Screening of clubroot-resistant varieties and transfer of clubroot resistance genes to Brassica napus using distant hybridization

Pyramiding of triple Clubroot resistance loci conferred superior resistance without negative effects on agronomic traits in Brassica napus

Clubroot disease caused by Plasmodiophora brassicae is becoming a serious threat to rapeseed (Brassica napus) production worldwide. Breeding resistant varieties using CR (clubroot resistance) loci is the most promising solution. Using marker-assisted selection and speed-breeding technologies, we generated Brassica napus materials in homozygous or heterozygous states using CRA3.7, CRA08.1, and CRA3.2 loci in the elite parental line of the Zhongshuang11 background. We developed three elite lines with two CR loci in different combinations and one line with three CR loci at the homozygous state. In our study, we used six different clubroot strains (Xinmin, Lincang, Yuxi, Chengdu, Chongqing, and Jixi) which are categorized into three groups based on our screening results. The newly pyramided lines with two or more CR loci displayed better disease resistance than the parental lines carrying single CR loci. There is an obvious gene dosage effect between CR loci and disease resistance levels. For example, pyramided lines with triple CR loci in the homozygous state showed superior resistance for all pathogens tested. Moreover, CR loci in the homozygous state are better on disease resistance than the heterozygous state. More importantly, no negative effect was observed on agronomic traits for the presence of multiple CR loci in the same background. Overall, these data suggest that the pyramiding of triple clubroot resistance loci conferred superior resistance with no negative effects on agronomic traits in Brassica napus.

Embryo Rescue in Plant Breeding

Embryo rescue (ER) techniques are among the oldest and most successful in vitro tissue culture protocols used with plant species. ER refers to a series of methods that promote the development of an immature or lethal embryo into a viable plant. Intraspecific, interspecific, or intergeneric crosses allow the introgression of important alleles of agricultural interest from wild species, such as resistance or tolerance to abiotic and biotic stresses or morphological traits in crops. However, pre-zygotic and post-zygotic reproductive barriers often present challenges in achieving successful hybridization. Pre-zygotic barriers manifest as incompatibility reactions that hinder pollen germination, pollen tube growth, or penetration into the ovule occurring in various tissues, such as the stigma, style, or ovary. To overcome these barriers, several strategies are employed, including cut-style or graft-on-style techniques, the utilization of mixed pollen from distinct species, placenta pollination, and in vitro ovule pollination. On the other hand, post-zygotic barriers act at different tissues and stages ranging from early embryo development to the subsequent growth and reproduction of the offspring. Many crosses among different genera result in embryo abortion due to the failure of endosperm development. In such cases, ER techniques are needed to rescue these hybrids. ER holds great promise for not only facilitating successful crosses but also for obtaining haploids, doubled haploids, and manipulating the ploidy levels for chromosome engineering by monosomic and disomic addition as well substitution lines. Furthermore, ER can be used to shorten the reproductive cycle and for the propagation of rare plants. Additionally, it has been repeatedly used to study the stages of embryonic development, especially in embryo-lethal mutants. The most widely used ER procedure is the culture of immature embryos taken and placed directly on culture media. In certain cases, the in vitro culture of ovule, ovaries or placentas enables the successful development of young embryos from the zygote stage to maturity.

Research progress and applications of colorful Brassica crops

MAIN CONCLUSION

We review the application and the molecular regulation of anthocyanins in colorful Brassica crops, the creation of new germplasm resources, and the development and utilization of colorful Brassica crops. Brassica crops are widely cultivated: these include oilseed crops, such as rapeseed, mustards, and root, leaf, and stem vegetable types, such as turnips, cabbages, broccoli, and cauliflowers. Colorful variants exist of these crop species, and asides from increased aesthetic appeal, these may also offer advantages in terms of nutritional content and improved stress resistances. This review provides a comprehensive overview of pigmentation in Brassica as a reference for the selection and breeding of new colorful Brassica varieties for multiple end uses. We summarize the function and molecular regulation of anthocyanins in Brassica crops, the creation of new colorful germplasm resources via different breeding methods, and the development and multifunctional utilization of colorful Brassica crop types.

Key Advances in the New Era of Genomics-Assisted Disease Resistance Improvement of Brassica Species

Disease resistance improvement remains a major focus in breeding programs as diseases continue to devastate Brassica production systems due to intensive cultivation and climate change. Genomics has paved the way to understand the complex genomes of Brassicas, which has been pivotal in the dissection of the genetic underpinnings of agronomic traits driving the development of superior cultivars. The new era of genomics-assisted disease resistance breeding has been marked by the development of high-quality genome references, accelerating the identification of disease resistance genes controlling both qualitative (major) gene and quantitative resistance. This facilitates the development of molecular markers for marker assisted selection and enables genome editing approaches for targeted gene manipulation to enhance the genetic value of disease resistance traits. This review summarizes the key advances in the development of genomic resources for Brassica species, focusing on improved genome references, based on long-read sequencing technologies and pangenome assemblies. This is further supported by the advances in pathogen genomics, which have resulted in the discovery of pathogenicity factors, complementing the mining of disease resistance genes in the host. Recognizing the co-evolutionary arms race between the host and pathogen, it is critical to identify novel resistance genes using crop wild relatives and synthetic cultivars or through genetic manipulation via genome-editing to sustain the development of superior cultivars. Integrating these key advances with new breeding techniques and improved phenotyping using advanced data analysis platforms will make disease resistance improvement in Brassica species more efficient and responsive to current and future demands.

The clubroot pathogen Plasmodiophora brassicae: A profile update

BACKGROUND

Plasmodiophora brassicae is the causal agent of clubroot disease of cruciferous plants and one of the biggest threats to the rapeseed (Brassica napus) and brassica vegetable industry worldwide.

DISEASE SYMPTOMS

In the advanced stages of clubroot disease wilting, stunting, yellowing, and redness are visible in the shoots. However, the typical symptoms of the disease are the presence of club-shaped galls in the roots of susceptible hosts that block the absorption of water and nutrients.

HOST RANGE

Members of the family Brassicaceae are the primary host of the pathogen, although some members of the family, such as Bunias orientalis, Coronopus squamatus, and Raphanus sativus, have been identified as being consistently resistant to P. brassicae isolates with variable virulence profile.

TAXONOMY

Class: Phytomyxea; Order: Plasmodiophorales; Family: Plasmodiophoraceae; Genus: Plasmodiophora; Species: Plasmodiophora brassicae (Woronin, 1877).

DISTRIBUTION

Clubroot disease is spread worldwide, with reports from all continents except Antarctica. To date, clubroot disease has been reported in more than 80 countries.

PATHOTYPING

Based on its virulence on different hosts, P. brassicae is classified into pathotypes or races. Five main pathotyping systems have been developed to understand the relationship between P. brassicae and its hosts. Nowadays, the Canadian clubroot differential is extensively used in Canada and has so far identified 36 different pathotypes based on the response of a set of 13 hosts.

EFFECTORS AND RESISTANCE

After the identification and characterization of the clubroot pathogen SABATH-type methyltransferase PbBSMT, several other effectors have been characterized. However, no avirulence gene is known, hindering the functional characterization of the five intercellular nucleotide-binding (NB) site leucine-rich-repeat (LRR) receptors (NLRs) clubroot resistance genes validated to date.

IMPORTANT LINK

Canola Council of Canada is constantly updating information about clubroot and P. brassicae as part of their Canola Encyclopedia: https://www.canolacouncil.org/canola-encyclopedia/diseases/clubroot/.

PHYTOSANITARY CATEGORIZATION

PLADBR: EPPO A2 list; Annex designation 9E.

Functional Characterization of the Cystine-Rich-Receptor-like Kinases (CRKs) and Their Expression Response to Sclerotinia sclerotiorum and Abiotic Stresses in Brassica napus

Cysteine-rich receptor-like kinases (CRKs) are transmembrane proteins that bind to the calcium ion to regulate stress-signaling and plant development-related pathways, as indicated by several pieces of evidence. However, the CRK gene family hasn’t been inadequately examined in Brassica napus. In our study, 27 members of the CRK gene family were identified in Brassica napus, which are categorized into three phylogenetic groups and display synteny relationship to the Arabidopsis thaliana orthologs. All the CRK genes contain highly conserved N-terminal PKINASE domain; however, the distribution of motifs and gene structure were variable conserved. The functional divergence analysis between BnaCRK groups indicates a shift in evolutionary rate after duplication events, demonstrating that BnaCRKs might direct a specific function. RNA-Seq datasets and quantitative real-time PCR (qRT-PCR) exhibit the complex expression profile of the BnaCRKs in plant tissues under multiple stresses. Nevertheless, BnaA06CRK6-1 and BnaA08CRK8 from group B were perceived to play a predominant role in the Brassica napus stress signaling pathway in response to drought, salinity, and Sclerotinia sclerotiorum infection. Insights gained from this study improve our knowledge about the Brassica napus CRK gene family and provide a basis for enhancing the quality of rapeseed.

Predicting Cloned Disease Resistance Gene Homologs (CDRHs) in Radish, Underutilised Oilseeds, and Wild Brassicaceae Species

Brassicaceae crops, including Brassica, Camelina and Raphanus species, are among the most economically important crops globally; however, their production is affected by several diseases. To predict cloned disease resistance (R) gene homologs (CDRHs), we used the protein sequences of 49 cloned R genes against fungal and bacterial diseases in Brassicaceae species. In this study, using 20 Brassicaceae genomes (17 wild and 3 domesticated species), 3172 resistance gene analogs (RGAs) (2062 nucleotide binding-site leucine-rich repeats (NLRs), 497 receptor-like protein kinases (RLKs) and 613 receptor-like proteins (RLPs)) were identified. CDRH clusters were also observed in Arabis alpina, Camelina sativa and Cardamine hirsuta with assigned chromosomes, consisting of 62 homogeneous (38 NLR, 17 RLK and 7 RLP clusters) and 10 heterogeneous RGA clusters. This study highlights the prevalence of CDRHs in the wild relatives of the Brassicaceae family, which may lay the foundation for rapid identification of functional genes and genomics-assisted breeding to develop improved disease-resistant Brassicaceae crop cultivars.

Genetic and molecular analysis reveals that two major loci and their interaction confer clubroot resistance in canola introgressed from rutabaga

Clubroot disease caused by Plasmodiophora brassicae is one of the serious threats to canola (Brassica napus L. subsp. napus) production. The evolution of new pathotypes rendering available resistances ineffective compel the introgression of new resistance into canola and extend our understanding of the genetic and molecular basis of the resistance. In this paper, we report the genetic and molecular basis of clubroot resistance in canola, introgressed from a rutabaga (B. napus L. subsp. rapifera Metzg. 'Polycross'), by using a doubled-haploid (DH) mapping population. Whole-genome resequencing (WGRS)-based bulked segregant analysis followed by genetic mapping and expression analysis of the genes in resistant and susceptible DH lines at 7 and 14 d after inoculation were carried out. Following this approach, two major quantitative trait loci (QTL) located at 14.41-15.44 Mb of A03 and at 9.96-11.09 Mb of A08 chromosomes and their interaction was observed to confer resistance to pathotypes 3H, 3A, and 3D. Analysis of the genes from the two QTL regions suggested that decreased expression of sugar transporter genes (BnaA03g29290D and BnaA03g29310D) may play an important role in resistance conferred by the A03 QTL, while increased expression of the toll/interleukin-1 receptor (TIR)-nucleotide binding (NB)-leucine rich repeat (LRR) (TNL) genes (BnaA08g10100D, BnaA08g09220D, and BnaA08g10540D) could be the major determinant of the resistance conferred by the A08 QTL. Single-nucleotide polymorphism (SNP) allele-specific polymerase chain reaction (PCR)-based markers, which could be detected by agarose gel electrophoresis, were also developed from the two QTL regions for use in breeding including pyramiding of multiple clubroot resistance genes.

Introgression of clubroot resistant gene into L. from based on homoeologous exchange

Clubroot is a soil-borne disease in cabbage (Brassica oleracea L. var. capitata L.) caused by Plasmodiophora brassicae, which poses a great threat to cabbage production. However, clubroot resistance (CR) genes in Brassica rapa could be introduced into the cabbage via breeding to make it clubroot resistant. In this study, CR genes from B. rapa were introduced into the cabbage genome and the mechanism of gene introgression was explored. Two methods were used to create CR materials: (i) The fertility of CR Ogura CMS cabbage germplasms containing CRa was restored by using an Ogura CMS restorer. After cytoplasmic replacement and microspore culture, CRa-positive microspore individuals were obtained. (ii) Distant hybridization was performed between cabbage and B. rapa, which contained three CR genes (CRa, CRb, and Pb8.1). Finally, BC2 individuals containing all three CR genes were obtained. Inoculation results showed that both CRa-positive microspore individuals and BC2 individuals containing three CR genes were resistant to race 4 of P. brassicae. Sequencing results from CRa-positive microspore individuals with specific molecular markers and genome-wide association study (GWAS) showed penetration at the homologous position of the cabbage genome by a 3.42 Mb CRa containing a fragment from B. rapa; indicating homoeologous exchange (HE) as the theoretical basis for the introgression of CR resistance. The successful introduction of CR into the cabbage genome in the present study can provide useful clues for creating introgression lines within other species of interest.

Multi-Omics Approaches to Improve Clubroot Resistance in Brassica with a Special Focus on Brassica oleracea L

Brassica oleracea is an agronomically important species of the Brassicaceae family, including several nutrient-rich vegetables grown and consumed across the continents. But its sustainability is heavily constrained by a range of destructive pathogens, among which, clubroot disease, caused by a biotrophic protist Plasmodiophora brassicae, has caused significant yield and economic losses worldwide, thereby threatening global food security. To counter the pathogen attack, it demands a better understanding of the complex phenomenon of Brassica-P. brassicae pathosystem at the physiological, biochemical, molecular, and cellular levels. In recent years, multiple omics technologies with high-throughput techniques have emerged as successful in elucidating the responses to biotic and abiotic stresses. In Brassica spp., omics technologies such as genomics, transcriptomics, ncRNAomics, proteomics, and metabolomics are well documented, allowing us to gain insights into the dynamic changes that transpired during host-pathogen interactions at a deeper level. So, it is critical that we must review the recent advances in omics approaches and discuss how the current knowledge in multi-omics technologies has been able to breed high-quality clubroot-resistant B. oleracea. This review highlights the recent advances made in utilizing various omics approaches to understand the host resistance mechanisms adopted by Brassica crops in response to the P. brassicae attack. Finally, we have discussed the bottlenecks and the way forward to overcome the persisting knowledge gaps in delivering solutions to breed clubroot-resistant Brassica crops in a holistic, targeted, and precise way.

Interspecific Hybridization of Transgenic Brassica napus and Brassica rapa-An Overview

In nature, interspecific hybridization occurs frequently and can contribute to the production of new species or the introgression of beneficial adaptive features between species. It has great potential in agricultural systems to boost the process of targeted crop improvement. In the advent of genetically modified (GM) crops, it has a disadvantage that it involves the transgene escaping to unintended plants, which could result in non-specific weedy crops. Several crop species in the Brassica genus have close kinship: canola (Brassica napus) is an ancestral hybrid of B. rapa and B. oleracea and mustard species such as B. juncea, B. carinata, and B. nigra share common genomes. Hence, intraspecific hybridization among the Brassica species is most common, especially between B. napus and B. rapa. In general, interspecific hybrids cause numerous genetic and phenotypic changes in the parental lines. Consequently, their fitness and reproductive ability are also highly varied. In this review, we discuss the interspecific hybridization and reciprocal hybridization studies of B. napus and B. rapa and their potential in the controlled environment. Further, we address the fate of transgenes (herbicide resistance) and their ability to transfer to their progenies or generations. This could help us to understand the environmental influence of interspecific hybrids and how to effectively manage their transgene escape in the future.

Development of Intron Polymorphism Markers and Their Association With Fatty Acid Component Variation in Oil Palm

Oil palm (Elaeis guineensis Jacq.) is a tropical woody oil crop of the palm family and is known as "the oil king of the world," but its palm oil contains about 50% palmitic acid, which is considered unhealthy for humans. Intron polymorphisms (IP) are highly efficient and easily examined molecular markers located adjacent to exon regions of functional genes, thus may be associated with targeted trait variation. In order to speed up the breeding of oil palm fatty acid composition, the current study identified a total of 310 introns located within 52 candidate genes involved in fatty acid biosynthesis in the oil palm genome. Based on the intron sequences, 205 primer pairs were designed, 64 of which showed polymorphism among 70 oil palm individuals. Phenotypic variation of fatty acid content in the 70 oil palm individuals was also investigated. Association analysis revealed that 13 IP markers were significantly associated with fatty acid content variation, and these IP markers were located on chromosomes 2, 5, 6, 8, 9, and 10 of oil palm. The development of such IP markers may be useful for the genetic improvement of fatty acid composition in oil palm.

A Loop-Mediated Isothermal DNA Amplification (LAMP) Assay for Detection of the Clubroot Pathogen Plasmodiophora brassicae

Clubroot caused by Plasmodiophora brassicae is a serious threat to cruciferous crops around the world. The resting spores of P. brassicae are a primary source of infection and can survive in soil for many years. Detection of resting spores in soil is essential for forecasting clubroot prevalence. Detection of P. brassicae has been relying on plant bioassays or PCR-based methods. The loop-mediated isothermal DNA amplification (LAMP) is a promising approach for microorganism detection with the advantage of high sensitivity, accuracy, and convenience in viewing. In this study, we developed a LAMP assay for detection of P. brassicae in soil, roots, and seeds. This method can detect P. brassicae at a minimal amount of 1 fg of plasmid DNA or 10 resting spores in the soil. Compared with conventional PCR, the LAMP was more sensitive in detection of P. brassicae at the lower levels in soil samples. In conclusion, we elaborated a sensitive, accurate, and easy-to-use LAMP assay to detect P. brassicae, which will facilitate sustainable clubroot management and planning.

Transferring of clubroot-resistant locus CRd from Chinese cabbage (Brassica rapa) to canola (Brassica napus) through interspecific hybridization

Clubroot, caused by Plasmodiophora brassicae is one of the most severe threats to brassica species in China and worldwide. Breeding for clubroot resistant varieties is one of the best ways to overcome this disease. In this study, we introduced clubroot resistance (CR) gene CRd from Chinese cabbage (85-74) into elite Brassica napus inbred line Zhongshuang 11 through interspecific hybridization and subsequent backcrossing with whole-genome molecular marker-assisted selection (MAS). The resistant test of CRd to P. brassicae isolates was evaluated in the greenhouse as well as in field conditions. Close linkage markers and the whole-chromosome background marker selection approach improved the recovery rate from 78.3% in BC₁ to 100% in BC₃F₁. The improved clubroot-resistant variety, Zhongshuang11R, was successfully selected in the BC₃F₂ generation. The greenhouse and field resistant tests revealed that Zhongshuang11R was resistant to P. brassicae pathotypes. The agronomic characteristics of Zhongshuang11R were similar to those of its recurrent parental line, including oil content, composition of fatty acid, plant height, primary effective branches, grain yield per plant and thousand-seed weight. In addition, the oil quality could satisfy the quality requirements for commercial rapeseed oil. Our results will enrich the resistant resources of canola and will certainly accelerate clubroot resistance breeding programs in B. napus.

Identification and Characterization of Circular RNAs in Brassica rapa in Response to Plasmodiophora brassicae

Plasmodiophora brassicae is a soil-borne pathogen that attacks the roots of cruciferous plants and causes clubroot disease. CircRNAs are noncoding RNAs, widely existing in plant and animal species. Although knowledge of circRNAs has been updated continuously and rapidly, information about circRNAs in the regulation of clubroot disease resistance is extremely limited in Brassica rapa. Here, Chinese cabbage (BJN 222) containing clubroot resistance genes (CRa) against P. brassicae Pb4 was susceptible to PbE. To investigate the mechanism of cicRNAs responsible for clubroot disease resistance in B. rapa, circRNA-seq was performed with roots of 'BJN 222' at 0, 8, and 23 days post-inoculated (dpi) with Pb4 and PbE. A total of 231 differentially expressed circRNAs were identified between the groups. Based on the differentially expressed circRNAs, the circRNA-miRNA-mRNA network was constructed using the target genes directly or indirectly related to plant resistance. Upregulated novel_circ_000495 suppressed the expression of miR5656-y, leading to the upregulation of Bra026508, which might cause plant resistance. Our results provide new insights into clubroot resistance mechanisms and lay a foundation for further studies exploring complex gene regulation networks in B. rapa.

The Transcriptome and Metabolome Reveal the Potential Mechanism of Lodging Resistance in Intergeneric Hybrids between Brassica napus and Capsella bursa-pastoris

Lodging is one of the main reasons for the reduction in seed yield and is the limitation of mechanized harvesting in B. napus. The dissection of the regulatory mechanism of lodging resistance is an important goal in B. napus. In this study, the lodging resistant B. napus line, YG689, derived from the hybridization between B. napus cv. Zhongyou 821 (ZY821) and Capsella bursa-pastoris, was used to dissect the regulation mechanism of hard stem formation by integrating anatomical structure, transcriptome and metabolome analyses. It was shown that the lignocellulose content of YG689 is higher than that of ZY821, and some differentially expressed genes (DEGs) involved in the lignocellulose synthesis pathway were revealed by transcriptome analyses. Meanwhile, GC–TOF–MS and UPLC–QTOF–MS identified 40, 54, and 31 differential metabolites in the bolting stage, first flower stage, and the final flower stage. The differential accumulation of these metabolites might be associated with the lignocellulose biosynthesis in B. napus. Finally, some important genes that regulate the metabolic pathway of lignocellulose biosynthesis, such as BnaA02g18920D, BnaA10g15590D, BnaC05g48040D, and NewGene_216 were identified in B. napus through the combination of transcriptomics and metabolomics data. The present results explored the potential regulatory mechanism of lignocellulose biosynthesis, which provided a new clue for the breeding of B. napus with lodging resistance in the future.

A Novel Target (Oxidation Resistant 2) in Arabidopsis thaliana to Reduce Clubroot Disease Symptoms via the Salicylic Acid Pathway without Growth Penalties

The clubroot disease (Plasmodiophora brassicae) is one of the most damaging diseases worldwide among brassica crops. Its control often relies on resistant cultivars, since the manipulation of the disease hormones, such as salicylic acid (SA) alters plant growth negatively. Alternatively, the SA pathway can be increased by the addition of beneficial microorganisms for biocontrol. However, this potential has not been exhaustively used. In this study, a recently characterized protein Oxidation Resistant 2 (OXR2) from Arabidopsis thaliana is shown to increase the constitutive pathway of SA defense without decreasing plant growth. Plants overexpressing AtOXR2 (OXR2-OE) show strongly reduced clubroot symptoms with improved plant growth performance, in comparison to wild type plants during the course of infection. Consequently, oxr2 mutants are more susceptible to clubroot disease. P. brassicae itself was reduced in these galls as determined by quantitative real-time PCR. Furthermore, we provide evidence for the transcriptional downregulation of the gene encoding a SA-methyltransferase from the pathogen in OXR2-OE plants that could contribute to the phenotype.

Identification of Candidate Genes for Clubroot-Resistance in Brassica oleracea Using Quantitative Trait Loci-Sequencing

Clubroot caused by Plasmodiophora brassicae is a devastating disease of cabbage (Brassica oleracea). To identify quantitative trait loci (QTLs) for clubroot resistance (CR) in B. oleracea, genomic resequencing was carried out in two sets of extreme pools, group I and group II, which were constructed separately from 110 and 74 F2 cloned lines derived from the cross between clubroot-resistant (R) cabbage "GZ87" (against race 4) and susceptible (S) cabbage "263." Based on the QTL-sequencing (QTL-Seq) analysis of group I and group II, three QTLs (i.e., qCRc7-2, qCRc7-3, and qCRc7-4) were determined on the C07 chromosome. RNA-Seq and qRT-PCR were conducted in the extreme pools of group II before and after inoculation, and two potential candidate genes (i.e., Bol037115 and Bol042270), which exhibiting upregulation after inoculation in the R pool but downregulation in the S pool, were identified from the three QTLs on C07. A functional marker "SWU-OA" was developed from qCRc7-4 on C07, exhibiting ∼95% accuracy in identifying CR in 56 F2 lines. Our study will provide valuable information on resistance genes against P. brassicae and may accelerate the breeding process of B. oleracea with CR.

Exploring the gene pool of Brassica napus by genomics-based approaches

De novo allopolyploidization in Brassica provides a very successful model for reconstructing polyploid genomes using progenitor species and relatives to broaden crop gene pools and understand genome evolution after polyploidy, interspecific hybridization and exotic introgression. B. napus (AACC), the major cultivated rapeseed species and the third largest oilseed crop in the world, is a young Brassica species with a limited genetic base resulting from its short history of domestication, cultivation, and intensive selection during breeding for target economic traits. However, the gene pool of B. napus has been significantly enriched in recent decades that has been benefit from worldwide effects by the successful introduction of abundant subgenomic variation and novel genomic variation via intraspecific, interspecific and intergeneric crosses. An important question in this respect is how to utilize such variation to breed crops adapted to the changing global climate. Here, we review the genetic diversity, genome structure, and population-level differentiation of the B. napus gene pool in relation to known exotic introgressions from various species of the Brassicaceae, especially those elucidated by recent genome-sequencing projects. We also summarize progress in gene cloning, trait-marker associations, gene editing, molecular marker-assisted selection and genome-wide prediction, and describe the challenges and opportunities of these techniques as molecular platforms to exploit novel genomic variation and their value in the rapeseed gene pool. Future progress will accelerate the creation and manipulation of genetic diversity with genomic-based improvement, as well as provide novel insights into the neo-domestication of polyploid crops with novel genetic diversity from reconstructed genomes.

A Modified Meiotic Recombination in Brassica napus Largely Improves Its Breeding Efficiency

Simple Summary

The selection of varieties more resilient to disease and climate change requires generating new genetic diversity for breeding. The main mechanism for reshuffling genetic information is through the recombination of chromosomes during meiosis. We showed in oilseed rape (Brassica napus, AACC, 2n = 4x = 38), which is a natural hybrid formed from a cross between turnip (B. rapa, AA, 2n = 2x = 20) and cabbage (B. oleracea, CC, 2n = 2x = 18), that there is significantly more crossovers occurring along the entire A chromosomes in allotriploid AAC (crossbetween B. napus and B. rapa) than in diploid AA or allotetraploid AACC hybrids. We demonstrated that these allotriploid AAC hybrids are highly efficient to introduce new variability within oilseed rape varieties, notably by enabling the introduction of small genomic regions carrying genes controlling agronomically interesting traits.

Abstract

Meiotic recombination is the main tool used by breeders to generate biodiversity, allowing genetic reshuffling at each generation. It enables the accumulation of favorable alleles while purging deleterious mutations. However, this mechanism is highly regulated with the formation of one to rarely more than three crossovers, which are not randomly distributed. In this study, we showed that it is possible to modify these controls in oilseed rape (Brassica napus, AACC, 2n = 4x = 38) and that it is linked to AAC allotriploidy and not to polyploidy per se. To that purpose, we compared the frequency and the distribution of crossovers along A chromosomes from hybrids carrying exactly the same A nucleotide sequence, but presenting three different ploidy levels: AA, AAC and AACC. Genetic maps established with 202 SNPs anchored on reference genomes revealed that the crossover rate is 3.6-fold higher in the AAC allotriploid hybrids compared to AA and AACC hybrids. Using a higher SNP density, we demonstrated that smaller and numerous introgressions of B. rapa were present in AAC hybrids compared to AACC allotetraploid hybrids, with 7.6 Mb vs. 16.9 Mb on average and 21 B. rapa regions per plant vs. nine regions, respectively. Therefore, this boost of recombination is highly efficient to reduce the size of QTL carried in cold regions of the oilseed rape genome, as exemplified here for a QTL conferring blackleg resistance.

Multi-omics-based prediction of hybrid performance in canola

Complementing or replacing genetic markers with transcriptomic data and use of reproducing kernel Hilbert space regression based on Gaussian kernels increases hybrid prediction accuracies for complex agronomic traits in canola. In plant breeding, hybrids gained particular importance due to heterosis, the superior performance of offspring compared to their inbred parents. Since the development of new top performing hybrids requires labour-intensive and costly breeding programmes, including testing of large numbers of experimental hybrids, the prediction of hybrid performance is of utmost interest to plant breeders. In this study, we tested the effectiveness of hybrid prediction models in spring-type oilseed rape (Brassica napus L./canola) employing different omics profiles, individually and in combination. To this end, a population of 950 F₁ hybrids was evaluated for seed yield and six other agronomically relevant traits in commercial field trials at several locations throughout Europe. A subset of these hybrids was also evaluated in a climatized glasshouse regarding early biomass production. For each of the 477 parental rapeseed lines, 13,201 single nucleotide polymorphisms (SNPs), 154 primary metabolites, and 19,479 transcripts were determined and used as predictive variables. Both, SNP markers and transcripts, effectively predict hybrid performance using (genomic) best linear unbiased prediction models (gBLUP). Compared to models using pure genetic markers, models incorporating transcriptome data resulted in significantly higher prediction accuracies for five out of seven agronomic traits, indicating that transcripts carry important information beyond genomic data. Notably, reproducing kernel Hilbert space regression based on Gaussian kernels significantly exceeded the predictive abilities of gBLUP models for six of the seven agronomic traits, demonstrating its potential for implementation in future canola breeding programmes.

Clubroot in Brassica: recent advances in genomics, breeding, and disease management

Clubroot disease, caused by Plasmodiophora brassicae, affects Brassica oilseed and vegetable production worldwide. This review is focused on various aspects of clubroot disease and its management, including understanding the pathogen and resistance in the host plants. Advances in genetics, molecular biology techniques, and omics research have helped to identify several major loci, QTL, and genes from the Brassica genomes involved in the control of clubroot resistance. Transcriptomic studies have helped to extend our understanding of the mechanism of infection by the pathogen and the molecular basis of resistance/susceptibility in the host plants. A comprehensive understanding of the clubroot disease and host resistance would allow developing a better strategy by integrating the genetic resistance with cultural practices to manage this disease from a long-term perspective.

Investigating genetic relationship of Brassica juncea with B. nigra via virtual allopolyploidy and hexaploidy strategy

Brassica juncea is an important economic crop of the world; however, the narrow genetic base of this crop has tremendously decreased its crop productivity. As an ancestral species of B. juncea, B. nigra is of great importance in widening the genetic diversity of B. juncea. In the present study, 42 SSR markers were employed to screen the genetic diversity among 83 B. nigra, 16 B. juncea, and other Brassica accessions. The molecular characteristics of 498 virtual B. juncea lines were deduced based on the bands of B. nigra and B. rapa via a virtual allopolyploid strategy, and then compared with natural B. juncea accessions. It was found that B. nigra had rich genetic diversity and could be classified into four subgroups, of which subgroup B-III and subgroup B-IV exhibited the closest and the most distant genetic relationship with B. juncea, respectively. To verify this, a hexaploidy strategy was applied to generated synthetic B. juncea from 20 B. nigra accessions, resulting in 45 new-type B. juncea genotypes. The genetic analyses detected that synthetic B. juncea derived from B. nigra in subgroup B-III was close to natural B. juncea, while B. juncea synthesized with B. nigra from subgroup B-IV exhibited wide genetic diversity and was most distant with current B. juncea. This study revealed a great potential of B. nigra in widening genetic diversity of B. juncea particularly using B. nigra in subgroup B-IV, and is helpful in better understanding of the genetic relationship between B. nigra and B. juncea.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-020-01197-7.

Towards breeding of rapeseed (Brassica napus) with alien cytoplasm and powdery mildew resistance from Ethiopian mustard (Brassica carinata)

Powdery mildew (PM), caused by Erysiphe cruciferarum, is an epidemic of oil rapeseed (Brassica napus L.) growing worldwide, but PM resistant germplasm is rare in this species. We screened 102 accessions of B. napus and other cruciferous species and found an Ethiopian mustard (Brassica carinata) cultivar 'White flower' immune to PM in both the field and greenhouse. Outcrossing in the female parent 'White flower' was promoted by using a chemical gametocide tribenuron-methyl, to obtain hybrid seeds of distant hybridization with an elite B. napus cultivar 'Zhongshuang11'. Three true F₁ hybrids with B. carinata cytoplasm were obtained without using embryo rescue, which showed complete male sterility and light yellow petals. The hybrid plants and the progenies derived from backcrossing were validated using morphological traits, seed quality, and molecular markers. Five lines in the BC₁F₃ generation, named 'W7-1', 'W7-4', 'W7-6', 'W8-1', and 'W8-3', and one BC₂F₂ line 'W3PS-1', whose young leaf was yellow green, were identified to be resistant or moderately resistant to PM. The seed quality and some morphological traits of these lines resembled the parent 'Zhongshuang11', indicating that the resistance gene(s) has been preliminarily introduced into B. napus.

An update on the arsenal: mining resistance genes for disease management of Brassica crops in the genomic era

Brassica species include many economically important crops that provide nutrition and health-promoting substances to humans worldwide. However, as with all crops, their production is constantly threatened by emerging viral, bacterial, and fungal diseases, whose incidence has increased in recent years. Traditional methods of control are often costly, present limited effectiveness, and cause environmental damage; instead, the ideal approach is to mine and utilize the resistance genes of the Brassica crop hosts themselves. Fortunately, the development of genomics, molecular genetics, and biological techniques enables us to rapidly discover and apply resistance (R) genes. Herein, the R genes identified in Brassica crops are summarized, including their mapping and cloning, possible molecular mechanisms, and application in resistance breeding. Future perspectives concerning how to accurately discover additional R gene resources and efficiently utilize these genes in the genomic era are also discussed.