Gene Pyramiding For Bacterial Blight(BB)Resistance And The QTL Mapping For Lodging Resistance In Rice(Oryza Sativa L.)

As a biotic stress,Bacterial Blight(BB),caused by Xanthomonas oryzae pv.oryzae,is one of the most economically destructive plant diseases decreasing rice production in rice growing regions of the world.Pyramiding effective BB resistance genes into popular but susceptible varieties is the most economical approach to manage this disease.RILs population of two crosses viz.Ciherang/IRBB60 and F88/IRBB60 were used to develop elite breeding lines with broad-spectrum resistance to bacterial blight in order to identify BB pyramided lines.IRBB60 is the donar parent containing four(Xa4,xa5,xa13 and Xa21)BB resistance genes where as Ciherang and F88 are two popular rice varieties grown in Indonesia and China respectively.To identify BB resistance genotypes 524 RILs(265 RILs of population 1 and 259 RILs of population 2)along with respective gene specific BB resistant checks(IRBB4,IRBB5,IRBB13 and IRBB21)and a BB susceptible check IR24were screened against three virulence strains(C5,P6 and V)up to F6 generations.Marker-assisted selection for confirmation of BB genes was done at F5 and F7generations using gene based markers.Sixty six and 82 pyramided lines with good agronomic characters from population 1 and population 2 respectively were identified through phenotypic and molecular selections.Among the pyramided lines,11 lines from both populations were identified having all four resistance genes whereas 25&53 lines having three resistance genes and 30&18 lines having two resistance genes were identified from population 1 and population 2 respectively.The multi genes having pyramided lines can be used as an important source for developing advance breeding lines against BB to meet the future demand of extra food grains.Genotyping of both population using 138 and 135 polymorphic SSR markers respectively,showed that the allelic contributions of parents to their progenies were varied from chromosome to chromosome.Recipient parent’s allelic contribution was ranging from21.59%to 44.41%and 36.79%to 61.52%,whereas donor parent’s allelic contribution was ranging from 53.69%to 76.99%&36.72%to 62.20%for the two populations respectively.Donor parent’s allelic contribution was higher in population 1 while recipient parent’s allelic contribution was higher in population 2.These result didn’t follow the Hardy–Weinberg law,because trait based selection was done in each generation through a combination of phenotypic and molecular tools which obviously deny the assumptions of this law.One hundred forty eight selected pyramided lines from both the populations along with respective parents,gene specific BB resistant checks and a susceptible check were evaluated in three and five seasons over two and three different locations in China,respectively.Stability parameters were estimated for grain yield,based on significant variation in the mean squares for genotypes and environments and their interactions of eight important agronomical traits in combined ANOVA.Most of the pyramided lines showed higher yield in Hainan followed by Beijing and Anhui.Hainan showed positive environmental index for all the pyramided lines whereas it was negative in Beijing and Anhui.Five pyramided lines(RIL 118,RIL 144,RIL 202,RIL 232&RIL 235)from population 1 and,two pyramid lines(RIL 9&RIL 26)from population 2 were found as the most stable with the changes of environments due to their higher grain yield across all the locations with non significant bi value(around unity),S~2d value(near to zero)and positive phenotypic index.The two RILs population were further investigated by dissecting QTL(s)affecting 16important traits related to lodging resistance using both SSR and SNP markers.Both the populations showed tremendous transgressive segregation for the measured traits and a total of 100 QTLs(55 QTLs in population1 and 45 QTLs in population 2)in 57 genomic regions(25 in population1 and 32 in population 2)were identified.Among these identified QTLs,three QTLs(one from population 1&two from population 2)were found for top first internode length,six QTLs(two from population 1&four from population 2)were mapped for top second internode length and four QTLs(two from population 1&two from population 2)were detected for top third internode length.Hence the QTL QIl1.3 and QIl3.2 on chromosome 3&chromosome 2 were common for both the populations.A total of 19 QTLs(12 from population 1 and seven QTLs for population 2)were identified for top three internodes diameter.A set of eight QTLs(five from population 1 and three QTLs for population 2)were mapped for pushing resistance,of which the QTL QPu.6 was common for both the populations.Eighteen QTLs(13 from population 1 and five QTLs for population 2)were found for top three internodes breaking strength.Eight QTLs(three from population 1 and five from population 2)were observed for both the trait FL&FW.Again six QTLs(four from population 1 and two from population 2)were found for HD,seven QTLs were detected for both the traits PH&PL whereas 6 QTLs(two from population 1 and four from population 2)were observed for GY.The QTLs mapped for the traits such as top three internodes diameter((ID1,ID2 and ID3)and their breaking strength(BK1,BK2 and BK3)and,pushing resistance of the lower part were identified as more important because these traits were highly correlated with each other for lodging resistance.Using a combination of phenotypic and molecular techniques under this study a panel of bacterial blight resistance rice lines having multi genes along with wider adaptability has been identified which can be step forward to the development of advanced breeding lines.The identified QTLs for lodging resistant through evaluation of several lodging related traits would offer important information for marker assisted improvement of the target trait in rice.