Delimitation of the fertility restorer locus Rfk1 to a 43-kb contig in Kosena radish (Raphanus sativus L.)

Mechanism and Utilization of Ogura Cytoplasmic Male Sterility in Cruciferae Crops

Hybrid production using lines with cytoplasmic male sterility (CMS) has become an important way to utilize heterosis in vegetables. Ogura CMS, with the advantages of complete pollen abortion, ease of transfer and a progeny sterility rate reaching 100%, is widely used in cruciferous crop breeding. The mapping, cloning, mechanism and application of Ogura CMS and fertility restorer genes in Brassica napus, Brassica rapa, Brassica oleracea and other cruciferous crops are reviewed herein, and the existing problems and future research directions in the application of Ogura CMS are discussed.

Functions of PPR Proteins in Plant Growth and Development

Pentatricopeptide repeat (PPR) proteins form a large protein family in land plants, with hundreds of different members in angiosperms. In the last decade, a number of studies have shown that PPR proteins are sequence-specific RNA-binding proteins involved in multiple aspects of plant organellar RNA processing, and perform numerous functions in plants throughout their life cycle. Recently, computational and structural studies have provided new insights into the working mechanisms of PPR proteins in RNA recognition and cytidine deamination. In this review, we summarized the research progress on the functions of PPR proteins in plant growth and development, with a particular focus on their effects on cytoplasmic male sterility, stress responses, and seed development. We also documented the molecular mechanisms of PPR proteins in mediating RNA processing in plant mitochondria and chloroplasts.

Fine Mapping and Identification of BnaC06.FtsH1, a Lethal Gene That Regulates the PSII Repair Cycle in Brassica napus

Photosystem II (PSII) is an important component of the chloroplast. The PSII repair cycle is crucial for the relief of photoinhibition and may be advantageous when improving stress resistance and photosynthetic efficiency. Lethal genes are widely used in the efficiency detection and method improvement of gene editing. In the present study, we identified the naturally occurring lethal mutant 7-521Y with etiolated cotyledons in Brassica napus, controlled by double-recessive genes (named cyd1 and cyd2). By combining whole-genome resequencing and map-based cloning, CYD1 was fine-mapped to a 29 kb genomic region using 15,167 etiolated individuals. Through cosegregation analysis and functional verification of the transgene, BnaC06.FtsH1 was determined to be the target gene; it encodes an filamentation temperature sensitive protein H 1 (FtsH1) hydrolase that degrades damaged PSII D1 in Arabidopsis thaliana. The expression of BnaC06.FtsH1 was high in the cotyledons, leaves, and flowers of B. napus, and localized in the chloroplasts. In addition, the expression of EngA (upstream regulation gene of FtsH) increased and D1 decreased in 7-521Y. Double mutants of FtsH1 and FtsH5 were lethal in A. thaliana. Through phylogenetic analysis, the loss of FtsH5 was identified in Brassica, and the remaining FtsH1 was required for PSII repair cycle. CYD2 may be a homologous gene of FtsH1 on chromosome A07 of B. napus. Our study provides new insights into lethal mutants, the findings may help improve the efficiency of the PSII repair cycle and biomass accumulation in oilseed rape.

Mapping of the New Fertility Restorer Gene Rf-PET2 Close to Rf1 on Linkage Group 13 in Sunflower (Helianthus annuus L.)

The PET2-cytoplasm represents a well characterized new source of cytoplasmic male sterility (CMS) in sunflower. It is distinct from the PET1-cytoplasm, used worldwide for commercial hybrid breeding, although it was, as PET1, derived from an interspecific cross between Helianthus. petiolaris and H. annuus. Fertility restoration is essential for the use of CMS PET2 in sunflower hybrid breeding. Markers closely linked to the fertility restorer gene are needed to build up a pool of restorer lines. Fertility-restored F₁-hybrids RHA 265(PET2) × IH-51 showed pollen viability of 98.2% ± 1.2, indicating a sporophytic mode of fertility restoration. Segregation analyses in the F₂-population of the cross RHA 265(PET2) × IH-51 revealed that this cross segregated for one major restorer gene Rf-PET2. Bulked-segregant analyses investigating 256 amplified fragment length polymorphism (AFLP) primer combinations revealed a high degree of polymorphism in this cross. Using a subset of 24 AFLP markers, three sequence-tagged site (STS) markers and three microsatellite markers, Rf-PET2 could be mapped to the distal region of linkage group 13 between ORS1030 and ORS630. Three AFLP markers linked to Rf-PET2 were cloned and sequenced. Homology search against the sunflower genome sequence of HanXRQ v1r1 confirmed the physical location of Rf-PET2 close to the restorer gene Rf1 for CMS PET1. STS markers were mapped that can now be used for marker-assisted selection.

Molecular breeding in Brassica for salt tolerance: importance of microsatellite (SSR) markers for molecular breeding in Brassica

Salinity is one of the important abiotic factors for any crop management in irrigated as well as rainfed areas, which leads to poor harvests. This yield reduction in salt affected soils can be overcome by improving salt tolerance in crops or by soil reclamation. Salty soils can be reclaimed by leaching the salt or by cultivation of salt tolerance crops. Salt tolerance is a quantitative trait controlled by several genes. Poor knowledge about mechanism of its inheritance makes slow progress in its introgression into target crops. Brassica is known to be a good reclamation crop. Inter and intra specific variation within Brassica species shows potential of molecular breeding to raise salinity tolerant genotypes. Among the various molecular markers, SSR markers are getting high attention, since they are randomly sparsed, highly variable and show co-dominant inheritance. Furthermore, as sequencing techniques are improving and softwares to find SSR markers are being developed, SSR markers technology is also evolving rapidly. Comparative SSR marker studies targeting Arabidopsis thaliana and Brassica species which lie in the same family will further aid in studying the salt tolerance related QTLs and subsequent identification of the "candidate genes" and finding out the origin of important QTLs. Although, there are a few reports on molecular breeding for improving salt tolerance using molecular markers in Brassica species, usage of SSR markers has a big potential to improve salt tolerance in Brassica crops. In order to obtain best harvests, role of SSR marker driven breeding approaches play important role and it has been discussed in this review especially for the introgression of salt tolerance traits in crops.

Heterozygous alleles restore male fertility to cytoplasmic male-sterile radish (Raphanus sativus L.): a case of overdominance

The practice of hybridization has greatly contributed to the increase in crop productivity. A major component that exploits heterosis in crops is the cytoplasmic male sterility (CMS)/nucleus-controlled fertility restoration (Rf) system. Through positional cloning, it is shown that heterozygous alleles (RsRf3-1/RsRf3-2) encoding pentatricopeptide repeat (PPR) proteins are responsible for restoring fertility to cytoplasmic male-sterile radish (Raphanus sativus L.). Furthermore, it was found that heterozygous alleles (RsRf3-1/RsRf3-2) show higher expression and RNA polymerase II occupancy in the CMS cytoplasmic background compared with their homozygous alleles (RsRf3-1/RsRf3-1 or RsRf3-2/RsRf3-2). These data provide new insights into the molecular mechanism of fertility restoration to cytoplasmic male-sterile plants and illustrate a case of overdominance.

Precise mapping Fhb5, a major QTL conditioning resistance to Fusarium infection in bread wheat (Triticum aestivum L.)

Qfhi.nau-5A is a major quantitative trait locus (QTL) against Fusarium graminearum infection in the resistant wheat germplasm Wangshuibai. Genetic analysis using BC(3)F(2) and BC(4)F(2) populations, derived from selfing two near-isogenic lines (NIL) heterozygous at Qfhi.nau-5A that were developed, respectively, with Mianyang 99-323 and PH691 as the recurrent parent, showed that Qfhi.nau-5A inherited like a single dominant gene. This QTL was thus designated as Fhb5. To fine map it, these two backcross populations and a recombinant inbred line (RIL) population derived from Nanda2419 × Wangshuibai were screened for recombinants occurring between its two flanking markers Xbarc56 and Xbarc100. Nineteen NIL recombinants were identified from the two backcross populations and nine from the RIL population. In the RIL recombinant selection process, selection against Fhb4 present in the RIL population was incorporated. Genotyping these recombinant lines with ten markers mapping to the Xbarc56-Xbarc100 interval revealed four types of Mianyang 99-323-derived NIL recombinants, three types of PH691-derived NIL recombinants, and four types of RIL recombinants. In different field trials, the percentage of infected spikes of these lines displayed a distinct two-peak distribution. The more resistant class had over 55% less infection than the susceptible class. Common to these resistant genotypes, the 0.3-cM interval flanked by Xgwm304 and Xgwm415 or one of these two loci was derived from Wangshuibai, while none of the susceptible recombinants had Wangshuibai chromatin in this interval. This interval harboring Fhb5 was mapped to the pericentromeric C-5AS3-0.75 bin through deletion bin mapping. The precise localization of Fhb5 will facilitate its utilization in marker-assisted wheat breeding programs.

A new fertility restorer locus linked closely to the Rfo locus for cytoplasmic male sterility in radish

In this study, we have investigated a new fertility restorer (Rf) locus for cytoplasmic male sterility (CMS) in radish. We have obtained a CMS-Rf system consisting of sterile line '9802A1', maintainer line '9802B1' and restorer line '9802H'. F(1) plants from cross between sterile line '9802A1' and restorer line '9802H' were all male fertile, self pollination of F(1) plants produced an F(2) segregating population consisting of 600 individuals. The segregating population was found to fit a segregation ratio 3:1 for male fertile and sterile types, indicating that male fertility is restored by a single dominant gene (termed Rfo2) in the CMS-Rf system. Based on the DNA sequence of Rfo/Rfk1 (AJ535623), just one full length gene in the sterile line '9802A1', in the restorer line '9802H' and in the male fertile line '2006H', was cloned, respectively. The three sequences correspond to the same gene with two alleles: Rfob in '9802H' and rfob in '9802A1' and '2006H'. These two alleles differ from Rfo/Rfk1 and rfk1 (AJ535624) alleles by two synonymous base substitutions, respectively. Based on the differences between the Rfob and rfob genes, one PCR-based marker was developed, and designated Marker 1, which is identical to the corresponding region of Rfob by sequence analysis. In the F(2) segregating population described above, the Marker 1 was present in 5 sterile plants and in 453 fertile plants, absent in 4 fertile plants and in 138 sterile plants, and was found to fit a segregation ratio 3:1 indicating that Rfob was single copy in '9802H'. Linkage analysis showed that the Rfo2 locus for our CMS-Rf system was distant from the Rfo locus by about 1.6 cM. The sterile line '9802A1' was pollinated by the male fertile line '2006H' and the resulting F(1) plants were all male fertile. These results indicated that the male fertility of radish CMS can be restored by a new Rf locus, which linked tightly to the Rfo locus.

Fine mapping of a recessive genic male sterility gene (Bnms3) in rapeseed (Brassica napus) with AFLP- and Arabidopsis-derived PCR markers

9012AB, a recessive genic male sterility (RGMS) line developed from spontaneous mutation in Brassica napus (Chen et al. in Acta Agron Sin 24:431-438, 1998), has been playing an increasing role in hybrid cultivar development in China. The male sterility of 9012AB is controlled by two recessive genes (designated Bnms3 and Bnms4) interacting with one recessive epistatic suppressor gene (esp). Previous study has identified seven AFLP markers, six of which were co-segregated with the Bnms3 gene in a small population (Ke et al. in Plant Breed 124:367-370, 2005). By cloning these AFLP markers and their flanking sequences, five of the six co-segregated markers were successfully converted into sequence characterized amplified region (SCAR) markers. For fine mapping of the Bnms3 gene, these SCAR markers were analyzed in a NIL population of 4,136 individuals. The Bnms3 gene was then genetically mapped to a region of 0.56 cM, with 0.15 cM from marker SEP8 and 0.41 from marker SEP4, respectively. BLAST analysis with these SCAR marker sequences identified a collinear genomic region in Arabidopsis chromosome 5, from which two specific PCR markers further narrowed the Bnms3 locus from an interval of 0.56 to 0.14 cM. These results provide additional information for map-based cloning of the Bnms3 gene and will be helpful for marker-assisted selection (MAS) of elite RGMS lines and maintainers.

Towards map-based cloning: fine mapping of a recessive genic male-sterile gene (BnMs2) in Brassica napus L. and syntenic region identification based on the Arabidopsis thaliana genome sequences

S45AB, a recessive genic male sterile (RGMS) line, originated as a spontaneous mutant in Brassica napus cv. Oro. The genotypes of sterile (S45A) and fertile plants (S45B) are Bnms1ms1ms2ms2 and BnMs1ms1ms2ms2, respectively. In our previous studies, Yi et al. (Theor Appl Genet 113:643-650, 2006) mapped the BnMs1 locus to a region of 0.4 cM, candidates of which have been identified and genetic transformation is in progress. We describe the fine mapping of BnMs2 exploiting amplified fragment length polymorphism (AFLP) and amplified consensus genetic marker (ACGM) methodologies, and the identification of a collinear region probably containing BnMs2 orthologue in Arabidopsis thaliana. A near isogenic line (NIL) population S4516AB which segregated for BnMs2 locus was generated by crossing, allelism testing and repeated full-sib mating. From the survey of 1,024 AFLP primer combinations, 12 tightly linked AFLP markers were obtained and five of them were successfully converted into co-dominant or dominant sequence characterized amplified region (SCAR) markers. A population of 2,650 sterile plants was screened using these markers and a high-resolution map surrounding BnMs2 was constructed. The closest AFLP markers flanking BnMs2 were 0.038 and 0.075 cM away, respectively. Subsequently, an ACGM marker was developed to delimit the BnMs2 locus at an interval of 0.075 cM. We extended marker sequences to perform BlastN searches against the Arabidopsis genome and identified a collinear region containing 68 Arabidopsis genes, in which the orthologue of BnMs2 might be included. We further integrated BnMs2 linked AFLP or SCAR markers to two doubled-haploid (DH) populations derived from the crosses Tapidor x Ningyou7 (Qiu et al., Theor Appl Genet 114:67-80, 2006) and Quantum x No.2127-17 (available in our laboratory), and BnMs2 was mapped on N16. Molecular markers developed from these investigations will facilitate the marker-assisted selection (MAS) of RGMS lines, and the fine map and syntenic region identified will greatly hasten the process of positional cloning of BnMs2 gene.

Identification of AFLP markers linked to the male fertility restorer gene of CMS (Moricandia arvensis) Brassica juncea and conversion to SCAR marker

We have developed a cytoplasmic male sterile (CMS) line of Brassica juncea through somatic hybridization with Moricandia arvensis and introgressed the fertility restorer gene into B. juncea. This fertility restorer locus is unique in that it is capable of restoring male fertility to two other alloplasmic CMS systems of B. juncea. As a first step toward cloning of this restorer gene we attempted molecular tagging of the Rf locus using the amplified fragment length polymorphism (AFLP) technique. A BC(1)F(1) population segregating for male sterility/fertility was used for tagging using the bulk segregant analysis method. Out of 64 primer combinations tested in the bulks, 5 combinations gave polymorphic amplification patterns. Further testing of these primers in individual plants showed four amplicons associated with the male fertility trait. Polymorphic amplicons were cloned and used for designing SCAR primers. One of the SCAR primers generated amplicons mostly in the fertile plants. Linkage analysis using MAPMAKER showed two AFLP and one SCAR markers linked to the male fertility gene with a map distance ranging from 0.6 to 2.9 cM. All the markers are located on one side of the Rf locus.

A new recombined double low restorer line for the Ogu-INRA cms in rapeseed (Brassica napus L.)

A major objective of breeders using the Ogu-INRA cytoplasmic male sterility (cms) system in rapeseed (Brassica napus L.) is to obtain double low restorer lines with a shorter introgression and a good agronomic value. The development of low glucosinolate content (low GC) restorer lines often occurs through the deletion of a part of the introgression. One of these lines has lost the radish Pgi-2 allele expression, without recovering that of the rapeseed Pgi-2 allele. This line shows a defect in the meiotic transmission of the restorer gene Rfo and a very poor agronomic value. We initiated a programme to force non-spontaneous recombination between this Rfo-carrying introgression and the rapeseed homologous chromosome from a low GC B. napus line. Gamma ray irradiation was used to induce chromosome breakage just prior meiosis aiming at just such a recombination. Low GC cms plants were crossed with the pollen of irradiated plants that were heterozygous for this introgression. The F(2) families were scored for their vigour, transmission rate of Rfo and female fertility. One family of plants, R2000, showed an improved behaviour for these three traits. This family presented a unique combination of molecular markers when compared to other rapeseed restorers analysed, which suggests that the recombination event allowed the recovery of B. oleracea genetic information that was originally replaced by the radish introgression in the original restorers. This resulted in a duplicated region (originating from radish and B. oleracea) on the chromosome carrying the introgression in the R2000 family.