Quantitative trait loci that control the oil content variation of rapeseed (Brassica napus L.)

Key message

This report describes an integrative analysis of seed-oil-content quantitative trait loci (QTL) in Brassica napus , using a high-density genetic map to align QTL among different populations.

Abstract

Rapeseed (Brassica napus) is an important source of edible oil and sustainable energy. Given the challenge involved in using only a few genes to substantially increase the oil content of rapeseed without affecting the fatty acid composition, exploitation of a greater number of genetic loci that regulate the oil content variation among rapeseed germplasm is of fundamental importance. In this study, we investigated variation in the seed-oil content among two related genetic populations of Brassica napus, the TN double-haploid population and its derivative reconstructed-F₂ population. Each population was grown in multiple experiments under different environmental conditions. Mapping of quantitative trait loci (QTL) identified 41 QTL in the TN populations. Furthermore, of the 20 pairs of epistatic interaction loci detected, approximately one-third were located within the QTL intervals. The use of common markers on different genetic maps and the TN genetic map as a reference enabled us to project QTL from an additional three genetic populations onto the TN genetic map. In summary, we used the TN genetic map of the B. napus genome to identify 46 distinct QTL regions that control seed-oil content on 16 of the 19 linkage groups of B. napus. Of these, 18 were each detected in multiple populations. The present results are of value for ongoing efforts to breed rapeseed with high oil content, and alignment of the QTL makes an important contribution to the development of an integrative system for genetic studies of rapeseed.     

Identification of genomic regions involved in resistance against Sclerotinia sclerotiorum from wild Brassica oleracea

The lack of resistant source has greatly restrained resistance breeding of rapeseed (Brassica napus, AACC) against Sclerotinia sclerotiorum which causes severe yield losses in rapeseed production all over the world. Recently, several wild Brassica oleracea accessions (CC) with high level of resistance have been identified (Mei et al. in Euphytica 177:393–400, 2011), bringing a new hope to improve Sclerotinia resistance of rapeseed. To map quantitative trait loci (QTL) for Sclerotinia resistance from wild B. oleracea, an F2 population consisting of 149 genotypes, with several clones of each genotypes, was developed from one F1 individual derived from the cross between a resistant accession of wild B. oleracea (B. incana) and a susceptible accession of cultivated B. oleracea var. alboglabra. The F2 population was evaluated for Sclerotinia reaction in 2009 and 2010 under controlled condition. Significant differences among genotypes and high heritability for leaf and stem reaction indicated that genetic components accounted for a large portion of the phenotypic variance. A total of 12 QTL for leaf resistance and six QTL for stem resistance were identified in 2 years, each explaining 2.2–28.4 % of the phenotypic variation. The combined effect of alleles from wild B. oleracea reduced the relative susceptibility by 22.5 % in leaves and 15 % in stems on average over 2 years. A 12.8-cM genetic region on chromosome C09 of B. oleracea consisting of two major QTL intervals for both leaf and stem resistance was assigned into a 2.7-Mb genomic region on chromosome A09 of B. rapa, harboring about 30 putative resistance-related genes. Significant negative corrections were found between flowering time and relative susceptibility of leaf and stem. The association of flowering time with Sclerotinia resistance is discussed.     

Analysis of QTLs for erucic acid and oil content in seeds on A8 chromosome and the linkage drag between the alleles for the two traits in Brassica napus

The history of canola breeding began with the discovery of germplasm with low erucic acid content in seeds of spring forage cultivar in tbe 1950's.FAEI,mutations led to a dramatic decrease of the seed erucic acid content in Arabidopsis thaliana.The products of the two FAEI loci.BnA8.FAEI and BnC3.FAEI,showed additive effects to the level of erucic acid content in oilseed rape.Previous research believed that the pleiotropy of FAEI was responsible for the decrease in seed oil content along with the reduction of seed erucic acid content in the modern cultivars.TN DH population was developed from a canola cultivar Tapidor and a Chinese traditional cultivar Ningyou7.The population had been tested in 10 and 11 environments to map QTLs for the erucic acid content and oil content in seeds.As the map resolution increased,a novel QTL for seed erucic acid content was revealed,after Meta-analysis,7 cM away from the most significant seed erucic acid content QTL where BnA8.FAEI is located.Seven independent QTLs for seed oil content(qOC) were detected around the two seed erucic acid content QTLs(qEA)across 39.20 cM on linkage group A8.Two of the qOCs co-localized with the two qEAs,respectively,and were detected in a single environment.The otherfive qOCs were detected in 10 of ll environments independent of qEAs.Alleles from Tapidor in all the QTLs at the 0-39.20 cM region contributed negative effects to either erucic acid content or oil content in seeds.Parallel,genocontent source.Through rounds of crossbreeding with oil-cropped cultivars and intensive selection for multi generations,Tapidor still had the controlled by the five qEA-independent qOCs,with low seed erucic acid content.Ninety cultivars of B.napus from 8 countries were used to analyze the genetic drag with 9 molecular markers located in the QTL confidence intervals (24.04cM) on linkage group A8.It was noticed that more than 46% of the cultivars with low seed erucic acid content trait remained the genotype of low seed oil content at least in one locus.Backcross and marker-assisted selection could break the genetic drag between the low oil content and erucic acid in seeds in the process for breeding modern high seed oil content canola cultivars.     

Characterization of Pectobacterium carotovorum proteins differentially expressed during infection of Zantedeschia elliotiana in vivo and in vitro which are essential for virulence

The identification of phytopathogen proteins that are differentially expressed during the course of the establishment of an infection is important to better understand the infection process. In vitro approaches, using plant extracts added to culture medium, have been used to identify such proteins, but the biological relevance of these findings for in planta infection are often uncertain until confirmed by in vivo studies. Here, we compared the proteins of Pectobacterium carotovorum ssp. carotovorum strain PccS1 differentially expressed in Luria–Bertani medium supplemented with extracts of the ornamental plant Zantedeschia elliotiana cultivar ‘Black Magic’ (in vitro) and in plant tissues (in vivo) by two‐dimensional electrophoresis coupled with mass spectrometry. A total of 53 differentially expressed proteins (>1.5‐fold) were identified (up‐regulated or down‐regulated in vitro, in vivo or both). Proteins that exhibited increased expression in vivo but not in vitro, or in both conditions, were identified, and deletions were made in a number of genes encoding these proteins, four of which (clpP, mreB, flgK and eda) led to a loss of virulence on Z. elliotiana, although clpP and mreB were later also shown to be reduced in growth in rich and minimal media. Although clpP, flgK and mreB have previously been reported as playing a role in virulence in plants, this is the first report of such a role for eda, which encodes 2‐keto‐3‐deoxy‐6‐phosphogluconate (KDPG) aldolase, a key enzyme in Entner–Doudoroff metabolism. The results highlight the value of undertaking in vivo as well as in vitro approaches for the identification of new bacterial virulence factors.     

Maternal control of seed weight in rapeseed (Brassica napus L.): the causal link between the size of pod (mother,source) and seed (offspring,sink)

Seed size/weight is one of the key traits related to plant domestication and crop improvement. In rapeseed (Brassica napus L.) germplasm, seed weight shows extensive variation, but its regulatory mechanism is poorly understood. To identify the key mechanism of seed weight regulation, a systematic comparative study was performed. Genetic, morphological and cytological evidence showed that seed weight was controlled by maternal genotype, through the regulation of seed size mainly via cell number. The physiological evidence indicated that differences in the pod length might result in differences in pod wall photosynthetic area, carbohydrates and the final seed weight. We also identified two pleiotropic major quantitative trait loci that acted indirectly on seed weight via their effects on pod length. RNA‐seq results showed that genes related to pod development and hormones were significantly differentially expressed in the pod wall; genes related to development, cell division, nutrient reservoir and ribosomal proteins were all up‐regulated in the seeds of the large‐seed pool. Finally, we proposed a potential seed weight regulatory mechanism that is specific to rapeseed and novel in plants. The results demonstrate a causal link between the size of the pod (mother, source) and the seed (offspring, sink) in rapeseed, which provides novel insight into the maternal control of seed weight and will open a new research field in plants.