Association of Clubroot Resistance Locus PbBa8.1 With a Linkage Drag of High Erucic Acid Content in the Seed of the European Turnip

Integrated Analysis of lncRNA–mRNA Regulatory Networks Related to Lipid Metabolism in High-Oleic-Acid Rapeseed

A high oleic acid content is considered an essential characteristic in the breeding of high-quality rapeseed in China. Long-chain non-coding RNA (lncRNA) molecules play an important role in the plant’s growth and its response to stress. To better understand the role of lncRNAs in regulating plant reproductive development, we analyzed whole-transcriptome and physiological data to characterize the dynamic changes in lncRNA expression during the four representative times of seed development of high- and low-oleic-acid rapeseed in three regions. We identified 21 and 14 lncRNA and mRNA modules, respectively. These modules were divided into three types related to region, development stages, and material. Next, we analyzed the key modules related to the oil content and the oleic acid, linoleic acid, and linolenic acid contents with physiological data and constructed the key functional network analysis on this basis. Genes related to lipid metabolism, such as 3-ketoacyl-CoA synthase 16 (KCS16) and acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1), were present in the co-expression network, suggesting that the effect of these genes on lipid metabolism might be embodied by the expression of these lncRNAs. Our results provide a fresh insight into region-, development-stage-, and material-biased changes in lncRNA expression in the seeds of Brassica napus. Some of these lncRNAs may participate in the regulatory network of lipid accumulation and metabolism, together with regulated genes. These results may help elucidate the regulatory system of lncRNAs in the lipid metabolism of high-oleic-acid rapeseed seeds.

Mapping of a novel clubroot disease resistance locus in Brassica napus and related functional identification

Clubroot disease, caused by Plasmodiophora brassicae, is a devastating disease that results in substantial yield loss in Brassicaceae crops worldwide. In this study, we identified a clubroot disease resistance (CR) Brassica napus, "Kc84R," which was obtained by mutation breeding. Genetic analysis revealed that the CR trait of "Kc84R" was controlled by a single dominant locus. We used the bulked segregant analysis sequencing (BSA-seq) approach, combined with genetic mapping based on single nucleotide polymorphism (SNP) markers to identify CR loci from the F₂ population derived from crossing CR "Kc84R" and clubroot susceptible "855S." The CR locus was mapped to a region between markers BnSNP14198336 and BnSNP14462201 on the A03 chromosome, and this fragment of 267 kb contained 68 annotated candidate genes. Furthermore, we performed the CR relation screening of candidate genes with the model species Arabidopsis. An ERF family transcriptional activator, BnERF034, was identified to be associated with the CR, and the corresponding Arabidopsis homozygous knockout mutants exhibited more pronounced resistance compared with the wild-type Col-0 and the transgenic lines of BnERF034 in response to P. brassicae infection. Additionally, the expression analysis between resistant and susceptible materials indicated that BnERF034 was identified to be the most likely CR candidate for the resistance in Kc84R. To conclude, this study reveals a novel gene responsible for CR. Further analysis of BnERF034 may reveal the molecular mechanisms underlying the CR of plants and provide a theoretical basis for Brassicaceae resistance breeding.

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.

Harnessing the potential of bulk segregant analysis sequencing and its related approaches in crop breeding

Most plant traits are governed by polygenes including both major and minor genes. Linkage mapping and positional cloning have contributed greatly to mapping genomic loci controlling important traits in crop species. However, they are low-throughput, time-consuming, and have low resolution due to which their efficiency in crop breeding is reduced. In this regard, the bulk segregant analysis sequencing (BSA-seq) and its related approaches, viz., quantitative trait locus (QTL)-seq, bulk segregant RNA-Seq (BSR)-seq, and MutMap, have emerged as efficient methods to identify the genomic loci/QTLs controlling specific traits at high resolution, accuracy, reduced time span, and in a high-throughput manner. These approaches combine BSA with next-generation sequencing (NGS) and enable the rapid identification of genetic loci for qualitative and quantitative assessments. Many previous studies have shown the successful identification of the genetic loci for different plant traits using BSA-seq and its related approaches, as discussed in the text with details. However, the efficiency and accuracy of the BSA-seq depend upon factors like sequencing depth and coverage, which enhance the sequencing cost. Recently, the rapid reduction in the cost of NGS together with the expected cost reduction of third-generation sequencing in the future has further increased the accuracy and commercial applicability of these approaches in crop improvement programs. This review article provides an overview of BSA-seq and its related approaches in crop breeding together with their merits and challenges in trait mapping.

R gene triplication confers European fodder turnip with improved clubroot resistance

Clubroot is one of the most important diseases for many important cruciferous vegetables and oilseed crops worldwide. Different clubroot resistance (CR) loci have been identified from only limited species in Brassica, making it difficult to compare and utilize these loci. European fodder turnip ECD04 is considered one of the most valuable resources for CR breeding. To explore the genetic and evolutionary basis of CR in ECD04, we sequenced the genome of ECD04 using de novo assembly and identified 978 candidate R genes. Subsequently, the 28 published CR loci were physically mapped to 15 loci in the ECD04 genome, including 62 candidate CR genes. Among them, two CR genes, CRA3.7.1 and CRA8.2.4, were functionally validated. Phylogenetic analysis revealed that CRA3.7.1 and CRA8.2.4 originated from a common ancestor before the whole-genome triplication (WGT) event. In clubroot susceptible Brassica species, CR-gene homologues were affected by transposable element (TE) insertion, resulting in the loss of CR function. It can be concluded that the current functional CR genes in Brassica rapa and non-functional CR genes in other Brassica species were derived from a common ancestral gene before WGT. Finally, a hypothesis for CR gene evolution is proposed for further discussion.

Efficient marker-assisted breeding for clubroot resistance in elite Pol-CMS rapeseed varieties by updating the PbBa8.1 locus

Clubroot disease poses a severe threat to rapeseed (Brassica napus) production worldwide and has recently been spreading across China at an unprecedented pace. Breeding and cultivation of resistant varieties constitute a promising and environment-friendly approach to mitigating this threat. In this study, the clubroot resistance locus PbBa8.1 was successfully transferred into SC4, a shared paternal line of three elite varieties in five generations by marker-assisted backcross breeding. Kompetitive allele specific PCR (KASP) markers of clubroot resistance gene PbBa8.1 and its linked high erucic acid gene (FAE1) were designed and applied for foreground selection, and 1,000 single-nucleotide polymorphisms (SNPs) were selected and used for the background selection. This breeding strategy produced recombinants with the highest recovery ratio of the recurrent parent genome (> 95%) at BC2F2 while breaking the linkage with FAE1 during the selection. An updated version of the paternal line (SC4R) was generated at BC2F3, showing significantly improved clubroot resistance at the seedling stage via artificial inoculation, and was comparable to that of the donor parent. Field trials of the three elite varieties and their updated versions in five environments indicated similar agronomic appearance and final yield. The introduced breeding strategy precisely pyramids the PbBa8.1 and FAE1 loci with the assistance of technical markers in a shorter period and could be applied to other desirable traits for directional improvement in the future.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-022-01305-9.

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.

Fine Mapping of Clubroot Resistance Loci CRA8.1 and Candidate Gene Analysis in Chinese Cabbage (Brassica rapa L.)

Clubroot is caused by Plasmodiophora brassicae, which threatens Brassicaceae crop production worldwide. In recent years, there has been an outbreak and rapid spread of clubroot in many major cruciferous crop-producing areas of China. In this study, we identified a cabbage material DingWen (DW) with different resistant capabilities from Huashuang5R (H5R) and Huayouza62R of Brassica napus, which are currently used as the main resistant cultivars for clubroot management in China. We used a next-generation sequencing-based bulked segregant analysis approach, combined with genetic mapping to identify clubroot-resistant (CR) genes from F₁ population generated from a cross between the DW (CR) and HZSX (clubroot susceptible). The CR locus of DW (named CRA8.1) was mapped to a region between markers A08-4346 and A08-4853, which contains two different loci CRA8.1a and CRA8.1b after fine mapping. The CRA8.1b loci contain a fragment of 395 kb between markers A08-4624 and A08-4853 on A08 chromosome, and it is responsible for the resistance to PbZj and PbXm isolates. However, together with CRA8.1a, corresponding to a 765-kb region between markers A08-4346 and A08-4624, then it can confer resistance to PbXm ⁺. Finally, through expression analysis between resistant and susceptible materials, two genes encoding TIR-NBS-LRR proteins (BraA08g039211E and BraA08g039212E) and one gene encoding an RLP protein (BraA08g039193E) were identified to be the most likely CR candidates for the peculiar resistance in DW.

Development of novel clubroot resistant rapeseed lines (Brassica napus L.) effective against Japanese field isolates by marker assisted selection

Clubroot is an important disease infectible to cruciferous plants and a major threat to rapeseed production in Japan. However, no clubroot resistant rapeseed cultivars have been released. We surveyed pathotype variation of six isolates collected from rapeseed fields and found they were classified as pathotype groups 2 and 4 using Japanese F₁ Chinese cabbage cultivars. We produced the resynthesized clubroot resistant Brassica napus harboring two resistant loci, Crr1 and Crr2, by interspecific crossing and developed resistant rapeseed lines for southern and northern regions by marker-assisted selection and backcrossing. We improved the DNA marker for erucic acid content to remove linkage drag between Crr1 and high erucic acid content and successfully selected lines with clubroot resistance and zero erucic acid for northern regions. A novel line, 'Tohoku No. 106', suitable for southern regions showed stable resistance against all six isolates and high performance in infested fields. We conclude that Crr1 and Crr2 are important genes for CR rapeseed breeding and marker-assisted selection is effective in improving clubroot resistance.