomplished by male-specificity and female-specificity determinants (S-determinants), encoded at the S-locus. Recent studies using genetic, molecular biological and biochemical approaches have revealed that angiosperms utilize diverse self/non-self discrimination systems, which can be classified into two fundamentally different systems, self-recognition and non-self recognition systems. The self-recognition system, adopted by Brassicaceae and Papaveraceae, depends on a specific interaction between male and female S-determinants derived from the same S-haplotype. The non-self recognition system, found in Solanaceae, depends on non-self (different S-haplotype)-specific interaction between male and female S-determinants, and the male S-determinant genes are duplicated to recognize diverse non-self female S-determinants.     

The actin cytoskeleton is a target of the self-incompatibility response in Papaver rhoeas

The integration of signals received by a cell, and their transduction to targets, is essential for all cellular responses. The cytoskeleton has been identified as a major target of signalling cascades in both animal and plant cells. Self-incompatibility (SI) in Papaver rhoeas involves an allele-specific recognition between stigmatic S-proteins and pollen, resulting in the inhibition of incompatible pollen. This highly specific response triggers a Ca(2+)-dependent signalling cascade in incompatible pollen when a stigmatic S-protein interacts with it. It has been demonstrated recently that SI induces dramatic alterations in the organization of the pollen actin cytoskeleton. This implicates the actin cytoskeleton as a key target for the SI-stimulated signals. The cytological alterations to the actin cytoskeleton that are triggered in response to SI are described here and there seem to be several stages that are distinguishable temporally. Evidence was obtained that F-actin depolymerization is also stimulated. The current understanding that the actin cytoskeleton is a target for the signals triggered by the SI response is discussed. It is suggested that these F-actin alterations may be Ca(2+)-mediated and that this could be a mechanism whereby SI-induced tip growth inhibition is achieved. The potential for actin-binding proteins to act as key mediators of this response is discussed and the mechanisms that may be responsible for effecting these changes are described. In particular, the parallels between sustained actin rearrangements during SI and in apoptosis of animal cells are considered.     

Neuroimmunology: is the immune system a sense organ?

Through antigenic recognition, the immune system (SI) perceives an external information which is not directly perceptible by the central nervous system (SNC). So, two roles can be assigned SI: immuno-surveillance and information. Since 1980, it was proved that activated lymphocytes and others elements involved in the expression of immune response are able to secrete hormones, neuromediators and peculiar secretions. Moreover, the immune elements possess receptors for SNC secretions. A direct relations between lymphocytes, hypothalamus, hypophysis and endocrine glands has been suggested with implications in the understanding of some mental diseases and conditioning of the immune response. This view of immunoregulation confers a role of mobile brain or sensory organ to the SI.     

Isolation of S-RNase binding proteins from<Emphasis Type="Italic"> Solanum chacoense</Emphasis>: identification of an SBP1 (RING finger protein) orthologue

Currently, the most attractive working model of gametophytic self-incompatibility (SI) involving S-RNases postulates the presence of an inhibitor protein or complex expressed in pollen tubes that would counteract the cytotoxic effect of the ribonuclease activity of the S-RNase. Since it has been previously shown that allele-specific recognition is mediated through the hypervariable domain sequence of the S-RNase, we have targeted this region to isolate pollen-expressed interacting proteins in the yeast two-hybrid system. One of the isolated proteins corresponds to a RING finger protein highly similar to the previously isolated SBP1 protein from Petunia hybrida. This protein is postulated to be part of the RING finger E3 ligase family. The ScSBP1 gene is expressed in almost all tissues tested, suggesting a more general role than only being involved in SI. Although the ScSBP1 gene is polymorphic, linkage analysis showed that it was unlinked to the S-locus. The isolation of this S-RNase-binding protein in two different species and with four different S-RNase sequences as bait, strengthens its putative involvement in the SI response. Furthermore, comparison of the bait sequences used suggests that the SBP1 protein interacts with conserved sequences located between the HVa and HVb domains.Genbank accession numbers: ScSBP1, AY545464     

Plant systems for recognition of pathogen-associated molecular patterns

Research of the last decade has revealed that plant immunity consists of different layers of defense that have evolved by the co-evolutional battle of plants with its pathogens. Particular light has been shed on PAMP- (pathogen-associated molecular pattern) triggered immunity (PTI) mediated by pattern recognition receptors. Striking similarities exist between the plant and animal innate immune system that point for a common optimized mechanism that has evolved independently in both kingdoms. Pattern recognition receptors (PRRs) from both kingdoms consist of leucine-rich repeat receptor complexes that allow recognition of invading pathogens at the cell surface. In plants, PRRs like FLS2 and EFR are controlled by a co-receptor SERK3/BAK1, also a leucine-rich repeat receptor that dimerizes with the PRRs to support their function. Pathogens can inject effector proteins into the plant cells to suppress the immune responses initiated after perception of PAMPs by PRRs via inhibition or degradation of the receptors. Plants have acquired the ability to recognize the presence of some of these effector proteins which leads to a quick and hypersensitive response to arrest and terminate pathogen growth.     

In vivo imaging of the S-locus receptor kinase,the female specificity determinant of self-incompatibility,in transgenic self-incompatible Arabidopsis thaliana

Background and Aims The S-locus receptor kinase (SRK), which is expressed in stigma epidermal cells, is responsible for the recognition and inhibition of ‘self’ pollen in the self-incompatibility (SI) response of the Brassicaceae. The allele-specific interaction of SRK with its cognate pollen coat-localized ligand, the S-locus cysteine-rich (SCR) protein, is thought to trigger a signalling cascade within the stigma epidermal cell that leads to the arrest of ‘self’ pollen at the stigma surface. In addition to the full-length signalling SRK receptor, stigma epidermal cells express two other SRK protein species that lack the kinase domain and whose role in the SI response is not understood: a soluble version of the SRK ectodomain designated eSRK and a membrane-tethered form designated tSRK. The goal of this study was to describe the sub-cellular distribution of the various SRK protein species in stigma epidermal cells as a prelude to visualizing receptor dynamics in response to SCR binding.Methods The Arabidopsis lyrata SRKb variant was tagged with the Citrine variant of yellow fluorescent protein (cYFP) and expressed in A. thaliana plants of the C24 accession, which had been shown to exhibit a robust SI response upon transformation with the SRKb–SCRb gene pair. The transgenes used in this study were designed for differential production and visualization of the three SRK protein species in stigma epidermal cells. Transgenic stigmas were analysed by pollination assays and confocal microscopy.Key Results and Conclusions Pollination assays demonstrated that the cYFP-tagged SRK proteins are functional and that the eSRK is not required for SI. Confocal microscopic analysis of cYFP-tagged SRK proteins in live stigma epidermal cells revealed the differential sub-cellular localization of the three SRK protein species but showed no evidence for redistribution of these proteins subsequent to incompatible pollination.     

The Diversity of Self-Incompatibility Systems in Flowering Plants

Abstract: Flowering plants are the most successful group of land plants and dominate the earth's vegetation with around 300 000 species. This success is, in part, the consequence of a set of unique reproductive innovations that evolved with the flower. Most notable of these innovations were the closed carpel and double fertilization. Closed carpels permitted the evolution of effective mechanisms for pollen selection and discrimination, while double fertilization leading to endosperm formation allowed for more efficient utilization of resources because reserves are only allocated to the seed after fertilization. This review will focus on the most important and best understood mechanism of pollen discrimination, self-incompatibility (SI), a genetically determined pollen recognition system that prevents self-fertilization and fertilization by other individuals with the same incompatibility phenotype. In recent years much progress has been made towards elucidating the molecular mechanisms of SI operating in three distinct SI systems found in the Brassicaceae, Solanaceae and Papaveraceae, respectively. More recent molecular data obtained from the Poaceae, Convolvulaceae and Asteraceae, however, suggest that other molecular mechanisms of SI exist. A survey of classical genetic studies of SI predicts yet further potential molecular mechanisms of SI. We discuss the evolutionary implications of this apparent diversity in molecular pathways leading to SI and stress the need for more molecular studies of different SI systems.     

nhibiting Self-Pollen： Self-Incompatibility in Papaver Involves Integration of Several Signaling Events

Cellular responses rely on signal perception and integration. A nice example of this is self incompatibility （SI）, which is an important mechanism to prevent inbreeding. It prevents self-fertilization by using a highly discriminatory cellular recognition and rejection mechanism. Most Sl systems are genetically specified by the S-locus, which has a pollen and a pistil S-component. A receptor-ligand interaction is used by Papaver rhoeas to control SI. S proteins encoded by the pistil part of the S-locus interact with incompatible pollen to achieve rapid inhibition of tip growth. The incompatible Sl interaction triggers a Ca^2＋-dependent signaling cascade. A number of Sl-specific events are triggered in incompatible pollen, including rapid depolymerization of the actin cytoskeleton; phosphorylation of soluble inorganic pyrophosphatases （SPPases）, Prp26.1; activation of a mitogen activated protein kinase, p56; programmed cell death （PCD） involving a caspase-3-1ike activity. These events contribute to prevent self-fertilizaUon. We are attempting to establish the functional significance of these events, and their possible involvement in integrating a coordinated signaling response. Here we describe the identification of these components shown to be involved in Sl, together with recent progress in identifying links between some of them. These data constitute the first steps in elucidating how SI signaling is integrated.