Characteristics of Fruiting and Pollen Tube Growth of Apple Autotetraploid Cultivars Showing Self-compatibility

An S-locus F-box protein as pollen S determinant targets non-self S-RNase underlying self-incompatibility in Citrus

Self-incompatibility (SI) is a crucial mechanism that prevents self-fertilization and inbreeding in flowering plants. Citrus exhibits SI regulated by a polymorphic S-locus containing an S-RNase gene and multiple S-locus F-box (SLF) genes. It has been documented that S-RNase functions as the pistil S determinant, but there is no direct evidence that the SLF genes closely linked with S-RNase function as pollen S determinants in Citrus. This study assembled the genomes of two pummelo (Citrus grandis) plants, obtained three novel complete and well-annotated S-haplotypes, and isolated 36 SLF or SLF-like alleles on the S-loci. Phylogenetic analysis of 138 SLFs revealed that the SLF genes were classified into 12 types, including six types with divergent or missing alleles. Furthermore, transformation experiments verified that the conserved S6-SLF7a protein can lead to the transition of SI to self-compatibility by recognizing non-self S8-RNase in 'Mini-Citrus' plants (S7S8 and S8S29, Fortunella hindsii), a model plant for citrus gene function studies. In vitro assays demonstrated interactions between SLFs of different S haplotypes and the Skp1-Cullin1-F-box subunit CgSSK1 protein. This study provides direct evidence that SLF controls the pollen function in Citrus, demonstrating its role in the 'non-self recognition' SI system.

Studies on pollen micro-morphology, pollen storage methods, and cross-compatibility among grape (Vitis spp.) genotypes

The knowledge of pollen morphology, suitable storage condition, and species compatibility is vital for a successful grapevine improvement programme. Ten grape genotypes from three different species, viz., Vitis vinifera L., Vitis parviflora Roxb., and Vitis champini Planc., were studied for their pollen structure and pollen storage with the objective of determining their utilization in grape rootstock improvement programs. Pollen morphology was examined through the use of a scanning electron microscope (SEM). The viability of the pollen was assessed using 2,3,5-triphenyltetrazolium chloride (TTC). In vitro pollen germination was investigated using the semi-solid medium with 10 % sucrose, 100 mg/L boric acid, and 300 mg/L calcium nitrate. The results revealed variations in pollen micro-morphology in 10 genotypes, with distinct pollen dimensions, shapes, and exine ornamentation. However, species-wise, no clear difference was found for these parameters. Pollen of V. parviflora Roxb. and Dogridge was acolporated and did not germinate. The remaining eight genotypes exhibited tricolporated pollen and showed satisfactory in vitro pollen germination. Storage temperature and duration interactions showed that, at room temperature, pollen of most of the grape genotypes can be stored for up to 1 day only with an acceptable pollen germination rate (>30 %). However, storage for up to 7 days was successfully achieved at 4 °C, except for 'Pearl of Csaba'. The most effective storage conditions were found to be at -20 °C and -196 °C (in liquid N2), enabling pollen storage for a period of up to 30 days, and can be used for pollination to overcome the challenge of asynchronous flowering. Four interspecific combinations were studied for their compatibility, among which V. parviflora Roxb. × V. vinifera L. (Pusa Navrang) and V. parviflora Roxb. × V. champini Planc. (Salt Creek) showed high cross-compatibility, offering their potential use for grape rootstock breeding. However, V. parviflora Roxb. × V. vinifera L. (Male Hybrid) recorded the lowest compatibility index among studied crosses. In the case of self-pollinated flowers from V. parviflora Roxb. and V. parviflora Roxb. × V. champini Planc. (Dogridge), pollen failed to germinate on the stigma due to male sterility caused by acolporated pollen. As a result, the flowers of these genotypes functioned as females, which means they are ideal female parents for grape breeding without the need for the tedious process of emasculation.

Breeding an underutilized fruit crop: a long-term program for Hylocereus

This review describes three decades of introduction, agro-technology development, breeding and selection of Hylocereus species, known as pitaya or dragon fruit, as an example of a holistic program aimed to develop the horticultural potential of a perennial underutilized fruit crop. Interspecific homoploid and interploid crosses and embryo rescue procedures produced improved hybrids, some of which have been released to farmers. Molecular tools and morphological and phenological comparisons between the parental species and the resulting hybrids provided valuable information on dominant/recessive traits and on genetic relationships that could be exploited for further hybridizations. In addition, Hylocereus were crossed with species of the closely related genus Selenicereus, producing valuable intergeneric hybrids. In situ chromosome doubling resulted in the production of autopolyploid lines, from which an understanding of the effect of increased ploidy on fruit traits and metabolomic profiles was obtained. Gamete-derived lines were produced, adding to our biobank homozygote lines that were subsequently used for further hybridization. Spontaneous chromosome doubling occurred in haploid gamete-derived Hylocereus monacanthus lines and in interspecific interploid Hylocereus megalanthus × H. undatus hybrids obtained from an embryo rescue procedure, resulting in plants with double the expected ploidy. Challenging technical problems were addressed by the development of protocols for DNA isolation, flow cytometry, in situ chromosome doubling, androgenesis, gynogenesis and embryo rescue following interspecific and interploidy crosses. Current research leading to the development of genomics and molecular tools, including a draft genome of H. undatus, is also presented. Perspectives for further development of Hylocereus species and hybrids are discussed.

Identification and Molecular Analysis of Putative Self-Incompatibility Ribonuclease Alleles in an Extreme Polyploid Species, Prunus laurocerasus L

Cherry laurel (Prunus laurocerasus L.) is an extreme polyploid (2n = 22x) species of the Rosaceae family where gametophytic self-incompatibility (GSI) prevents inbreeding. This study was carried out to identify the S-ribonuclease alleles (S-RNases) of P. laurocerasus using PCR amplification of the first and second intron region of the S-RNase gene, cloning and sequencing. A total of 23 putative S-RNase alleles (S ₁-S ₂₀, S _{5 m}, S _{13 m}, and S _{18 m}) were sequenced from the second (C2) to the fifth conserved region (C5), and they shared significant homology to other Prunus S-RNases. The length of the sequenced amplicons ranged from 505 to 1,544 bp, and similar sizes prevented the proper discrimination of some alleles based on PCR analysis. We have found three putatively non-functional alleles (S _{5 m}, S _{18 m}, and S ₉) coding for truncated proteins. Although firm conclusions cannot be drawn, our data seem to support that heteroallelic pollen cannot induce self-compatibility in this polyploid Prunus species. The identities in the deduced amino acid sequences between the P. laurocerasus and other Prunus S-RNases ranged between 44 and 100%, without a discontinuity gap separating the identity percentages of trans-specific and more distantly related alleles. The phylogenetic position, the identities in nucleotide sequences of the second intron and in deduced amino acid sequences found one or more trans-specific alleles for all but S ₁₀, S ₁₄, S ₁₈, and S ₂₀ cherry laurel RNases. The analysis of mutational frequencies in trans-specific allele pairs indicated the region RC4-C5 accepts the most amino acid replacements and hence it may contribute to allele-specificity. Our results form the basis of future studies to confirm the existence and function of the GSI system in this extreme polyploid species and the alleles identified will be also useful for phylogenetic studies of Prunus S-RNases as the number of S-RNase sequences was limited in the Racemose group of Prunus (where P. laurocerasus belongs to).

Association of T2/S-RNase With Self-Incompatibility of Japanese Citrus Accessions Examined by Transcriptomic, Phylogenetic, and Genetic Approaches

Several citrus varieties show gametophytic self-incompatibility (GSI), which can contribute to seedless fruit production in several cultivars. This study investigated the genes regulating this trait through RNA-seq performed using styles collected from the flowers of Japanese citrus cultivars 'Hyuganatsu,' 'Tosabuntan,' 'Hassaku,' 'Banpeiyu,' and 'Sweet Spring'. We screened the transcripts of putative T2 RNases, i.e., the protein family including all S-RNases from S-RNase-based GSI plants, and constructed a phylogenetic tree using the screened T2 RNases and S-RNases retrieved from citrus genome databases and a public database. Three major clusters (class I-III) were formed, among which, the class III cluster contained family specific subclusters formed by S-RNase and a citrus-specific cluster monophyletic to the S-RNase clusters. From the citrus class III cluster, six transcripts were consistent with the S haplotypes previously determined in Japanese citrus accessions, sharing characteristics such as isoelectric point, extracellular localization, molecular weight, intron number and position, and tissue-specific expression with S-RNases. One T2 RNase gene in self-incompatible Hyuganatsu was significantly down-regulated in the styles of a self-compatible mutant of Hyuganatsu in RNA-seq and qPCR analyses. In addition, the inheritance pattern of some T2 RNase genes was consistent with the pattern of the S haplotype in the progeny population of Hyuganatsu and Tosabuntan. As all results supported citrus self-incompatibility being based on S-RNase, we believe that six T2 RNase genes were S-RNases. The homology comparison between the six T2 RNases and S-RNases recently reported in Chinese citrus revealed that three out of six T2 RNases were identical to S-RNases from Chinese citrus. Thus, the other three T2 RNases were finally concluded to be novel citrus S-RNases involved in self-incompatibility.

Is selfing a reproductive assurance promoting polyploid establishment? Reduced fitness, leaky self-incompatibility and lower inbreeding depression in neotetraploids

PREMISE

Newly formed polyploids face significant obstacles to persistence and population establishment because of fitness costs of intercytotype mating. Selfing provides the opportunity to escape mate limitation, enabling production of new individuals and increasing the likelihood of fixation of new polyploid lineages. Still, association between self-compatibility and polyploidy is not always clear. We compared self-incompatibility and inbreeding depression in neotetraploids and their diploid progenitor to explore the direct effects of whole genome duplications on self-incompatibility and the implications of ploidy-driven changes for polyploid establishment.

METHODS

Outcross and self-pollinations were performed in diploids and synthetic neotetraploids of Jasione maritima var. maritima, and reproductive success was measured through fruit and seed production and seed germination. Self- and outcross offspring were grown under controlled conditions, and plant performance was measured through several fitness parameters.

RESULTS

Neotetraploids showed an overall lower performance than diploids. Reproductive success was negatively affected by selfing in both cytotypes. However, greater variation in the expression of self-incompatibility was observed in neotetraploids; additionally, developmental and physiological parameters were not affected by selfing on neotetraploids, with an overall similar fitness of outcrossed and selfed individuals, resulting in lower inbreeding depression indexes.

CONCLUSIONS

Neotetraploids might have benefited from selfing at initial stages after their formation. Genome duplications resulted in leaky self-incompatibility, enabling the production of offspring under minority cytotype disadvantage with similar fitness as outcrossed offspring. Our results support theoretical assumptions that selfing might be important for neopolyploid establishment, although changes in self-incompatibility might not be abrupt.

Finding a Compatible Partner: Self-Incompatibility in European Pear (Pyrus communis); Molecular Control, Genetic Determination, and Impact on Fertilization and Fruit Set

Pyrus species display a gametophytic self-incompatibility (GSI) system that actively prevents fertilization by self-pollen. The GSI mechanism in Pyrus is genetically controlled by a single locus, i.e., the S-locus, which includes at least two polymorphic and strongly linked S-determinant genes: a pistil-expressed S-RNase gene and a number of pollen-expressed SFBB genes (S-locus F-Box Brothers). Both the molecular basis of the SI mechanism and its functional expression have been widely studied in many Rosaceae fruit tree species with a particular focus on the characterization of the elusive SFBB genes and S-RNase alleles of economically important cultivars. Here, we discuss recent advances in the understanding of GSI in Pyrus and provide new insights into the mechanisms of GSI breakdown leading to self-fertilization and fruit set. Molecular analysis of S-genes in several self-compatible Pyrus cultivars has revealed mutations in both pistil- or pollen-specific parts that cause breakdown of self-incompatibility. This has significantly contributed to our understanding of the molecular and genetic mechanisms that underpin self-incompatibility. Moreover, the existence and development of self-compatible mutants open new perspectives for pear production and breeding. In this framework, possible consequences of self-fertilization on fruit set, development, and quality in pear are also reviewed.

Non-self- and self-recognition models in plant self-incompatibility

The mechanisms by which flowering plants choose their mating partners have interested researchers for a long time. Recent findings on the molecular mechanisms of non-self-recognition in some plant species have provided new insights into self-incompatibility (SI), the trait used by a wide range of plant species to avoid self-fertilization and promote outcrossing. In this Review, we compare the known SI systems, which can be largely classified into non-self- or self-recognition systems with respect to their molecular mechanisms, their evolutionary histories and their modes of evolution. We review previous controversies on haplotype evolution in the gametophytic SI system of Solanaceae species in light of a recently elucidated non-self-recognition model. In non-self-recognition SI systems, the transition from self-compatibility (SC) to SI may be more common than previously thought. Reversible transition between SI and SC in plants may have contributed to their adaptation to diverse and fluctuating environments.

Molecular mechanism of the S-RNase-based gametophytic self-incompatibility in fruit trees of Rosaceae

Self-incompatibility (SI) is a major obstacle for stable fruit production in fruit trees of Rosaceae. SI of Rosaceae is controlled by the S locus on which at least two genes, pistil S and pollen S, are located. The product of the pistil S gene is a polymorphic and extracellular ribonuclease, called S-RNase, while that of the pollen S gene is a protein containing the F-box motif, SFB (S haplotype-specific F-box protein)/SFBB (S locus F-box brothers). Recent studies suggested that SI of Rosaceae includes two different systems, i.e., Prunus of tribe Amygdaleae exhibits a self-recognition system in which its SFB recognizes self-S-RNase, while tribe Pyreae (Pyrus and Malus) shows a non-self-recognition system in which many SFBB proteins are involved in SI, each recognizing subset of non-self-S-RNases. Further biochemical and biological characterization of the S locus genes, as well as other genes required for SI not located at the S locus, will help our understanding of the molecular mechanisms, origin, and evolution of SI of Rosaceae, and may provide the basis for breeding of self-compatible fruit tree cultivars.

Identification of SFBB-containing canonical and noncanonical SCF complexes in pollen of apple (Malus × domestica)

Gametophytic self-incompatibility (GSI) of Rosaceae, Solanaceae and Plantaginaceae is controlled by a single polymorphic S locus. The S locus contains at least two genes, S-RNase and F-box protein encoding gene SLF/SFB/SFBB that control pistil and pollen specificity, respectively. Generally, the F-box protein forms an E3 ligase complex, SCF complex with Skp1, Cullin1 (CUL1) and Rbx1, however, in Petunia inflata, SBP1 (S-RNase binding protein1) was reported to play the role of Skp1 and Rbx1, and form an SCFSLF-like complex for ubiquitination of non-self S-RNases. On the other hand, in Petunia hybrida and Petunia inflata of Solanaceae, Prunus avium and Pyrus bretschneideri of Rosaceae, SSK1 (SLF-interacting Skp1-like protein1) is considered to form the SCFSLF/SFB complex. Here, we isolated pollen-expressed apple homologs of SSK1 and CUL1, and named MdSSK1, MdCUL1A and MdCUL1B. MdSSK1 was preferentially expressed in pollen, but weakly in other organs analyzed, while, MdCUL1A and MdCUL1B were almost equally expressed in all the organs analyzed. MdSSK1 transcript abundance was significantly (>100 times) higher than that of MdSBP1. In vitro binding assays showed that MdSSK1 and MdSBP1 interacted with MdSFBB1-S9 and MdCUL1, and MdSFBB1-S9 interacted more strongly with MdSSK1 than with MdSBP1. The results suggest that both MdSSK1-containing SCFSFBB1 and MdSBP1-containing SCFSFBB1-like complexes function in pollen of apple, and the former plays a major role.

A segmental duplication encompassing S-haplotype triggers pollen-part self-compatibility in Japanese pear (Pyrus pyrifolia)

Self-compatible mutants of self-incompatible crops have been extensively studied for research and agricultural purposes. Until now, the only known pollen-part self-compatible mutants in Rosaceae subtribe Pyrinae, which contains many important fruit trees, were polyploid. This study revealed that the pollen-part self-compatibility of breeding selection 415-1, a recently discovered mutant of Japanese pear (Pyrus pyrifolia) derived from γ-irradiated pollen, is caused by a duplication of an S-haplotype. In the progeny of 415-1, some plants had three S-haplotypes, two of which were from the pollen parent. Thus, 415-1 was able to produce pollen with two S-haplotypes, even though it was found to be diploid: the relative nuclear DNA content measured by flow cytometry showed no significant difference from that of a diploid cultivar. Inheritance patterns of simple sequence repeat (SSR) alleles in the same linkage group as the S-locus (LG 17) showed that some SSRs closely linked to S-haplotypes were duplicated in progeny containing the duplicated S-haplotype. These results indicate that the pollen-part self-compatibility of 415-1 is not caused by a mutation of pollen S factors in either one of the S-haplotypes, but by a segmental duplication encompassing the S-haplotype. Consequently, 415-1 can produce S-heteroallelic pollen grains that are capable of breaking down self-incompatibility (SI) by competitive interaction between the two different S factors in the pollen grain. 415-1 is the first diploid pollen-part self-compatible mutant with a duplicated S-haplotype to be discovered in the Pyrinae. The fact that 415-1 is not polyploid makes it particularly valuable for further studies of SI mechanisms.

Genetic features of the spontaneous self-compatible mutant, 'Jin Zhui' (Pyrus bretschneideri Rehd.)

‘Jin Zhui’ is a spontaneous self-compatible mutant of ‘Ya Li’ (Pyrus bretschneideri Rehd. S₂₁S₃₄), the latter displaying a typical S-RNase-based gametophytic self-incompatibility (GSI). The pollen-part mutation (PPM) of ‘Jin Zhui’ might be due to a natural mutation in the pollen-S gene (S₃₄ haplotype). However, the molecular mechanisms behind these phenotypic changes are still unclear. In this study, we identified five SLF (S-Locus F-box) genes in ‘Ya Li’, while no nucleotide differences were found in the SLF genes of ‘Jin Zhui’. Further genetic analysis by S-RNase PCR-typing of selfed progeny of ‘Jin Zhui’ and ‘Ya Li’ × ‘Jin Zhui’ progeny showed three progeny classes (S₂₁S₂₁, S₂₁S₃₄ and S₃₄S₃₄) as opposed to the two classes reported previously (S₂₁S₃₄ and S₃₄S₃₄), indicating that the pollen gametes of ‘Jin Zhui’, bearing either the S₂₁- or S₃₄-haplotype, were able to overcome self-incompatibility (SI) barriers. Moreover, no evidence of pollen-S duplication was found. These findings support the hypothesis that loss of function of S-locus unlinked PPM expressed in pollen leads to SI breakdown in ‘Jin Zhui’, rather than natural mutation in the pollen-S gene (S₃₄ haplotype). Furthermore, abnormal meiosis was observed in a number of pollen mother cells (PMCs) in ‘Jin Zhui’, but not in ‘Ya Li’. These and other interesting findings are discussed.

Identification of a canonical SCF(SLF) complex involved in S-RNase-based self-incompatibility of Pyrus (Rosaceae)

S-RNase-based self-incompatibility (SI) is an intraspecific reproductive barrier to prevent self-fertilization found in many species of the Solanaceae, Plantaginaceae and Rosaceae. In this system, S-RNase and SLF/SFB (S-locus F-box) genes have been shown to control the pistil and pollen SI specificity, respectively. Recent studies have shown that the SLF functions as a substrate receptor of a SCF (Skp1/Cullin1/F-box)-type E3 ubiquitin ligase complex to target S-RNases in Solanaceae and Plantaginaceae, but its role in Rosaceae remains largely undefined. Here we report the identification of two pollen-specific SLF-interacting Skp1-like (SSK) proteins, PbSSK1 and PbSSK2, in Pyrus bretschneideri from the tribe Pyreae of Rosaceae. Both yeast two-hybrid and pull-down assays demonstrated that they could connect PbSLFs to PbCUL1 to form a putative canonical SCF(SLF) (SSK/CUL1/SLF) complex in Pyrus. Furthermore, pull-down assays showed that the SSK proteins could bind SLF and CUL1 in a cross-species manner between Pyrus and Petunia. Additionally, phylogenetic analysis revealed that the SSK-like proteins from Solanaceae, Plantaginaceae and Rosaceae form a monoclade group, hinting their shared evolutionary origin. Taken together, with the recent identification of a canonical SCF(SFB) complex in Prunus of the tribe Amygdaleae of Rosaceae, our results show that a conserved canonical SCF(SLF/SFB) complex is present in Solanaceae, Plantaginaceae and Rosaceae, implying that S-RNase-based self-incompatibility shares a similar molecular and biochemical mechanism.

Identification of a Skp1-like protein interacting with SFB, the pollen S determinant of the gametophytic self-incompatibility in Prunus

Many species in Rosaceae, Solanaceae, and Plantaginaceae exhibit S-RNase-based self-incompatibility (SI). In this system, the pistil and pollen specificities are determined by S-RNase and the S locus F-box protein, respectively. The pollen S determinant F-box protein in Prunus (Rosaceae) is referred to by two different terms, SFB (for S-haplotype-specific F-box protein) and SLF (for S locus F box), whereas it is called SLF in Solanaceae and Plantaginaceae. Prunus SFB is thought to be a molecule indispensable for its cognate S-RNase to exert cytotoxicity and to arrest pollen tube growth in incompatible reactions. Although recent studies have demonstrated the molecular function of SCF(SLF) in the SI reaction of Solanaceae and Plantaginaceae, how SFB participates in the Prunus SI mechanism remains to be elucidated. Here we report the identification of sweet cherry (Prunus avium) SFB (PavSFB)-interacting Skp1-like1 (PavSSK1) using a yeast (Saccharomyces cerevisiae) two-hybrid screening against the pollen cDNA library. Phylogenetic analysis showed that PavSSK1 belongs to the same clade as Antirrhinum hispanicum SLF-interacting Skp1-like1 and Petunia hybrida SLF-interacting Skp1-like1 (PhSSK1). In yeast, PavSSK1 interacted not only with PavSFBs from different S haplotypes and Cullin1-likes (PavCul1s), but also with S-locus F-box-likes. A pull-down assay confirmed the interactions between PavSSK1 and PavSFB and between PavSSK1 and PavCul1s. These results collectively indicate that PavSSK1 could be a functional component of the SCF complex and that PavSFB may function as a component of the SCF complex. We discuss the molecular function of PavSFB in self-/nonself-recognition in the gametophytic SI of Prunus.

Molecular bases and evolutionary dynamics of self-incompatibility in the Pyrinae (Rosaceae)

The molecular bases of the gametophytic self-incompatibility (GSI) system of species of the subtribe Pyrinae (Rosaceae), such as apple and pear, have been widely studied in the last two decades. The characterization of S-locus genes and of the mechanisms underlying pollen acceptance or rejection have been topics of major interest. Besides the single pistil-side S determinant, the S-RNase, multiple related S-locus F-box genes seem to be involved in the determination of pollen S specificity. Here, we collect and review the state of the art of GSI in the Pyrinae. We emphasize recent genomic data that have contributed to unveiling the S-locus structure of the Pyrinae, and discuss their consistency with the models of self-recognition that have been proposed for Prunus and the Solanaceae. Experimental data suggest that the mechanism controlling pollen-pistil recognition specificity of the Pyrinae might fit well with the collaborative 'non-self' recognition system proposed for Petunia (Solanaceae), whereas it presents relevant differences with the mechanism exhibited by the species of the closely related genus Prunus, which uses a single evolutionarily divergent F-box gene as the pollen S determinant. The possible involvement of multiple pollen S genes in the GSI system of Pyrinae, still awaiting experimental confirmation, opens up new perspectives to our understanding of the evolution of S haplotypes, and of the evolution of S-RNase-based GSI within the Rosaceae family. Whereas S-locus genes encode the players determining self-recognition, pollen rejection in the Pyrinae seems to involve a complex cascade of downstream cellular events with significant similarities to programmed cell death.

Sequence divergence and loss-of-function phenotypes of S locus F-box brothers genes are consistent with non-self recognition by multiple pollen determinants in self-incompatibility of Japanese pear (Pyrus pyrifolia)

The S-RNase-based gametophytic self-incompatibility (SI) of Rosaceae, Solanaceae, and Plantaginaceae is controlled by at least two tightly linked genes located at the complex S locus; the highly polymorphic S-RNase for pistil specificity and the F-box gene (SFB/SLF) for pollen. Self-incompatibility in Prunus (Rosaceae) is considered to represent a 'self recognition by a single factor' system, because loss-of-function of SFB is associated with self-compatibility, and allelic divergence of SFB is high and comparable to that of S-RNase. In contrast, Petunia (Solanaceae) exhibits 'non-self recognition by multiple factors'. However, the distribution of 'self recognition' and 'non-self recognition' SI systems in different taxa is not clear. In addition, in 'non-self recognition' systems, a loss-of-function phenotype of pollen S is unknown. Here we analyze the divergence of SFBB genes, the multiple pollen S candidates, of a rosaceous plant Japanese pear (Pyrus pyrifolia) and show that intrahaplotypic divergence is high and comparable to the allelic diversity of S-RNase while interhaplotypic divergence is very low. Next, we analyzed loss-of-function of the SFBB1 type gene. Genetic analysis showed that pollen with the mutant haplotype S(4sm) lacking SFBB1-S(4) is rejected by pistils with an otherwise compatible S(1) while it is accepted by other non-self pistils. We found that the S(5) haplotype encodes a truncated SFBB1 protein, even though S(5) pollen is accepted normally by pistils with S(1) and other non-self haplotypes. These findings suggest that Japanese pear has a 'non-self recognition by multiple factors' SI system, although it is a species of Rosaceae to which Prunus also belongs.

S-RNase-based self-incompatibility in Petunia inflata

BACKGROUND

For the Solanaceae-type self-incompatibility, also possessed by Rosaceae and Plantaginaceae, the specificity of self/non-self interactions between pollen and pistil is controlled by two polymorphic genes at the S-locus: the S-locus F-box gene (SLF or SFB) controls pollen specificity and the S-RNase gene controls pistil specificity.

SCOPE

This review focuses on the work from the authors' laboratory using Petunia inflata (Solanaceae) as a model. Here, recent results on the identification and functional studies of S-RNase and SLF are summarized and a protein-degradation model is proposed to explain the biochemical mechanism for specific rejection of self-pollen tubes by the pistil.

CONCLUSIONS

The protein-degradation model invokes specific degradation of non-self S-RNases in the pollen tube mediated by an SLF, and can explain compatible versus incompatible pollination and the phenomenon of competitive interaction, where SI breaks down in pollen carrying two different S-alleles. In Solanaceae, Plantaginaceae and subfamily Maloideae of Rosaceae, there also exist multiple S-locus-linked SLF/SFB-like genes that potentially function as the pollen S-gene. To date, only three such genes, all in P. inflata, have been examined, and they do not function as the pollen S-gene in the S-genotype backgrounds tested. Interestingly, subfamily Prunoideae of Rosaceae appears to possess only a single SLF/SFB gene, and competitive interaction, observed in Solanaceae, Plantaginaceae and subfamily Maloideae, has not been observed. Thus, although the cytotoxic function of S-RNase is an integral part of SI in Solanaceae, Plantaginaceae and Rosaceae, the function of SLF/SFB may have diverged. This highlights the complexity of the S-RNase-based SI mechanism. The review concludes by discussing some key experiments that will further advance our understanding of this self/non-self discrimination mechanism.

Apple S locus region represents a large cluster of related, polymorphic and pollen-specific F-box genes

Gametophytic self-incompatibility (GSI) of Rosaceae, Solanaceae and Plantaginaceae is controlled by a complex S locus that encodes separate proteins for pistil and pollen specificities, extracellular ribonucleases (S-RNases) and F-box proteins SFB/SLF, respectively. SFB/SLFs of Prunus (subfamily Prunoideae of Rosaceae), Solanaceae and Plantaginaceae are single copy in each S haplotype, while recently identified pollen S candidates SFBBs of subfamily Maloideae of Rosaceae, apple and Japanese pear, are multiple; two and three related SFBBs were isolated from each S haplotype of apple and Japanese pear, respectively. Here, we show that apple (Malus x domestica) SFBBs constitute a gene family that is much larger than initially thought. Twenty additional SFBB-like genes/alleles were isolated by screening of a BAC library derived from S (3) S (9) genotype, and tentatively named MdFBX1-20. All but one MdFBX showed S haplotype-specific polymorphisms. All the polymorphic MdFBXs were completely linked to S-RNase in 239 segregants. In addition, FISH revealed that the monomorphic gene MdFBX11 is also located near S-RNase, and the S locus is located in a subtelomeric region of a chromosome and is not close to the centromere. All MdFBXs were specifically expressed in pollen, except for a pseudogene MdFBX4 that showed no expression in any organs analyzed. Phylogenetic analysis revealed that the closest relatives of most MdFBXs were from a different S haplotype, suggesting that proliferation of MdSFBB/FBXs predates diversification of the S haplotypes.