Pyramiding Sclerotinia Stem Rot Resistance Of Wild Brassica Oleracea Into Brassica Rapa To Improve Its Resistance

Oilseed rape(Brassica napus L.)is the very important oil-producing and economic crop worldwide.It suffers heavily from sclerotinia stem rot which is caused by the necrotrophic fungal pathogen Sclerotinia sclerotiorum,inducing serious yield losses and oil quality decrease.It is the most economical and sustainable method to control disease by breeding rapeseed varieties with high level of resistance.However,the situation that no source of complete or high level resistance was identified within B.napus,has severely limited the process of resistance breeding to S.sclerotinia.So the researchers try to find good resistance in the closely related species to improve the resistance of S.sclerotiorum.In previous studies,5 resistance locis on C1(q LR10-1,q LR10-7),C3(q LR10-8)and C9(q SR09-1,q SR09-2)chromosomes were identified in a F2 population derived from a cross between a wild resistance B.oleracea accession,“C01”(B.incana)and susceptible accession “C41”,and the resistance locis on C9 chromosome were successfully transferred into B.rapa accession “6Y733” by backcrossing “6Y733” with a haploid between “6Y733” and “C01”,producing an individual of B.rapa with 1.7-fold stem resistance of “6Y733”.In this study,another resistant locis on C1 and C3 chromosomes from wild B.oleracea was pyramided into B.rapa by molecular-assisted selection(MAS).The results were listed below.1 S.sclerotiorum-specific molecular marker developmentThe QTL region on C1 chromosome was aligned to a 1.56 Mb genomic region of B.oleracea by mapping the sequences of markers linked with the QTL to the reference genome of B.oleracea v1.0.Based on the genome sequence differences between “C01” and “C41”,40 In Del molecular markers with insertion-deletion size of at least 20 bp were developed.Of which 9 markers exhibited polymorphisms between “C01”,“C41” and “6Y733”.Eventually,only one marker was successfully added to the C1 chromosome linkage group.2 Pyramiding resistance by MASIn this study,to ensure the B.rapa progenies with the QTLs on the C9 chromosome,we used the markers linked with the QTLs on C9 chromosome to screen the B.rapa progenies.Three simple sequence repeat(SSR)markers(2 and 1 on C1 and C3 chromosome,respectively),that linked with the QTLs on C1 and C3 exhibited polymorphisms among “C01”,“6Y733” and six individuals,which were randomly selected from the backcrossing progenies with B.rapa.Polymorphic markers were subsequently employed to track the resistance loci in the progenies.The QTLs on C1 and C3 chromosomes were successfully transferred into B.rapa with the QTLs on the C9 chromosome after two years of pyramiding breeding.Every year the sclerotinia resistance of stem was evaluated among the B.rapa progenies with the markers linked with the QTLs(regarded as the New-QTL group),apart from C9 chromosome,the B.rapa progenies without the markers linked with the QTLs(regarded as the Non-QTL group)and “6Y733”.A significant difference(P < 0.0001)was detected between the New-QTL group and Non-QTL group for stem resistance in the first year,and the resistance values of two groups are 0.43±0.10 and 0.68±0.09 for NewQTL group and Non-QTL group,respectively.We found three individuals with 3-fold stem resistance of “6Y733”(Sstem = 0.34±0.032,0.37±0.059 and 0.32±0.119,respectively)on New-QTL group,and the significant differences were also detected between the three individuals and their parents,respectively.Three individuals were self-intersection and hybridization,and the progenies were screened by MAS with the markers linked with the QTLs on C1,C3 and C9 chromosomes again.In second year,the assessment of stem resistance between New-QTL group and Non-QTL group were finished as the first year.Similarly,the significant difference(P < 0.0001)was detected between the New-QTL group and Non-QTL group for stem resistance,and the resistance values of two groups are 0.55±0.181 and 0.90±0.132 for New-QTL group and Non-QTL group,respectively.Two individuals with the highest resistance level(Sstem = 0.24±0.016 and 0.25±0.024,respectively)were selected in the New-QTL group,and there are more 4.0-fold higher resistance than “6Y733”.Eventually,we succeeded in polymerizing QTLs on C1 and C3 chromosomes into B.rapa progenies with QTLs on C9 chromosome.