Improvement Of Clubroot Resistance In Cabbage By Using Molecular Marker Assisted Selection

Cabbage（Brassica oleracea var.capitata）is one of the most important cruciferous vegetable crops in the world.Clubroot is one of the main soilborne disease in cruciferous crops,which might seriously decrease cabbage yield and quality.Basically,breeding varieties resistance to clubroot is an effective mean to prevent its damage in cabbage.Through backcrossing,clubroot resistance genes/loci from other materials can be transferred into the inbred line.However,cabbage is a typical plant-vernalization-responsive plant,it need strictly environment to flower,which leads to extreme difficult breeding of cabbage inbred lines,and needs very long time to breed an inbred line.Therefore,in the process of backcrossing,if the individuals with the most genetic background of the recipient inbreed line but minimal genetic background of the donor material are precisely screeded from the early backcross generation,it will be beneficial to introduce the resistance gene into the target recipient inbred line in a short generation,and maximized the genetic background of the recipient inbred line at the same time,thereby improving the breeding efficiency effectively.In the present study,a cabbage inbred line‘F416’with excellent traits but sensitive to clubroot,was used as the recipient parent,and a material‘9LQ18-5’with high resistance to clubroot,was used as the donor parent.Based on 3～4 generations of backcrossing population,in order to obtain a individual with a genetic background highly similar to‘F416’and high resistance to clubroot from the backcrossing population,molecular marker assistant foreground selection combined with clubroot disease resistance identification was used to achieve directional selection of target traits,at the same time,the genetic similarity between the offspring of disease-resistant individuals and the recipient inbred line‘F416’was identified by molecular marker assistant background selection.The results of the study are as follows:1.18 lines with clubroot resistance（CCR1 and CCR2）were screened from 55individual plants of BC3F1generation by using clubroot assistant selective molecular markers.And their genetic background was carried out using SSR markers.We found5 individual plants with highly genetic similarity to‘F416’and 3 individual plants with lowly genetic similarity to‘F416’were screened by SSR polymorphic molecular markers.The morphological features were further analyzed using the clonal population of 8 individual plants,among the 5 lines with high genetic similarity to‘F416’based on SSR markers,3 lines showed the same"flat"leaf tip as‘F416’,2lines showed the same‘oval’leaf shape as‘F416’,but their plant height,plant expansion and leaf length were significantly different from‘F416’.Among the 3 lines with low genetic similarity to‘F416’,2 lines showed the same‘flat’leaf tip as‘F416’,and their average plant height,plant expansion and leaf width were similar to‘F416’.This results indicated that a great difference between the results of background selection using SSR molecular markers and phenotype based screening and it is necessary to develop new background selection markers.2.The eight BC₃F₁ lines were re-sequenced together with the recurrent parent（F416）,the non-recurrent parent（9LQ18-5）and the clubroot resistant control variety（Xiangan 336）.Comparing to parental lines and the control,21-9,21-1,21-2 were highly similar to the recurrent parent（F416）in terms of SNPs and indel variation,and‘21-9’was the most similar to‘F416’genetic background.In addition,no any reads in the clubroot candidate QTL region（42986635-43033590）of about 47kb in size in all the clubroot resistant lines were detected,in comparison the sequencing depth of 8BC₃F₁ lines with‘F416’,‘9LQ18-5’and‘Xiangan 336’.It is inferred that this region might be a potential functional segment or a linked segment for clubroot resistance.3.The 8 BC₃F₁ lines were backcrossed with‘F416’to obtain BC₄F₁.36 lines with CCR1 and CCR2 markers were selected from the 72 BC₄F₁ lines.49 pairs of indel-specific primers based on the indel mutation sites obtained from whole-genome resequencing were designed and screened for background selection in the 36 BC₄F₁lines.The BC₄F₁ lines‘21-9’showed similar genetic background and agronomic traits to‘F416’,and their average backcross recovery rate was as high as 85.25%.Among them,the 90%backcross recovery rates of 21-9-16 and 21-9-19 individuals is close to the 93.8%theoretical expectation of the background recovery rate of BC₄F₁backcrossing.In addition,the‘21-9’lines were resistant to clubroot,and they were very important materials for backcrossing breeding in the future.In conclusion,the markers developed and screened by whole genome re sequencing in this study can be effectively used for the selection of backcross progeny population of elite inbreeding line‘F416’.And the the background markers of‘F416’developed in this study have important application value to the efficient selection of backcross progeny population for other important agronomic traits introgressing into the elite inbreeding line‘F416’.