| IBacterial Blight (BB) caused by Xanthomonas oryzae pv. Oryzae (Xoo) is one of the most destructive diseases of rice throughout the world. The most effective and economical approach to combat BB is breeding and use resistant varieties and one way to delay breakdown of rice resistance caused by BB and increase BB resistance of varieties is to pyramid multiple resistance genes into one rice variety . Hybrid rice is widely used in China but most of them are susceptible to BB, one of the most effective method to improve their resistance BB is to introgress two or more dominant resistant BB genes. Xa21 is one of the broad spectrum resistance gene Xa4 can control most of BB race in China, the molecular markers tightly linked with the two genes are developed, which make it possible to pyramid the two genes by molecular marker- assisted selection (MAS).Shuhui527 (527R) is a promising restorer line bred by Rice Research Institute of Sichuan Agricultural University in recent years. However, it is susceptible to Bacterial Blight (BB) ,which limits its use widely. The objective of this study is improvement its resistance to BB using IRBB60,carrying resistant genes, Xa21,Xa4,xal3 and xa5,as donor parent by MAS. The main results are following:pTA248 and Mpl2,tightly linked with Xa21 and Xa4,respectively,have polymorphism between 527R and IRBB60, and the segregate ratio of pTA248 in population of (527R/IRBB21)F2,MP12 in (527R/IRBB4)F2 all fit to 1 : 2 : 1,which indicate they are co-dominant markers. The selection accuracies of pTA248 and Mp12 are 97.1% and 83.3%,respectively.Using 527R as recurrent receiptor parent, IRBB60 as donor parent,through three times backcrosses and one time self-cross, 10 plants.Linel-Line 10, were obtained which their loci Xa21 and Xa4 are homozygous and field phenotype are similar to those of 527R,using the MAS together with field phenotype selection. The PCR analysis with RM264 and RM611, tightly linked with xa13 and xa5, respectively, shows that the 10 plants have not xa13 and xa5. The resistance analysis indicates that the 10 plants(lines) are resistance to race P1,P6 and C I ~CVII.The genetic background analysis with SSR markers which have polymorphism between 527R and IRBB60 and 100 RAPD markers shows that the Line3, Line5, Line8, Line9 and Line10 are the most similar to 527R, which demonstrate that the genetic background recover rapidly through MAS coupled with strict field phenotype selection in every generation.Under no diseased condition in Wenjiang, the agronomic traits of 10 improved lines and 527R and their hybrid combinations crossing with CMS D62A and G46A ,were investigated, which include days to heading ,1000-grain weight, plant height, panicles per plant, spikelets per panicle, grain per panicle , setting percentage and yield. The results show that setting percentage is significant at 0.05 level in 527R and 10 improved lines. The variation of the traits observed the hybrid combinations that each improved lines cross with D62A are not significant, but the analysis of the hybrid combinations that improved lines cross with G46A show that 1000-grain weight is significant at 0.05 level and the spikletes per panicles and yield are significant at 0.01 level. From the analysis of above ,the Line5,Line9 and Line 10 are the most similar to 527R.The Line5,Line9 and LinelO were more evaluated under heavily diseased conditions caused by artificial inoculation P6 and mixed races of C I ~CVII in Hai'nan. The results show that their combinations which the 3 lines crossed with susceptible CMS D62A and G46A resistance to P6 and mixed race of C I ~CVII .The agronomic traits ,1000-gain weight, and yield per plant of the above combinations,are better than those of controls 'Gangyou527' and 'Dyou527' .Then the Line5,Line9 and Line10 can be used as the improved 527R resistant to BB ,and the more test are on the way.The above analysis indicate it is success to improve the resistance to BB of 527R by pyramiding Xa21 and Xa4 using MAS.
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