Fine Mapping Clubroot Disease Resistance Gene QS＿B3.1 In Chinese Cabbage

Clubroot is a specific soil-borne disease,which is caused by the obligate plant pathogen（Plasmodiophora brassicae Wor.）.It’s one of the main diseases of crucifer.In a previous study,QS＿B3.1 gene was preliminarily mapped in a 2.89Mb region of A3chromosome in Chinese cabbage.In this study,’CR510’and a clubroot-susceptible’59-1’were used as parental lines to construct the population.Fine mapping and screening of candidate genes were carried out.The main results are as follows: 1.F1 was obtained by crossing the disease-resistant parent’CR510’and the susceptible parent ’59-1’,’CR510’is a near-isogenic line of’59-1’.It is a Chinese cabbage near-isogenic line which only contains QS＿B3.1 locus in BC₄F3 generation through screening molecular markers of backcross population and selfing population by using turnip inbred line’Siloga’ containing clubroot resistance gene as male parent and clubroot sensitive Chinese cabbage inbred line’59-1’as female parent.F2 was harvested by selfing of F1individuals.F2 population containing 3000 individuals were screened by using the flanking markers cnu-ssr316,syau-In Del3008 and sau＿um026 of QS＿B3.1 gene,and 51 recombinant plants were screened out,with 12 genotypes.TheF3 population was harvested by selfing the recombinant plants,and theF3 population（30 plants in each family）was inoculated with Plasmodiophora brassicae.LNND-2 for disease resistance test.Finally,8 families showed complete disease resistance,18 families showed complete disease susceptibility, and 25 families showed character segregation with segregation ratio of 3:1,which was in line with QS＿B3.1 dominant single gene inheritance of disease resistance site. 2.25 pairs of In Del markers were developed between cnu-ssr316 and sau＿um026,of which 19 pairs were polymorphic.These polymorphic primers were used for PCR amplification of recombinant plants.Genotypes of F2 population（180 plants）were identified by using the flanking markers of QS＿B3.1 and newly developed markers.According to the genotype of F2 population,the genetic map of QS＿B3.1 was constructed by using Join Map4.0.Use Map Chart2.2 to construct physical map.Fally,QS＿B3.1 gene was narrowed to a 753.4kb region on A3 chromosome of Chinese cabbage. 3.There are 109 genes in the mapping region（753.4kb）of QS＿B3.1 gene on chromosome A3 of Chinese cabbage,among which 5 genes with disease resistance include Bra019407, Bra019409,Bra019410,Bra019412 and Bra019413,and the length of these 5 genes amplified in’CR510’is 669bp,respectively The five candidate genes were sequenced, and the sequence difference analysis showed that the sequences of Bra019407 and Bra019412 genes in’CR510’were identical to with the reference genome sequence of Chinese cabbage,which could exclude these two candidate genes.The other three genes Bra019409,Bra019410 and Bra019413,which have typical disease resistance gene domains,are quite different from the reference genome sequences,and there are large fragment insertions and deletions and a large number of SNP variations in introns and exons（TIR and LRR domains）.Therefore the candidate genes of QS＿B3.1 disease resistance gene are Bra019409,Bra019410 and Bra01941.