Meta-analysis Of Rice Sheath Blight Resistance QTLs And Identification Of Candidate Genes

Rice sheath blight caused by Rhizoctonia solani is a worldwide disease of rice.The resistance of rice to this disease is partial resistance and is controlled by multiple genes or quantitative trait locis(QTLs).So far,no completely immune rice germplasms and highly resistant quality genes have been found.Up to now,more than 300 QTLs for rice sheath blight resistance have been reported to be distributed on all 12 rice chromosomes.However,because of the differences among the adopted parents,population types,genetic markers,identification methods and environmental conditions,most QTLs locations could not be compared with each other directly,and the larger interval of them affected the breeding application.The QTLs meta-analysis strategy proposed in recent years uses the relevant information about the known QTLs,integrates and reanalyzes data,and then deduces the meta QTLs(Meta-QTLs)with higher reliability and more accurate location intervals.In this study,we adopted QTLs meta analysis strategy to analyze Meta-QTLs for rice sheath blight resistance and plant height and heading date affecting sheath blight resistance.Chromosome partial substitution lines and high backcross populations were used to verify and localize some of the major QTL.Meanwhile,the whole genome chromosome segment substitution line of the new rice germplasm YSBR1 with high resistance to sheath blight was constructed under the background of the japonica rice variety TJ394 popularized in Jiangsu Province in recent years.The main results are as follows:1.We collected many QTLs for rice sheath blight from previous studies and carried out meta analysis of 188 qualified QTLs.Eventually,we obtained 34 consistent QTLs named MQSBR(Meta-QTLs of Sheath blight resistance)for rice sheath blight resistance.These MQSBRs are mainly distributed in 1,2,3,5,7,8,9,11 and 12 chromosomes,respectively.The physical interval of them are in the range of 0.2-4.2Mb and the physical interval of nearly half of them are in the range of 1Mb,which is clearly smaller than the previously reported confidence intervals of the primary location.Through the comparison of MQSBRs and MQHD(Meta QTLs of Heading Date),MQPH(Meta QTLs of Plant Height),we found that 8 MQSBRs was linkage or overlapped with MQHD and MQPH,and the remaining 26 MQSBRs could not be linkage with plant height and heading date characters.2.In combination with the location information of the 5 previously reported higher reliable QTLs for rice sheath blight(qSB-9TQ?qSB-11LE?qSB-11HJX74?qSB-11Tetep?qSB-12YSBR1),we found that MQSBR-25 is located in the range of qSB-9TQ,qSB-11LE in the range of MQSBR-27.MQSBR-33 and MQSBR-34 in the range of qSB-12YSBR1.These results show that the results of meta analysis are better.The field resistance of 2 YSBR1/Lemont chromosome segment substitution lines(Lemont background)are significantly higher than that of control Lemont.,which contain MQSBR-7,MQSBR-8 and MQSBR-33,MQSBR-34,respectively.to analyze whether the 2 lines carry other MQSBRs.Through detecting 236 polymorphic molecular markers distributed on 12 rice chromosomes.We found that the genetic background of these 2 lines contained 7 and 2 MQSBRs,respectively.3.In order to find out more useful QTLs for sheath blight resistance and establish the resistant breeding intermediate materials,we constructed the whole genome chromosome substitution line of YSBR1/TJ394.A total of 266 pairs of polymorphic molecular markers were screened and were distributed evenly among 12 chromosomes of rice.A total of 111 YSBR1 chromosome segment substitution lines with TJ394 background were obtained.Among them,BC6F1 generation substitution lines were 14,BC5F1 generation substitution lines 49,BC4F1 generation substitution lines 37,and BC3F1 generation substitution lines 11.The selected substitution lines carry 95%of the genome of the rice genome.We identified a new QTL for rice sheath blight by the method of composite interval mapping using BC4F2 backcross segregating population(TJ394/YSBR1)that covered the location interval of qSB-12YSBR1.The LOD value of this QTL is up toll.2 and the confidence interval contains the most of area of MQSBR-34,MQPH6 and MQSBR-33.4.The QTLs in middle and lower ends of the long arm of rice ninth chromosome were detected by many researchers.In this region,5 MQSBRs named MQSBR-22-MQSBR-26 were detected.In combination with differentially expressed genes induced by Rhizoctonia solani,OsPIP(Plasma membrane protein 2C)and OsEXPR(Expansin-related protein 2 precursor)were found to be significantly induced by Rhizoctonia solani in MQSBR-24 and MQSBR-25,respectively.We obtained RNAi transgenic lines of each gene in the YSBR1 background using? the RNAi method.The result of resistance identification showed that there was no significant difference in disease level between the RNAi lines of each gene and the control YSBR1,indicating that they might not be candidate genes for MQSBR-24 and MQSBR-25.In order to analyze whether these two genes are candidate genes for qSB-9TQ,we hybridized RNAi lines of each gene(OsPIP-Ri,OsEXPR-Ri/YSBR1 background)with LE-qSB-9TQ(Lemont background)and obtained a large number of F1 seeds.At the same time,we made up F1(YSBR1*LE-qSB-9rQ)seeds as control.These three F1 hybrid plants were inoculated with Rhizoctonia solani.The results showed that the difference between different materials was not significant,suggesting that these two genes might not be candidate genes for qSB-9TQ.The results will provide more valuable disease resistant gene resources and breeding intermediate materials for molecular breeding of rice sheath blight resistance.