Identification of candidate genes associated with fertility restoration of cytoplasmic male-sterility in onion (Allium cepa L.) using a combination of bulked segregant analysis and RNA-seq

Identification of a gene coding for a pentatricopeptide repeat protein as a candidate responsible for the Ms2, a novel restorer-of-fertility locus in onion (Allium cepa L.)

Ms and Ms2 are restorer-of-fertility loci in onion (Allium cepa L.); additionally, Ms2 is responsible for unstable male fertility in some accessions. Although a candidate gene was previously reported for the Ms locus, the gene responsible for the Ms2 locus remains unidentified. A 12.5 Mb genomic region harboring the Ms2 locus was initially obtained from onion whole genome sequences using two flanking markers to identify candidates. This region was further delimited to 3.19 Mb via fine mapping using 12 recombinants and 11 additional markers. A gene coding for a pentatricopeptide repeat (PPR) protein was identified within the 3.19 Mb region and designated AcPPR876. Phylogenetic analysis showed that AcPPR876 and four homologs belonged to the Rf-like PPR gene family. Polymorphic sequences between male fertile (MF) and male sterile (MS) AcPPR876 alleles were concentrated in the 5' region of the gene. Among them, a 446 bp insertion was identified at the putative promoter region of the MF allele. Although overall AcPPR876 transcription levels were very low, transcription levels of the MF allele were generally higher than those of the MS allele. A simple PCR marker was developed using the 446 bp insertion to perform Ms2 locus genotyping in the diverse onion germplasm. Although the dominant Ms2 allele was not found in any of the 250 domestic breeding lines, 29 out of 108 exotic accessions were shown to contain the dominant Ms2 alleles. Overall, the AcPPR876 gene is proposed as a strong candidate for the Ms2 locus.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-025-01561-5.

Fertility restorer gene CaRf and PepperSNP50K provide a promising breeding system for hybrid pepper

Cytoplasmic male sterility (CMS) is pivotal in plant breeding and widely employed in various crop hybrids, including pepper. However, the functional validation of the restorer of fertility (Rf) gene in pepper has been lacking until now. This study identifies and characterizes CaRf, a single dominant locus crucial for restoring CMS in the pepper strong recovery inbred line Zhangshugang. The CaRf gene encodes a mitochondria-targeted pentatricopeptide repeat protein, validated through the induction of male sterility upon its silencing in hybrid F1 plants. To enhance pepper breeding efficiency, 176 important pepper breeding parent materials were resequenced, and a PepperSNP50K liquid-phase breeding chip was developed, comprising 51 172 markers. Integration of CaRf functional characterization and PepperSNP50K facilitated the development of a high-quality red pepper hybrid. These findings provide significant insights and practical strategies for advancing molecular-designed breeding in peppers.

Combined BSA-Seq and RNA-Seq Reveal Genes Associated with the Visual Stay-Green of Maize (Zea mays L.)

Maize has become one of the most widely grown grains in the world, and the stay-green mutant allows these plants to maintain their green leaves and photosynthetic potential for longer following anthesis than in non-mutated plants. As a result, stay-green plants have a higher production rate than non-stay-green varieties due to their prolonged grain-filling period. In this study, the candidate genes related to the visual stay-green at the maturation stage of maize were investigated. The F2 population was derived from the T01 (stay-green) and the Xin3 (non-stay-green) cross. Two bulked segregant analysis pools were constructed. According to the method of combining ED (Euclidean distance), Ridit (relative to an identified distribution unit), SmoothG, and SNP algorithms, a region containing 778 genes on chromosome 9 was recognized as the candidate region associated with the visual stay-green in maize. A total of eight modules were identified using WGCNA (weighted correlation network analysis), of which green, brown, pink, and salmon modules were significantly correlated with visual stay-green. BSA, combined with the annotation function, discovered 7 potential candidate genes, while WGCNA discovered 11 stay-green potential candidate genes. The candidate range was further reduced due through association analysis of BSA-seq and RNA-seq. We identified Zm00001eb378880, Zm00001eb383680, and Zm00001eb384100 to be the most likely candidate genes. Our results provide valuable insights into this new germplasm resource with reference to increasing the yield for maize.

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.

Fine mapping and causal gene identification of a novel QTL for early flowering by QTL-seq, Target-seq and RNA-seq in spring oilseed rape

A novel quantitative trait locus for early flowering in spring oilseed rape, BnaC08cqDTF, was mapped to an 86-kb region on chromosome C08, and its causal gene, CRY2, was uncovered. Days to flowering is a very important agronomic and adaptive trait of Brassica napus oilseed rape (AACC, 2n = 38). We previously identified BnaC08cqDTF as a novel candidate quantitative trait locus (QTL) for early flowering in spring oilseed rape. Here, we present fine mapping of the locus and a study of its causal gene. Initial mapping was performed by QTL sequencing of DNA pools of BC₃F₂ plants with extreme flowering times derived from crosses between the spring-type cv. No. 4512 (early flowering) and cv. No. 5246 (late flowering), along with fine mapping by target sequencing of the BC₃F₂ and BC₄F₂ populations. Fine mapping narrowed down BnaC08cqDTF to an 86-kb region on chromosome C08. The region harbored fifteen genes. After comparative analyses of the DNA sequences for mutation between A and C syntenic regions and detected by RNA-seq and qRT-PCR between the two parents, we found that BnaC08G0010400ZS harbors an A/G nonsynonymous mutation in exon 3. This single nucleotide polymorphism (SNP) haplotype was also correlated with early flowering in a 256 accession panel. BnaC08G0010400ZS is a homolog of the AT1G04400 gene (CRY2) in Arabidopsis. The analyses of transgenic Arabidopsis verified that BnaC08G0010400ZS is responsible for early flowering. Our results contribute to a better understanding of the genetic control mechanism of early flowering in spring Brassica napus and will promote the breeding for early mature varieties.

Construction of a high-resolution genetic map and identification of single nucleotide polymorphism markers relevant to flower stalk height in onion

INTRODUCTION

Onion (Allium cepa L., 2n=16) is an economically and nutritionally important vegetable crop worldwide. Construction of a high-resolution genetic map and map-based gene mining in onion have lagged behind other vegetable crops such as tomato and pepper.

METHODS

In this study, we constructed a high-resolution genetic map of onion using 321 F2 individuals from a cross between two double haploid lines DH-1×DH-17 and employing specific length amplified fragment (SLAF)-seq technology. The genetic map containing 10,584 polymorphic SLAFs with 21,250 single nucleotide polymorphism (SNP) markers and 8 linkage groups was developed for onion, which spanned 928.32 cM, with an average distance of 0.09 cM between adjacent markers.

RESULTS

Using this map, we carried out QTL mapping of Ms locus related to the male-fertile trait and reproduced previous mapping results, which proved that this map was of good quality. Then, four QTLs (located on LG2, LG5, and LG8) were detected for flower stalk height, explaining 26.60% of the phenotypic variance. Among them, we proposed that 20 SLAF markers (in three QTLs) of flower stalk height trait were effective favorable allelic variant markers associated with heterosis.

DISCUSSION

Overall, the genetic map was structured using SLAF-seq based on DH lines, and it is the highest-quality and highest-resolution linkage map of onion to date. It lays a foundation for the fine mapping and candidate gene identification of flower stalk height, and provides new insights into the developmental genetic mechanisms in onion breeding.

Two-Step Identification of N-, S-, R- and T-Cytoplasm Types in Onion Breeding Lines Using High-Resolution Melting (HRM)-Based Markers

High-resolution melting (HRM) analysis is a powerful detection method for fast, high-throughput post-PCR analysis. A two-step HRM marker system was developed for identification of the N-, S-, R- and T-cytoplasms of onion. In the first step for the identification of N-, S- and R-cytoplasms, one forward primer was designed to the identical sequences of both cox1 and orf725 genes, and two reverse primers specific to the polymorphic sequences of cox1 and orf725 genes were used. For the second step, breeding lines with N-cytoplasm were evaluated with primers developed from the orfA501 sequence to distinguish between N- and T-cytoplasms. An amplicon with primers to the mitocondrial atp9 gene was used as an internal control. The two-step HRM marker system was tested using 246 onion plants. HRM analysis showed that the most common source of CMS, often used by Russian breeders, was S-cytoplasm; the rarest type of CMS was R-cytoplasm; and the proportion of T-cytoplasm among the analyzed breeding lines was 20.5%. The identification of the cytoplasm of a single plant by phenotype takes from 4 to 8 years. The HRM-based system enables quick and easy distinguishing of the four types of onion cytoplasm.

Comparative phylogenomic analyses and co-expression gene network reveal insights in flowering time and aborted meiosis in woody bamboo, Bambusa oldhamii 'Xia Zao' ZSX

Woody bamboos have peculiar flowering characteristics with intervals ranging from several years to more than 100 years. Elucidating flowering time and reproductive development in bamboo could be beneficial for both humans and wildlife. To identity the mechanisms responsible for flowering time and embryo abortion in Bambusa oldhamii 'Xia Zao' ZSX, a transcriptome sequencing project was initiated to characterize the genes involved in developing flowers in this bamboo species. Morphological studies showed that pollen abortion in this bamboo species was mainly caused by a delay in tapetum degradation and abnormal meiotic process. Differential expression (DE) and optimized hierarchical clustering analyses identified three of nine gene expression clusters with decreasing expression at the meiosis of flowering stages. Together with enriched Gene Ontology Biological Process terms for meiosis, this suggests that their expression pattern may be associated with aborted meiosis in B. oldhamii 'Xia Zao'. Moreover, our large-scale phylogenomic analyses comparing meiosis-related transcripts of B. oldhamii 'Xia Zao' with well annotated genes in 22 representative angiosperms and sequence evolution analyses reveal two core meiotic genes NO EXINE FORMATION 1 (NFE1) and PMS1 with nonsense mutations in their coding regions, likely providing another line of evidence supporting embryo abortion in B. oldhamii 'Xia Zao'. Similar analyses, however, reveal conserved sequence evolution in flowering pathways such as LEAFY (LFY) and FLOWERING LOCUS T (FT). Seventeen orthogroups associated with flowering were identified by DE analyses between nonflowering and flowering culm buds. Six regulators found primarily in several connected network nodes of the photoperiod pathway were confirmed by mapping to the flowering time network in rice, such as Heading date (Hd3a) and Rice FT-like 1 (RFT1) which integrate upstream signaling into the downstream effectors. This suggests the existence of an intact photoperiod pathway is likely the key regulators that switch on/off flowering in B. oldhamii 'Xia Zao'.

Fine mapping of Rf2, a minor Restorer-of-fertility (Rf) gene for cytoplasmic male sterility in chili pepper G164 (Capsicum annuum L.)

Genome re-sequencing and recombination analyses identified Capana06g000193 as a strong candidate for the minor male fertility restoration locus Rf2 in chili pepper G164 harboring two dominant male fertility restoration genes. Male fertility restoration genes of chili pepper restorer line G164 (Capsicum annuum L.) were studied using molecular marker genotypes of an F₂ population (7G) of G164 crossed with the cytoplasmic male sterility line 77013A. The ratio of sterile to fertile single plants in the F₂ population was 1:15. This result indicates that chili pepper G164 has two dominant restoration genes, which we designated as Rf1 and Rf2. An individual plant recessive for Rf1 and heterozygous for Rf2, 7G-112 (rf1rf1Rf2rf2), was identified by molecular marker selection and genetic analysis, and a single Rf2 gene-segregating population with a 3:1 ratio of fertile to sterile plants was developed from the self-pollination of male fertile individuals of 77013A and 7G-112 hybrid progeny. Bulk segregant analysis of fertile and sterile pools from the segregating populations was used to genetically map Rf2 to a 3.1-Mb region on chromosome 6. Rf2 was further narrowed to a 179.3-kb interval through recombination analysis of molecular markers and obtained the most likely candidate gene, Capana06g000193.

Using Transcriptome Analysis to Explore Gray Mold Resistance-Related Genes in Onion (Allium cepa L.)

Gray mold disease caused by Botrytis in onions (Allium cepa L.) during growth and storage negatively affects their yield and quality. Exploring the genes related to gray mold resistance in onion and their application to the breeding of resistant onion lines will support effective and ecological control methods of the disease. Here, the genetic relationship of 54 onion lines based on random amplified polymorphic DNA (RAPD) and in vitro-cultured onion lines infected with gray mold were used for screening resistance and susceptibility traits. Two genetically related onion lines were selected, one with a resistant and one with a susceptible phenotype. In vitro gray mold infection was repeated with these two lines, and leaf samples were collected for gene expression studies in time series. Transcript sequences obtained by RNA sequencing were subjected to DEG analysis, variant analysis, and KEGG mapping. Among the KEGG pathways, ‘α-linoleic acid metabolism’ was selected because the comparison of the time series expression pattern of Jasmonate resistant 1 (JAR1), Coronatine-insensitive protein 1 (COI 1), and transcription factor MYC2 (MYC2) genes between the resistant and susceptible lines revealed its significant relationship with gray-mold-resistant phenotypes. Expression pattern and SNP of the selected genes were verified by quantitative real-time PCR and high-resolution melting (HRM) analysis, respectively. The results of this study will be useful for the development of molecular marker and finally breeding of gray-mold-resistant onions.

Bulk segregation analysis in the NGS era: a review of its teenage years

Bulk segregation analysis (BSA) utilizes a strategy of pooling individuals with extreme phenotypes to conduct economical and rapidly linked marker screening or quantitative trait locus (QTL) mapping. With the development of next-generation sequencing (NGS) technology in the past 10 years, BSA methods and technical systems have been gradually developed and improved. At the same time, the ever-decreasing costs of sequencing accelerate NGS-based BSA application in different species, including eukaryotic yeast, grain crops, economic crops, horticultural crops, trees, aquatic animals, and insects. This paper provides a landscape of BSA methods and reviews the BSA development process in the past decade, including the sequencing method for BSA, different populations, different mapping algorithms, associated region threshold determination, and factors affecting BSA mapping. Finally, we summarize related strategies in QTL fine mapping combining BSA.

Exploiting sterility and fertility variation in cytoplasmic male sterile vegetable crops

Cytoplasmic male sterility (CMS) has long been used to economically produce hybrids that harness growth vigor through heterosis. Yet, how CMS systems operate within commercially viable seed production strategies in various economically important vegetable crops, and their underlying molecular mechanisms, are often overlooked details that could expand the utility of CMS as a cost-effective and stable system. We provide here an update on the nature of cytoplasmic-nuclear interplay for pollen sterility and fertility transitions in vegetable crops, based on the discovery of components of nuclear fertility restoration and reversion determinants. Within plant CMS systems, pollen fertility can be rescued by the introduction of nuclear fertility restorer genes (Rfs), which operate by varied mechanisms to countermand the sterility phenotype. By understanding these systems, it is now becoming feasible to achieve fertility restoration with Rfs designed for programmable CMS-associated open reading frames (ORFs). Likewise, new opportunities exist for targeted disruption of CMS-associated ORFs by mito-TALENs in crops where natural Rfs have not been readily identified, providing an alternative approach to recovering fertility of cytoplasmic male sterile lines in crops. Recent findings show that facultative gynodioecy, as a reproductive strategy, can coordinate the sterility and fertility transition in response to environmental cues and/or metabolic signals that reflect ecological conditions of reproductive isolation. This information is important to devising future systems that are more inherently stable.

Development and application of KASP markers associated with Restorer-of-fertility gene in Capsicum annuum L

Fertility restoration of cytoplasmic male sterility (CMS) in Capsicum annuum is controlled by multiple alleles of Restorer-of-fertility (Rf) genes. The isolation of additional Rf genes should therefore enrich the knowledge of CMS/Rf systems and accelerate their exploitation in hybrid seed production. In this study, the fertility restorer gene CaRfm of '0601 M', a non-pungent bell pepper, was genetically mapped to a 1.2-cM region flanked by KASP markers S761 and S183. CaRfm was then physically mapped to a 128.96-Kb interval predicted from 24 recombinants with two co-segregated markers, S423 and S424. CaPPR6 encoding a pentatricopeptide repeat (PPR) protein was suggested as the most likely candidate gene for the CaRfm locus on the basis of sequence alignment as well as genotyping of tightly linked markers. In addition, molecular markers S1597 and S1609, which are immediately adjacent to CaRfm at 15.7 and 57.8-Kb respectively, were developed and applied to marker-assisted selection. The results provided friendly markers for breeding pepper restorer lines and laid the foundation for elucidating the male fertility restoration mechanism.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01109-9.

Fine-mapping of the BjPur gene for purple leaf color in Brassica juncea

Purple leaves are rich in health-protecting anthocyanins and food colorants in Brassica juncea. But the causal gene, which is related to leaf color formation, have not been reported in B. juncea. Anthocyanins mainly accumulated throughout the adaxial and abaxial epidermal leaf cells of purple leaves. A genetic analysis indicated that an incompletely dominant gene controls the purple leaf trait in B. juncea. Furthermore, the BjPur gene, which increased anthocyanin accumulation in purple-leaf mustard, was cloned. Blast and phylogenetic analyses revealed that BjPur encodes a new R2R3-MYB transcription factor. Sequence analysis of two alleles revealed a DNA sequence insertion in the first intron of BjPur in green leaves parent line (LY) when compared with the BjPur gene in the purple-leaf parent line (ZY). And this insertion greatly reduced the transcription of BjPur in green leaves. In purple-leaf plants, the transcript level of BjPur was significantly higher in leaves than in roots, stems, siliques, and flower buds. Additionally, molecular markers linked to leaf color were developed to distinguish different genotypes of B. juncea. These results will be helpful for the genetic improvement of the purple leaf color in B. juncea.

The loss of function of HEL, which encodes a cellulose synthase interactive protein, causes helical and vine-like growth of tomato

Helical growth is an economical way for plant to obtain resources. The classic microtubule-microfibril alignment model of Arabidopsis helical growth involves restriction of the appropriate orientation of cellulose microfibrils appropriately in the cell walls. However, the molecular mechanism underlying tomato helical growth remains unknown. Here, we identified a spontaneous tomato helical (hel) mutant with right-handed helical cotyledons and petals but left-handed helical stems and true leaves. Genetic analysis revealed that the hel phenotype was controlled by a single recessive gene. Using map-based cloning, we cloned the HEL gene, which encodes a cellulose interacting protein homologous to CSI1 of Arabidopsis. We identified a 27 bp fragment replacement that generated a premature stop codon. Transgenic experiments showed that the helical growth phenotype could be restored by the allele of this gene from wild-type Pyriforme. In contrast, the knockout mutation of HEL in Pyriforme via CRISPR/Cas9 resulted in helical growth. These findings shed light on the molecular control of the helical growth of tomato.

Omics approaches in Allium research: Progress and way ahead

BACKGROUND

The genus Allium (Family: Amaryllidaceae) is an economically important group of crops cultivated worldwide for their use as a vegetable and spices. Alliums are also well known for their nutraceutical properties. Among alliums, onion, garlic, leek, and chives cultivated worldwide. Despite their substantial economic and medicinal importance, the genome sequence of any of the Allium is not available, probably due to their large genome sizes. Recently evolved omics technologies are highly efficient and robust in elucidating molecular mechanisms of several complex life processes in plants. Omics technologies, such as genomics, transcriptomics, proteomics, metabolomics, metagenomics, etc. have the potential to open new avenues in research and improvement of allium crops where genome sequence information is limited. A significant amount of data has been generated using these technologies for various Allium species; it will help in understanding the key traits in Allium crops such as flowering, bulb development, flavonoid biosynthesis, male sterility and stress tolerance at molecular and metabolite level. This information will ultimately assist us in speeding up the breeding in Allium crops.

METHOD

In the present review, major omics approaches, and their progress, as well as potential applications in Allium crops, could be discussed in detail.

RESULTS

Here, we have discussed the recent progress made in Allium research using omics technologies such as genomics, transcriptomics, micro RNAs, proteomics, metabolomics, and metagenomics. These omics interventions have been used in alliums for marker discovery, the study of the biotic and abiotic stress response, male sterility, organ development, flavonoid and bulb color, micro RNA discovery, and microbiome associated with Allium crops. Further, we also emphasized the integrated use of these omics platforms for a better understanding of the complex molecular mechanisms to speed up the breeding programs for better cultivars.

CONCLUSION

All the information and literature provided in the present review throws light on the progress and potential of omics platforms in the research of Allium crops. We also mentioned a few research areas in Allium crops that need to be explored using omics technologies to get more insight. Overall, alliums are an under-studied group of plants, and thus, there is tremendous scope and need for research in Allium species.

Identification of a Major QTL and Candidate Gene Analysis of Salt Tolerance at the Bud Burst Stage in Rice (Oryza sativa L.) Using QTL-Seq and RNA-Seq

BACKGROUND

Salt stress is one of the main abiotic stresses that limits rice production worldwide. Rice salt tolerance at the bud burst stage directly affects the seedling survival rate and the final yield in the direct seeding cultivation model. However, the reports on quantitative trait locus (QTL) mapping and map-based cloning for salt tolerance at the bud burst stage are limited.

RESULTS

Here, an F_2:3 population derived from a cross between IR36 (salt-sensitive) and Weiguo (salt-tolerant) was used to identify salt-tolerant QTL interval at the bud burst stage using a whole-genome sequencing-based QTL-seq containing 40 extreme salt-tolerant and 40 extreme salt-sensitive individuals. A major QTL, qRSL7, related to relative shoot length (RSL) was detected on chromosome 7 using ΔSNP index algorithms and Euclidean Distance (ED) algorithms. According to single nucleotide polymorphisms (SNPs) between the parents, 25 Kompetitive allele-specific PCR (KASP) markers were developed near qRSL7, and regional QTL mapping was performed using 199 individuals from the F_2:3 population. We then confirmed and narrowed down qRSL7 to a 222 kb genome interval. Additionally, RNA sequencing (RNA-seq) was performed for IR36 and Weiguo at 36 h after salt stress and control condition at the bud burst stage, and 5 differentially expressed genes (DEGs) were detected in the candidate region. The qRT-PCR results showed the same expression patterns as the RNA-seq data. Furthermore, sequence analysis revealed a 1 bp Indel difference in Os07g0569700 (OsSAP16) between IR36 and Weiguo. OsSAP16 encodes a stress-associated protein whose expression is increased under drought stress.

CONCLUSION

These results indicate that OsSAP16 was the candidate gene of qRSL7. The results is useful for gene cloning of qRSL7 and for improving the salt tolerance of rice varieties by marker assisted selection (MAS).

Mapping of QTL and identification of candidate genes conferring spontaneous haploid genome doubling in maize (Zea mays L.)

In vivo doubled haploid (DH) technology is widely used in commercial maize (Zea mays L.) breeding. Haploid genome doubling is a critical step in DH breeding. In this study, inbred lines GF1 (0.65), GF3(0.29), and GF5 (0) with high, moderate, and poor spontaneous haploid genome doubling (SHGD), respectively, were selected to develop mapping populations for SHGD. Three QTL, qshgd1, qshgd2, and qshgd3, related to SHGD were identified by selective genotyping. With the exception of qshgd3, the source of haploid genome doubling alleles were derived from GF1. Furthermore, RNA-Seq was conducted to identify putative candidate genes between GF1 and GF5 within the qshgd1 region. A differentially expressed formin-like protein 5 transcript was identified within the qshgd1 region.

Identification and Molecular Mapping of a Gummy Stem Blight Resistance Gene in Wild Watermelon (Citrullus amarus) Germplasm PI 189225

Gummy stem blight (GSB), caused by Stagonosporopsis cucurbitacearum (syn. Didymella bryoniae), is a destructive foliar disease of watermelon in areas with hot and humid climates. The wild watermelon germplasm PI 189225 is a known source of resistance to GSB. The identification and use of molecular markers linked to resistance genes in the wild-type germplasm will speed up the introgression of GSB resistance into new watermelon varieties. An F₂ segregating population was obtained from a cross between the resistant wild watermelon genotype PI 189225 and the susceptible genotype K3. The F₂-derived F₃ families were inoculated with a single isolate of S. cucurbitacearum (JS002) from Jiangsu Academy of Agricultural Sciences. The results of the genetic analysis demonstrated that GSB resistance in PI 189225 was controlled by a major quantitative trait locus (QTL), temporarily designated Qgsb8.1. Based on the results of bulk sergeant analysis and sequencing, one associated region spanning 5.7 Mb (10,358,659 to 16,101,517) on chromosome 8 was identified as responsible for the resistance to GSB using the Δ(single-nucleotide polymorphism [SNP]-index) method. The result of a QTL linkage analysis with Kompetitive allele-specific PCR (KASP) SNP markers further mapped the GSB resistance locus between the SNP markers KASP_JS9383 and KASP_JS9168 in a region of 571.27 kb on chromosome 8. According to the watermelon gene annotation database, the region contains approximately 19 annotated genes and, of these 19 genes, 2 are disease resistance gene analogs: Cla001017 (coiled-coil nucleotide-binding site leucine-rich repeat resistance protein) and Cla001019 (pathogenesis related). Reverse-transcription quantitative PCR demonstrated that the expression of the two genes changed following S. cucurbitacearum infection, suggesting that they play important roles in GSB resistance in watermelon. This result will facilitate fine mapping and cloning of the Qgsb8.1 locus, and the linked markers will further provide a useful tool for marker-assisted selection of this locus in watermelon breeding programs.

A predicted NEDD8 conjugating enzyme gene identified as a Capsicum candidate Rf gene using bulk segregant RNA sequencing

Cytoplasmic male sterility (CMS) is an important tool for producing F₁ hybrids, which can exhibit heterosis. The companion system, restorer-of-fertility (Rf), is poorly understood at the molecular level and would be valuable in producing restorer lines for hybrid seed production. The identity of the Rf gene in Capsicum (pepper) is currently unclear. In this study, using bulked segregant RNA sequencing (BSR-seq), a strong candidate Rf gene, Capana06g002866, which is annotated as a NEDD8 conjugating enzyme E2, was identified. Capana06g002866 has an ORF of 555 bp in length encoding 184 amino acids; it can be cloned from F₁ plants from the hybridization of the CMS line 8A and restorer line R1 but is not found in CMS line 8A. With qRT-PCR validation, Capana06g002866 was found to be upregulated in restorer accessions compared to sterile accessions. The relative expression in flower buds increased with the developmental stage in F₁ plants, while the expression was very low in all flower bud stages of the CMS lines. These results provide new insights into the Rf gene in pepper and will be useful for other crops utilizing the CMS system.

Rapid identification of a candidate nicosulfuron sensitivity gene (Nss) in maize (Zea mays L.) via combining bulked segregant analysis and RNA-seq

A candidate nicosulfuron sensitivity gene Nss was identified by combining bulked segregant analysis and RNA-seq. Multiple mutations of this gene were discovered in nicosulfuron-sensitive maize compared with the tolerant. It has been demonstrated that variabilities exist in maize response to nicosulfuron. Two nicosulfuron-sensitive inbred lines (HB39, HB41) and two tolerant inbred lines (HB05, HB09) were identified via greenhouse and field trials. Genetic analysis indicated that the sensitivity to nicosulfuron in maize was controlled by a single, recessive gene. To precisely and rapidly map the nicosulfuron sensitivity gene (Nss), two independent F₂ segregating populations, Population A (HB41 × HB09) and Population B (HB39 × HB05), were constructed. By applying bulked segregant RNA-Seq (BSR-Seq), the Nss gene was, respectively, mapped on the short arm of chromosome 5 (chr5: 1.1-15.3 Mb) and (chr5: 0.5-18.2 Mb) using two populations, with 14.2 Mb region in common. Further analysis revealed that there were 43 and 119 differentially expressed genes in the mapping intervals, with 18 genes in common. Gene annotation results showed that a cytochrome P450 gene (CYP81A9) appeared to be the candidate gene of Nss associated with nicosulfuron sensitivity in maize. Sequence analysis demonstrated that two common deletion mutations existed in the sensitive maize, which might lead to the nicosulfuron sensitivity in maize. The results will make valuable contributions to the understanding of molecular mechanism of herbicide sensitivity in maize.

Identification of a gene responsible for cytoplasmic male-sterility in onions (Allium cepa L.) using comparative analysis of mitochondrial genome sequences of two recently diverged cytoplasms

Almost identical mitochondrial genome sequences of two recently diverged male-fertile normal and male-sterile CMS-T-like cytoplasms were obtained in onions. A chimeric gene, orf725 , was found to be a CMS-inducing gene. In onions (Allium cepa L.), cytoplasmic male-sterility (CMS) has been widely used in hybrid seed production. Two types of CMS (CMS-S and CMS-T) have been reported in onions. A complete mitochondrial genome sequence of the CMS-S cytoplasm has been reported in our previous study. Draft mitochondrial genome sequences of male-fertile normal and CMS-T-like cytoplasms are reported in this study. Raw reads obtained from normal and CMS-T-like cytoplasms were assembled into eight and nine almost identical contigs, respectively. After connection and reorganization of contigs by PCR amplification and genome walking, four scaffold sequences with total length of 339 and 180 bp were produced for the normal cytoplasm. A mitochondrial genome sequence of the CMS-T-like cytoplasm was obtained by mapping trimmed reads of CMS-T onto scaffold sequences of the normal cytoplasm. Compared with the CMS-S mitochondrial genome, the normal mitochondrial genome was highly rearranged with 31 syntenic blocks. A total of 499 single nucleotide polymorphisms (SNPs) or insertions/deletions were identified in these syntenic regions. On the other hand, normal and CMS-T-like mitochondrial genome sequences were almost identical except for orf725, a chimeric gene consisting of cox1 with other sequences. Only three SNPs were identified between normal and CMS-T-like syntenic sequences. These results indicate that orf725 is likely to be the casual gene for CMS induction in onions and that CMS-T-like cytoplasm has recently diverged from the normal cytoplasm by introduction of orf725.

Sequence-tagged site-based diagnostic markers linked to a novel anthracnose resistance gene RCt1 in chili pepper (Capsicum annuum L.)

Anthracnose, caused by Colletotrichum spp. is the most devastating disease of chili (Capsicum annuum) in the tropical and subtropical regions of the world. The present study aimed at molecular mapping and development of markers linked to a new gene for anthracnose resistance in the chili cultivar 'Punjab Lal'. Phenotypic evaluation of F1, F2, and BC1F1 populations derived from a cross between 'Punjab Lal' and susceptible cultivar 'Arka Lohit' against a virulent isolate of C. truncatum revealed that anthracnose resistance in Punjab Lal is governed by a monogenic-dominant gene designated as RCt1. Forty-four (28 ISSRs and 16 AFLPs) out of 201 markers exhibited parental polymorphism and were used in bulk segregant analysis. Three ISSRs (ISSR411493, ISSR581485, and ISSR1121857) and one AFLP marker (E-ACA/M-CTG516) showed precise polymorphism between resistant and susceptible bulks, and were used for genotyping F2 and BC1 populations. The four putative fragments were converted into sequence-tagged site (STS) markers and southern blotting confirmed their association with the resistance locus. Molecular mapping revealed that the STS markers CtR-431 and CtR-594 were closely linked to the RCt1 locus in coupling at distances of 1.8 and 2.3 cM, respectively. Furthermore, both of these markers showed the presence of resistance-linked allele in seven genotypes including the highly resistant C. chinnese 'PBC932' and C. baccatum 'PBC80' while negatively validated in 32 susceptible genotypes. Therefore, CtR431 and CtR-594 could be recommended as efficient diagnostic markers to facilitate the introgression of RCt1 locus into susceptible chili variants towards the development of high-yielding anthracnose resistance genotypes in C. annuum background.

Differentially Expressed Genes Associated with the Cabbage Yellow-Green-Leaf Mutant in the ygl-1 Mapping Interval with Recombination Suppression

Although the genetics and preliminary mapping of the cabbage yellow-green-leaf mutant YL-1 has been extensively studied, transcriptome profiling associated with the yellow-green-leaf mutant of YL-1 has not been discovered. Positional mapping with two populations showed that the yellow-green-leaf gene ygl-1 is located in a recombination-suppressed genomic region. Then, a bulk segregant RNA-seq (BSR) was applied to identify differentially expressed genes (DEGs) using an F₃ population (YL-1 × 11-192) and a BC₂ population (YL-1 × 01-20). Among the 37,286 unique genes, 5730 and 4118 DEGs were detected between the yellow-leaf and normal-leaf pools from the F₃ and BC₂ populations. BSR analysis with four pools greatly reduced the number of common DEGs from 4924 to 1112. In the ygl-1 gene mapping region with suppressed recombination, 43 common DEGs were identified. Five of the DEGs were related to chloroplasts, including the down-regulated Bo1g087310, Bo1g094360, and Bo1g098630 and the up-regulated Bo1g059170 and Bo1g098440. The Bo1g098440 and Bo1g098630 genes were excluded by qRT-PCR. Hence, we inferred that these three DEGs (Bo1g094360, Bo1g087310, and Bo1g059170) in the mapping interval may be tightly associated with the development of the yellow-green-leaf mutant phenotype.

Apple fruit acidity is genetically diversified by natural variations in three hierarchical epistatic genes: MdSAUR37, MdPP2CH and MdALMTII

Many efforts have been made to map quantitative trait loci (QTLs) to facilitate practical marker-assisted selection (MAS) in plants. In the present study, using MapQTL and BSA-seq (bulk segregant analysis using next generation sequencing) with two independent pedigree-based populations, we identified four major genome-wide QTLs responsible for apple fruit acidity. Candidate genes were screened in major QTL regions, and three functional gene markers, including a non-synonymous A/G single-nucleotide polymorphism (SNP) in the coding region of MdPP2CH, a 36-bp insertion in the promoter of MdSAUR37 and a previously reported SNP in MdALMTII, were validated to influence the malate content of apple fruits. In addition, MdPP2CH inactivated three vacuolar H⁺ -ATPases (MdVHA-A3, MdVHA-B2 and MdVHA-D2) and one aluminium-activated malate transporter (MdALMTII) via dephosphorylation and negatively influenced fruit malate accumulation. The dephosphotase activity of MdPP2CH was suppressed by MdSAUR37, which implied a higher hierarchy of genetic interaction. Therefore, the MdSAUR37/MdPP2CH/MdALMTII chain cascaded hierarchical epistatic genetic effects to precisely determine apple fruit malate content. An A/G SNP (-1010) on the MdMYB44 promoter region from a major QTL (qtl08.1) was closely associated with fruit malate content. The predicted phenotype values (PPVs) were estimated using the tentative genotype values of the gene markers, and the PPVs were significantly correlated with the observed phenotype values. Our findings provide an insight into plant genome-based selection in apples and will aid in conducting research to understand the fundamental physiological basis of quantitative genetics.

Comparative transcript profiling reveals the mechanism of female sterility associated with seedless Ponkan mandarin (Citrus reticulata Blanco)

Seedlessness is a highly desirable trait in citrus varieties. Sterility is the key determination for seedlessness formation. However, the molecular basis for female sterility in seedless mandarin remains unclear. Thus, a seedless Ponkan mandarin (Citrus reticulata Blanco 'Lipeng No.2'), considered the bud mutation of normal seedy Ponkan, was collected to identify candidate genes involved in seedless variation. Suppression subtractive hybridization (SSH) screened 1091 uniESTs related to seedy and seedless Ponkan (727 singlets and 364 contigs), which mainly governed catalytic activity, transferase activity, and oxygen binding. By using RNA-Seq technology, 106 differentially expressed genes (DEGs) were captured, of which 74 were up-regulated and 32 were down-regulated. Gene Ontology and pathway analysis showed that six DEGs were enriched in the biosynthesis of secondary metabolite, whereas five DEGs were enriched in the signaling of plant hormones. The combined results of SSH and RNA-Seq indicated the importance of amino acid metabolism in seedless Ponkan. Our findings revealed that the mechanism of seedless Ponkan generation may be related to gene regulation, signal cascade, and hormone levels. This study provided a solid foundation for functional gene identification in seedless Ponkan and a good reference for relevant research on molecular mechanisms of female sterility in Ponkan mandarin.

Exploration of miRNAs and target genes of cytoplasmic male sterility line in cotton during flower bud development

Cytoplasmic male sterility (CMS) lines provide crucial material to harness heterosis for crop plants, which serves as an important strategy for hybrid seed production. However, the molecular mechanism remains obscure. Although microRNAs (miRNAs) play important roles in vegetative growth and reproductive growth, there are few reports on miRNAs regulating the development of male sterility in Upland cotton. In present study, 12 small RNA libraries were constructed and sequenced for two development stages of flower buds from a CMS line and its maintainer line. Based on the results, 256 novel miRNAs were allocated to 141 new miRNA families, and 77 known miRNAs belonging to 54 conserved miRNA families were identified as well. Comparative analysis revealed that 61 novel and 10 conserved miRNAs were differentially expressed. Further transcriptome analysis identified 232 target genes for these miRNAs, which participated in cellular developmental process, cell death, pollen germination, and sexual reproduction. In addition, expression patterns of typical miRNA and the negatively regulated target genes, such as PPR, ARF, AP2, and AFB, were verified by qRT-PCR in cotton flower buds. These targets were previously reported to be related to reproduction development and male sterility, suggesting that miRNAs might act as regulators of CMS occurrence. Some miRNAs displayed specific expression profiles in special developmental stages of CMS line and its fertile hybrid (F₁). Present study offers new information on miRNAs and their related target genes in exploiting CMS mechanism, and revealing the miRNA regulatory networks in Upland cotton.

Physical mapping and candidate gene prediction of fertility restorer gene of cytoplasmic male sterility in cotton

Background

Cytoplasmic male sterility (CMS) is a maternally inherited trait failing to produce functional pollen. It plays a pivotal role in the exploitation of crop heterosis. The specific locus amplified fragment sequencing (SLAF-seq) as a high-resolution strategy for the identification of new SNPs on a large-scale is gradually applied for functional gene mining. The current study combined the bulked segregant analysis (BSA) with SLAF-seq to identify the candidate genes associated with fertility restorer gene (Rf) in CMS cotton.

Methods

Illumina sequencing systematically investigated the parents. A segregating population comprising of 30 + 30 F₂ individuals was developed using 3096A (female parent) as sterile and 866R (male parent) as a restorer. The original data obtained by dual-index sequencing were analyzed to obtain the reads of each sample that were compared to the reference genome in order to identify the SLAF tag with a polymorphism in parent lines and the SNP with read-associated coverage. Based on SLAF tags, SNP-index analysis, Euclidean distance (ED) correlation analysis, and whole genome resequencing, the hot regions were annotated.

Results

A total of 165,007 high-quality SLAF tags, with an average depth of 47.90× in the parents and 50.78× in F₂ individuals, were sequenced. In addition, a total of 137,741 SNPs were detected: 113,311 and 98,861 SNPs in the male and female parent, respectively. A correlation analysis by SNP-index and ED initially located the candidate gene on 1.35 Mb of chrD05, and 20 candidate genes were identified. These genes were involved in genetic variations, single base mutations, insertions, and deletions. Moreover, 42 InDel markers of the whole genome resequencing were also detected.

Conclusions

In this study, associated markers identified by super-BSA could accelerate the study of CMS in cotton, and as well as in other crops. Some of the 20 genes’ preliminary characteristics provided useful information for further studies on CMS crops.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4406-y) contains supplementary material, which is available to authorized users.

Sequencing of bulks of segregants allows dissection of genetic control of amylose content in rice

Amylose content (AC) is a key quality trait in rice. A cross between Oryza glaberrima (African rice) and Oryza sativa (Asian rice) segregating for AC was analysed by sequencing bulks of individuals with high and low AC. SNP associated with the granule bound starch synthase (GBSS1) locus on chromosome 6 were polymorphic between the bulks. In particular, a G/A SNP that would result in an Asp to Asn mutation was identified. This amino acid substitution may be responsible for differences in GBSS activity as it is adjacent to a disulphide linkage conserved in all grass GBSS proteins. Other polymorphisms in genomic regions closely surrounding this variation may be the result of linkage drag. In addition to the variant in the starch biosynthesis gene, SNP on chromosomes 1 and 11 linked to AC was also identified. SNP was found in the genes encoding the NAC and CCAAT-HAP5 transcription factors that have previously been linked to starch biosynthesis. This study has demonstrated that the approach of sequencing bulks was able to identify genes on different chromosomes associated with this complex trait.

Functional natural allelic variants of flavonoid 3',5'-hydroxylase gene governing catechin traits in tea plant and its relatives

Functional allelic variants of the flavonoid 3',5'-hydroxylase (F3'5'H) gene provides new information of F3'5'H function of tea plant and its relatives. This insight may serve as the foundation upon which to advance molecular breeding in the tea plant. Catechins are the active components of tea that determine its quality and health attributes. This study established the first integrated genomic strategy for deciphering the genetic basis of catechin traits of tea plant. With the RNA-sequencing analysis of bulked segregants representing the tails of a F1 population segregated for total catechin content, we identified a flavonoid 3',5'-hydroxylase (F3'5'H) gene. F3'5'H had one copy in the genomic DNA of tea plant. Among 202 tea accessions, we identified 120 single nucleotide polymorphisms (SNPs) at F3'5'H locus. Seventeen significant marker-trait associations were identified by association mapping in multiple environments, which were involved in 10 SNP markers, and the traits including the ratio of di/tri-hydroxylated catechins and catechin contents. The associated individual and combination of SNPs explained 4.5-25.2 and 53.0-63.0% phenotypic variations, respectively. In the F1 population (validation population), the catechin trait variation percentages explained by F3'5'H diplotype were 6.9-74.3%. The genotype effects of ten functional SNPs in the F1 population were all consistent with the association population. Furthermore, the function of SNP-711/-655 within F3'5'H was validated by gene expression analysis. Altogether, our work indicated functional SNP allelic variants within F3'5'H governing the ratio of di/tri-hydroxylated catechins and catechin contents. The strong catechin-associated SNPs identified in this study can be used for future marker-assisted selection to improve tea quality.

Searching for an Accurate Marker-Based Prediction of an Individual Quantitative Trait in Molecular Plant Breeding

Molecular plant breeding with the aid of molecular markers has played an important role in modern plant breeding over the last two decades. Many marker-based predictions for quantitative traits have been made to enhance parental selection, but the trait prediction accuracy remains generally low, even with the aid of dense, genome-wide SNP markers. To search for more accurate trait-specific prediction with informative SNP markers, we conducted a literature review on the prediction issues in molecular plant breeding and on the applicability of an RNA-Seq technique for developing function-associated specific trait (FAST) SNP markers. To understand whether and how FAST SNP markers could enhance trait prediction, we also performed a theoretical reasoning on the effectiveness of these markers in a trait-specific prediction, and verified the reasoning through computer simulation. To the end, the search yielded an alternative to regular genomic selection with FAST SNP markers that could be explored to achieve more accurate trait-specific prediction. Continuous search for better alternatives is encouraged to enhance marker-based predictions for an individual quantitative trait in molecular plant breeding.

Doubled Haploid 'CUDH2107' as a Reference for Bulb Onion (Allium cepa L.) Research: Development of a Transcriptome Catalogue and Identification of Transcripts Associated with Male Fertility

Researchers working on model plants have derived great benefit from developing genomic and genetic resources using ‘reference’ genotypes. Onion has a large and highly heterozygous genome making the sharing of germplasm and analysis of sequencing data complicated. To simplify the discovery and analysis of genes underlying important onion traits, we are promoting the use of the homozygous double haploid line ‘CUDH2107’ by the onion research community. In the present investigation, we performed transcriptome sequencing on vegetative and reproductive tissues of CUDH2107 to develop a multi-organ reference transcriptome catalogue. A total of 396 million 100 base pair paired reads was assembled using the Trinity pipeline, resulting in 271,665 transcript contigs. This dataset was analysed for gene ontology and transcripts were classified on the basis of putative biological processes, molecular function and cellular localization. Significant differences were observed in transcript expression profiles between different tissues. To demonstrate the utility of our CUDH2107 transcriptome catalogue for understanding the genetic and molecular basis of various traits, we identified orthologues of rice genes involved in male fertility and flower development. These genes provide an excellent starting point for studying the molecular regulation, and the engineering of reproductive traits.

Bulked sample analysis in genetics, genomics and crop improvement

Biological assay has been based on analysis of all individuals collected from sample populations. Bulked sample analysis (BSA), which works with selected and pooled individuals, has been extensively used in gene mapping through bulked segregant analysis with biparental populations, mapping by sequencing with major gene mutants and pooled genomewide association study using extreme variants. Compared to conventional entire population analysis, BSA significantly reduces the scale and cost by simplifying the procedure. The bulks can be built by selection of extremes or representative samples from any populations and all types of segregants and variants that represent wide ranges of phenotypic variation for the target trait. Methods and procedures for sampling, bulking and multiplexing are described. The samples can be analysed using individual markers, microarrays and high-throughput sequencing at all levels of DNA, RNA and protein. The power of BSA is affected by population size, selection of extreme individuals, sequencing strategies, genetic architecture of the trait and marker density. BSA will facilitate plant breeding through development of diagnostic and constitutive markers, agronomic genomics, marker-assisted selection and selective phenotyping. Applications of BSA in genetics, genomics and crop improvement are discussed with their future perspectives.

Completion of the mitochondrial genome sequence of onion (Allium cepa L.) containing the CMS-S male-sterile cytoplasm and identification of an independent event of the ccmF N gene split

Cytoplasmic male-sterility (CMS) conferred by the CMS-S cytoplasm has been most commonly used for onion (Allium cepa L.) F₁ hybrid seed production. We first report the complete mitochondrial genome sequence containing CMS-S cytoplasm in this study. Initially, seven contigs were de novo assembled from 150-bp paired-end raw reads produced from the total genomic DNA using the Illumina NextSeq500 platform. These contigs were connected into a single circular genome consisting of 316,363 bp (GenBank accession: KU318712) by PCR amplification. Although all 24 core protein-coding genes were present, no ribosomal protein-coding genes, except rps12, were identified in the onion mitochondrial genome. Unusual trans-splicing of the cox2 gene was verified, and the cox1 gene was identified as part of the chimeric orf725 gene, which is a candidate gene responsible for inducing CMS. In addition to orf725, two small chimeric genes were identified, but no transcripts were detected for these two open reading frames. Thirteen chloroplast-derived sequences, with sizes of 126-13,986 bp, were identified in the intergenic regions. Almost 10 % of the onion mitochondrial genome was composed of repeat sequences. The vast majority of repeats were short repeats of <100 base pairs. Interestingly, the gene encoding ccmF_N was split into two genes. The ccmF _N gene split is first identified outside the Brassicaceae family. The breakpoint in the onion ccmF _N gene was different from that of other Brassicaceae species. This split of the ccmF _N gene was also present in 30 other Allium species. The complete onion mitochondrial genome sequence reported in this study would be fundamental information for elucidation of onion CMS evolution.

Involvement of a universal amino acid synthesis impediment in cytoplasmic male sterility in pepper

To explore the mechanisms of pepper (Capsicum annuum L.) cytoplasmic male sterility (CMS), we studied the different maturation processes of sterile and fertile pepper anthers. A paraffin section analysis of the sterile anthers indicated an abnormality of the tapetal layer and an over-vacuolization of the cells. The quantitative proteomics results showed that the expression of histidinol dehydrogenase (HDH), dihydroxy-acid dehydratase (DAD), aspartate aminotransferase (ATAAT), cysteine synthase (CS), delta-1-pyrroline-5-carboxylate synthase (P5CS), and glutamate synthetase (GS) in the amino acid synthesis pathway decreased by more than 1.5-fold. Furthermore, the mRNA and protein expression levels of DAD, ATAAT, CS and P5CS showed a 2- to 16-fold increase in the maintainer line anthers. We also found that most of the amino acid content levels decreased to varying degrees during the anther tapetum period of the sterile line, whereas these levels increased in the maintainer line. The results of our study indicate that during pepper anther development, changes in amino acid synthesis are significant and accompany abnormal tapetum maturity, which is most likely an important cause of male sterility in pepper.