Fine mapping of a dominant thermo-sensitive genic male sterility gene (BntsMs) in rapeseed (Brassica napus) with AFLP- and Brassica rapa-derived PCR markers

Applications of Molecular Markers for Developing Abiotic-Stress-Resilient Oilseed Crops

Globally, abiotic stresses, such as temperature (heat or cold), water (drought and flooding), and salinity, cause significant losses in crop production and have adverse effects on plant growth and development. A variety of DNA-based molecular markers, such as SSRs, RFLPs, AFLPs, SNPs, etc., have been used to screen germplasms for stress tolerance and the QTL mapping of stress-related genes. Such molecular-marker-assisted selection strategies can quicken the development of tolerant/resistant cultivars to withstand abiotic stresses. Oilseeds such as rapeseed, mustard, peanuts, soybeans, sunflower, safflower, sesame, flaxseed, and castor are the most important source of edible oil worldwide. Although oilseed crops are known for their capacity to withstand abiotic challenges, there is a significant difference between actual and potential yields due to the adaptation and tolerance to severe abiotic pressures. This review summarizes the applications of molecular markers to date to achieve abiotic stress tolerance in major oilseed crops. The molecular markers that have been reported for genetic diversity studies and the mapping and tagging of genes/QTLs for drought, heavy metal stress, salinity, flooding, cold and heat stress, and their application in the MAS are presented.

Development of the PARMS Marker of the Dominant Genic Male Sterility (DGMS) Line and Its Utilization in Rapeseed (Brassica napus L.) Breeding

The 8029AB line is a dominant genic male sterility (DGMS) two-type line in Brassica napus L., which can be used in a three-line approach for the seed production of rapeseed hybrids. Genetic analyses have demonstrated that the sterility of 8029A is controlled by a single dominant nuclear gene (BnMS5^e) interacting with one recessive gene (BnMS5^c). Six pairs of penta-primer amplification refractory mutation system (PARMS) markers were designed according to the sequence of BnMS5^a, BnMS5^c and BnMS5^e. Two pairs of these PARMS markers were successfully identified and validated. The PARMS markers MS5-1Fc/MS5-1Ft/MS5-1R12 could distinguish BnMS5^c from BnMS5^a/BnMS5^e, and the PARMS markers MS5-2Ft/MS5-2Fa/MS5-1R12 could genotype BnMS5^a and BnMS5^c/BnMS5^e. The combination of these two pairs of PARMS markers could be used to identify the presence or absence of BnMS5^a/BnMS5^c/BnMS5^e effectively. Consequently, marker-assisted selection can be carried out in the early generation to shorten the breeding period and improve the breeding efficiency.

Cytological and genetic characterisation of dominant GMS line Shaan-GMS in Brassica napus L

Genic male sterility (GMS) is an effective pollination control system applied in the hybrid breeding of Brassica napus L. Shaan-GMS is a spontaneous mutant of dominant GMS in B. napus. In this research, anther abortion in the homozygous two-type line 9A15AB derived from Shaan-GMS was characterised with the combined use of light microscopy and transmission electron microscopy. Results indicated that the most striking differences between the fertile and sterile plants occurred in the tapetum in the early microsporocyte stage. In sterile plants, the tapetal cells were irregularly arranged, multi-layered and occupied the growing space of microsporocytes. When entering into meiosis, the tapetum cells degraded and the cytoplasm fused. Some oval monolayer or bilayer membrane organelles existed in the tapetal cells in sterile anthers. Mitochondria in the tapetal cells were abnormal, and middle layer cells degraded early. Pollen mother cells of Shaan-GMS degenerated at the start of meiosis and ceased at the anaphase I stage, with no dyads or tetrads formed. The combined effects of the abnormal development of the tapetum, the middle layer cells and meiosis lead to male sterility in Shaan-GMS. Inheritance of male sterility of Shaan-GMS is controlled by a monogenically multiallelic locus with three different alleles (Ms, ms and Mf), with a relationship expressed as Mf > Ms and Ms > ms. The findings help lay the foundation for illustrating the mechanism of male sterility and the utilisation of Shaan-GMS in rapeseed.

Mapping of the male sterile mutant gene ftms in Brassica rapa L. ssp. pekinensis via BSR-Seq combined with whole-genome resequencing

A male sterile mutant was created by ⁶⁰Co γ-rays of microspores isolated from Chinese cabbage DH line 'FT'. A candidate gene for the male sterile trait was identified as Bra010198. Male sterility is used for hybrid seed production in Chinese cabbage. In this study, we derived a male sterile mutant (ftms) from Chinese cabbage DH line 'FT' by irradiating microspores with ⁶⁰Co γ-rays and realized the rapid trait transformation from male fertility to sterility for creating valuable breeding materials. Genetic analysis indicated that the male sterile trait is controlled by a single recessive nuclear gene, ftms. Microspore development in mutant ftms was aborted at the tetrad stage and associated with severely retarded degeneration and vacuolation of tapetum. Using BSR-seq analysis, the candidate region for ftms was mapped on chromosome A05. A large F₂ population was created, and the region was narrowed to approximately 1.7-Mb between markers Indel20 and Indel14 via linkage analysis. The recombination frequency was extremely suppressed because the region was located on the chromosome A05 centromere. Whole-genome resequencing of mutant ftms and wild-type 'FT' aligned only one nonsynonymous SNP to Bra010198; this gene is a homolog of Arabidopsis KNS4/UPEX1, which encodes a putative β-(1,3)-galactosyltransferase that controls pollen exine development. Comparative sequencing verified the SNP position on the fifth exon of Bra010198 in mutant ftms. Further genotyping revealed that the male sterile phenotype was fully co-segregated with this SNP. Quantitative real-time PCR indicated that Bra0101918 specifically expressed in stamen. The data presented herein suggested that Bra010198 is a strong candidate gene for ftms. Hence, we developed a male sterile line for potential application in breeding and expanded the knowledge about the molecular mechanism underlying male sterility in Chinese cabbage.

Identification of candidate genes underlying genic male-sterile msc-1 locus via genome resequencing in Capsicum annuum L

Based on genome resequencing, a strong candidate gene Capana02g002096 was identified in this study. Capana02g002096 encodes a homolog of AtDYT1 which is a bHLH transcription factor and involves in the early tapetal development. Genic male-sterile line is an efficient tool for commercial hybrid seed production in pepper; however, so far, only few genes controlling this trait have been cloned. A spontaneous genic male-sterile mutant, msc-1, had been identified and widely used in China, of which the male-sterile trait was proved to be controlled by a single recessive locus. For cloning the gene(s) underlying the msc-1 locus, genome resequencing and comparison analyses were performed between male-sterile and male-fertile lines. According to the genomic variations and genes' annotations, Capana02g002096 was selected as a candidate gene underlying the msc-1 locus. Capana02g002096 encodes a homolog of AtDYT1, which is a bHLH transcription factor and involves in the early tapetal development. Moreover, a 7-bp deletion was identified in the exon of Capana02g002096, which led to a premature stop codon and may cause a loss-of-function mutation. Further genotyping in the 16C1369AB population containing 1110 plants, a F₂ population consisting of 510 plants and 46 inbreed lines revealed that the male-sterile phenotype was co-segregated with the 7-bp deletion. Additionally, real-time PCR analysis revealed that Capana02g002096 was an anther-specific gene and repression of the gene's expression through VIGS led to male-sterile phenotype. Therefore, based on the evidence at genetic, genomic, transcriptional and posttranscriptional levels, Capana02g002096 was considered as a strong candidate gene underlying the msc-1 locus in pepper and was renamed Msc-1.

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.

microRNAs involved in auxin signalling modulate male sterility under high-temperature stress in cotton (Gossypium hirsutum)

Male sterility caused by long-term high-temperature (HT) stress occurs widely in crops. MicroRNAs (miRNAs), a class of endogenous non-coding small RNAs, play an important role in the plant response to various abiotic stresses. To dissect the working principle of miRNAs in male sterility under HT stress in cotton, a total of 112 known miRNAs, 270 novel miRNAs and 347 target genes were identified from anthers of HT-insensitive (84021) and HT-sensitive (H05) cotton cultivars under normal-temperature and HT conditions through small RNA and degradome sequencing. Quantitative reverse transcriptase-polymerase chain reaction and 5'-RNA ligase-mediated rapid amplification of cDNA ends experiments were used to validate the sequencing data. The results show that miR156 was suppressed by HT stress in both 84021 and H05; miR160 was suppressed in 84021 but induced in H05. Correspondingly, SPLs (target genes of miR156) were induced both in 84021 and H05; ARF10 and ARF17 (target genes of miR160) were induced in 84021 but suppressed in H05. Overexpressing miR160 increased cotton sensitivity to HT stress seen as anther indehiscence, associated with the suppression of ARF10 and ARF17 expression, thereby activating the auxin response that leads to anther indehiscence. Supporting this role for auxin, exogenous Indole-3-acetic acid (IAA) leads to a stronger male sterility phenotype both in 84021 and H05 under HT stress. Cotton plants overexpressing miR157 suppressed the auxin signal, and also showed enhanced sensitivity to HT stress, with microspore abortion and anther indehiscence. Thus, we propose that the auxin signal, mediated by miRNAs, is essential for cotton anther fertility under HT stress.

Identification and Analysis of MS5d: A Gene That Affects Double-Strand Break (DSB) Repair during Meiosis I in Brassica napus Microsporocytes

Here, we report the identification of the Brassica-specific gene MS5d, which is responsible for male sterility in Brassica napus. The MS5d gene is highly expressed in the microsporocyte and encodes a protein that localizes to the nucleus. Light microscopy analyses have demonstrated that the MS5d gene affects microsporocyte meiosis in the thermosensitive genic male sterility line TE5A. Sequence comparisons and genetic complementation revealed a C-to-T transition in MS5d, encoding a Leu-to-Phe (L281F) substitution and causing abnormal male meiosis in TE5A. These findings suggest arrested meiotic chromosome dynamics at pachytene. Furthermore, immunofluorescence analyses showed that double-strand break (DSB) formation and axial elements were normal but that DSB repair and spindle behavior were aberrant in TE5A meiocytes. Collectively, our results indicate that MS5d likely encodes a protein required for chromosomal DSB repair at early stages of meiosis in B. napus.

Aberrant Meiotic Prophase I Leads to Genic Male Sterility in the Novel TE5A Mutant of Brassica napus

Genic male sterility (GMS) has already been extensively utilized for hybrid rapeseed production. TE5A is a novel thermo-sensitive dominant GMS line in Brassica napus, however, its mechanisms of GMS remain largely unclear. Histological and Transmission electron microscopy (TEM) analyses of anthers showed that the male gamete development of TE5A was arrested at meiosis prophase I. EdU uptake of S-phase meiocytes revealed that the TE5A mutant could accomplish DNA replication, however, chromosomal and fluorescence in situ hybridization (FISH) analyses of TE5A showed that homologous chromosomes could not pair, synapse, condense and form bivalents. We then analyzed the transcriptome differences between young floral buds of sterile plants and its near-isogenic fertile plants through RNA-Seq. A total of 3,841 differentially expressed genes (DEGs) were obtained, some of which were associated with homologous chromosome behavior and cell cycle control during meiosis. Dynamic expression changes of selected candidate DEGs were then analyzed at different anther developmental stages. The present study not only demonstrated that the TE5A mutant had defects in meiotic prophase I via detailed cytological analysis, but also provided a global insight into GMS-associated DEGs and elucidated the mechanisms of GMS in TE5A through RNA-Seq.