Single-seed PCR-RFLP analysis for the identification of S haplotypes in commercial F1 hybrid cultivars of broccoli and cabbage

An Identification System Targeting the SRK Gene for Selecting S-Haplotypes and Self-Compatible Lines in Cabbage

Cabbage (Brassica oleracea L. var. capitata) self-incompatibility is important for heterosis. However, the seed production of elite hybrid cannot be facilitated by honey bees due to the cross-incompatibility of the two parents. In this study, the self-compatibility of 58 winter cabbage inbred lines was identified by open-flower self-pollination (OS) and molecular techniques. Based on the NCBI database, a new class I S-haplotype-specific marker, PKC6F/PKC6R, was developed. Verification analyses revealed 9 different S-haplotypes in the 58 cabbage inbred lines; of these lines, 46 and 12 belonged to class I (S6, S7, S12, S14, S33, S45, S51, S68) and class II (S15) S-haplotypes, respectively. The coincidence rate between the self-compatibility index and S-haplotype was 91%. This study developed a Tri-Primer-PCR amplification method to rapidly select plants with specific S-haplotypes in biased segregated S-locus populations. Furthermore, it established an S-haplotype identification system based on these nine S-haplotypes. To overcome parental cross-incompatibility (18-503 and 18-512), an inbred line (18-2169) with the S15 haplotype was selected from the sister lines of self-incompatible 18-512 (S68, class I S-haplotype). The inbred line (18-2169) showed self-compatibility and cross-compatibility with 18-503. This study provides guidance for self-compatibility breeding in cabbage and predicts parental cross-incompatibility in elite combinations.

Advances in Understanding and Harnessing the Molecular Regulatory Mechanisms of Vegetable Quality

The quality of vegetables is facing new demands in terms of diversity and nutritional health. Given the improvements in living standards and the quality of consumed products, consumers are looking for vegetable products that maintain their nutrition, taste, and visual qualities. These requirements are directing scientists to focus on vegetable quality in breeding research. Thus, in recent years, research on vegetable quality has been widely carried out, and many applications have been developed via gene manipulation. In general, vegetable quality traits can be divided into three parts. First, commodity quality, which is most related to the commerciality of plants, refers to the appearance of the product. The second is flavor quality, which usually represents the texture and flavor of vegetables. Third, nutritional quality mainly refers to the contents of nutrients and health ingredients such as soluble solids (sugar), vitamin C, and minerals needed by humans. With biotechnological development, researchers can use gene manipulation technologies, such as molecular markers, transgenes and gene editing to improve the quality of vegetables. This review attempts to summarize recent studies on major vegetable crops species, with Brassicaceae, Solanaceae, and Cucurbitaceae as examples, to analyze the present situation of vegetable quality with the development of modern agriculture.

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.

Quantitative Trait Loci (QTLs) Associated with Microspore Culture in Raphanus sativus L. (Radish)

The radish is a highly self-incompatible plant, and consequently it is difficult to produce homozygous lines. Bud pollination in cross-fertilization plants should be done by opening immature pollen and attaching pollen to mature flowers. It accordingly takes a lot of time and effort to develop lines with fixed alleles. In the current study, a haploid breeding method has been applied to obtain homozygous plants in a short period of time by doubling chromosomes through the induction of a plant body in the haploid cells, in order to shorten the time to breed inbred lines. We constructed genetic maps with an F1 population derived by crossing parents that show a superior and inferior ability to regenerate microspores, respectively. Genetic maps were constructed from the maternal and parental maps, separately, using the two-way pseudo-testcross model. The phenotype of the regeneration rate was examined by microspore cultures and a quantitative trait loci (QTL) analysis was performed based on the regeneration rate. From the results of the culture of microspores in the F1 population, more than half of the group did not regenerate, and only a few showed a high regeneration rate. A total of five significant QTLs were detected in the F1 population, and five candidate genes were found based on the results. These candidate genes are divided into two classes, and appear to be related to either PRC2 subunits or auxin synthesis.

Progress on deciphering the molecular aspects of cell-to-cell communication in Brassica self-incompatibility response

The sporophytic system of self-incompatibility is a widespread genetic phenomenon in plant species, promoting out-breeding and maintaining genetic diversity. This phenomenon is of commercial importance in hybrid breeding of Brassicaceae crops and is controlled by single S locus with multiple S haplotypes. The molecular genetic studies of Brassica 'S' locus has revealed the presence of three tightly linked loci viz. S-receptor kinase (SRK), S-locus cysteine-rich protein/S-locus protein 11 (SCR/SP11), and S-locus glycoprotein (SLG). On self-pollination, the allele-specific ligand-receptor interaction activates signal transduction in stigma papilla cells and leads to rejection of pollen tube on stigmatic surface. In addition, arm-repeat-containing protein 1 (ARC1), M-locus protein kinase (MLPK), kinase-associated protein phosphatase (KAPP), exocyst complex subunit (Exo70A1) etc. has been identified in Brassica crops and plays a key role in self-incompatibility signaling pathway. Furthermore, the cytoplasmic calcium (Ca2+) influx in papilla cells also mediates self-incompatibility response in Brassicaceae, but how this cytoplasmic Ca2+ influx triggers signal transduction to inhibit pollen hydration is still obscure. There are many other signaling components which are not well characterized yet. Much progress has been made in elucidating the downstream multiple pathways of Brassica self-incompatibility response. Hence, in this review, we have made an effort to describe the recent advances made on understanding the molecular aspects of genetic mechanism of self-incompatibility in Brassicaceae.

Identification of species in tribe Brassiceae by dot-blot hybridization using species-specific ITS1 probes

Simple, reliable methods for identification of species are required for management of many species and lines in a plant gene bank. Species-specific probes were designed from published sequences of the ITS1 region in rDNA of 16 species in Brassica and its related genera, and used as probes for dot-blot hybridization with plant genomic DNA. All the probes detected species-specific signals at dot-blots of genomic DNAs of the 16 species in Brassica, Diplotaxis, Eruca, and Raphanus. Signals of the Brassica digenomic species in the U's triangle, i.e., B. napus, B. juncea, and B. carinata, were detected by the probes of their parental monogenomic species, i.e., B. rapa, B. nigra, and B. oleracea. The probe for B. oleracea showed signals of B. balearica, B. cretica, B. incana, B. insularis, and B. macrocarpa, which have the C genome as B. oleracea. Eruca vesicaria DNA was detected by the probe for E. sativa, which has been classified as a subspecies of E. vescaria. DNA of leaf tissue extracted by an alkaline solution and seed DNA prepared by the NaI method can be used directly for dot-blotting. Misidentification of species was revealed in 20 accessions in the Tohoku University Brassica Seed Bank. These results indicate dot-blot hybridization to be a simple and efficient technique for identification of plant species in a gene bank.

Assessment of genetic diversity of accessions in Brassicaceae genetic resources by frequency distribution analysis of S haplotypes

Plant genetic resources are important sources of genetic variation for improving crop varieties as breeding materials. Conservation of such resources of allogamous species requires maintenance of the genetic diversity within each accession to avoid inbreeding depression and loss of rare alleles. For assessment of genetic diversity in the self-incompatibility locus (S locus), which is critically involved in the chance of mating, we developed a dot-blot genotyping method for self-incompatibility (S) haplotypes and applied it to indigenous, miscellaneous landraces of Brassica rapa, provided by the IPK Gene Bank (Gatersleben, Germany) and the Tohoku University Brassica Seed Bank (Sendai, Japan), in which landraces are maintained using different population sizes. This method effectively determined S genotypes of more than 500 individuals from the focal landraces. Although our results suggest that these landraces might possess sufficient numbers of S haplotypes, the strong reduction of frequencies of recessive S haplotypes occurred, probably owing to genetic drift. Based on these results, we herein discuss an appropriate way to conserve genetic diversity of allogamous plant resources in a gene bank.

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

Brassica napus (AACC, 2n = 38) is a self-compatible amphidiploid plant that arose from the interspecies hybridization of two self-incompatible species, B. rapa (AA, 2n = 20) and B. oleracea (CC, 2n = 18). Self-incompatibility (S) haplotypes in one self-incompatible line and 124 cultivated B. napus lines were detected using S-locus-specific primers, and their relationships with restorer-maintainers were investigated. Two class I (S-I ( SLG ) a and S-I ( SLG ) b) and four class II (S-II ( SLG ) a, S-II ( SLG ) b, S-II ( SP11 ) a and S-II ( SP11 ) b) S haplotypes were observed, of which S-II ( SP11 ) b was newly identified. The nucleotide sequence of SP11 showed little similarity to the reported SP11 alleles. The lines were found to express a total of eleven S genotypes. The self-incompatible line had a specific genotype consisting of S-II ( SP11 ) a, similar to B. rapa S-60, and S-II ( SLG ) a, similar to B. oleracea S-15. Restorers expressed six genotypes: the most common genotype contained S-I ( SLG ) a, similar to B. rapa S-47, and S-II ( SLG ) b, similar to B. oleracea S-15. Maintainers expressed nine genotypes: the predominant genotype was homozygous for two S haplotypes, S-II ( SLG ) a and S-II ( SP11 ) b. One genotype was specific to restorers and four genotypes were specific to maintainers, whereas five genotypes were expressed in both restorers and maintainers. This suggests that there is no definitive correlation between the distribution of S genotypes and restorer-maintainers of self-incompatibility. The finding that restorers and maintainers express unique genotypes, and share some common genotypes, would be valuable for detecting the interaction of S haplotypes in inter- or intra-genomes as well as for developing markers-assisted selection in self-incompatibility hybrid breeding.

The pattern of amplification and differentiation of Ty1-copia and Ty3-gypsy retrotransposons in Brassicaceae species

One of the causes of genome size expansion is considered to be amplification of retrotransposons. We determined nucleotide sequences of 24 PCR products for each of six retrotransposons in Brassica rapa and Brassica oleracea. Phylogenetic trees of these sequences showed species-specific clades. We also sequenced STF7a homologs and Tto1 homologs, 24 PCR products each, in nine diploids and three allopolyploids, and constructed phylogenetic trees. In these phylogenetic trees, species-specific clades of diploid species were also formed, but retrotransposons of allopolyploids were clustered into the clades of their original genomes, indicating that these two retrotransposons amplified after speciation of the nine diploids. Genetic variation in these retrotransposons may have arisen before emergence of allopolyploid species. There was a positive correlation between the genome size and the average number of substitutions of STF7a and Tto1 homologs in at least seven diploids. The implications of these results in the genome evolution of Brassicaceae are herein discussed.

Self-compatibility in Brassica napus is caused by independent mutations in S-locus genes

Brassica napus is an amphidiploid species with the A genome from Brassica rapa and the C genome from Brassica oleracea. Although B. rapa, B. oleracea and artificially synthesized amphidiploids with the AC genome are self-incompatible, B. napus is self-compatible. Six S genotypes were identified in B. napus, five of which had class I S haplotypes from one species and a class II S haplotype from the other species, and mutations causing self-compatibility were identified in three of these S genotypes. The most predominant S genotype (BnS-1;BnS-6), which is that of cv. 'Westar', had a class I S haplotype similar to B. rapa S-47 (BrS-47) and a class II S haplotype similar to B. oleracea S-15 (BoS-15). The stigmas of 'Westar' rejected the pollen grains of both BrS-47 and BoS-15, while reciprocal crossings were compatible. Insertion of a DNA fragment of about 3.6 kb was found in the promoter region of the SP11/SCR allele of BnS-1, and transcripts of SP11/SCR were not detected in 'Westar'. The nucleotide sequence of the SP11 genomic DNA of BnS-6 was 100% identical to that of SP11 of BoS-15. Class I SP11 alleles from one species showed dominance over class II SP11 alleles from the other species in artificially synthesized B. napus lines, suggesting that the non-functional dominant SP11 allele suppressed the expression of the recessive SP11 allele in 'Westar'. Two other S genotypes in B. napus also had non-functional class I S haplotypes together with recessive BnS-6. These observations suggest independent origins of self-compatibility in B. napus.

Gene conversion from SLG to SRK resulting in self-compatibility in Brassica rapa

Self-compatible S-54 homozygotic plants were found in progenies of an F(1) hybrid cultivar in Chinese cabbage. Pollination tests revealed that this self-compatibility is controlled by the S locus and caused by the loss of the recognition function of the stigma. SRK, the gene for the recognition molecule in the stigma, was normally transcribed and translated in the self-compatible plants. The 1034-bp region in the receptor domain of SRK in the self-compatible plants was 100% identical to SLG in S-54, while that in self-incompatible S-54 homozygotic plants was 95.1% identical. These results suggest that the self-compatibility of the S-54 homozygotes is due to amino-acid changes caused by gene conversion from SLG to SRK.

Identification of S haplotypes in Brassica by dot-blot analysis of SP11 alleles

A self-incompatibility system is used for F(1) hybrid breeding in Brassicaceae vegetables. The determinants of recognition specificity of self-incompatibility in Brassica are SRK in the stigma and SP11/SCR in the pollen. Nucleotide sequences of SP11 alleles are more highly variable than those of SRK. We analyzed the S haplotype specificity of SP11 DNA by Southern-blot analysis and dot-blot analysis using 16 S haplotypes in Brassica oleracea, and found that DNA fragments of a mature protein region of SP11 cDNA, SP11(m), of eight S haplotypes can detect only the SP11 alleles of the same S haplotypes. This specificity makes these methods useful for S haplotype identification. Therefore, we developed two methods of dot-blot analysis for SP11. One is dot blotting of DNA samples, i.e. plant genomic DNA probed with labeled SP11(m), and the other is dot blotting of SP11(m) DNA fragments probed with labeled DNA samples, i.e. the SP11 coding region labeled by PCR using a template of plant genomic DNA. The former is useful for testing many plant materials. The latter is suitable, if there is no previous information on the S haplotypes of plant materials.