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

The self-compatibility is acquired after polyploidization: a case study of Brassica napus self-incompatible trilinear hybrid breeding system

Self-incompatibility plays a vital role in angiosperms, by preventing inbreeding depression and maintaining genetic diversity within populations. Following polyploidization, many angiosperm species transition from self-incompatibility to self-compatibility. Here, we investigated the S-locus in Brassicaceae and identified distinct origins for the sRNA loci, SMI and SMI2 (SCR Methylation Inducer 1 and 2), within the S-locus. The SMI loci were found to be widespread in Cruciferae, whereas the SMI2 loci were exclusive to Brassica species. Additionally, we discovered four major S-haplotypes (BnS-1, BnS-6, BnS-7, and BnS-1300) in rapeseed. Overexpression of BnSMI-1 in self-incompatible Brassica napus ('S-70S1300S6 ') resulted in a significant increase in DNA methylation in the promoter regions of BnSCR-6 and BnSCR-1300, leading to self-compatibility. Conversely, by overexpressing a point mutation of BnSmi-1 in the 'S-70S1300S6 ' line, we observed lower levels of DNA methylation in BnSCR-6 and BnSCR-1300 promoters. Furthermore, the overexpression of BnSMI2-1300 in the 'SI-326S7S6 ' line inhibited the expression of BnSCR-7 through transcriptional repression of the Smi2 sRNA from the BnS-1300 haplotype. Our study demonstrates that the self-compatibility of rapeseed is determined by S-locus sRNA-mediated silencing of SCR after polyploidization, which helps to further breed self-incompatible or self-compatible rapeseed lines, thereby facilitating the utilization of heterosis.

Characterization of a Common S Haplotype BnS-6 in the Self-Incompatibility of Brassica napus

Self-incompatibility (SI) is a pollen-stigma recognition system controlled by a single and highly polymorphic genetic locus known as the S-locus. The S-locus exists in all Brassica napus (B. napus, AACC), but natural B. napus accessions are self-compatible. About 100 and 50 S haplotypes exist in Brassica rapa (AA) and Brassica oleracea (CC), respectively. However, S haplotypes have not been detected in B. napus populations. In this study, we detected the S haplotype distribution in B. napus and ascertained the function of a common S haplotype BnS-6 through genetic transformation. BnS-1/BnS-6 and BnS-7/BnS-6 were the main S haplotypes in 523 B. napus cultivars and inbred lines. The expression of SRK in different S haplotypes was normal (the expression of SCR in the A subgenome affected the SI phenotype) while the expression of BnSCR-6 in the C subgenome had no correlation with the SI phenotype in B. napus. The BnSCR-6 protein in BnSCR-6 overexpressed lines was functional, but the self-compatibility of overexpressed lines did not change. The low expression of BnSCR-6 could be a reason for the inactivation of BnS-6 in the SI response of B. napus. This study lays a foundation for research on the self-compatibility mechanism and the SI-related breeding in B. napus.

Characterization of self-incompatible Brassica napus lines lacking SP11 expression

Recognition of self-incompatibility (SI) is regulated by the SRK and SP11 genes in Brassicaceae. Brassica rapa and B. oleracea are self-incompatible, while most cultivated species of B. napus, which arose from hybridization between B. rapa and B. oleracea, are self-compatible. Various studies of the SRK and SP11 genes in self-compatible B. napus have been reported, but details of the mechanism in different B. napus lines are not fully understood. In this study, we confirmed the S haplotypes, SI phenotypes and SP11 expression in 10 representative lines of B. napus, and identified two SI lines (N110 and N343) lacking SP11 expression. In N343 (with BnS¹ and BnS⁶ haplotypes), we confirmed that there is a 3.6-kb insertion in the promoter region of BnSP11-1, and that BnSP11-1 and BnSP11-6 are not expressed, as reported previously (expression of BnSP11-6 is suppressed by the BnS¹ haplotype), although this line is self-incompatible. Similarly, in N110, with two novel S haplotypes (BnS⁸ and BnS⁹) in addition to BnS⁶, a 4.3-kb insertion was identified in the promoter region of BnSP11-9, and expression levels of BnSP11-6, BnSP11-8 and BnSP11-9 were all suppressed (BnSP11-6 and BnSP11-8 may be suppressed by BnS⁸ and BnS⁹, respectively), although the phenotype was self-incompatible. This observation of an SI phenotype without SP11 expression suggests the existence of unknown factor(s) that induce pollen-stigma incompatibility in B. napus.

Mechanism of Salt-Induced Self-Compatibility Dissected by Comparative Proteomic Analysis in Brassica napus L

Self-incompatibility (SI) in plants genetically prevents self-fertilization to promote outcrossing and genetic diversity. Its hybrids in Brassica have been widely cultivated due to the propagation of SI lines by spraying a salt solution. We demonstrated that suppression of Brassica napus SI from edible salt solution treatment was ascribed to sodium chloride and independent of S haplotypes, but it did not obviously change the expression of SI-related genes. Using the isobaric tags for relative and absolute quantitation (iTRAQ) technique, we identified 885 differentially accumulated proteins (DAPs) in Brassica napus stigmas of un-pollinated (UP), pollinated with compatible pollen (PC), pollinated with incompatible pollen (PI), and pollinated with incompatible pollen after edible salt solution treatment (NA). Of the 307 DAPs in NA/UP, 134 were unique and 94 were shared only with PC/UP. In PC and NA, some salt stress protein species, such as glyoxalase I, were induced, and these protein species were likely to participate in the self-compatibility (SC) pathway. Most of the identified protein species were related to metabolic pathways, biosynthesis of secondary metabolites, ribosome, and so on. A systematic analysis implied that salt treatment-overcoming SI in B.napus was likely conferred by at least five different physiological mechanisms: (i) the use of Ca2+ as signal molecule; (ii) loosening of the cell wall to allow pollen tube penetration; (iii) synthesis of compatibility factor protein species for pollen tube growth; (iv) depolymerization of microtubule networks to facilitate pollen tube movement; and (v) inhibition of protein degradation pathways to restrain the SI response.

Helitron-like transposons contributed to the mating system transition from out-crossing to self-fertilizing in polyploid Brassica napus L

The mating system transition in polyploid Brassica napus (AACC) from out-crossing to selfing is a typical trait to differentiate it from their diploid progenitors. Elucidating the mechanism of mating system transition has profound consequences for understanding the speciation and evolution in B. napus. Functional complementation experiment has shown that the insertion of 3.6 kb into the promoter of self-incompatibility male determining gene, BnSP11-1 leads to its loss of function in B. napus. The inserted fragment was found to be a non-autonomous Helitron transposon. Further analysis showed that the inserted 3.6 kb non-autonomous Helitron transposon was widely distributed in B. napus accessions which contain the S haplotype BnS-1. Through promoter deletion analysis, an enhancer and a putative cis-regulatory element (TTCTA) that were required for spatio-temporal specific expression of BnSP11-1 were identified, and both might be disrupted by the insertion of Helitron transposon. We suggested that the insertion of Helitron transposons in the promoter of BnSP11-1 gene had altered the mating system and might facilitated the speciation of B. napus. Our findings have profound consequences for understanding the self-compatibility in B. napus as well as for the trait variations during evolutionary process of plant polyploidization.