Variability of the self-incompatibility reaction in Brassica oleracea L. with S 15 haplotype

Genetic Mechanisms for Hybrid Breeding in Vegetable Crops

To address the complex challenges faced by our planet such as rapidly changing climate patterns, food and nutritional insecurities, and the escalating world population, the development of hybrid vegetable crops is imperative. Vegetable hybrids could effectively mitigate the above-mentioned fundamental challenges in numerous countries. Utilizing genetic mechanisms to create hybrids not only reduces costs but also holds significant practical implications, particularly in streamlining hybrid seed production. These mechanisms encompass self-incompatibility (SI), male sterility, and gynoecism. The present comprehensive review is primarily focused on the elucidation of fundamental processes associated with floral characteristics, the genetic regulation of floral traits, pollen biology, and development. Specific attention is given to the mechanisms for masculinizing and feminizing cucurbits to facilitate hybrid seed production as well as the hybridization approaches used in the biofortification of vegetable crops. Furthermore, this review provides valuable insights into recent biotechnological advancements and their future utilization for developing the genetic systems of major vegetable crops.

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.

Impact of whole genome triplication on the evolutionary history and the functional dynamics of regulatory genes involved in Brassica self-incompatibility signalling pathway

Polyploidy or whole genome duplication is a frequent and recurrent phenomenon in flowering plants that has played a major role in their diversification, adaptation and speciation. The adaptive success of polyploids relates to the different evolutionary fates of duplicated genes. In this study, we explored the impact of the whole genome triplication (WGT) event in the Brassiceae tribe on the genes involved in the self-incompatibility (SI) signalling pathway, a mechanism allowing recognition and rejection of self-pollen in hermaphrodite plants. By taking advantage of the knowledge acquired on this pathway as well as of several reference genomes in Brassicaceae species, we determined copy number of the different genes involved in this pathway and investigated their structural and functional evolutionary dynamics. We could infer that whereas most genes involved in the SI signalling returned to single copies after the WGT event (i.e. ARC1, JDP1, THL1, THL2, Exo70A01) in diploid Brassica species, a few were retained in duplicated (GLO1 and PLDα) or triplicated copies (MLPK). We also carefully studied the gene structure of these latter duplicated genes (including the conservation of functional domains and active sites) and tested their transcription in the stigma to identify which copies seem to be involved in the SI signalling pathway. By taking advantage of these analyses, we then explored the putative origin of a contrasted SI phenotype between two Brassica rapa varieties that have been fully sequenced and shared the same S-allele (S60).

Overcoming Self-Incompatibility in Diploid Potato Using CRISPR-Cas9

Potato breeding can be redirected to a diploid inbred/F1 hybrid variety breeding strategy if self-compatibility can be introduced into diploid germplasm. However, the majority of diploid potato clones (Solanum spp.) possess gametophytic self-incompatibility that is primarily controlled by a single multiallelic locus called the S-locus which is composed of tightly linked genes, S-RNase (S-locus RNase) and multiple SLFs (S-locus F-box proteins), which are expressed in the style and pollen, respectively. Using S-RNase genes known to function in the Solanaceae gametophytic SI mechanism, we identified S-RNase alleles with flower-specific expression in two diploid self-incompatible potato lines using genome resequencing data. Consistent with the location of the S-locus in potato, we genetically mapped the S-RNase gene using a segregating population to a region of low recombination within the pericentromere of chromosome 1. To generate self-compatible diploid potato lines, a dual single-guide RNA (sgRNA) strategy was used to target conserved exonic regions of the S-RNase gene and generate targeted knockouts (KOs) using a Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated protein 9 (Cas9) approach. Self-compatibility was achieved in nine S-RNase KO T₀ lines which contained bi-allelic and homozygous deletions/insertions in both genotypes, transmitting self compatibility to T₁ progeny. This study demonstrates an efficient approach to achieve stable, consistent self-compatibility through S-RNase KO for use in diploid potato breeding approaches.

A Comprehensive Study of Molecular Evolution at the Self-Incompatibility Locus of Rosaceae

The family Rosaceae includes a range of important fruit trees, most of which have the S-RNase-based self-incompatibility (SI). Several models have been developed to explain how pollen (SLF) and pistil (S-RNase) components of the S-locus interact. It was discovered in 2010 that additional SLF proteins are involved in pollen specificity, and a Collaborative Non-Self Recognition model has been proposed for SI in Solanaceae; however, the validity of such model remains to be elucidated for other species. The results of this study support the divergent evolution of the S-locus genes from two Rosaceae subfamilies, Prunoideae/Amygdaloideae and Maloideae, The difference identified in the selective pressures between the two lineages provides evidence for positive selection at specific sites in both the S-RNase and the SLF proteins. The evolutionary findings of this study support the role of multiple SLF proteins leading to a Collaborative Non-Self Recognition model for SI in the Maloideae. Furthermore, the identification of the sites responsible for SI specificity determination and the mapping of these sites onto the modelled tertiary structure of ancestor proteins provide useful information for rational functional redesign and protein engineering for the future engineering of new functional alleles providing increased diversity in the SI system in the Maloideae.

Genetic variation of temperature-regulated curd induction in cauliflower: elucidation of floral transition by genome-wide association mapping and gene expression analysis

Cauliflower (Brassica oleracea var. botrytis) is a vernalization-responsive crop. High ambient temperatures delay harvest time. The elucidation of the genetic regulation of floral transition is highly interesting for a precise harvest scheduling and to ensure stable market supply. This study aims at genetic dissection of temperature-dependent curd induction in cauliflower by genome-wide association studies and gene expression analysis. To assess temperature-dependent curd induction, two greenhouse trials under distinct temperature regimes were conducted on a diversity panel consisting of 111 cauliflower commercial parent lines, genotyped with 14,385 SNPs. Broad phenotypic variation and high heritability (0.93) were observed for temperature-related curd induction within the cauliflower population. GWA mapping identified a total of 18 QTL localized on chromosomes O1, O2, O3, O4, O6, O8, and O9 for curding time under two distinct temperature regimes. Among those, several QTL are localized within regions of promising candidate flowering genes. Inferring population structure and genetic relatedness among the diversity set assigned three main genetic clusters. Linkage disequilibrium (LD) patterns estimated global LD extent of r(2) = 0.06 and a maximum physical distance of 400 kb for genetic linkage. Transcriptional profiling of flowering genes FLOWERING LOCUS C (BoFLC) and VERNALIZATION 2 (BoVRN2) was performed, showing increased expression levels of BoVRN2 in genotypes with faster curding. However, functional relevance of BoVRN2 and BoFLC2 could not consistently be supported, which probably suggests to act facultative and/or might evidence for BoVRN2/BoFLC-independent mechanisms in temperature-regulated floral transition in cauliflower. Genetic insights in temperature-regulated curd induction can underpin genetically informed phenology models and benefit molecular breeding strategies toward the development of thermo-tolerant cultivars.