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

Specific functions of single pistil S-RNases in S-gene homozygous Pyrus germplasm

Gametophytic self-incompatibility (SI) is regulated by S-allele recognition; that is, pollen in a style with the same S-genotype will undergo programmed cell death and stop growing so that it is unable to complete double fertilization, ultimately resulting in the SI response. S-RNase is the female determinant of SI in pear (Pyrus). In the Pyrus genome, there are two different S-RNase alleles at the S-locus, which generate two different S-RNase products in the pistil. The extracted S-glycoprotein is actually a protein complex. In this study, artificial self-pollination was conducted at the bud stage to overcome SI in 'Huanghua' (S1S2) pear. Seven plants homozygous for S1-RNase and four homozygous for S2-RNase were selected from the selfed progeny of 'Huanghua' by S-gene molecular identification biotechnology. We investigated the function of single S-RNases isolated from the pistils of S-gene homozygous Pyrus germplasm. The pollen of 'Huanghua' could smoothly pass through the style of the S-gene homozygous germplasm and complete fertilization. S-RNases were extracted from flower styles of different genotypes and used to treat different types of pollen. The S-RNase from 'Huanghua' completely inhibited the growth of S1S2, S1S1, and S2S2 pollen, while the S-RNase from homozygous germplasm allowed some S1S2 pollen and different single genotypes of pollen to continue growing. These results further validate the core events of SI including cytoskeleton depolymerization and programmed cell death. By iTRAQ-based proteomic analysis of style proteins, a total of 13 S-RNase-related proteins were identified. In summary, we have created reliable S-RNase gene homozygous germplasm, which will play a crucial role in further research on SI in pear and in the development of the pear industry.

Transcriptome Analysis of the Late-Acting Self-Incompatibility Associated with RNase T2 Family in Camellia oleifera

The Camellia oil tree (Camellia oleifera Abel.) is an important nonwood forest species in China, and the majority of its cultivars are late-acting self-incompatibility (LSI) types. Although several studies have examined the mechanism of LSI, the process is quite complicated and unclear. In this study, pollen tube growth and fruit setting of two Camellia oil tree cultivars Huashuo (HS) and Huajin (HJ) were investigated after non and self-pollination, and transcriptomic analysis of the ovaries was performed 48 h after self-pollination to identify the potential genes implicated in the LSI of Camellia oil trees. The results showed that the fruit set of HS was significantly higher than that of HJ after self-pollination. Transcriptomic analysis revealed that plant hormone signal transduction, the phosphatidylinositol signaling system, ATP-binding cassette (ABC) transporters, reactive oxygen species (ROS) metabolism, and Ca2+ signaling were mainly contributed in the LSI of reaction of Camellia oil tree. Moreover, nine RNase T2 genes were identified from the transcriptome analysis, which also showed that CoRNase7 participated in the self-incompatibility reaction in HS. Based on phylogenetic analysis, CoRNase6 was closely related to S-RNase from coffee, and CoRNase7 and CoRNase8 were closely related to S-RNase from Camellia sinensis. The 9 RNase T2 genes successfully produced proteins in prokaryotes. Subcellular localization indicated that CoRNase1 and CoRNase5 were cytoplasmic proteins, while CoRNase7 was a plasma membrane protein. These results screened the main metabolic pathways closely related to LSI in Camellia oil tree, and SI signal transduction might be regulated by a large molecular regulatory network. The discovery of T2 RNases provided evidence that Camellia oil tree might be under RNase-based gametophytic self-incompatibility.

Self S-RNase inhibits ABF-LRX signaling to arrest pollen tube growth to achieve self-incompatibility in pear

Gametophytic self-incompatibility (GSI) has been widely studied in flowering plants, but studies of the mechanisms underlying pollen tube growth arrest by self S-RNase in GSI species are limited. In the present study, two leucine-rich repeat extensin genes in pear (Pyrus bretschneideri), PbLRXA2.1 and PbLRXA2.2, were identified based on transcriptome and quantitative real-time PCR analyses. The expression levels of these two LRX genes were significantly higher in the pollen grains and pollen tubes of the self-compatible cultivar 'Jinzhui' (harboring a spontaneous bud mutation) than in those of the self-incompatible cultivar 'Yali'. Both PbLRXA2.1 and PbLRXA2.2 stimulated pollen tube growth and attenuated the inhibitory effects of self S-RNase on pollen tube growth by stabilizing the actin cytoskeleton and enhancing cell wall integrity. These results indicate that abnormal expression of PbLRXA2.1 and PbLRXA2.2 is involved in the loss of self-incompatibility in 'Jinzhui'. The PbLRXA2.1 and PbLRXA2.2 promoters were directly bound by the ABRE-binding factor PbABF.D.2. Knockdown of PbABF.D.2 decreased PbLRXA2.1 and PbLRXA2.2 expression and inhibited pollen tube growth. Notably, the expression of PbLRXA2.1, PbLRXA2.2, and PbABF.D.2 was repressed by self S-RNase, suggesting that self S-RNase can arrest pollen tube growth by restricting the PbABF.D.2-PbLRXA2.1/PbLRXA2.2 signal cascade. These results provide novel insight into pollen tube growth arrest by self S-RNase.

Gametophytic self-incompatibility in Andean capuli (Prunus serotina subsp. capuli): allelic diversity at the S-RNase locus influences normal pollen-tube formation during fertilization

Capuli (Prunus serotina subsp. capuli) is a tree species that is widely distributed in the northern Andes. In Prunus, fruit set and productivity appears to be limited by gametophytic self-incompatibility (GSI) which is controlled by the S-Locus. For the first time, this research reveals the molecular structure of the capuli S-RNase (a proxy for S-Locus diversity) and documents how S-Locus diversity influences GSI in the species. To this end, the capuli S-RNase gene was amplified and sequenced in order to design a CAPS (Cleaved Amplified Polymorphic Sequence) marker system that could unequivocally detect S-alleles by targeting the highly polymorphic C2-C3 S-RNase intra-genic region. The devised system proved highly effective. When used to assess S-Locus diversity in 15 P. serotina accessions, it could identify 18 S-alleles; 7 more than when using standard methodologies for the identification of S-alleles in Prunus species. CAPS marker information was subsequently used to formulate experimental crosses between compatible and incompatible individuals (as defined by their S-allelic identity). Crosses between heterozygote individuals with contrasting S-alleles resulted in normal pollen tube formation and growth. In crosses between individuals with exactly similar S-allele identities, pollen tubes often showed morphological alterations and arrested development, but for some (suspected) incompatible crosses, pollen tubes could reach the ovary. The latter indicates the possibility of a genotype-specific breakdown of GSI in the species. Overall, this supports the notion that S-Locus diversity influences the reproductive patterns of Andean capuli and that it should be considered in the design of orchards and the production of basic propagation materials.

A mutation near the active site of S-RNase causes self-compatibility in S-RNase-based self-incompatible plants

The structurally simplest amino acid glycine could make contribution to nuclease activity of S-RNase and self-incompatibility in S-RNase-based plants. S-RNase is regarded as inhibitor of self-pollen tube in S-RNase-based self-incompatibility plants. Certain residues like histidine are necessary for RNase activity and self-incompatibility; however, it is unknown whether any other residues contribute to this. Previously, we identified an association between the self-compatible Chinese pear (Pyrus × bretschneideri) cultivar 'Yanzhuang' (YZ) and a mutation causing a residue shift (glycine-to-valine) in the 2nd conserved region (C2) of S₂₁-RNase; however, it was unclear how this nonpolar aliphatic amino acid substitution caused self-compatibility. In this study, we observed that 'YZ' offspring were self-compatible when S₂₁-RNases were all mutated. In vitro pollen tube (S₂₁S₂₁) growth was not completely arrested by the mutated S₂₁-RNase. Residue frequency analysis showed that the glycine residue is highly conserved in diverse S-RNases across many plant species. We therefore generated a mutated petunia S_V'-RNase (glycine to valine) and transformed it into S_3LS_3L petunia. The transformed pistil could not inhibit S_V pollen tubes. Three-dimensional protein prediction suggested that the glycine-to-valine mutation alters the spatial structure near the active site, and RNase activity of mutated S-RNase was reducing. Thus, the glycine residue in the C2 is essential for RNase activity, substitution of this residue leads to a failure of self-incompatibility.

RNA-Seq analysis of compatible and incompatible styles of Pyrus species at the beginning of pollination

At the early stage of pollination, the difference in gene expression between compatibility and incompatibility is highly significant about the pollen-specific expression of the LRR gene, resistance, and defensin genes. In Rosaceae, incompatible pollen can penetrate into the style during the gametophytic self-incompatibility response. It is therefore considered a stylar event rather than a stigmatic event. In this study, we explored the differences in gene expression between compatibility and incompatibility in the early stage of pollination. The self-compatible pear variety "Jinzhuili" is a naturally occurring bud mutant from "Yali", a leading Chinese native cultivar exhibiting typical gametophytic self-incompatibility. We collected the styles of 'Yali' and 'Jinzhuili' at 0.5 and 2 h after self-pollination and then performed high-throughput sequencing. According to the KEGG analysis of the differentially expressed genes, several metabolic pathways, such as "Plant hormone signal transduction", "Plant-pathogen interaction", are the main pathways was the most represented pathway. Quantitative PCR was used to validate these differential genes. The expression levels of genes related to pollen growth and disease inhibition, such as LRR (Leucine-rich repeat extensin), resistance, defensin, and auxin, differed significantly between compatible and incompatible pollination. Interestingly, at 0.5 h, most of these genes were upregulated in the compatible pollination system compared with the incompatible pollination system. Calcium transport, which requires ATPase, also demonstrated upregulated expression. In summary, the self-incompatibility reaction was initiated when the pollen land on the stigma.

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.

The unique evolutionary pattern of the Hydroxyproline-rich glycoproteins superfamily in Chinese white pear (Pyrus bretschneideri)

BACKGROUND

The hydroxyproline-rich glycoprotein (HRGP) superfamily, comprising three families (arabinogalactan-proteins, AGPs; extensins, EXTs; proline-rich proteins, PRPs), is a class of proline-rich proteins that exhibit high diversity and are involved in many aspects of plant biology.

RESULTS

In this study, 838 HRGPs were identified from Chinese white pear (Pyrus bretschneideri) by searching for biased amino acid composition and conserved motifs. 405 HRGPs were derived from whole genome duplication (WGD) events which is suggested to be the major force of driving HRGPs expansion and the recent WGD event shared by apple and pear generated most duplicated HRGPs in pear. This duplication event drived the structural variation of the HRGPs encoding hydroxyproline (Hyp)-rich motifs. The rate of HRGPs evolution mainly impacted the Hyp-rich motifs even in chimeric HRGPs. During the evolution of 53 PRPs that are also typified by 7-deoxyloganetin glucosyltransferase-like genes, the duplication from PRP to non-PRP was indirectly modified by positive selection. These results suggested that the rate of HRGP evolution mainly influenced the Hyp-rich motifs even in chimeric HRGPs. The expression divergence of HRGPs was higher than that of other commonly duplicated genes. In pear pistil, 601 HRGPs exhibited expression, while in pear pollen, 285 HRGPs were expressed. The qPCR results revealed that Pbr036330.1 and Pbr010506.1 showed different expression profile in self-incompatibility of pear pistil.

CONCLUSIONS

The researches indicated that WGD events was the main duplication type during the evolution of HRGPs, and the highly variable Hyp-motifs might be accountable for the expansion, evolution and expression divergence of HRGPs and that this divergence may be responsible for the gain of new functions in plants.

PLC-Mediated Signaling Pathway in Pollen Tubes Regulates the Gametophytic Self-incompatibility of Pyrus Species

Among the Rosaceae species, the gametophytic self-incompatibility (GSI) is controlled by a single multi-allelic S locus, which is composed of the pistil-S and pollen-S genes. The pistil-S gene encodes a polymorphic ribonuclease (S-RNase), which is essential for identifying self-pollen. However, the S-RNase system has not been fully characterized. In this study, the self-S-RNase inhibited the Ca²⁺-permeable channel activity at pollen tube apices and the selectively decreased phospholipase C (PLC) activity in the plasma membrane of Pyrus pyrifolia pollen tubes. Self-S-RNase decreased the Ca²⁺ influx through a PLC-mediated signaling pathway. Phosphatidylinositol-specific PLC has a 26-amino acid insertion in pollen tubes of the 'Jinzhuili' cultivar, which is a spontaneous self-compatible mutant of the 'Yali' cultivar. 'Yali' plants exhibit a typical S-RNase-based GSI. Upon self-pollination, PLC gene expression is significantly higher in 'Jinzhuili' pollen tubes than that in 'Yali' pollen tubes. Moreover, the PLC in pollen tubes can only interact with one of the two types of S-RNase from the style. In the Pyrus x bretschneideri Rehd, the PLC directly interacted with the S₇-RNase in the pollen tube, but not with the S₃₄-RNase. Collectively, our results reveal that the effects of S-RNase on PLC activity are required for S-specific pollen rejection, and that PLC-IP₃ participates in the self-incompatibility reaction of Pyrus species.

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.