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Brassica rapa is an important oilseed crop species next to B. napus in Brassicaceae. However, genetic improvement of B. rapa oilseeds has not been intensively conducted compared to B. napus, which resulted in the limited number of low erucic acid (LEA) cultivars, which is an essential trait for edible oil materials. Candle and Tobin are LEA B. rapa cultivars bred in Canada that are closely related to each other; however, the causal mutation has not been identified. This study was initially aimed to investigate whether the alleles of BrFAE1, the gene encoding a key enzyme for erucic acid synthesis, in Candle and Tobin have mutations impairing their functions. An insertion of the long terminal repeat (LTR) retrotransposon, designated as BRACOPIA, was identified in the 5′ end of the coding region of the gene in both Candle and Tobin. BRACOPIA disrupted the transcription in developing seeds, resulting in a loss of function allele designated as brfae1re. Next, we found that Span, the world’s first LEA B. rapa cultivar developed in Canada, has brfae1re, suggesting that the BRACOPIA insertion is the founder mutation of Canadian LEA B. rapa cultivars. Finally, we investigated the distribution of BRACOPIA family retrotransposons in the Brassica genus, and found that they are present in both the A and C genomes, but the activity has been kept rather modest. Since brfae1re is easily distinguishable from the wild-type allele by PCR, the identification of this mutation could enhance LEA breeding in B. rapa.

Throughout the world, clubroot disease is one of the most damaging diseases affecting Brassica oleracea. To develop marker-assisted selection (MAS) that could assist the incorporation of durable clubroot resistance (CR) into cultivars, previous genetic analyses have identified several CR quantitative trait loci (CR–QTL). However, the independent and cumulative effects of each CR locus against various isolates have rarely been tested. Previously, we identified one major CR–QTL and four minor CR–QTL in the F2 plants from broccoli doubled haploid (DH) line × cabbage DH line of B. oleracea. In the present study, to clarify their effectiveness for controlling disease involving various isolates, inoculation testing was conducted in genotypes with various combinations of the CR genes, which were selected using the DNA markers closely associated with each CR–QTL. In exploring the overall disease incidence, it was apparent that a single involvement of the major CR gene located in the PbBo(Anju)1 locus, or accumulation of CR genes in the minor CR–QTL, is not enough to confer sufficient resistance. One major CR gene in the QTL PbBo(Anju)1 locus plus two to three minor CR genes conferred moderate resistance. The genotype in which all of the CR genes locating in the five QTL including PbBo(Anju)1 were accumulated showed the highest resistance, and it was broadly resistant against six isolates. Accumulation of several CR genes by MAS is necessary to conduct CR breeding in B. oleracea. Our developed DNA markers can be used efficiently to make selections of required loci for the acquisition of resistance, and use of these markers will be a powerful tool for CR breeding in B. oleracea.

Fusarium wilt caused by Fusarium oxysporum f. sp. conglutinans is one of the most important diseases of Brassica crops, resulting in severe reductions in yield and quality. To characterize the inheritance pattern of fusarium resistance, a cross between a susceptible broccoli and a resistant cabbage was subjected to segregation analysis. Results indicated that resistance was controlled by a single dominant allele. This gene was named Foc-Bo1 and mapped to linkage group seven (O7) by both the segregation test and quantitative trait locus (QTL) analysis. The QTL on O7 was detected with a logarithm of odds score (LOD) of 19.5, which was above the threshold value with genome-wide 1% significance level (2.01). A minor QTL was also detected on O4 with a LOD score of 2.06. Inoculation tests indicated that stable expression of fusarium resistance at high temperatures required Foc-Bo1 homozygosity. The association between Foc-Bo1 and the closest simple sequence repeat marker (KBrS003O1N10) was analyzed in three F3 populations. Based on these studies, KBrS003O1N10 represents an effective marker-assisted selection (MAS) tool for breeding fusarium wilt resistance into Brassica oleracea crops. To our knowledge, this is the first paper to map the fusarium-resistance gene in Brassica species and to validate the effectiveness of MAS in improving fusarium resistance in these important plants.

The tuberous stem of kohlrabi is an important quantitative trait, which affects its yield and quality. Genetic control of this trait has not yet been unveiled. To identify the QTLs controlling stem swelling of kohlrabi, a BC1 population of 92 plants was developed from a cross of broccoli DH line GCP04 and kohlrabi var. Seine. A wide range of variation in tuberous stem diameter was observed among the mapping populations. We constructed a genetic map of nine linkage groups (LGs) with different types of markers, spanning a total length of 913.5 cM with an average marker distance of 7.55 cM. Four significant QTLs for radial enlargement of kohlrabi stem, namely, REnBo1, REnBo2, REnBo3, and REnBo4 were detected on C02, C03, C05, and C09, respectively, and accounted for the phenotypic variation of 59% for the stem diameter and 55% for the qualitative grading of tuberous stem in classes. Then, we confirmed the stability of identified QTLs using BC1S1 populations derived from the BC1 plants having heterozygous alleles at the target QTL and homozygous kohlrabi alleles at the remaining QTLs. REnBo1and REnBo2 using 128 plants of BC168S1 and 94 plants of BC143S1, respectively, and REnBo3 and REnBo4 using 152 plants of BC157S1 were detected at the same positions as the respective QTLs of the BC1 population. Confirmation of QTLs in two successive generations indicates that the QTLs are persistent. The QTLs obtained in this study could be useful in marker-assisted selection of kohlrabi breeding, and to understand the genetic mechanisms of stem swelling and storage organ development in kohlrabi and other Brassica species.