Fertility of CMS wheat is restored by two Rf loci located on a recombined acrocentric chromosome

Allelic haplotype combinations at the MS-P1 region, including P-class pentatricopeptide repeat family genes, influence wide phenotypic variation in pollen grain number through a cytoplasmic male sterility model in citrus

In citrus breeding programs, male sterility is an important trait for developing seedless varieties. Sterility associated with the male sterile cytoplasm of Kishu mandarin (Kishu-cytoplasm) has been proposed to fit the cytoplasmic male sterility (CMS) model. However, it remains undetermined whether CMS in citrus is controlled by interactions between sterile cytoplasm and nuclear restorer-of-fertility (Rf) genes. Accordingly, mechanisms underlying the control of the wide phenotypic variation in pollen number for breeding germplasm should be elucidated. This study aimed to identify complete linkage DNA markers responsible for male sterility at the MS-P1 region based on fine mapping. Two P-class pentatricopeptide repeat (PPR) family genes were identified as candidates for Rf based on predicted mitochondrial localization and higher expression in a male fertile variety/selected strain than in a male sterile variety. Eleven haplotypes (HT1-HT11) at the MS-P1 region were defined based on genotyping of DNA markers. Association analysis of diplotypes at the MS-P1 region and the number of pollen grains per anther (NPG) in breeding germplasms harboring Kishu-cytoplasm revealed that the diplotypes in this region influenced NPG. Among these haplotypes, HT1 is a non-functional restorer-of-fertility (rf) haplotype; HT2, a less-functional Rf; HT3-HT5 are semi-functional Rfs; and HT6 and HT7 are functional Rfs. However, the rare haplotypes HT8-HT11 could not be characterized. Therefore, P-class PPR family genes in the MS-P1 region may constitute the nuclear Rf genes within the CMS model, and a combination of the seven haplotypes could contribute to phenotypic variation in the NPG of breeding germplasms. These findings reveal the genomic mechanisms of CMS in citrus and will contribute to seedless citrus breeding programs by selecting candidate seedless seedlings using the DNA markers at the MS-P1 region.

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.

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.

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

Identification of Rf Genes in Hexaploid Wheat (Triticumaestivum L.) by RNA-Seq and Paralog Analyses

Among the natural mechanisms used for wheat hybrid breeding, the most desirable is the system combining the cytoplasmic male sterility (cms) of the female parent with the fertility-restoring genes (Rf) of the male parent. The objective of this study was to identify Rf candidate genes in the wheat genome on the basis of transcriptome sequencing (RNA-seq) and paralog analysis data. Total RNA was isolated from the anthers of two fertility-restorer (Primépi and Patras) and two non-restorer (Astoria and Grana) varieties at the tetrad and late uninucleate microspore stages. Of 36,912 differentially expressed genes (DEGs), 21 encoding domains in known fertility-restoring proteins were selected. To enrich the pool of Rf candidates, 52 paralogs (PAGs) of the 21 selected DEGs were included in the analyses. The expression profiles of most of the DEGs and PAGs determined bioinformatically were as expected (i.e., they were overexpressed in at least one fertility-restorer variety). However, these results were only partially consistent with the quantitative real-time PCR data. The DEG and PAG promoters included cis-regulatory elements common among PPR-encoding genes. On the basis of the obtained results, we designated seven genes as Rf candidate genes, six of which were identified for the first time in this study.

Genetic mapping reveals BjRf as a candidate gene controlling fertility restoration of the oxa CMS in Brassica juncea

A candidate gene for male fertility restoration in Brassica juncea, BjRf, was isolated from a 23-kb interval on chromosome A05 using map-based cloning and BSA methods. The cytoplasmic male sterility/fertility restoration (CMS/Rf) system has been extensively used for heterosis in plants. It also provides valuable resources for studying mitochondrial-nuclear coevolution and interaction. The oxa CMS, which is a new CMS type reported in Brassica juncea (B. juncea), has been broadly used in the exploitation and application of heterosis in this species. However, the oxa CMS fertility restorer gene BjRf has not been reported. In this study, a stable restorer line was successfully constructed via continuous testcross and artificial selection. Besides, a new Rf gene was mapped in a 23-kb region on chromosome A05 in B. juncea with a genetic distance of 0.5 cM by the method incorporating bulk segregant analysis (BSA) and conventional map-based cloning. Finally, BjuA017917, a non-PPR Rf gene encoding a guanosine nucleotide diphosphate dissociation inhibitor (GDI), is proposed to be the candidate gene for fertility restoration of the oxa CMS line in B. juncea. Moreover, a functional marker, CRY3, was developed for marker-assisted selection for Brassica juncea breeding.

Tritordeum: Creating a New Crop Species-The Successful Use of Plant Genetic Resources

Hexaploid tritordeum is the amphiploid derived from the cross between the wild barley Hordeum chilense and durum wheat. This paper reviews the main advances and achievements in the last two decades that led to the successful development of tritordeum as a new crop. In particular, we summarize the progress in breeding for agronomic performance, including the potential of tritordeum as a genetic bridge for wheat breeding; the impact of molecular markers in genetic studies and breeding; and the progress in quality and development of innovative food products. The success of tritordeum as a crop shows the importance of the effective utilization of plant genetic resources for the development of new innovative products for agriculture and industry. Considering that wild plant genetic resources have made possible the development of this new crop, the huge potential of more accessible resources, such as landraces conserved in gene banks, goes beyond being sources of resistance to biotic and abiotic stresses. In addition, the positive result of tritordeum also shows the importance of adequate commercialization strategies and demonstrative experiences aimed to integrate the whole food chain, from producers to end-point sellers, in order to develop new products for consumers.

A mitochondria-localized pentatricopeptide repeat protein is required to restore hau cytoplasmic male sterility in Brassica napus

A mitochondria-localized pentatricopeptide repeat protein was identified by positional cloning and transferred into the hau CMS line, where it successfully restored fertility Cytoplasmic male sterility (CMS) is a maternally inherited trait that can be controlled by restorer-of-fertility (Rf) genes present in the nucleus. The hau CMS was identified as a new form of CMS associated with the mitochondrial transcript orf288; however, a lack of a restorer gene has limited its utilization in Brassica crops. Here, the combination of Brassica 60 K array with bulk segregant analysis and map-based cloning was used to delimit the Rfh locus to an 82.2-kb region on chromosome A09. A candidate gene encoding a mitochondria-localized pentatricopeptide repeat (PPR) protein was identified and transferred into the hau CMS line, where it successfully restored the fertility of the hau CMS plants. Furthermore, the expression analysis showed that Rfh was highly expressed in the flower buds, and the sequence analysis results implied that functional divergence between RFH and rfh could be due to 59 amino acid residue differences in the deduced protein sequences. In addition, a co-separated molecular marker was developed based on the divergent sequences between the dominant and recessive alleles. These results will help enable the heterosis of Brassica crops in the future.

Wheat, Rye, and Barley Genomes Can Associate during Meiosis in Newly Synthesized Trigeneric Hybrids

Polyploidization, or whole genome duplication (WGD), has an important role in evolution and speciation. One of the biggest challenges faced by a new polyploid is meiosis, in particular, discriminating between multiple related chromosomes so that only homologs recombine to ensure regular chromosome segregation and fertility. Here, we report the production of two new hybrids formed by the genomes of species from three different genera: a hybrid between Aegilops tauschii (DD), Hordeum chilense (H^chH^ch), and Secale cereale (RR) with the haploid genomic constitution H^chDR (n = 7× = 21); and a hybrid between Triticum turgidum spp. durum (AABB), H. chilense, and S. cereale with the constitution ABH^chR (n = 7× = 28). We used genomic in situ hybridization and immunolocalization of key meiotic proteins to establish the chromosome composition of the new hybrids and to study their meiotic behavior. Interestingly, there were multiple chromosome associations at metaphase I in both hybrids. A high level of crossover (CO) formation was observed in H^chDR, which shows the possibility of meiotic recombination between the different genomes. We succeeded in the duplication of the ABH^chR genome, and several amphiploids, AABBH^chH^chRR, were obtained and characterized. These results indicate that recombination between the genera of three economically important crops is possible.

Identification of Rf9, a Gene Contributing to the Genetic Complexity of Fertility Restoration in Hybrid Wheat

Wheat (Triticum aestivum L.) is a self-pollinating crop whose hybrids offer the potential to provide a major boost in yield. Male sterility induced by the cytoplasm of Triticum timopheevii is a powerful method for hybrid seed production. Hybrids produced by this method are often partially sterile, and full fertility restoration is crucial for wheat production using hybrid cultivars. To identify the genetic loci controlling fertility restoration in wheat, we produced two cytoplasmic male-sterile (CMS) backcross (BC1) mapping populations. The restorer lines Gerek 79 and 71R1203 were used to pollinate the male-sterile winter wheat line CMS-Sperber. Seed set and numbers of sterile spikelets per spike were evaluated in 340 and 206 individuals of the populations derived from Gerek 79 and 71R1203, respectively. Genetic maps were constructed using 930 and 994 single nucleotide polymorphism (SNP) markers, spanning 2,160 and 2,328 cM over 21 linkage groups in the two populations, respectively. Twelve quantitative trait loci (QTL) controlled fertility restoration in both BC1 populations, including a novel restorer-of-fertility (Rf) locus flanked by the SNP markers IWB72413 and IWB1550 on chromosome 6AS. The locus was mapped as a qualitative trait in the BC1 Gerek 79 population and was designated Rf9. One hundred-nineteen putative candidate genes were predicted within the QTL region on chromosome 6AS. Among them were genes encoding mitochondrial transcription termination factor and pentatricopeptide repeat-containing proteins that are known to be associated with fertility restoration. This finding is a promising step to better understand the functions of genes for improving fertility restoration in hybrid wheat.

Hybrid breeding in wheat: how shaping floral biology can offer new perspectives

Hybrid breeding in wheat (Triticum aestivum L.) has the potential to deliver major yield increases. This is a requisite to guarantee food security for increasing population demands and to counterbalance the effects of extreme environmental conditions. Successful hybrid breeding in wheat relies on forced outcrossing while preventing self-pollination. To achieve this, research has been directed towards identifying and improving fertility control systems. To maximise cross-pollination and seed set, however, fertility control systems need to be complemented by breeding phenotypically distinct male and female lines. This review summarises existing and novel male sterility systems for wheat hybridisation. We also consider the genetic resources that can be used to alter wheat's floral development and spike morphology, with a focus on the genetic variation already available. Exploiting these resources can lead to enhanced outcrossing, a key requirement in the progress towards hybrid wheat breeding.

Fine mapping of the male fertility restoration gene CaRf032 in Capsicum annuum L

A novel strong candidate gene CA00g82510 for the male fertility restoration locus CaRf032 in Capsicum annuum was identified by genome re-sequencing and recombination analysis. A single dominant locus (CaRf032) for fertility restoration of cytoplasmic male sterility was identified in the strong restorer inbred line IVF2014032 of chili pepper (Capsicum annuum L.). CaRf032 was localized within an 8.81-Mb candidate intervals on chromosome 6 using bulked segregant analysis based on high-throughput sequencing data. Subsequently, the candidate interval was genetically mapped and defined to a 249.41-kb region using an F₂ population of 441 individuals generated by crossing the male-sterile line 77013A and the restorer line IVF2014032. To fine map CaRf032, eight newly developed KASP markers were used to genotype 23 recombinants screened from a larger F₂ population of 2877 individuals. The CaRf032 locus was localized to a 148.05-kb region between the KASP markers S1402 and S1354, which was predicted to contain 22 open reading frames (ORFs). One ORF with an incomplete sequence was predicted to contain a PPR motif, and its physical position overlapped with the Rf candidate gene CaPPR6_46. The PPR ORF sequence before the gap showed 100% identity with the CA00g82510 locus of the CM334 reference genome. CA00g82510 encodes a protein of 583 amino acids, containing 14 PPR motifs, and shows significantly differential expression between the flower buds of the maintainer line 77013 and the restorer line IVF2014032. These results indicated that CA00g82510 is a strong candidate gene for CaRf032. Five KASP markers, which detected single-nucleotide polymorphisms in CA00g82510 of 77013 and IVF2014032, co-segregated with CaRf032 and showed 64.4% successful genotyping of 38 maintainer and 63 restorer lines.

Transcriptomics, chromosome engineering and mapping identify a restorer-of-fertility region in the CMS wheat system msH1

An original RNA-seq mapping strategy, validated with chromosome engineering and physical mapping, identifies candidate genes for fertility restoration in the 6H^chS chromosome of Hordeum chilense in the wheat msH1 system. Cytoplasmic male sterility (CMS) is a valuable trait for hybrid seed production. The msH1 CMS system in common wheat results from the incompatibility between the nuclear genome of wheat and the cytoplasm of the wild barley Hordeum chilense. This work aims to identify H. chilense candidate genes for fertility restoration in the msH1 system with a multidisciplinary strategy based on chromosome engineering, differential expression analysis and genome mapping. Alloplasmic isogenic wheat lines differing for fertility, associated with the presence of an acrocentric chromosome H^chac resulting from the rearrangement of the short arms of H. chilense chromosomes 1H^ch and 6H^ch, were used for transcriptome sequencing. Two novel RNA-seq mapping approaches were designed and compared to identify differentially expressed genes of H. chilense associated with male fertility restoration. Minichromosomes (H^chmi), new smaller reorganizations of the H^chac also restoring fertility, were obtained and used to validate the candidate genes. This strategy was successful identifying a putative restorer-of-fertility region on 6H^chS, with six candidate genes, including the ortholog of the barley restorer gene Rfm1. Additionally, transcriptomics gave preliminary insights on sterility and restoration networks showing the importance of energy supply, stress, protein metabolism and RNA processing.

Hybrid wheat: past, present and future

The review outlines past failures, present status and future prospects of hybrid wheat, and includes information on CMS/CHA/transgenic approaches for male sterility, heterotic groups and cost-effective hybrid seed production. Hybrid varieties give increased yield and improved grain quality in both cross- and self-pollinated crops. However, hybrid varieties in self-pollinated crops (particularly cereals) have not been very successful, except for hybrid rice in China. In case of hybrid wheat, despite the earlier failures, renewed efforts in recent years have been made and hybrid varieties with desirable attributes have been produced and marketed in some European countries. This review builds upon previous reviews, with a new outlook and improved knowledge base, not covered in earlier reviews. New technologies have been described, which include the Hordeum chilense-based CMS-fertility restorer system, chromosomal XYZ-4E-ms system and the following transgenic technologies: (1) conditional male sterility involving use of tapetum-specific expression of a gene that converts a pro-toxin into a phytotoxin causing male sterility; (2) barnase-barstar SeedLink system of Bayer CropScience; (3) split-barnase system that obviates the need of a barstar-based male restorer line; and (4) seed production technology of DuPont-Pioneer that makes use of transgenes in production of male-sterile lines, but gives hybrid seed with no transgenes. This review also includes a brief account of studies for discovery of heterotic QTL, genomic prediction of hybrid vigour and the development of heterotic groups/patterns and their importance in hybrid wheat production. The problem of high cost of hybrid seed due to required high seed rate in wheat relative to hybrid rice has also been addressed. The review concludes with a brief account of the current efforts and future possibilities in making hybrid wheat a commercial success.

Identification and comparison of individual chromosomes of three accessions of Hordeum chilense, Hordeum vulgare, and Triticum aestivum by FISH

Karyotypes of three accessions of Hordeum chilense (H1, H16, and H7), Hordeum vulgare, and Triticum aestivum were characterized by physical mapping of several repetitive sequences. A total of 14 repetitive sequences were used as probes for fluorescence in situ hybridization (FISH) with the aim of identifying inter- and intraspecies polymorphisms. The (AG)12 and 4P6 probes only produced hybridization signals in wheat, the BAC7 probe only hybridized to the centromeric region of H. vulgare, and the pSc119.2 probe hybridized to both wheat and H. chilense, but not to H. vulgare. The remaining repetitive sequences used in this study produced a hybridization signal in all the genotypes. Probes pAs1, pTa-535, pTa71, CCS1, and CRW were much conserved, showing no significant polymorphism among the genotypes studied. Probes GAA, (AAC)5, (CTA)5, HvT01, and pTa794 produced the most different hybridization pattern. We identified large polymorphisms in the three accessions of H. chilense studied, supporting the proposal of the existence of different groups inside species of H. chilense. The set of probes described in this work allowed the identification of every single chromosome in all three species, providing a complete cytogenetic karyotype of H. chilense, H. vulgare, and T. aestivum chromosomes, which could be useful in wheat and tritordeum breeding programs.

iTRAQ-Based Proteomics Analyses of Sterile/Fertile Anthers from a Thermo-Sensitive Cytoplasmic Male-Sterile Wheat with Aegilops kotschyi Cytoplasm

A “two-line hybrid system” was developed, previously based on thermo-sensitive cytoplasmic male sterility in Aegilops kotschyi (K-TCMS), which can be used in wheat breeding. The K-TCMS line exhibits complete male sterility and it can be used to produce hybrid wheat seeds during the normal wheat-growing season; it propagates via self-pollination at high temperatures. Isobaric tags for relative and absolute quantification-based quantitative proteome and bioinformatics analyses of the TCMS line KTM3315A were conducted under different fertility conditions to understand the mechanisms of fertility conversion in the pollen development stages. In total, 4639 proteins were identified, the differentially abundant proteins that increased/decreased in plants with differences in fertility were mainly involved with energy metabolism, starch and sucrose metabolism, phenylpropanoid biosynthesis, protein synthesis, translation, folding, and degradation. Compared with the sterile condition, many of the proteins that related to energy and phenylpropanoid metabolism increased during the anther development stage. Thus, we suggest that energy and phenylpropanoid metabolism pathways are important for fertility conversion in K-TCMS wheat. These findings provide valuable insights into the proteins involved with anther and pollen development, thereby, helping to further understand the mechanism of TCMS in wheat.

Non-coding RNAs and plant male sterility: current knowledge and future prospects

Latest outcomes assign functional role to non-coding (nc) RNA molecules in regulatory networks that confer male sterility to plants. Male sterility in plants offers great opportunity for improving crop performance through application of hybrid technology. In this respect, cytoplasmic male sterility (CMS) and sterility induced by photoperiod (PGMS)/temperature (TGMS) have greatly facilitated development of high-yielding hybrids in crops. Participation of non-coding (nc) RNA molecules in plant reproductive development is increasingly becoming evident. Recent breakthroughs in rice definitively associate ncRNAs with PGMS and TGMS. In case of CMS, the exact mechanism through which the mitochondrial ORFs exert influence on the development of male gametophyte remains obscure in several crops. High-throughput sequencing has enabled genome-wide discovery and validation of these regulatory molecules and their target genes, describing their potential roles performed in relation to CMS. Discovery of ncRNA localized in plant mtDNA with its possible implication in CMS induction is intriguing in this respect. Still, conclusive evidences linking ncRNA with CMS phenotypes are currently unavailable, demanding complementing genetic approaches like transgenics to substantiate the preliminary findings. Here, we review the recent literature on the contribution of ncRNAs in conferring male sterility to plants, with an emphasis on microRNAs. Also, we present a perspective on improved understanding about ncRNA-mediated regulatory pathways that control male sterility in plants. A refined understanding of plant male sterility would strengthen crop hybrid industry to deliver hybrids with improved performance.

Molecular Control of Male Fertility for Crop Hybrid Breeding

In many plant species, male-sterile female lines with cytoplasmic male sterility (CMS) or nuclear-controlled environment-sensitive genic male sterility (EGMS) have long been used to efficiently produce hybrids that harness hybrid vigor or heterosis. However, the underlying molecular mechanisms for these applications have only recently been uncovered in a few species. We provide here an update on the understanding of cytoplasmic-nuclear communication based on the discovery of mitochondrial CMS genes and their corresponding nuclear fertility determinants. Recent findings that uncover diverse mechanisms such as epigenetic, transcriptional, and post-transcriptional controls of EGMS by temperature and photoperiod signals are also reviewed. Furthermore, translational research that applies basic knowledge of plant male fertility control to hybrid seed production practice is highlighted.

Cytoplasmic male sterility (CMS) in hybrid breeding in field crops

A comprehensive understanding of CMS/Rf system enabled by modern omics tools and technologies considerably improves our ability to harness hybrid technology for enhancing the productivity of field crops. Harnessing hybrid vigor or heterosis is a promising approach to tackle the current challenge of sustaining enhanced yield gains of field crops. In the context, cytoplasmic male sterility (CMS) owing to its heritable nature to manifest non-functional male gametophyte remains a cost-effective system to promote efficient hybrid seed production. The phenomenon of CMS stems from a complex interplay between maternally-inherited (mitochondrion) and bi-parental (nucleus) genomic elements. In recent years, attempts aimed to comprehend the sterility-inducing factors (orfs) and corresponding fertility determinants (Rf) in plants have greatly increased our access to candidate genomic segments and the cloned genes. To this end, novel insights obtained by applying state-of-the-art omics platforms have substantially enriched our understanding of cytoplasmic-nuclear communication. Concomitantly, molecular tools including DNA markers have been implicated in crop hybrid breeding in order to greatly expedite the progress. Here, we review the status of diverse sterility-inducing cytoplasms and associated Rf factors reported across different field crops along with exploring opportunities for integrating modern omics tools with CMS-based hybrid breeding.

Contribution of Chromosomes 1HchS and 6HchS to Fertility Restoration in the Wheat msH1 CMS System under Different Environmental Conditions

Exploiting hybrid wheat heterosis has been long pursued to increase crop yield, stability and uniformity. Cytoplasmic male sterility (CMS) systems based in the nuclear-cytoplasmic incompatible interactions are a classic way for hybrid seed production, but to date, no definitive system is available in wheat. The msH1 CMS system results from the incompatibility between the nuclear genome of wheat and the cytoplasmic genome of the wild barley Hordeum chilense. Fertility restoration of the CMS phenotype was first associated with the disomic addition of the short arm of chromosome 6H from H. chilense. In further studies it was observed that chromosome arm 1HchS was also implicated, and the combination of genes in both chromosome arms restored fertility more efficiently. In this work we aim to dissect the effect of each chromosome in fertility restoration when combined in different genomic backgrounds and under different environmental conditions. We propose a model to explain how restoration behaves in the msH1 system and generate valuable information necessary to develop an efficient system for hybrid wheat production.