Distribution of S haplotypes and its relationship with restorer-maintainers of self-incompatibility in cultivated Brassica napus

Genetic and Molecular Characterization of a Self-Compatible Brassica rapa Line Possessing a New Class II S Haplotype

Most flowering plants have evolved a self-incompatibility (SI) system to maintain genetic diversity by preventing self-pollination. The Brassica species possesses sporophytic self-incompatibility (SSI), which is controlled by the pollen- and stigma-determinant factors SP11/SCR and SRK. However, the mysterious molecular mechanism of SI remains largely unknown. Here, a new class II S haplotype, named BrS-325, was identified in a pak choi line '325', which was responsible for the completely self-compatible phenotype. To obtain the entire S locus sequences, a complete pak choi genome was gained through Nanopore sequencing and de novo assembly, which provided a good reference genome for breeding and molecular research in B. rapa. S locus comparative analysis showed that the closest relatives to BrS-325 was BrS-60, and high sequence polymorphism existed in the S locus. Meanwhile, two duplicated SRKs (BrSRK-325a and BrSRK-325b) were distributed in the BrS-325 locus with opposite transcription directions. BrSRK-325b and BrSCR-325 were expressed normally at the transcriptional level. The multiple sequence alignment of SCRs and SRKs in class II S haplotypes showed that a number of amino acid variations were present in the contact regions (CR II and CR III) of BrSCR-325 and the hypervariable regions (HV I and HV II) of BrSRK-325s, which may influence the binding and interaction between the ligand and the receptor. Thus, these results suggested that amino acid variations in contact sites may lead to the SI destruction of a new class II S haplotype BrS-325 in B. rapa. The complete SC phenotype of '325' showed the potential for practical breeding application value in B. rapa.

Self-(In)compatibility Systems: Target Traits for Crop-Production, Plant Breeding, and Biotechnology

Self-incompatibility (SI) mechanisms prevent self-fertilization in flowering plants based on specific discrimination between self- and non-self pollen. Since this trait promotes outcrossing and avoids inbreeding it is a widespread mechanism of controlling sexual plant reproduction. Growers and breeders have effectively exploited SI as a tool for manipulating domesticated crops for thousands of years. However, only within the past thirty years have studies begun to elucidate the underlying molecular features of SI. The specific S-determinants and some modifier factors controlling SI have been identified in the sporophytic system exhibited by Brassica species and in the two very distinct gametophytic systems present in Papaveraceae on one side and in Solanaceae, Rosaceae, and Plantaginaceae on the other. Molecular level studies have enabled SI to SC transitions (and vice versa) to be intentionally manipulated using marker assisted breeding and targeted approaches based on transgene integration, silencing, and more recently CRISPR knock-out of SI-related factors. These scientific advances have, in turn, provided a solid basis to implement new crop production and plant breeding practices. Applications of self-(in)compatibility include widely differing objectives such as crop yield and quality improvement, marker-assisted breeding through SI genotyping, and development of hybrids for overcoming intra- and interspecific reproductive barriers. Here, we review scientific progress as well as patented applications of SI, and also highlight future prospects including further elucidation of SI systems, deepening our understanding of SI-environment relationships, and new perspectives on plant self/non-self recognition.

Gene expression and genetic analysis reveal diverse causes of recessive self-compatibility in Brassica napus L

Background

Brassica napus (AACC) is self-compatible, although its ancestor species Brassica rapa (AA) and Brassica oleracea (CC) are self-incompatible. Most B.napus accessions have dominant self-compatibility (SC) resulting from an insertion of 3.6 kb in the promoter region of BnSCR-1 on the A genome, while recessive SC in B.napus has rarely been observed. Expression and cloning of SRK and SCR genes and genetic analysis were carried out to dissect bases of recessive SC in B.napus.

Results

Eleven accessions were screened to identify stable recessive SC and had the S genotype BnS-7 on the A genome and BnS-6 on the C genome similarly to BrS-29 and BoS-15, respectively. In eight SC accessions, BnSCR-7 and BnSCR-6 were nearly undetectable and harbored no structural mutations in the promoters, while SRK genes were expressed at normal levels and contained intact CDS, with the exception of BnSRK-7 in line C32. SRK and SCR genes were expressed normally but their CDSs had no mutations in three SC accessions. In self-incompatible S-1300 and 11 F₁ hybrids, SRK genes and BnSCR-1300 transcripts were present at high levels, while expression of the BnSCR-7 and BnSCR-6 were absent. Plants of S genotype S₁₃₀₀S₁₃₀₀ were completely SI, while SI phenotypes of S_BnS-7S_BnS-7 and S₁₃₀₀S_BnS-7 plants were segregated in BC₁ and F₂ populations.

Conclusions

The recessive SC in eight accessions is caused by the loss of function of BnSCR-7 and BnSCR-6 in pollen. Translational repression contributes to the recessive SC in three accessions, whose SRK and SCR genes were expressed normally and had identical CDSs to BrS-29 or BoS-15. SI in 11 F₁ hybrids relies on the expression of BnSCR-1300 rather than SRK genes. Other factor(s) independent of the S locus are involved in recessive SC. Therefore, diverse causes underlie recessive SC in B. napus, yielding insight into these complex mechanisms.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1037) contains supplementary material, which is available to authorized users.

Detection of self-incompatible oilseed rape plants (Brassica napus L.) based on molecular markers for identification of the class I S haplotype

The selection of desirable genotypes with recessive characteristics, such as self-incompatible plants, is often difficult or even impossible and represents a crucial barrier in accelerating the breeding process. Molecular approaches and selection based on molecular markers can allow breeders to overcome this limitation. The use of self-incompatibility is an alternative in hybrid breeding of oilseed rape. Unfortunately, stable self-incompatibility is recessive and phenotype-based selection is very difficult and time-consuming. The development of reliable molecular markers for detecting desirable plants with functional self-incompatible genes is of great importance for breeders and allows selection at early stages of plant growth. Because most of these reliable molecular markers are based on discrimination of class I S-locus genes that are present in self-compatible plants, there is a need to use an internal control in order to detect possible PCR inhibition that gives false results during genotyping. In this study, 269 double haploid F2 oilseed rape plants obtained by microspore embryogenesis were used to verify the applicability of an improved PCR assay based on the detection of the class I SLG gene along with an internal control. Comparative analysis of the PCR genotyping results vs. S phenotype analysis confirmed the applicability of this molecular approach in hybrid breeding programs. This approach allows accurate detection of self-incompatible plants via a different amplification profile.

The self-compatibility mechanism in Brassica napus L. is applicable to F1 hybrid breeding

Brassica napus, an allopolyploid species having the A genome of B. rapa and the C genome of B. oleracea, is self-compatible, although both B. rapa and B. oleracea are self-incompatible. We have previously reported that SP11/SCR alleles are not expressed in anthers, while SRK alleles are functional in the stigma in B. napus cv. 'Westar', which has BnS-1 similar to B. rapa S-47 and BnS-6 similar to B. oleracea S-15. This genotype is the most frequent S genotype in B. napus, and we hypothesized that the loss of the function of SP11 is the primary cause of the self-compatibility of 'Westar'. To verify this hypothesis, we transformed 'Westar' plants with the SP11 allele of B. rapa S-47. All the transgenic plants and their progeny were completely self-incompatible, demonstrating self-compatibility to be due to the S haplotype having the non-functional SP11 allele in the A genome, which suppresses a functional recessive SP11 allele in the C genome. An artificially synthesized B. napus line having two recessive SP11 alleles was developed by interspecific hybridization between B. rapa and B. oleracea. This line was self-incompatible, but F(1) hybrids between this line and 'Westar' were self-compatible. These results suggest that the self-compatibility mechanism of 'Westar' is applicable to F(1) seed production in B. napus.