S-RNase and SLF determine S-haplotype-specific pollen recognition and rejection

A self-compatible pear mutant derived from γ-irradiated pollen carries an 11-Mb duplication in chromosome 17

Self-compatibility is a highly desirable trait for pear breeding programs. Our breeding program previously developed a novel self-compatible pollen-part Japanese pear mutant (Pyrus pyrifolia Nakai), '415-1', by using γ-irradiated pollen. '415-1' carries the S-genotype S4dS5S5, with "d" indicating a duplication of S 5 responsible for breakdown of self-incompatibility. Until now, the size and inheritance of the duplicated segment was undetermined, and a reliable detection method was lacking. Here, we examined genome duplications and their inheritance in 140 F1 seedlings resulting from a cross between '515-20' (S1S3) and '415-1'. Amplicon sequencing of S-RNase and SFBB18 clearly detected S-haplotype duplications in the seedlings. Intriguingly, 30 partially triploid seedlings including genotypes S1S4dS5, S3S4dS5, S1S5dS5, S3S5dS5, and S3S4dS4 were detected among the 140 seedlings. Depth-of-coverage analysis using ddRAD-seq showed that the duplications in those individuals were limited to chromosome 17. Further analysis through resequencing confirmed an 11-Mb chromosome duplication spanning the middle to the end of chromosome 17. The duplicated segment remained consistent in size across generations. The presence of an S3S4dS4 seedling provided evidence for recombination between the duplicated S5 segment and the original S4haplotype, suggesting that the duplicated segment can pair with other parts of chromosome 17. This research provides valuable insights for improving pear breeding programs using partially triploid individuals.

Intraspecific breakdown of self-incompatibility in Physalis acutifolia (Solanaceae)

Variation in mating systems is prevalent throughout angiosperms, with many transitions between outcrossing and selfing above and below the species level. This study documents a new case of an intraspecific breakdown of self-incompatibility in a wild relative of tomatillo, Physalis acutifolia. We used controlled greenhouse crosses to identify self-incompatible (SI) and self-compatible (SC) individuals grown from seed sampled across seven sites across Arizona and New Mexico. We measured 14 flower and fruit traits to test for trait variation associated with mating system. We also quantified pollen tube growth in vivo and tested for the presence of the S-RNase proteins in SI and SC styles. We found that seed from six of the seven sites produced SI individuals that terminated self-pollen tubes in the style and showed detectable S-RNase expression. By contrast, seed from one Arizona site produced SC individuals with no S-RNase expression. These SC individuals displayed typical selfing-syndrome traits such as smaller corollas, reduced stigma-anther distances, and a smaller pollen-ovule ratio. We also found plasticity in self-incompatibility as most of the SI individuals became SC and lost S-RNase expression roughly after 6 months in the greenhouse. While fixed differences in mating systems are known among the SI wild species and the often SC domesticated tomatillos, our study is the first to demonstrate intraspecific variation in natural populations as well as variation in SI over an individual's lifespan.

S-RNase Alleles Associated With Self-Compatibility in the Tomato Clade: Structure, Origins, and Expression Plasticity

The self-incompatibility (SI) system in the Solanaceae is comprised of cytotoxic pistil S-RNases which are countered by S-locus F-box (SLF) resistance factors found in pollen. Under this barrier-resistance architecture, mating system transitions from SI to self-compatibility (SC) typically result from loss-of-function mutations in genes encoding pistil SI factors such as S-RNase. However, the nature of these mutations is often not well characterized. Here we use a combination of S-RNase sequence analysis, transcript profiling, protein expression and reproductive phenotyping to better understand different mechanisms that result in loss of S-RNase function. Our analysis focuses on 12 S-RNase alleles identified in SC species and populations across the tomato clade. In six cases, the reason for gene dysfunction due to mutations is evident. The six other alleles potentially encode functional S-RNase proteins but are typically transcriptionally silenced. We identified three S-RNase alleles which are transcriptionally silenced under some conditions but actively expressed in others. In one case, expression of the S-RNase is associated with SI. In another case, S-RNase expression does not lead to SI, but instead confers a reproductive barrier against pollen tubes from other tomato species. In the third case, expression of S-RNase does not affect self, interspecific or inter-population reproductive barriers. Our results indicate that S-RNase expression is more dynamic than previously thought, and that changes in expression can impact different reproductive barriers within or between natural populations.

The apple MdPTI1L kinase is phosphorylated by MdOXI1 during S-RNase-induced reactive oxygen species signaling in pollen tubes

Apple (Malus domestica) exhibits classic S-RNase-mediated gametophytic self-incompatibility. Previous studies have shown that the S-RNase secreted from style cells could trigger signal transduction and defense responses mediated by Ca²⁺ and reactive oxygen species (ROS) after entering into the pollen tube. In this study, we investigated the downstream genes activated by ROS during S-RNase-mediated gametophytic self-incompatibility in pollen tubes. A substantial increase in ROS, as well as up-regulated expression of a serine-threonine protein kinase gene, OXIDATIVE SIGNAL-INDUCIBLE1 (MdOXI1), was detected in apple pollen tubes treated with self-S-RNase. A kinase assay-linked phosphoproteomics (KALIP) analysis suggested that MdOXI1 could bind and phosphorylate the downstream protein kinase Pto-interacting protein 1-like (MdPTI1L). The phosphorylation level of MdPTI1L was significantly reduced after silencing MdOXI1 with antisense oligonucleotides in the pollen tube. Silencing of either MdOXI1 or MdPTI1L alleviated the inhibitory effect of self-S-RNase on pollen tube growth. Our results thus indicate that MdPTI1L is phosphorylated by MdOXI1 in the pollen tube and participates in the ROS signaling pathway triggered by S-RNase.

A gamma-thionin protein from apple, MdD1, is required for defence against S-RNase-induced inhibition of pollen tube prior to self/non-self recognition

Apple exhibits S-RNase-mediated self-incompatibility. Although the cytotoxic effect of S-RNase inside the self-pollen tube has been studied extensively, the underlying defence mechanism in pollen tube in Rosaceae remains unclear. On exposure to stylar S-RNase, plant defence responses are activated in the pollen tube; however, how these are regulated is currently poorly understood. Here, we show that entry of both self and non-self S-RNase into pollen tubes of apple (Malus domestica) stimulates jasmonic acid (JA) production, in turn inducing the accumulation of MdMYC2 transcripts, a transcription factor in the JA signalling pathway widely considered to be involved in plant defence processes. MdMYC2 acts as a positive regulator in the pollen tube activating expression of MdD1, a gene encoding a defence protein. Importantly, MdD1 was shown to bind to the RNase activity sites of S-RNase leading to inhibition of enzymatic activity. This work provides intriguing insights into an ancient defence mechanism present in apple pollen tubes where MdD1 likely acts as a primary line of defence to inhibit S-RNase cytotoxicity prior to self/non-self recognition.

Apple S-RNase triggers inhibition of tRNA aminoacylation by interacting with a soluble inorganic pyrophosphatase in growing self-pollen tubes in vitro

Apple exhibits S-RNase-based self-incompatibility (SI), in which S-RNase plays a central role in rejecting self-pollen. It has been proposed that the arrest of pollen growth in SI of Solanaceae plants is a consequence of the degradation of pollen rRNA by S-RNase; however, the underlying mechanism in Rosaceae is still unclear. Here, we used S₂ -RNase as a bait to screen an apple pollen cDNA library and characterized an apple soluble inorganic pyrophosphatase (MdPPa) that physically interacted with S-RNases. When treated with self S-RNases, apple pollen tubes showed a marked growth inhibition, as well as a decrease in endogenous soluble pyrophosphatase activity and elevated levels of inorganic pyrophosphate (PPi). In addition, S-RNase was found to bind to two variable regions of MdPPa, resulting in a noncompetitive inhibition of its activity. Silencing of MdPPa expression led to a reduction in pollen tube growth. Interestingly, tRNA aminoacylation was inhibited in self S-RNase-treated or MdPPa-silenced pollen tubes, resulting in the accumulation of uncharged tRNA. Furthermore, we provide evidence showing that this disturbance of tRNA aminoacylation is independent of RNase activity. We propose an alternative mechanism differing from RNA degradation to explain the cytotoxicity of the S-RNase apple SI process.

Reproduction in woody perennial Citrus: an update on nucellar embryony and self-incompatibility

Review on citrus reproduction. Citrus is one of the most important and widely grown fruit crops. It possesses several special reproductive characteristics, such as nucellar embryony and self-incompatibility. The special phenomenon of nucellar embryony in citrus, also known as the polyembryony, is a kind of sporophytic apomixis. During the past decade, the emergence of novel technologies and the construction of multiple citrus reference genomes have facilitated rapid advances to our understanding of nucellar embryony. Indeed, several research teams have preliminarily determined the genetic basis of citrus apomixis. On the other hand, the phenomenon of self-incompatibility that promotes genetic diversity by rejecting self-pollen and accepting non-self-pollen is difficult to study in citrus because the long juvenile period of citrus presents challenges to identifying candidate genes that control this phenomenon. In this review, we focus on advances to our understanding of reproduction in citrus from the last decade and discuss priorities for the coming decade.

Transcriptome and phytohormone analysis reveals a comprehensive phytohormone and pathogen defence response in pear self-/cross-pollination

Candidate genes were identified and the role of phytohormones such as JA-Me and ABA in the synthesis of S-RNase was emphasized in pear self-incompatibility. Self-incompatibility (SI) occurs widely in flowering plants as an intraspecific reproductive barrier. This phenomenon promotes variation within species, but for some species such as Pyrus, SI is a nuisance rather than a benefit in agricultural production. Although many studies have been conducted on SI in pears, its mechanism remains unclear. In this study, high-throughput Illumina RNA sequencing (RNA-seq) was used to identify SI-related genes in pear styles. Using transcriptome comparisons, differentially expressed genes of unpollinated (UP), cross-pollinated (CP), and self-pollinated (SP) styles were identified after 48 h. A total of 1796 and 1890 genes were identified in DSC (UP vs. CP) and DSI (UP vs. SP), respectively. KEGG analysis revealed that genes involved in the "plant hormone signal transduction pathway" and "plant-pathogen interaction pathway" were significantly enriched in DSI (UP vs. SP) compared to those in DSC (UP vs. CP). The expression level of S-glycoprotein ribonuclease (S-RNase) was dramatically reduced in cross-pollinated (CP) styles. To better understand the relationship between the expression patterns of S-RNase and two major KEGG pathways, the concentrations of phytohormones were measured, and the expression pattern of S-RNase was analysed using qRT-PCR. Our results demonstrate that methyl jasmonate and abscisic acid may enhance the expression level of S-RNase, and pollination can affect the synthesis of methyl jasmonate and abscisic acid in pear styles. Overall, this study is a global transcriptome analysis of SI in pear. A relationship between self-rejection, plant hormones, and pathogen defence was shown in pear.

Molecular and genetic characterization of a self-compatible apple cultivar, 'CAU-1'

In this study, we characterized a naturally occurring self-compatible apple cultivar, 'CAU-1' (S₁S₉), and studied the underlying mechanism that causes its compatibility. Analyses of both fruit set rate and seed number after self-pollination or cross-pollination with 'Fuji' (S₁S₉), and of pollen tube growth, demonstrated that 'CAU-1' is self-compatible. Genetic analysis by S-RNase PCR-typing of selfed progeny of 'CAU-1' revealed the presence of all progeny classes (S₁S₁, S₁S₉, and S₉S₉). Moreover, no evidence of S-allele duplication was found. These findings support the hypothesis that loss of function of an S-locus unlinked pollen-part mutation (PPM) expressed in pollen, rather than a natural mutation in the pollen-S gene (S₁- and S₉- haplotype), leads to SI breakdown in 'CAU-1'. In addition, there were no significant differences in pollen morphology or fertility between 'Fuji' and 'CAU-1'. However, we found that the effect of S₁- and S₉-RNase on the SI behavior of pollen could not be addressed better in 'CAU-1' than in 'Fuji'. Furthermore, we found that a pollen-expressed hexose transporter, MdHT1, interacted with S-RNases and showed significantly less expression in 'CAU-1' than in 'Fuji' pollen tubes. These findings support the hypothesis that MdHT1 may participate in S-RNase internalization during the SI process, and decrease of MdHT1 expression in 'CAU-1' hindered the release of self S-RNase into the cytoplasm of pollen tubes, thereby protecting pollen from the cytotoxicity of S-RNase, finally probably resulting in self-compatibility. Together, these findings indicate that S-locus external factors are required for gametophytic SI in the Rosaceae subtribe Pyrinae.

Testing the SI × SC rule: Pollen-pistil interactions in interspecific crosses between members of the tomato clade (Solanum section Lycopersicon, Solanaceae)

PREMISE OF THE STUDY

Interspecific reproductive barriers (IRBs) act to ensure species integrity by preventing hybridization. Previous studies on interspecific crosses in the tomato clade have focused on the success of fruit and seed set. The SI × SC rule (SI species × SC species crosses are incompatible, but the reciprocal crosses are compatible) often applies to interspecific crosses. Because SI systems in the Solanaceae affect pollen tube growth, we focused on this process in a comprehensive study of interspecific crosses in the tomato clade to test whether the SI × SC rule was always followed.

METHODS

Pollen tube growth was assessed in reciprocal crosses between all 13 species of the tomato clade using fluorescence microscopy.

KEY RESULTS

In crosses between SC and SI species, pollen tube growth follows the SI × SC rule: interspecific pollen tube rejection occurs when SI species are pollinated by SC species, but in the reciprocal crosses (SC × SI), pollen tubes reach ovaries. However, pollen tube rejection occurred in some crosses between pairs of SC species, demonstrating that a fully functional SI system is not necessary for pollen tube rejection in interspecific crosses. Further, gradations in the strength of both pistil and pollen IRBs were revealed in interspecific crosses using SC populations of generally SI species.

CONCLUSION

The SI × SC rule explains many of the compatibility relations in the tomato clade, but exceptions occur with more recently evolved SC species and accessions, revealing differences in strength of both pistil and pollen IRBs.

Strength, diversity and plasticity of postmating reproductive barriers between two hybridizing oak species (Quercus robur L. and Quercus petraea (Matt) Liebl.)

Very little is known about the nature and strength of reproductive isolation (RI) in Quercus species, despite extensive research on the estimation and evolutionary significance of hybridization rates. We characterized postmating pre- and postzygotic RI between two hybridizing oak species, Quercus robur and Quercus petraea, using a large set of controlled crosses between different genotypes. Various traits potentially associated with reproductive barriers were quantified at several life history stages, from pollen-pistil interactions to seed set and progeny fitness-related traits. Results indicate strong intrinsic postmating prezygotic barriers, with significant barriers also at the postzygotic level, but relatively weaker extrinsic barriers on early hybrid fitness measures assessed in controlled conditions. Using general linear modelling of common garden data with clonal replicates, we showed that most traits exhibited important genotypic differences, as well as different levels of sensitivity to micro-environmental heterogeneity. These new findings suggest a large potential genetic diversity and plasticity of reproductive barriers and are confronted with hybridization evidence in these oak species.

Characterization and mapping of non-S gametophytic self-compatibility in sweet cherry (Prunus avium L.)

Self-incompatibility in Prunus (Rosaceae) species, such as sweet cherry, is controlled by a multiallelic locus (S), in which two tightly linked genes, S-RNase and SFB (S haplotype-specific F-box), determine the specificity of the pollen and the style. Fertilization in these species occurs only if the S-specificities expressed in the pollen and the pistils are different. However, modifier genes have been proposed to be necessary for a full manifestation of the self-incompatibility response. 'Cristobalina' is a spontaneous self-compatible sweet cherry cultivar that originated in Eastern Spain. Previous studies with this genotype suggested that pollen modifier gene(s), not linked to the S-locus, may be the cause of self-incompatibility breakdown. In this work, an F(1) population from 'Cristobalina' that segregates for this trait was used to identify molecular markers linked to self-compatibility by bulked segregant analysis. One simple sequence repeat (SSR) locus (EMPaS02) was found to be linked to self-compatibility in this population at 3.2 cM. Two additional populations derived from 'Cristobalina' were used to confirm the linkage of this marker to self-compatibility. Since EMPaS02 has been mapped to the sweet cherry linkage group 3, other markers located on the same linkage group were analysed in these populations to confirm the location of the self-compatibility locus.

Functional gametophytic self-incompatibility in a peripheral population of Solanum peruvianum (Solanaceae)

The transition from self-incompatibility to self-compatibility is a common transition in angiosperms often reported in populations at the edge of species range limits. Geographically distinct populations of wild tomato species (Solanum section Lycopersicon (Solanaceae)) have been described as polymorphic for mating system with both self-incompatible and self-compatible populations. Using controlled pollinations and sequencing of the S-RNase mating system gene, we test the compatibility status of a population of S. peruvianum located near its southern range limit. Pollinations among plants of known genotypes revealed strong self-incompatibility; fruit set following compatible pollinations was significantly higher than following incompatible pollinations for all tested individuals. Sequencing of the S-RNase gene in parents and progeny arrays was also as predicted under self-incompatibility. Molecular variation at the S-RNase locus revealed a diverse set of alleles, and heterozygosity in over 500 genotyped individuals. We used controlled crosses to test the specificity of sequences recovered in this study; in all cases, results were consistent with a unique allelic specificity for each tested sequence, including two alleles sharing 92% amino-acid similarity. Site-specific patterns of selection at the S-RNase gene indicate positive selection in regions of the gene associated with allelic specificity determination and purifying selection in previously characterized conserved regions. Further, there is broad convergence between the present and previous studies in specific amino-acid positions inferred to be evolving under positive selection.

Evolutionary biology: the origins of two sexes

Evidence is accumulating that in the green algae the evolution of female and male gametes differing in size--anisogamy--involves genes linked to the mating-type locus, as was predicted theoretically.

Multiple features that distinguish unilateral incongruity and self-incompatibility in the tomato clade

Wild tomato species in Solanum Section Lycopersicon often exhibit two types of reproductive barriers: self-incompatibility (SI) and unilateral incompatibility or incongruity (UI), wherein the success of an inter-specific cross depends on the direction of the cross. UI pollen rejection often follows the 'SI × SC' rule, i.e. pistils of SI species reject the pollen of SC (self-compatible) species but not vice versa, suggesting that the SI and UI pollen rejection mechanisms may overlap. In order to address this question, pollen tube growth was measured after inter-specific crosses using wild tomato species as the female parents and pollen from cultivated tomato (Solanum lycopersicum). Two modes of UI pollen rejection, early and late, were observed, and both differed from SI pollen rejection. The structure and expression of known stylar SI genes were evaluated. We found that S-RNase expression is not required for either the early or late mode of UI pollen rejection. However, two HT family genes, HT-A and HT-B, map to a UI QTL. Surprisingly, we found that a gene previously implicated in SI, HT-B, is mutated in both SI and SC S. habrochaites accessions, and no HT-B protein could be detected. HT-A genes were detected and expressed in all species examined, and may therefore function in both SI and UI. We conclude that there are significant differences between SI and UI in the tomato clade, in that pollen tube growth differs between these two rejection systems, and some stylar SI factors, including S-RNase and HT-B, are not required for UI.

Genetic map-based location of the red clover (Trifolium pratense L.) gametophytic self-incompatibility locus

Red clover is a hermaphroditic allogamous diploid (2n = 2x = 14) with a homomorphic gametophytic self-incompatibility (GSI) system (Trifolium pratense L.). Red clover GSI has long been studied, and it is thought that the genetic control of GSI constitutes a single locus. Although GSI genes have been identified in other species, the genomic location of the red clover GSI-locus remains unknown. The objective of this study was to use a mapping-based approach to identify simple sequence repeats (SSR) that were closely linked to the GSI-locus. Previously published SSR markers were used in this effort (Sato et al. in DNA Res 12:301-364, 2005). A bi-parental cross was initiated in which the parents were known to have one self-incompatibility allele (S-allele) in common. S-allele genotypes of 100 progeny were determined through test crosses and pollen compatibility. Pseudo F(1) linkage analysis isolated the GSI-locus on red clover linkage-group one within 2.5 cM of markers RCS5615, RCS0810, and RCS3161. A second 256 progeny mapping testcross population of a heterozygous self-compatible mutant revealed that this specific self-compatible mutant mapped to the same location as the GSI-locus. Finally, 82 genotypes were identified whose parents putatively shared one S-allele in common from maternal halfsib families derived from two random mating populations in which paternal identity was determined using molecular markers. Unique S-allele identity in the two random mating populations was tentatively inferred based on haplotypes of two highly allelic linkage-group one SSR (RCS0810 and RCS4956), which were closely linked to each other and the GSI-locus. Paternally derived pollen haplotype linkage analysis of RCS0810 and RCS4956 SSR and the GSI-locus again revealed tight linkage at 2.5 and 4.7 cM between the GSI-locus and RCS0810 and RCS4956, respectively. The map-based location of the GSI-locus in red clover has many immediate applications to red clover plant breeding and could be useful in helping to sequence the GSI-locus.

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.

Sex determination in honeybees: two separate mechanisms induce and maintain the female pathway

Sex determination in honeybees is realized by the csd and the fem gene that establish and maintain, throughout development, sexual fates via the control of alternative splicing.

Organisms have evolved a bewildering diversity of mechanisms to generate the two sexes. The honeybee (Apis mellifera) employs an interesting system in which sex is determined by heterozygosity at a single locus (the Sex Determination Locus) harbouring the complementary sex determiner (csd) gene. Bees heterozygous at Sex Determination Locus are females, whereas bees homozygous or hemizygous are males. Little is known, however, about the regulation that links sex determination to sexual differentiation. To investigate the control of sexual development in honeybees, we analyzed the functions and the regulatory interactions of genes involved in the sex determination pathway. We show that heterozygous csd is only required to induce the female pathway, while the feminizer (fem) gene maintains this decision throughout development. By RNAi induced knockdown we show that the fem gene is essential for entire female development and that the csd gene exclusively processes the heterozygous state. Fem activity is also required to maintain the female determined pathway throughout development, which we show by mosaic structures in fem-repressed intersexuals. We use expression of Fem protein in males to demonstrate that the female maintenance mechanism is controlled by a positive feedback splicing loop in which Fem proteins mediate their own synthesis by directing female fem mRNA splicing. The csd gene is only necessary to induce this positive feedback loop in early embryogenesis by directing splicing of fem mRNAs. Finally, fem also controls the splicing of Am-doublesex transcripts encoding conserved male- and female-specific transcription factors involved in sexual differentiation. Our findings reveal how the sex determination process is realized in honeybees differing from Drosophila melanogaster.

Sexual differentiation is a fundamental process in the animal kingdom, and different species have evolved a bewildering diversity of mechanisms to generate the two sexes in the proper proportions. Sex determination in honeybees (Apis mellifera) provides an interesting and unusual system to study, as it is governed by heterozygosity of a single locus harbouring the complementary sex determiner gene (csd), in contrast to the well-studied sex chromosome system of Drosophila melanogaster. We show that the female sex determination pathway is exclusively induced by the csd gene in early embryogenesis. Later on and throughout development this inductive signal is maintained via a positive feedback loop of the feminizer (fem) gene, in which the Fem protein mediates its own synthesis. The findings reveal how the sex determination process in honeybees is realized by the regulation and function of two genes differing from Drosophila.

Darwin's foundation for investigating self-incompatibility and the progress toward a physiological model for S-RNase-based SI

Charles Darwin made extensive observations of the pollination biology of a wide variety of plants. He carefully documented the consequences of self-pollination and described species that were self-sterile but that could easily be crossed with other plants of the same species. He believed that compatibility was controlled by the 'mutual action' of pollen and pistil contents. A genetic model for self-sterility was developed in the early 1900 s based on studies of the compatibility relationships among, what are now referred to as, self-incompatible (SI) Nicotiana species. Today, it is believed that SI in these species is controlled by an interaction between S-RNases produced in the pistil and F-box proteins expressed in pollen and, moreover, that this S-RNase-based SI system is shared by a great diversity of other plant species. Current research is aimed at understanding how the mutual actions of these S-gene products function in the physiological context of pollen tube growth.

RNase-based self-incompatibility: puzzled by pollen S

Many plants have a genetically determined self-incompatibility system in which the rejection of self pollen grains is controlled by alleles of an S locus. A common feature of these S loci is that separate pollen- and style-expressed genes (pollen S and style S, respectively) determine S allele identity. The long-held view has been that pollen S and style S must be a coevolving gene pair in order for allelic recognition to be maintained as new S alleles arise. In at least three plant families, the Solanaceae, Rosaceae, and Plantaginaceae, the style S gene has long been known to encode an extracellular ribonuclease called the S-RNase. Pollen S in these families has more recently been identified and encodes an F-box protein known as either SLF or SFB. In this perspective, we describe the puzzling evolutionary relationship that exists between the SLF/SFB and S-RNase genes and show that in most cases cognate pairs of genes are not coevolving in the expected manner. Because some pollen S genes appear to have arisen much more recently than their style S cognates, we conclude that either some pollen S genes have been falsely identified or that there is a major problem with our understanding of how the S locus evolves.

New microsome-associated HT-family proteins from Nicotiana respond to pollination and define an HT/NOD-24 protein family

HT-family proteins have been identified in Nicotiana, Solanum, and Petunia. HT-B-type proteins are implicated in S-RNase-based self-incompatibility, but the functions of other family members are unknown. Screening for cDNA sequences with an expression pattern similar to HT-B in Nicotiana alata revealed a new group of small HT-family proteins, designated HT-M. HT-M proteins resemble HT-B in several respects: their pistil-specific expression pattern is indistinguishable from HT-B, they pellet with a microsome fraction, and their abundance decreases after pollination. Unlike HT-B, there is no S-specificity to this response, and RNAi experiments show that HT-M proteins are not necessary for self-incompatibility. Identification of a third group of pistil-specific HT-family proteins helps better define the characteristics of the family and allowed identification of putative new family members. By searching the databases with only the most conserved HT-family sequence elements, the signal sequence and cysteine motifs, we identified nodulin-24-like proteins and several small glycine-rich proteins as putative HT-family members. Like HT-M and HT-B, nodulin-24 is membrane associated. We propose that the conserved features in HT-family proteins are important for targeting or modification and refer to the broader family that includes both HT- and nodulin-24-like proteins as the HT/NOD-24-family.

Expression of 10 S-class SLF-like genes in Nicotiana alata pollen and its implications for understanding the pollen factor of the S locus

The S locus of Nicotiana alata encodes a polymorphic series of ribonucleases (S-RNases) that determine the self-incompatibility (SI) phenotype of the style. The pollen product of the S locus (pollen S) in N. alata is unknown, but in species from the related genus Petunia and in self-incompatible members of the Plantaginaceae and Rosaceae, this function has been assigned to an F-box protein known as SLF or SFB. Here we describe the identification of 10 genes (designated DD1-10) encoding SLF-related proteins that are expressed in N. alata pollen. Because our approach to cloning the DD genes was based on sequences of SLFs from other species, we presume that one of the DD genes encodes the N. alata SLF ortholog. Seven of the DD genes were exclusively expressed in pollen and a low level of sequence variation was found in alleles of each DD gene. Mapping studies confirmed that all 10 DD genes were linked to the S locus and that at least three were located in the same chromosomal segment as pollen S. Finally, the different topologies of the phylogenetic trees produced using available SLF-related sequences and those produced using S-RNase sequences suggests that pollen S and the S-RNase have different evolutionary histories.

Primary molecular features of self-incompatible and self-compatible F(1) seedling from apricot (Prunus armeniaca L.) Katy x Xinshiji

Expression of the S-RNase genes in the self-compatible (SC) apricot (Prunus armeniaca L.) cultivar Katy, the self-incompatible (SI) cultivar Xinshiji and their F(1) seedling was examined in this study. Three S-genotypes, S(9)Sc (Sc, self-compatibility S-gene absent from the style), S(8)S(9), and S(8)S(10), were obtained. Seedlings with S-RNase that migrated as a single band in gel electrophoresis were SC, despite high transcript abundance, and those with S-RNase that migrated as two bands were SI with high transcript abundance or SC with low transcript expression. S(8)-RNase was induced in SI cultivars only 24 h after self-pollination, indicating post-transcriptional regulation of S(8)-RNase in SI apricots. A Proteomic study showed that 35 protein spots were synthesized differently between SC and SI pistils. Fifteen of the 35 protein spots were identified; nine proteins, including receptor protein kinase-like protein, reversibly glycosylated polypeptide-2, and isoflavone reductase-like protein, were detected only in the SC pistils; while nine proteins, including actin 7, a putative serine/threonine kinase, and S-RNase, were detected only in the SI pistils. A mitochondrial NAD-dependent malate dehydrogenase and a probable elongation factor G were up-regulated, while heat shock cognate 70 was down-regulated in the SC pistils compared to those in the SI pistils. The results suggest that the proteins responsible for self-compatibility and self-incompatibility may be different.

AXL and AXR1 have redundant functions in RUB conjugation and growth and development in Arabidopsis

Cullin-RING ubiquitin-protein ligases such as the Skp1, cullin, F-box protein (SCF) have been implicated in many growth and developmental processes in plants. Normal SCF function requires that the CUL1 subunit be post-translationally modified by related to ubiquitin (RUB), a protein related to ubiquitin. This process is mediated by two enzymes: the RUB-activating and RUB-conjugating enzymes. In Arabidopsis, the RUB-activating enzyme is a heterodimer consisting of AXR1 and ECR1. Mutations in the AXR1 gene result in a pleiotropic phenotype that includes resistance to the plant hormone auxin. Here we report that the AXL (AXR1-like) gene also functions in the RUB conjugation pathway. Overexpression of AXL in the axr1-3 background complements the axr1-3 phenotype. Biochemical analysis indicates that AXL overexpression restores CUL1 modification to the wild-type level, indicating that AXR1 and AXL have the same biochemical activity. Although the axl mutant resembles wild-type plants, the majority of axr1 axl-1 double mutants are embryo or seedling lethal. Furthermore, the axl-1 mutation reveals novel RUB-dependent processes in embryo development. We conclude that AXR1 and AXL function redundantly in the RUB conjugating pathway.

Polymorphism of SFBB-gamma and its use for S genotyping in Japanese pear (Pyrus pyrifolia)

Japanese pear (Pyrus pyrifolia) exhibits the S-RNase-based gametophytic self-incompatibility where the pollen-part determinant, pollen S, had long remained elusive. Recent identification of S locus F-box brothers (SFBB) in Japanese pear and apple suggested that the multiple F-box genes are the pollen S candidates as they exhibited pollen specific expression, S haplotype-specific polymorphisms and linkage to the S locus. In Japanese pear, three SFBBs were identified from a single S haplotype, and they were more homologous to other haplotype genes of the same group (i.e., alpha-, beta- and gamma-groups). In this study, we isolated new seven PpSFBB(-gamma) genes from different S genotypes of Japanese pear. These genes showed S haplotype-specific polymorphisms, however, sequence similarities among them were very high. Based on the sequence polymorphisms of the PpSFBB(-gamma) genes, we developed a CAPS/dCAPS system for S genotyping of the Japanese pear cultivars. This new S genotyping system was found to not only be able to discriminate the S(1)-S(9), but also be suitable for identification of the mutant S(4sm) haplotype for the breeding of self-compatible cultivars, and detection of new S haplotypes such as S(k).

Self-incompatibility of Prunus tenella and evidence that reproductively isolated species of Prunus have different SFB alleles coupled with an identical S-RNase allele

Many species of Prunus display an S-RNase-based gametophytic self-incompatibility (SI), controlled by a single highly polymorphic multigene complex termed the S-locus. This comprises tightly linked stylar- and pollen-expressed genes that determine the specificity of the SI response. We investigated SI of Prunus tenella, a wild species found in small, isolated populations on the Balkan peninsula, initially by pollination experiments and identifying stylar-expressed RNase alleles. Nine P. tenella S-RNase alleles (S(1)-S(9)) were cloned; their sequence analysis showed a very high ratio of non-synonymous to synonymous nucleotide substitutions (K(a)/K(s)) and revealed that S-RNase alleles of P. tenella, unlike those of Prunus dulcis, show positive selection in all regions except the conserved regions and that between C2 and RHV. Remarkably, S(8)-RNase, was found to be identical to S(1)-RNase from Prunus avium, a species that does not interbreed with P. tenella and, except for just one amino acid, to S(11) of P. dulcis. However, the corresponding introns and S-RNase-SFB intergenic regions showed considerable differences. Moreover, protein sequences of the pollen-expressed SFB alleles were not identical, harbouring 12 amino-acid replacements between those of P. tenella SFB(8) and P. avium SFB(1). Implications of this finding for hypotheses about the evolution of new S-specificities are discussed.

New views of S-RNase-based self-incompatibility

S-RNase-based self-incompatibility (SI) is the most widespread form of genetically controlled mate selection in plants. S-RNase controls pollination specificity in the pistil, while the newly discovered SLF/SFB controls pollination specificity in the pollen. A widely discussed model suggests that compatibility is explained by ubiquitylation and degradation of nonself-S-RNase and that, conversely, incompatibility is caused by failure to degrade self-S-RNase. This model is consistent with the long-standing view that S-RNase inhibition is central to SI. Recent results show, however, that S-RNase is compartmentalized in pollen tubes and, significantly, that compatibility might not require SLF/SFB. S-RNase compartmentalization and dislocation into the pollen tube cytoplasm might be similar to the trafficking of other cytotoxins such as ricin.

Identification and characterization of components of a putative petunia S-locus F-box-containing E3 ligase complex involved in S-RNase-based self-incompatibility

Petunia inflata S-locus F-box (Pi SLF) is thought to function as a typical F-box protein in ubiquitin-mediated protein degradation and, along with Skp1, Cullin-1, and Rbx1, could compose an SCF complex mediating the degradation of nonself S-RNase but not self S-RNase. We isolated three P. inflata Skp1s (Pi SK1, -2, and -3), two Cullin-1s (Pi CUL1-C and -G), and an Rbx1 (Pi RBX1) cDNAs and found that Pi CUL1-G did not interact with Pi RBX1 and that none of the three Pi SKs interacted with Pi SLF(2). We also isolated a RING-HC protein, S-RNase Binding Protein1 (Pi SBP1), almost identical to Petunia hybrida SBP1, which interacts with Pi SLFs, S-RNases, Pi CUL1-G, and an E2 ubiquitin-conjugating enzyme, suggesting that Pi CUL1-G, SBP1, and SLF may be components of a novel E3 ligase complex, with Pi SBP1 playing the roles of Skp1 and Rbx1. S-RNases interact more with nonself Pi SLFs than with self Pi SLFs, and Pi SLFs also interact more with nonself S-RNases than with self S-RNases. Bacterially expressed S(1)-, S(2)-, and S(3)-RNases are degraded by the 26S proteasomal pathway in a cell-free system, albeit not in an S-allele-specific manner. Native glycosylated S(3)-RNase is not degraded to any significant extent; however, deglycosylated S(3)-RNase is degraded as efficiently as the bacterially expressed S-RNases. Finally, S-RNases are ubiquitinated in pollen tube extracts, but whether this is mediated by the Pi SLF-containing E3 complex is unknown.

Quality control in self/nonself discrimination

Unicellular eukaryotes primarily employ self/nonself discrimination to avoid self-mating, whereas multicellular organisms also use self/nonself discrimination in immune defense. Recent advances in understanding self/nonself discrimination in eukaryotes shed new light on the emergence of the most sophisticated self/nonself discrimination system known, the antigen receptors employed in the adaptive immune system.

Gametophytic self-incompatibility: understanding the cellular mechanisms involved in "self" pollen tube inhibition

Self-incompatibility (SI) prevents the production of "self" seed and inbreeding by providing a recognition and rejection system for "self," or genetically identical, pollen. Studies of gametophytic SI (GSI) species at a molecular level have identified two completely different S-genes and SI mechanisms. One GSI mechanism, which is found in the Solanaceae, Rosaceae and Scrophulariaceae, has S-RNase as the pistil S-component and an F-box protein as the pollen S-component. However, non-S-locus factors are also required. In an incompatible situation, the S-RNases degrade pollen RNA, thereby preventing pollen tube growth. Here, in the light of recent evidence, we examine alternative models for how compatible pollen escapes this cytotoxic activity. The other GSI mechanism, so far found only in the Papaveraceae, has a small secreted peptide, the S-protein, as its pistil S-component. The pollen S-component remains elusive, but it is thought to be a transmembrane receptor, as interaction of the S-protein with incompatible pollen triggers a signaling network, resulting in rapid actin depolymerization and pollen tube inhibition and programmed cell death (PCD). Here, we present an overview of what is currently known about the mechanisms involved in regulating pollen tube inhibition in these two GSI systems.

AhSSK1, a novel SKP1-like protein that interacts with the S-locus F-box protein SLF

The S-locus F-box (SLF/SFB) protein, recently identified as the pollen determinant of S-RNase-based self-incompatibility (SI) in Solanaceae, Scrophulariaceae and Rosaceae, has been proposed to serve as the subunit of an SCF (SKP1-CUL1-F-box) ubiquitin ligase and to target its pistil counterpart S-RNase during the SI response. However, the underlying mechanism is still in dispute, and the putative SLF-binding SKP1-equivalent protein remains unknown. Here, we report the identification of AhSSK1, Antirrhinum hispanicumSLF-interacting SKP1-like1, using a yeast two-hybrid screen against a pollen cDNA library. GST pull-down assays confirmed the SSK1-SLF interaction, and showed that AhSSK1 could connect AhSLF to a CUL1-like protein. AhSSK1, despite having a similar secondary structure to other SKP1-like proteins, appeared quite distinctive in sequence and unique in a phylogenetic analysis, in which no SSK1 ortholog could be predicted in the sequenced genomes of Arabidopsis and rice. Thus, our results suggest that the pollen-specific SSK1 could be recruited exclusively as the adaptor of putative SCF(SLF) in those plants with S-RNase-based SI, providing an important clue to dissecting the function of the pollen determinant.

Compartmentalization of S-RNase and HT-B degradation in self-incompatible Nicotiana

Pollen-pistil interactions are crucial for controlling plant mating. For example, S-RNase-based self-incompatibility prevents inbreeding in diverse angiosperm species. S-RNases are thought to function as specific cytotoxins that inhibit pollen that has an S-haplotype that matches one of those in the pistil. Thus, pollen and pistil factors interact to prevent mating between closely related individuals. Other pistil factors, such as HT-B, 4936-factor and the 120 kDa glycoprotein, are also required for pollen rejection but do not contribute to S-haplotype-specificity per se. Here we show that S-RNase is taken up and sorted to a vacuolar compartment in the pollen tubes. Antibodies to the 120 kDa glycoprotein label the compartment membrane. When the pistil does not express HT-B or 4936-factor, S-RNase remains sequestered, unable to cause rejection. Similarly, in wild-type pistils, compatible pollen tubes degrade HT-B and sequester S-RNase. We suggest that S-RNase trafficking and the stability of HT-B are central to S-specific pollen rejection.

Plant bioactive peptides: an expanding class of signaling molecules

Until recently, our knowledge of intercellular signaling in plants was limited to the so-called five classical plant hormones: auxin, cytokinin, gibberellin, ethylene, and abscissic acid. Other chemical compounds like sterols and lipids have also been recognized as signaling molecules in plants, but it was only recently discovered that peptides in plants, as in animal cells, play crucial roles in various aspects of growth and development, biotic and abiotic stress responses, and self/non-self recognition in sporophytic self-incompatibility. These peptides are often part of a very large gene family whose members show diverse, sometime overlapping spatial and temporal expression patterns, allowing them to regulate different aspects of plant growth and development. Only a handful of peptides have been linked to a bona fide receptor, thereby activating a cascade of events. Since these peptides have been thoroughly reviewed in the past few years, this review will focus on the small putative plant signaling peptides, some often disregarded in the plant peptide literature, which have been shown through biochemical or genetic studies to play important roles in plants.

Evolution of reproductive proteins from animals and plants

Sexual reproduction is a fundamental biological process common among eukaryotes. Because of the significance of reproductive proteins to fitness, the diversity and rapid divergence of proteins acting at many stages of reproduction is surprising and suggests a role of adaptive diversification in reproductive protein evolution. Here we review the evolution of reproductive proteins acting at different stages of reproduction among animals and plants, emphasizing common patterns. Although we are just beginning to understand these patterns, by making comparisons among stages of reproduction for diverse organisms we can begin to understand the selective forces driving reproductive protein diversity and the functional consequences of reproductive protein evolution.

Stylar glycoproteins bind to S-RNase in vitro

S-RNases determine the specificity of S-specific pollen rejection in self-incompatible plants of the Solanaceae, Rosaceae, and Scrophulariaceae. They are also implicated in at least two distinct types of unilateral interspecific incompatibility in Nicotiana. However, S-RNase itself is not sufficient for most types of pollen rejection, and evidence for its direct interaction with pollen tubes is limited. Thus, non-S-RNase factors also are required for pollen rejection. As one approach to identifying such factors, we tested whether SC10-RNase from Nicotiana alata would bind to other stylar proteins in vitro. SC10-RNase was immobilized on Affi-gel, and binding proteins were analyzed by SDS-PAGE and immunoblotting. In addition to SC10-RNase and a small protein similar to lily chemocyanin, the most prominent binding proteins include NaTTS, 120K, and NaPELPIII, these latter three being arabinogalactan proteins previously shown to interact directly with pollen tubes. We also show that SC10-RNase and these glycoproteins migrate as a complex in a native PAGE system. Our hypothesis is that S-RNase forms a complex with these glycoproteins in the stylar ECM, that the glycoproteins interact directly with the pollen tubes and thus that the initial interaction between the pollen tube and S-RNase is indirect.

Duplication of the S-locus F-box gene is associated with breakdown of pollen function in an S-haplotype identified in a natural population of self-incompatible Petunia axillaris

We previously identified both self-incompatible and self-compatible plants in a natural population of self-incompatible Petunia axillaris subsp. axillaris, and found that all the self-compatible plants studied carried either SC1- or SC2-haplotype. Genetic crosses showed that SC2 was identical to S17 identified from another natural population of P. axillaris, except that its pollen function was defective, and that the pollen-part mutation in SC2 was tightly linked to the S-locus. Recent identification of the S-locus F-box gene (SLF) as the gene that controls pollen specificity in S-RNase-based self-incompatibility has prompted us to examine the molecular basis of this pollen-part mutation. We cloned and sequenced the S17-allele of SLF of P. axillaris, named PaSLF17, and found that SC2SC2 plants contained extra restriction fragments that hybridized to PaSLF17 in addition to all of those observed in S17S17 plants. Moreover, these additional fragments co-segregated with SC2. We used the SC2-specific restriction fragments as templates to clone an allele of PaSLF by PCR. To determine the identity of this allele, named PaSLFx, primers based on its sequence were used to amplify PaSLF alleles from genomic DNA of 40 S-homozygotes of P. axillaris, S1S1 through S40S40. Sequence comparison revealed that PaSLFx was completely identical with PaSLF19 obtained from S19S19. We conclude that the S-locus of SC2 contained both S17-allele and the duplicated S19-allele of PaSLF. SC2 is the first naturally occurring pollen-part mutation of a solanaceous species that was shown to be associated with duplication of the pollen S. This finding lends support to the proposal, based on studies of irradiation-generated pollen-part mutants of solanaceous species, that duplication, but not deletion, of the pollen S, causes breakdown of pollen function.