Molecular mapping and candidate gene identification of the Rf2 gene for pollen fertility restoration in sorghum [Sorghum bicolor (L.) Moench]

Newly Developed Restorer Lines of Sorghum [Sorghum bicolor (L.) Moench] Resistant to Greenbug

Eight lines of grain sorghum [Sorghum bicolor (L.) Moench], which can be used as a promising source material in heterotic hybrid breeding as pollen fertility restorers and donors of resistance to the greenbug (Schizaphis graminum Rondani), are characterized. The new restorer lines (R-lines) were developed by crossing the maternal sterile line Nizkorosloe 81s (CMS A1) with two lines selected from the grain sorghum collection accessions VIR-928 and VIR-929 as the paternal forms. The R-lines were genotyped using PCR markers, and also characterized by height, duration of the seedling-flowering period, and some of the technological properties of flour. With the use of microsatellite markers linked to the Rf genes and by hybridological analysis, it was shown that the new lines carry the dominant allele of the gene Rf2. The PCoA analysis demonstrated clear differences of each R-line from the parents. The genotypes of the new lines and their parental forms for the Rf2 locus were confirmed by applying three allele-specific codominant CAPS markers which detected SNPs in the candidate Rf2 gene. All new lines were highly fertile, as demonstrated by cytological analysis of acetocarmine-stained pollen preparations. A high resistance to the greenbug was demonstrated for each new R-line both in the laboratory and field conditions against a severe aphid infestation. Grain quality parameters such as protein content and dough rheological properties varied widely and were quite satisfactory in some R-lines. Characteristics common to all eight sorghum lines studied, such as the ability to restore pollen fertility in the F1 generation, good pollen quality, greenbug resistance, early ripening, spreading panicle, and low stature, allow us to recommend them for producing commercial F1 hybrids with satisfactory grain quality for both fodder and food purposes.

Discovering useful genetic variation in the seed parent gene pool for sorghum improvement

Multi-parent populations contain valuable genetic material for dissecting complex, quantitative traits and provide a unique opportunity to capture multi-allelic variation compared to the biparental populations. A multi-parent advanced generation inter-cross (MAGIC) B-line (MBL) population composed of 708 F6 recombinant inbred lines (RILs), was recently developed from four diverse founders. These selected founders strategically represented the four most prevalent botanical races (kafir, guinea, durra, and caudatum) to capture a significant source of genetic variation to study the quantitative traits in grain sorghum [Sorghum bicolor (L.) Moench]. MBL was phenotyped at two field locations for seven yield-influencing traits: panicle type (PT), days to anthesis (DTA), plant height (PH), grain yield (GY), 1000-grain weight (TGW), tiller number per meter (TN) and yield per panicle (YPP). High phenotypic variation was observed for all the quantitative traits, with broad-sense heritabilities ranging from 0.34 (TN) to 0.84 (PH). The entire population was genotyped using Diversity Arrays Technology (DArTseq), and 8,800 single nucleotide polymorphisms (SNPs) were generated. A set of polymorphic, quality-filtered markers (3,751 SNPs) and phenotypic data were used for genome-wide association studies (GWAS). We identified 52 marker-trait associations (MTAs) for the seven traits using BLUPs generated from replicated plots in two locations. We also identified desirable allelic combinations based on the plant height loci (Dw1, Dw2, and Dw3), which influences yield related traits. Additionally, two novel MTAs were identified each on Chr1 and Chr7 for yield traits independent of dwarfing genes. We further performed a multi-variate adaptive shrinkage analysis and 15 MTAs with pleiotropic effect were identified. The five best performing MBL progenies were selected carrying desirable allelic combinations. Since the MBL population was designed to capture significant diversity for maintainer line (B-line) accessions, these progenies can serve as valuable resources to develop superior sorghum hybrids after validation of their general combining abilities via crossing with elite pollinators. Further, newly identified desirable allelic combinations can be used to enrich the maintainer germplasm lines through marker-assisted backcross breeding.

Current insights and advances into plant male sterility: new precision breeding technology based on genome editing applications

Plant male sterility (MS) represents the inability of the plant to generate functional anthers, pollen, or male gametes. Developing MS lines represents one of the most important challenges in plant breeding programs, since the establishment of MS lines is a major goal in F1 hybrid production. For these reasons, MS lines have been developed in several species of economic interest, particularly in horticultural crops and ornamental plants. Over the years, MS has been accomplished through many different techniques ranging from approaches based on cross-mediated conventional breeding methods, to advanced devices based on knowledge of genetics and genomics to the most advanced molecular technologies based on genome editing (GE). GE methods, in particular gene knockout mediated by CRISPR/Cas-related tools, have resulted in flexible and successful strategic ideas used to alter the function of key genes, regulating numerous biological processes including MS. These precision breeding technologies are less time-consuming and can accelerate the creation of new genetic variability with the accumulation of favorable alleles, able to dramatically change the biological process and resulting in a potential efficiency of cultivar development bypassing sexual crosses. The main goal of this manuscript is to provide a general overview of insights and advances into plant male sterility, focusing the attention on the recent new breeding GE-based applications capable of inducing MS by targeting specific nuclear genic loci. A summary of the mechanisms underlying the recent CRISPR technology and relative success applications are described for the main crop and ornamental species. The future challenges and new potential applications of CRISPR/Cas systems in MS mutant production and other potential opportunities will be discussed, as generating CRISPR-edited DNA-free by transient transformation system and transgenerational gene editing for introducing desirable alleles and for precision breeding strategies.

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.

The Unstable Restorer-of-fertility locus in pepper (Capsicum annuum. L) is delimited to a genomic region containing PPR genes

Unstable Restorer-of-fertility (Rf_u), conferring unstable fertility restoration in the pepper CGMS system, was delimited to a genomic region near Rf and is syntenic to the PPR-like gene-rich region in tomato. The use of cytoplasmic-genic male sterility (CGMS) systems greatly increases the efficiency of hybrid seed production. Although marker development and candidate gene isolation have been performed for the Restorer-of-fertility (Rf) gene in pepper (Capsicum annuum L.), the broad use of CGMS systems has been hampered by the instability of fertility restoration among pepper accessions, especially sweet peppers, due to the widespread presence of the Unstable Restorer-of-fertility (Rf_u) locus. Therefore, to investigate the genetic factors controlling unstable fertility restoration in sweet peppers, we developed a segregation population (BC₄F₅) from crosses using a male-sterile line and an Rf_u-containing line. Segregation did not significantly deviate from a 3:1 ratio for unstable fertility restoration to sterility, indicating single dominant locus control for unstable fertility restoration in this population. Genetic mapping delimited the Rf_u locus to a 398 kb genomic region on chromosome 6, which is close to but different from the previously identified Rf-containing region. The Rf_u-containing region harbors a pentatricopeptide repeat (PPR) gene, along with 10 other candidate genes. In addition, this region is syntenic to the genomic region containing the largest number of Rf-like PPR genes in tomato. Therefore, the dynamic evolution of PPR genes might be responsible for both the restoration and instability of fertility in pepper. During genetic mapping, we developed various molecular markers, including one that co-segregated with Rf_u. These markers showed higher accuracy for genotyping than previously developed markers, pointing to their possible use in marker-assisted breeding of sweet peppers.

Fine mapping of Rf5 region for a sorghum fertility restorer gene and microsynteny analysis across grass species

Cytoplasmic male sterility (CMS) is widely used to control pollination in the production of commercial F₁ hybrid seed in sorghum. So far, 6 major fertility restorer genes, Rf1 to Rf6, have been reported in sorghum. Here, we fine-mapped the Rf5 locus on sorghum chromosome 5 using descendant populations of a 'Nakei MS-3A' × 'JN43' cross. The Rf5 locus was narrowed to a 140-kb region in BTx623 genome (161-kb in JN43) with 16 predicted genes, including 6 homologous to the rice fertility restorer Rf1 (PPR.1 to PPR.6). These 6 homologs have tandem pentatricopeptide repeat (PPR) motifs. Many Rf genes encode PPR proteins, which bind RNA transcripts and modulate gene expression at the RNA level. No PPR genes were detected at the Rf5 locus on the corresponding homologous chromosome of rice, foxtail millet, or maize, so this gene cluster may have originated by chromosome translocation and duplication after the divergence of sorghum from these species. Comparison of the sequences of these genes between fertile and CMS lines identified PPR.4 as the most plausible candidate gene for Rf5.

Comparative Transcriptome Analysis of the Anthers from the Cytoplasmic Male-Sterile Pepper Line HZ1A and Its Maintainer Line HZ1B

Cytoplasmic male-sterility (CMS) is important for the utilization of crop heterosis and study of the molecular mechanisms involved in CMS could improve breeding programs. In the present study, anthers of the pepper CMS line HZ1A and its maintainer line HZ1B were collected from stages S1, S2, and S3 for transcriptome sequencing. A total of 47.95 million clean reads were obtained, and the reads were assembled into 31,603 unigenes. We obtained 42 (27 up-regulated and 15 down-regulated), 691 (346 up-regulated and 345 down-regulated), and 709 (281 up-regulated and 428 down-regulated) differentially expressed genes (DEGs) in stages S1, S2, and S3, respectively. Through Gene Ontology (GO) analysis, the DEGs were found to be composed of 46 functional groups. Two GO terms involved in photosynthesis, photosynthesis (GO:0015986) and photosystem I (GO:0009522), may be related to CMS. Through Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis, oxidative phosphorylation (ko00190) and phenylpropanoid biosynthesis (ko00940) were significantly enriched in the S1 and S2 stages, respectively. Through the analysis of 104 lipid metabolism-related DEGs, four significantly enriched KEGG pathways may help to regulate male sterility during anther development. The mitochondrial genes orf470 and atp6 were identified as candidate genes of male sterility for the CMS line HZ1A. Overall, the results will provide insights into the molecular mechanisms of pepper CMS.

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.

Sorghum mitigates climate variability and change on crop yield and quality

Global food insecurity concerns due to climate change, emphasizes the need to focus on the sensitivity of sorghum to climate change and potential crop improvement strategies available, which is discussed in the current review to promote climate-smart agriculture. Climate change effects immensely disturb the global agricultural systems by reducing crop production. Changes in extreme weather and climate events such as high-temperature episodes and extreme rainfalls events, droughts, flooding adversely affect the production of staple food crops, posing threat to ecosystem resilience. The resulting crop losses lead to food insecurity and poverty and question the sustainable livelihoods of small farmer communities, particularly in developing countries. In view of this, it is essential to focus and adapt climate-resilient food crops which need lower inputs and produce sustainable yields through various biotic and abiotic stress-tolerant traits. Sorghum, "the camel of cereals", is one such climate-resilient food crop that is less sensitive to climate change vulnerabilities and also an important staple food in many parts of Asia and Africa. It is a rainfed crop and provides many essential nutrients. Understanding sorghum's sensitivity to climate change provides scope for improvement of the crop both in terms of quantity and quality and alleviates food and feed security in future climate change scenarios. Thus, the current review focused on understanding the sensitivity of sorghum crop to various stress events due to climate change and throws light on different crop improvement strategies available to pave the way for climate-smart agriculture.

Identification and validation of genetic locus Rfk1 for wheat fertility restoration in the presence of Aegilops kotschyi cytoplasm

Major fertility restorer locus for Aegilops kotschyi cytoplasm in wheat, Rfk1, was mapped to chromosome arm 1BS. Most likely candidate gene is TraesCS1B02G197400LC, which is predicted to encode a pectinesterase/pectinesterase inhibitor. Cytoplasmic male sterility (CMS) is widely used for heterosis and hybrid seed production in wheat. Genes related to male fertility restoration in the presence of Aegilops kotschyi cytoplasm have been reported, but the fertility restoration-associated gene loci have not been investigated systematically. In this study, a BC₁F₁ population derived from a backcross between KTP116A, its maintainer line TP116B, and its restorer line LK783 was employed to map fertility restoration by bulked segregant RNA-Seq (BSR-Seq). A major fertility allele restorer locus for Ae. kotschyi cytoplasm in wheat, Rfk1, was mapped to chromosome arm 1BS, and it was contributed by LK783. Morphological and cytological studies showed that male fertility restoration occurred mainly after the late uninucleate stage. Based on simple sequence repeat and single-nucleotide polymorphism genotyping, the gene locus was located between Xnwafu_6 and Xbarc137 on chromosome arm 1BS. To further isolate the specific region, six Kompetitive allele-specific polymerase chain reaction markers derived from BSR-Seq were developed to delimit Rfk1 within physical intervals of 26.0 Mb. After searching for differentially expressed genes within the candidate interval in the anthers and sequencing analysis, TraesCS1B02G197400LC was identified as a candidate gene for Rfk1 and it was predicted to encode a pectinesterase/pectinesterase inhibitor. Expression analysis also confirmed that it was specifically expressed in the anthers, and its expression level was higher in fertile lines compared with sterile lines. Thus, TraesCS1B02G197400LC was identified as the most likely candidate gene for Rfk1, thereby providing insights into the fertility restoration mechanism for K-type CMS in wheat.

QTG-Finder2: A Generalized Machine-Learning Algorithm for Prioritizing QTL Causal Genes in Plants

Linkage mapping has been widely used to identify quantitative trait loci (QTL) in many plants and usually requires a time-consuming and labor-intensive fine mapping process to find the causal gene underlying the QTL. Previously, we described QTG-Finder, a machine-learning algorithm to rationally prioritize candidate causal genes in QTLs. While it showed good performance, QTG-Finder could only be used in Arabidopsis and rice because of the limited number of known causal genes in other species. Here we tested the feasibility of enabling QTG-Finder to work on species that have few or no known causal genes by using orthologs of known causal genes as the training set. The model trained with orthologs could recall about 64% of Arabidopsis and 83% of rice causal genes when the top 20% ranked genes were considered, which is similar to the performance of models trained with known causal genes. The average precision was 0.027 for Arabidopsis and 0.029 for rice. We further extended the algorithm to include polymorphisms in conserved non-coding sequences and gene presence/absence variation as additional features. Using this algorithm, QTG-Finder2, we trained and cross-validated Sorghum bicolor and Setaria viridis models. The S. bicolor model was validated by causal genes curated from the literature and could recall 70% of causal genes when the top 20% ranked genes were considered. In addition, we applied the S. viridis model and public transcriptome data to prioritize a plant height QTL and identified 13 candidate genes. QTL-Finder2 can accelerate the discovery of causal genes in any plant species and facilitate agricultural trait improvement.

Analysis of quantitative trait loci for fertility restoration in seven F2 populations derived from sorghum F1 hybrids bred in Japan

To clarify the genetic mechanisms of fertility restoration in sorghum F₁ hybrids produced in Japan ('Ryokuryu', 'Hazuki', 'Haretaka', 'Natsuibuki', 'Hanaaoba', 'Akidachi' and 'Kazetachi'), we analyzed QTLs for fertility restoration using seven F₂ populations derived from those hybrids. By QTL mapping with a series of SSR markers, we detected three major QTLs for fertility restoration. These data and the results of haplotype analysis of known fertility restorer (Rf) genes showed that qRf5, corresponding to the Rf5 locus, was the most widely used Rf gene for fertility restoration of sorghum F₁ hybrids among the lines tested. Other major Rf genes detected were qRf8, corresponding to Rf1, and qRf2, corresponding to Rf2. QTLs for grain weight also corresponded to these Rf loci. A minor QTL, qRf3, may also affect restoration of fertility. Our data show that three major Rfs-Rf1, Rf2, and Rf5-were used in F₁ hybrid sorghum production in Japan. This knowledge can be used to improve the efficiency of the F₁ sorghum breeding program.

Transmission Genetics of a Sorghum bicolor × S. halepense Backcross Populations

Despite a "ploidy barrier," interspecific crosses to wild and/or cultivated sorghum (Sorghum bicolor, 2n = 2x = 20) may have aided the spread across six continents of Sorghum halepense, also exemplifying risks of "transgene escape" from crops that could make weeds more difficult to control. Genetic maps of two BC₁F₁ populations derived from crosses of S. bicolor (sorghum) and S. halepense with totals of 722 and 795 single nucleotide polymorphism (SNP) markers span 37 and 35 linkage groups, with 2-6 for each of the 10 basic sorghum chromosomes due to fragments covering different chromosomal portions or independent segregation from different S. halepense homologs. Segregation distortion favored S. halepense alleles on chromosomes 2 (1.06-4.68 Mb, near a fertility restoration gene), 7 (1.20-6.16 Mb), 8 (1.81-5.33 Mb, associated with gene conversion), and 9 (47.5-50.1 Mb); and S. bicolor alleles on chromosome 6 (0-40 Mb), which contains both a large heterochromatin block and the Ma1 gene. Regions of the S. halepense genome that are recalcitrant to gene flow from sorghum might be exploited as part a multi-component system to reduce the likelihood of spread of transgenes or other modified genes. Its SNP profile suggests that chromosome segments from its respective progenitors S. bicolor and Sorghum propinquum have extensively recombined in S. halepense. This study reveals genomic regions that might discourage crop-to-weed gene escape, and provides a foundation for marker-trait association analysis to determine the genetic control of traits contributing to weediness, invasiveness, and perenniality of S. halepense.

Undesired fertility restoration in msm1 barley associates with two mTERF genes

The novel Rfm3 locus causing undesired fertility restoration in the msm1 cytoplasm of winter barley is located on the short arm of chromosome 6H. Undesired fertility restoration of cytoplasmic male sterile (CMS) mother lines in absence of the functional Rfm1 restorer gene is a significant problem for hybrid breeding in winter barley. Here, we describe that a novel restorer locus on the short arm of chromosome 6H, designated Rfm3, is closely linked to two mitochondrial transcription termination factor family (mTERF) protein coding genes. Genome-wide association studies in a multiparental mapping population revealed that two of the most significantly associated markers are located very close to these genes, with one marker lying directly within one mTERF gene sequence. Sequences of the candidate genes in the parental lines, segregating individuals and an independent set of breeding lines clearly revealed haplotypes discriminating completely sterile, partially fertile and Rfm1-restorer lines. The haplotypes segregate for several single nucleotide polymorphisms, a 6 bp insertion-deletion (InDel) polymorphism and another 2 bp InDel. CMS-unstable genotypes carrying haplotypes associated with undesired fertility restoration showed significantly higher grain setting on bagged spikes when plants were subjected to elevated temperatures during anthesis, indicating a temperature influence on pollen fertility. SNPs associated with desirable Rfm3 haplotypes can be implemented in marker-assisted selection of stable CMS mother lines.

Genetic and genomic resources of sorghum to connect genotype with phenotype in contrasting environments

With the recent development of genomic resources and high-throughput phenotyping platforms, the 21st century is primed for major breakthroughs in the discovery, understanding and utilization of plant genetic variation. Significant advances in agriculture remain at the forefront to increase crop production and quality to satisfy the global food demand in a changing climate all while reducing the environmental impacts of the world's food production. Sorghum, a resilient C₄ grain and grass important for food and energy production, is being extensively dissected genetically and phenomically to help connect the relationship between genetic and phenotypic variation. Unlike genetically modified crops such as corn or soybean, sorghum improvement has relied heavily on public research; thus, many of the genetic resources serve a dual purpose for both academic and commercial pursuits. Genetic and genomic resources not only provide the foundation to identify and understand the genes underlying variation, but also serve as novel sources of genetic and phenotypic diversity in plant breeding programs. To better disseminate the collective information of this community, we discuss: (i) the genomic resources of sorghum that are at the disposal of the research community; (ii) the suite of sorghum traits as potential targets for increasing productivity in contrasting environments; and (iii) the prospective approaches and technologies that will help to dissect the genotype-phenotype relationship as well as those that will apply foundational knowledge for sorghum improvement.

Exploration of miRNA-mediated fertility regulation network of cytoplasmic male sterility during flower bud development in soybean

Cytoplasmic male sterility (CMS) plays an important role in the production of soybean hybrid seeds. MicroRNAs (miRNAs) are a class of non-coding endogenous ~ 21 nt small RNAs that play crucial roles in flower and pollen development by targeting genes in plants. To dissect the function of miRNAs in soybean CMS, a total of 558 known miRNAs, 10 novel miRNAs, and 466 target genes were identified in flower buds of the soybean CMS line NJCMS1A and its restorer line NJCMS1C through small RNA sequencing and degradome analysis. In addition, miRNA-mediated editing events were also observed, and the two most frequently observed editing types (A → G and C → U) were validated by cloning and sequencing. And as the base editing occurred, some targets were filtered, such as gma-miR2118b-P6GT with Glyma.08G122000.2. Further integrated analysis of transcriptome and small RNA found some miRNAs and their targets' expression patterns showing a negative correlation, such as gma-miR156b/GmSPL9a and gma-miR4413b/GmPPR. Furthermore, opposite expression pattern was observed between gma-miR156b and GmSPL9 during early stage of flower bud development. Taken together, the regulatory network of gma-miR156b/GmSPL9 and gma-miR4413b/GmPPR with flower bud development in soybean CMS was developed. Most importantly, previous reports showed that these targets might be related to pollen development and male sterility, suggesting that both conserved and species-specific miRNAs might act as regulators of flower bud development in soybean CMS. These findings may provide a better understanding of the miRNA-mediated regulatory networks of CMS mechanisms in soybean.

QTL mapping and validation of fertility restoration in West African sorghum A1 cytoplasm and identification of a potential causative mutation for Rf2

Key message Major A ₁ cytoplasm fertility restoration loci, Rf ₂ and Rf ₅ , were found in the West African sorghum. A potential causative mutation for Rf ₂ was identified. KASP markers were validated on independent material. To accelerate the identification and development of hybrid parental lines in West African (WA) sorghum, this study aimed to understand the genetics underlying the fertility restoration (Rf) in WA A₁ cytoplasmic male sterility system and to develop markers for a routine use in WA breeding programs. We genotyped by sequencing three F₂ populations to map the Rf quantitative trait loci (QTL), validated the molecular KASP markers developed from those QTL in two F_2:3 populations, and assessed the most promising markers on a set of 95 R- and B-lines from WA breeding programs. Seven QTL were found across the three F₂ populations. On chromosome SBI-05, we found a major fertility restorer locus (Rf₅) for two populations with the same male parent, explaining 19 and 14% of the phenotypic variation in either population. Minor QTL were detected in these two populations on chromosomes SBI-02, SBI-03, SBI-04 and SBI-10. In the third population, we identified one major fertility restorer locus on chromosome SBI-02, Rf₂, explaining 31% of the phenotypic variation. Pentatricopeptide repeat genes in the Rf₂ QTL region were sequenced, and we detected in Sobic.002G057050 a missense mutation in the first exon, explaining 81% of the phenotypic variation in a F_2:3 population and clearly separating B- from R-lines. The KASP marker developed from this mutation stands as a promising tool for routine use in WA breeding programs.

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.

Comparative Analysis of the Cytology and Transcriptomes of the Cytoplasmic Male Sterility Line H276A and Its Maintainer Line H276B of Cotton (Gossypium barbadense L.)

In this study, the tetrad stage of microspore development in a new cotton (Gossypium barbadense L.) cytoplasmic male sterility (CMS) line, H276A, was identified using paraffin sections at the abortion stage. To explore the molecular mechanism underlying CMS in cotton, a comparative transcriptome analysis between the CMS line H276A and its maintainer line H276B at the tetrad stage was conducted using an Illumina HiSeq 4000 platform. The comparison of H276A with H276B revealed a total of 64,675 genes, which consisted of 59,255 known and 5420 novel genes. An analysis of the two libraries with a given threshold yielded a total of 3603 differentially expressed genes (DEGs), which included 1363 up- and 2240 down-regulated genes. Gene Ontology (GO) annotation showed that 2171 DEGs were distributed into 38 categories, and a Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that 2683 DEGs were classified into 127 groups. Thirteen DEGs were randomly selected and detected by quantitative reverse-transcribed PCR (qRT-PCR), and the results indicated that the transcriptome sequencing results were reliable. The bioinformatic analysis results in conjunction with previously reported data revealed key DEGs that might be associated with the male sterility features of H276A. Our results provide a comprehensive foundation for understanding anther development and will accelerate the study of the molecular mechanisms of CMS in cotton.

A genome-wide identification and analysis of the DYW-deaminase genes in the pentatricopeptide repeat gene family in cotton (Gossypium spp.)

The RNA editing occurring in plant organellar genomes mainly involves the change of cytidine to uridine. This process involves a deamination reaction, with cytidine deaminase as the catalyst. Pentatricopeptide repeat (PPR) proteins with a C-terminal DYW domain are reportedly associated with cytidine deamination, similar to members of the deaminase superfamily. PPR genes are involved in many cellular functions and biological processes including fertility restoration to cytoplasmic male sterility (CMS) in plants. In this study, we identified 227 and 211 DYW deaminase-coding PPR genes for the cultivated tetraploid cotton species G. hirsutum and G. barbadense (2n = 4x = 52), respectively, as well as 126 and 97 DYW deaminase-coding PPR genes in the ancestral diploid species G. raimondii and G. arboreum (2n = 26), respectively. The 227 G. hirsutum PPR genes were predicted to encode 52-2016 amino acids, 203 of which were mapped onto 26 chromosomes. Most DYW deaminase genes lacked introns, and their proteins were predicted to target the mitochondria or chloroplasts. Additionally, the DYW domain differed from the complete DYW deaminase domain, which contained part of the E domain and the entire E+ domain. The types and number of DYW tripeptides may have been influenced by evolutionary processes, with some tripeptides being lost. Furthermore, a gene ontology analysis revealed that DYW deaminase functions were mainly related to binding as well as hydrolase and transferase activities. The G. hirsutum DYW deaminase expression profiles varied among different cotton tissues and developmental stages, and no differentially expressed DYW deaminase-coding PPRs were directly associated with the male sterility and restoration in the CMS-D2 system. Our current study provides an important piece of information regarding the structural and evolutionary characteristics of Gossypium DYW-containing PPR genes coding for deaminases and will be useful for characterizing the DYW deaminase gene family in cotton biology and breeding.

Allelism analysis of BrRfp locus in different restorer lines and map-based cloning of a fertility restorer gene, BrRfp1, for pol CMS in Chinese cabbage (Brassica rapa L.)

In Chinese cabbage, there are two Rf loci for pol CMS and one of them was mapped to a 12.6-kb region containing a potential candidate gene encoding PPR protein. In Chinese cabbage (Brassica rapa), polima cytoplasmic male sterility (pol CMS) is an important CMS type and is widely used for hybrid breeding. By extensive test crossing in Chinese cabbage, four restorer lines (92s105, 01s325, 00s109, and 88s148) for pol CMS were screened. By analyzing the allelism of the four restorer lines, it was found that 92s105, 01s325, and 00s109 had the same "restorers of fertility" (Rf) locus (designated as BrRfp1), but 88s148 had a different Rf locus (designated as BrRfp2). For fine mapping the BrRfp1 locus of 92s105, a BC₁F₁ population with 487 individuals and a BC₁F₂ population with 2485 individuals were successively constructed. Using simple sequence repeat (SSR) markers developed from Brassica rapa reference genome and InDel markers derived from whole-genome resequencing data of 94c9 and 92s105, BrRfp1 was mapped to a 12.6-kb region containing a potential candidate gene encoding pentatricopeptide repeat-containing protein. Based on the nucleotide polymorphisms of the candidate gene sequence between the restoring and nonrestoring alleles, a co-segregating marker SC718 was developed, which would be helpful for hybrid breeding by marker-assisted screening and for detecting new restorer lines.

Evolutionary plasticity of restorer-of-fertility-like proteins in rice

Hybrid seed production in rice relies on cytoplasmic male sterility (CMS) induced by specific mitochondrial proteins, whose deleterious effects are suppressed by nuclear Restorer of Fertility (RF) genes. The majority of RF proteins belong to a specific clade of the RNA-binding pentatricopeptide repeat protein family. We have characterised 'restorer-of-fertility-like' (RFL) sequences from 13 Oryza genomes and the Brachypodium distachyon genome. The majority of the RFL sequences are found in genomic clusters located at two or three chromosomal loci with only a minor proportion being present as isolated genes. The RFL genomic cluster located on Oryza chromosome 10, the location of almost all known active rice RF genes, shows extreme variation in structure and gene content between species. We show evidence for homologous recombination events as an efficient mechanism for generating the huge repertoire of RNA sequence recognition motifs within RFL proteins and a major driver of RFL sequence evolution. The RFL sequences identified here will improve our understanding of the molecular basis of CMS and fertility restoration in plants and will accelerate the development of new breeding strategies.

A Gene Encoding Pentatricopeptide Repeat Protein Partially Restores Fertility in RT98-Type Cytoplasmic Male-Sterile Rice

Cytoplasmic male sterility (CMS) lines in rice, which have the cytoplasm of a wild species and the nuclear genome of cultivated rice, are of value for the study of genetic interactions between the mitochondrial and nuclear genomes. The RT98-type CMS line RT98A and the fertility restorer line RT98C carry the cytoplasm of the wild species Oryza rufipogon and the nuclear genome of the Taichung 65 cultivar (Oryza sativa L.). Based on a classical crossing experiment, fertility is reported to be restored gametophytically by the presence of a tentative single gene, designated Rf98, which is derived from the cytoplasm donor. Fine mapping of Rf98 revealed that at least two genes, which are closely positioned, are required for complete fertility restoration in RT98A. Here, we identified seven pentatricopeptide repeat (PPR) genes that are located within a 170 kb region as candidates for Rf98 Complementation tests revealed that the introduction of one of these PPR genes, PPR762, resulted in the partial recovery of fertility with a seed setting rate up to 9.3%. We conclude that PPR762 is an essential fertility restorer gene for RT98-type CMS. The low rate of seed setting suggested that some other genes near the Rf98 locus are also necessary for the full recovery of seed setting.

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.

The Propensity of Pentatricopeptide Repeat Genes to Evolve into Restorers of Cytoplasmic Male Sterility

Cytoplasmic male sterility (CMS) is a widespread phenotype in plants, which present a defect in the production of functional pollen. The male sterilizing factors usually consist of unusual genes or open reading frames encoded by the mitochondrial genome. CMS can be suppressed by specific nuclear genes called restorers of fertility (Rfs). In the majority of cases, Rf genes produce proteins that act directly on the CMS conferring mitochondrial transcripts by binding them specifically and promoting processing events. In this review, we explore the wide array of mechanisms guiding fertility restoration. PPR proteins represent the most frequent protein class among identified Rfs and they exhibit ideal characteristics to evolve into restorer of fertility when the mechanism of restoration implies a post-transcriptional action. Here, we review the literature that highlights those characteristics and help explain why PPR proteins are ideal for the roles they play as restorers of fertility.

Molecular Breeding of Sorghum bicolor, A Novel Energy Crop

Currently, molecular breeding is regarded as an important tool for the improvement of many crop species. However, in sorghum, recently heralded as an important bioenergy crop, progress in this field has been relatively slow and limited. In this review, we present existing efforts targeted at genetic characterization of sorghum mutants. We also comprehensively review the different attempts made toward the isolation of genes involved in agronomically important traits, including the dissection of some sorghum quantitative trait loci (QTLs). We also explore the current status of the use of transgenic techniques in sorghum, which should be crucial for advancing sorghum molecular breeding. Through this report, we provide a useful benchmark to help assess how much more sorghum genomics and molecular breeding could be improved.

Inheritance and molecular mapping of Rf6 locus with pollen fertility restoration ability on A1 and A2 cytoplasms in sorghum

Of the several male sterility cytoplasms available as an alternative to the widely exploited A1 (milo) cytoplasm in sorghum, A2 is more suitable for commercial exploitation. Diversification of genetic and cytoplasmic base of hybrids involving A2 cytoplasm necessitates mapping of fertility restorer (Rf) genes for use in marker-assisted restorer development. We mapped a major male fertility restoration locus on sorghum chromosome 4 tightly linked with SSR markers, SB2387 and SB2388. This new fertility locus, Rf6, was able to restore male fertility on both A1 and A2 cytoplasms. Analysis of the genomic region around the Rf6 locus identified six genes including a pentatricopeptide repeat (PPR) gene, Sobic.004G004100. With its similar restoration ability to Rf1, Rf2 and Rf5 loci in sorghum, it is most likely that the Rf6 is a member of the PPR gene family, and the PPR gene Sobic.004G004100 could be a candidate for fertility restoration on A1 and A2 cytoplasms.

Genetic differentiation analysis for the identification of complementary parental pools for sorghum hybrid breeding in Ethiopia

The potential for exploiting heterosis for sorghum hybrid production in Ethiopia with improved local adaptation and farmers preferences has been investigated and populations suitable for initial hybrid development have been identified. Hybrids in sorghum have demonstrated increased productivity and stability of performance in the developed world. In Ethiopia, the uptake of hybrid sorghum has been limited to date, primarily due to poor adaptation and absence of farmer's preferred traits in existing hybrids. This study aimed to identify complementary parental pools to develop locally adapted hybrids, through an analysis of whole genome variability of 184 locally adapted genotypes and introduced hybrid parents (R and B). Genetic variability was assessed using genetic distance, model-based STRUCTURE analysis and pair-wise comparison of groups. We observed a high degree of genetic similarity between the Ethiopian improved inbred genotypes and a subset of landraces adapted to lowland agro-ecology with the introduced R lines. This coupled with the genetic differentiation from existing B lines, indicated that these locally adapted genotype groups are expected to have similar patterns of heterotic expression as observed between introduced R and B line pools. Additionally, the hybrids derived from these locally adapted genotypes will have the benefit of containing farmers preferred traits. The groups most divergent from introduced B lines were the Ethiopian landraces adapted to highland and intermediate agro-ecologies and a subset of lowland-adapted genotypes, indicating the potential for increased heterotic response of their hybrids. However, these groups were also differentiated from the R lines, and hence are different from the existing complementary heterotic pools. This suggests that although these groups could provide highly divergent parental pools, further research is required to investigate the extent of heterosis and their hybrid performance.

Mitochondrion role in molecular basis of cytoplasmic male sterility

Cytoplasmic male sterility and its fertility restoration via nuclear genes offer the possibility to understand the role of mitochondria during microsporogenesis. In most cases rearrangements in the mitochondrial DNA involving known mitochondrial genes as well as unknown sequences result in the creation of new chimeric open reading frames, which encode proteins containing transmembrane domains. So far, most of the CMS systems have been characterized via restriction fragment polymorphisms followed by transcript analysis. However, whole mitochondrial genome sequence analyses comparing male sterile and fertile cytoplasm open options for deeper insights into mitochondrial genome rearrangements. We more and more start to unravel how mitochondria are involved in triggering death of the male reproductive organs. Reduced levels of ATP accompanied by increased concentrations of reactive oxygen species, which are produced more under conditions of mitochondrial dysfunction, seem to play a major role in the fate of pollen production. Nuclear genes, so called restorer-of-fertility are able to restore the male fertility. Fertility restoration can occur via pentatricopeptide repeat (PPR) proteins or via different mechanisms involving non-PPR proteins.

Male sterility and fertility restoration in crops

In plants, male sterility can be caused either by mitochondrial genes with coupled nuclear genes or by nuclear genes alone; the resulting conditions are known as cytoplasmic male sterility (CMS) and genic male sterility (GMS), respectively. CMS and GMS facilitate hybrid seed production for many crops and thus allow breeders to harness yield gains associated with hybrid vigor (heterosis). In CMS, layers of interaction between mitochondrial and nuclear genes control its male specificity, occurrence, and restoration of fertility. Environment-sensitive GMS (EGMS) mutants may involve epigenetic control by noncoding RNAs and can revert to fertility under different growth conditions, making them useful breeding materials in the hybrid seed industry. Here, we review recent research on CMS and EGMS systems in crops, summarize general models of male sterility and fertility restoration, and discuss the evolutionary significance of these reproductive systems.

The Rf and Rf-like PPR in higher plants, a fast-evolving subclass of PPR genes

In the last years, a number of nuclear genes restoring cytoplasmic male sterility (CMS) have been cloned in various crop species. The majority of these genes have been shown to encode pentatricopeptide repeat proteins (PPR) that act by specifically suppressing the expression of sterility-causing mitochondrial transcripts. Functional analysis of these proteins has indicated that the inhibitory effects of restoring PPR (Rf-PPR) proteins involve various mechanisms, including RNA cleavage, RNA destabilization, or translation inhibition. Cross-species sequence comparison of PPR protein complements revealed that most plant genomes encode 10-30 Rf-like (RFL) proteins sharing high-sequence similarity with the identified Rf-PPRs from crops. Evolutionary analyses further showed that they constitute a monophyletic group apart in the PPR family, with peculiar evolution dynamic and constraints. Here we review recent data on RF-PPRs and present the latest discoveries on the RFL family, with prospects on the functionality and evolution of this peculiar subclass of PPR.

Unusual and typical features of a novel restorer-of-fertility gene of sugar beet (Beta vulgaris L.)

Male gametogenesis in plants can be impaired by an incompatibility between nuclear and mitochondrial genomes, termed cytoplasmic male sterility (CMS). A sterilizing factor resides in mitochondria, whereas a nuclear factor, Restorer-of-fertility (Rf), restores male fertility. Although a majority of plant Rf genes are thought to encode a family of RNA-binding proteins called pentatrico-peptide repeat (PPR) proteins, we isolated a novel type of Rf from sugar beet. Two BACs and one cosmid clone that constituted a 383-kbp contig covering the sugar beet Rf1 locus were sequenced. Of 41 genes borne by the contig, quadruplicated genes were found to be associated with specific transcripts in Rf1 flower buds. The quadruplicated genes encoded a protein resembling OMA1, a protein known from yeast and mammals to be involved in mitochondrial protein quality control. Construction of transgenic plants revealed that one of the four genes (bvORF20) was capable of restoring partial pollen fertility to CMS sugar beet; the level of restoration was comparable to that evaluated by a crossing experiment. However, the other genes lacked such a capability. A GFP-fusion experiment showed that bvORF20 encoded a mitochondrial protein. The corresponding gene was cloned from rf1rf1 sugar beet and sequenced, and a solitary gene that was similar but not identical to bvORF20 was found. Genetic features exhibited by sugar beet Rf1, such as gene clustering and copy-number variation between Rf1 and rf, were reminiscent of PPR-type Rf, suggesting that a common evolutionary mechanism(s) operates on plant Rfs irrespective of the translation product.

Identification of QTLs for eight agronomically important traits using an ultra-high-density map based on SNPs generated from high-throughput sequencing in sorghum under contrasting photoperiods

The productivity of sorghum is mainly determined by agronomically important traits. The genetic bases of these traits have historically been dissected and analysed through quantitative trait locus (QTL) mapping based on linkage maps with low-throughput molecular markers, which is one of the factors that hinder precise and complete information about the numbers and locations of the genes or QTLs controlling the traits. In this study, an ultra-high-density linkage map based on high-quality single nucleotide polymorphisms (SNPs) generated from low-coverage sequences (~0.07 genome sequence) in a sorghum recombinant inbred line (RIL) population was constructed through new sequencing technology. This map consisted of 3418 bin markers and spanned 1591.4 cM of genome size with an average distance of 0.5 cM between adjacent bins. QTL analysis was performed and a total of 57 major QTLs were detected for eight agronomically important traits under two contrasting photoperiods. The phenotypic variation explained by individual QTLs varied from 3.40% to 33.82%. The high accuracy and quality of this map was evidenced by the finding that genes underlying two cloned QTLs, Dw3 for plant height (chromosome 7) and Ma1 for flowering time (chromosome 6), were localized to the correct genomic regions. The close associations between two genomic regions on chromosomes 6 and 7 with multiple traits suggested the existence of pleiotropy or tight linkage. Several major QTLs for heading date, plant height, numbers of nodes, stem diameter, panicle neck length, and flag leaf width were detected consistently under both photoperiods, providing useful information for understanding the genetic mechanisms of the agronomically important traits responsible for the change of photoperiod.

Two non-allelic nuclear genes restore fertility in a gametophytic pattern and enhance abiotic stress tolerance in the hybrid rice plant

In indica rice, the HongLian (HL)-type combination of cytoplasmic male sterility (CMS) and fertility restoration (Rf) is widely used for the production of commercial hybrid seeds in China, Laos, Vietnam and other Southeast Asian countries. Generally, any member of the gametophytic fertility restoration system, 50% of the pollen in hybrid F(1) plants displays recovered sterility. In this study, however, a HL-type hybrid variety named HongLian You6 had approximately 75% normal (viable) pollen rather than the expected 50%. To resolve this discrepancy, several fertility segregation populations, including F(2) and BC(1)F(1) derived from the HL-CMS line Yuetai A crossed with the restorer line 9311, were constructed and subjected to genetic analysis. A gametophytic restoration model was discovered to involve two non-allelic nuclear restorer genes, Rf5 and Rf6. The Rf5 had been previously identified using a positional clone strategy. The Rf6 gene represents a new restorer gene locus, which was mapped to the short arm of chromosome 8. The hybrid F(1) plants containing one restorer gene, either Rf5 or Rf6, displayed 50% normal pollen grains with I(2)-KI solution; however, those with both Rf5 and Rf6 displayed 75% normal pollens. We also established that the hybrid F(1) plants including both non-allelic restorer genes exhibited an increased stable seed setting when subjected to stress versus the F(1) plants with only one restorer gene. Finally, we discuss the breeding scheme for the plant gametophytic CMS/Rf system.

Mapping and characterization of Rf 5: a new gene conditioning pollen fertility restoration in A1 and A2 cytoplasm in sorghum (Sorghum bicolor (L.) Moench)

With an aim to further characterize the cytoplasmic male sterility-fertility restoration system in sorghum, a major fertility restoration gene was mapped along with a second locus capable of partial restoration of pollen fertility. The major fertility restoration gene, Rf(5), was located on sorghum chromosome SBI-05, and was capable of restoring pollen fertility in both A(1) and A(2) male sterile cytoplasms. Depending on the restorer parent, mapping populations exhibited fertility restoration phenotypes that ranged from nearly bimodal distribution due to the action of Rf(5), to a more normalized distribution reflecting the action of Rf(5) and additional modifier/partial restoration genes. A second fertility restoration locus capable of partially restoring pollen fertility in A(1) cytoplasm was localized to chromosome SBI-04. Unlike Rf(5), this modifier/partial restorer gene acting alone resulted in less than 10% seed set in both A(1) and A(2) cytoplasms, and modified the extent of restoration conditioned by the major restorer Rf(5) in A(1) cytoplasm. In examining the genomic regions spanning the Rf(5) locus, a cluster of pentatricopeptide gene family members with high homology to rice Rf (1) and sorghum Rf (2) were identified as potential candidates encoding Rf(5).

Location of major effect genes in sorghum (Sorghum bicolor (L.) Moench)

Major effect genes are often used for germplasm identification, for diversity analyses and as selection targets in breeding. To date, only a few morphological characters have been mapped as major effect genes across a range of genetic linkage maps based on different types of molecular markers in sorghum (Sorghum bicolor (L.) Moench). This study aims to integrate all available previously mapped major effect genes onto a complete genome map, linked to the whole genome sequence, allowing sorghum breeders and researchers to link this information to QTL studies and to be aware of the consequences of selection for major genes. This provides new opportunities for breeders to take advantage of readily scorable morphological traits and to develop more effective breeding strategies. We also provide examples of the impact of selection for major effect genes on quantitative traits in sorghum. The concepts described in this paper have particular application to breeding programmes in developing countries where molecular markers are expensive or impossible to access.