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SUMMARY: 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. Significance Statement: PbLRXA2.1 and PbLRX.A2.2 mediate pollen tube growth and attenuate the inhibitory effects of self S‐RNase on pollen tube growth by stabilizing the actin cytoskeleton and enhancing cell wall integrity. Self S‐RNase can arrest pollen tube growth by restricting the PbABF.D.2–PbLRXA2.1/PbLRXA2.2 signal cascade. [ABSTRACT FROM AUTHOR]

Pear is one of the most important fruits in the world. In recent years, many new varieties have been bred. However, little is known about these new varieties, including pollen germination, self-(in)compatibility, and the best pollinator for the varieties. In this study, by surveying the length of the pollen tubes that grew into the self styles, pollen germination and self-(in)compatibility of the 127 pear cultivars were determined. All tested pollen germinated on the self stigmas, but the lengths of pollen tube growth into the self styles were different among the pear cultivars. Of these 127 cultivars, 56, 58, and 8 cultivars presented strong, intermediate, and weak self-incompatibility, respectively. Moreover, 5 cultivars presented self-compatibility because their pollen tubes grew to the bottom of the self styles and higher fruit sets with self-pollination were verified in the field. Furthermore, to assign the best pollinator for fruit production and/or new variety breeding, the S-RNase alleles that shared almost identical sequences were integrated together and then re-designated to discern different function-specificity. As a result, the original 68 S-RNase alleles in eastern pear were integrated into 48 re-designated function-specific S-RNase alleles. This integration is promising for assigning the best pollinator for the 36 main pear cultivars in China. Unless the integrated S-RNase alleles, in addition, the S-RNase alleles, which have extremely high identities in amino acid sequences, should be excluded to avoid cross-incompatibility between two parents, leading to poor fruit production and the loss of genetic information in this chromosome [ABSTRACT FROM AUTHOR]

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Self-incompatibility is an important genetic mechanism that prevents inbreeding and promotes genetic polymorphism and heterosis in flowering plants. Many fruit species in the Rosaceae, including apple, pear, plum, apricot, sweet cherry, Japanese apricot, and almond, exhibit typical gametophytic self-incompatibility (GSI) controlled by an apparently single multi-allelic locus. This locus encodes at least two components from both the pollen and the pistil, and controls recognition of self- and non-self pollen. Recently, the GSI system has been investigated at the molecular and cellular levels in Rosaceae, and findings have provided some important insights as to how these two genes interact within pollen tubes that lead to specific inhibition of germination and/or growth of self-pollen tubes. In this review, molecular features of S-determinants of both pistil and pollen, identification of S-alleles, mechanisms of self-incompatibility break-down, and evolution of S-alleles are presented. Moreover, hypothetical signal transduction models in a self-incompatible system in Rosaceae are proposed based on recent findings that indicate that several signal factors are involved in GSI responses. [ABSTRACT FROM PUBLISHER]

Abstract: In this study, eight S-RNase alleles were isolated from two Prunus pseudocerasus and two Prunus speciosa accessions by PCR amplification from genomic DNA. Analysis of the amino acid sequences revealed five novel and three published S-alleles. These S-RNases share typical structural features with S-RNases from other Prunus species exhibiting gametophytic self-incompatibility. The deduced amino acid identities ranged from 60.8 to 75.6% among four S-RNase alleles in Prunus speciosa and ranged from 73 to 81.4% among four S-RNase alleles in Prunus pseudocerasus. The size of the first introns ranged from 197 to 341bp, and the size of second introns ranged from 81 to 1182bp. Sequence analysis demonstrated that the deduced amino acid identities, by comparison with other Prunus species, were often higher than those of intraspecific identities. Moreover, exceptionally high identities were found between Pspe-S 1 and Pd-S 28 ; between Pspe-S 31 and Pm-S 6 ; among Pspe-S 51 , Pa-S 29 and Pweb-S 7 ; and among Pps-S 13 , Psim-S 4 and Ps-S b , indicating that the S-RNase alleles evolved before Prunus species divergence. Interestingly, the similarities of the first and second introns were also high between the two S-RNase alleles, which range from 83.63 to 100% among the first introns and from 46.13 to 100% among the second introns. These information could not increase our knowledge on the S-alleles of Prunus species, but is available for molecular breeding of fruit trees, to avoid cross-incompatibility. [Copyright &y& Elsevier]

Self-incompatibility is a genetic mechanism in flowering plants that promotes outcrossing and prevents inbreeding. In this study, the reported 15 S-RNase alleles were isolated from 18 pear accessions that contained 14 different S -genotypes. The S-RNase alleles were only expressed in the style, not in the root, stem, leaf, fruit, sepal, and pollen. Pollen tube grew into self-styles and self-pollinated fruit set involved the breakdown of self-incompatibility in cv. Xinxue. However, the two S-RNase alleles in cv. Xinxue had identical amino acid sequences to those self-incompatible cultivars, and S 5 - and S 6 -RNase were normally expressed in style, indicating that the loss of self-incompatibility in cv. Xinxue likely resulted from pollen-part mutation. Further, S 5 S 5 and S 6 S 6 genotyped individuals were identified in self-pollinated progeny, indicating that S 5 and S 6 genotyped pollen were compatible with the self-styles of cv. Xinxue. A genetic analysis showed that the segregation ratio of S 5 S 5 , S 5 S 6 , and S 6 S 6 was approximately 0:2:1 (χ 2  = 3.505 < χ 2 0.05, 2  = 5.99), which did not fit the theoretical ratio of 1:2:1 (χ 2  = 13.340 > χ 2 0.05, 2  = 5.99), suggesting that the breakdown of self-incompatibility could be caused by modified factor(s) located outside of the S -locus. These results are useful for parental assignment and understanding self-incompatibility reaction. [ABSTRACT FROM AUTHOR]

• The expression pattern of eight ABF genes was characterized in various tissues at different stages. • RNA-Seq analysis of the self- and cross-pollinated styles revealed the 278 differentially expressed genes (DEGs) responsive to self-incompatibility reaction. • The ABRE-including promoter of six DEGs was activated by PbABF.E.1, PbABF.E.2, and/or PbABF.B. Self-incompatibility (SI) facilitates the rejection of pollen by self S-RNase. It is believed that a large number of genes are responsive to SI reactions; however, little is known about the transcriptional regulation of these genes. Herein, we explored the role of ABRE-binding factor (ABF) in gametophytic self-incompatibility (GSI). Phylogenetic analysis showed that eight ABF genes in pear clustered with five ABF genes in Arabidopsis. Of these ABF genes, PbABF.E.1 and PbABF.E.2 were expressed in all tissues, and PbABF.B was expressed in most tissues. These three ABF genes were also detected in the self- and cross-pollinated styles. Additionally, RNA-Seq analysis revealed 11,476 genes that were differentially expressed between the self- and cross-pollinated styles at 24, 48, and/or 72 h after pollination. Of these differentially expressed genes (DEGs), 278 were differentially expressed between the self- and cross-pollinated styles at any stage and thus are responsive to the GSI reaction. Notably, abscisic acid-responsive elements were detected in the promoter sequences of the 1213 DEGs including the six genes negatively correlated with pollen tube growth. The dual-luciferase assay showed that the promoter of these six genes was activated by PbABF.B, PbABF.E.1 , and/or PbABF.E.2. Therefore, PbABF.B, PbABF.E.1 , and PbABF.E.2 were involved in the GSI reaction by mediating the expression of genes responsive to the GSI reaction. [ABSTRACT FROM AUTHOR]