| Molecular marker-assisted selection (MAS) provides a very efficient tool for backcross breeding because it can offer rapid and precise selection of the targeted gene. The objectives of this study were: to improve the bacterial blight (BB) resistance of two restorer lines of rice hybrids by introgressing Xa21, a gene that is highly resistant to a broad spectrum of the pathogen races, using MAS in the process of recurrent backcrossing; and to evaluate the effects of such improvement on the agronomic performance of the restorer lines and their hybrids under BB stressed and non-stressed conditions. The main results are as follows:1. A total of 17 BB isolates were used to identify the resistance of one rice variety IRBB21, the donor parent ofXa21, two restorer lines of rice hybrids, Minghui 63 and 6078, and several related rice combination. The results showed that: (1) IRBB21 was resistant to all the strains except T7147, a strain from Japan belonging to the Japanese racial group 2, whereas Minghui 63 and 6078 were moderately susceptible to most of the strains tested; (2) The resistance reactions of the hybrids derived from IRBB21 appeared to be the same as that of IRBB21, whereas that of the hybrids derived from Minghui 63 and 6078 were more susceptible to BB than the two restorers. Thus, Xa21 would be very useful for improving BB resistance of the rice hybrids in China.2. A local linkage map of the Xa2I genomic region on chromosome 11 was constructed based on 200 individuals of the F2 population from the Minghui 63/IRBB21 cross, from which a MAS system was established. Two Xa21 cosegregated markers were used for selecting for the presence of the Xa21 gene, and two additional markers flanked both sides of the Xa21 locus were chosen to select for recombination between the Xa21 locus and the flanking markers. This MAS system would ensure that the introgressed segment surrounding the Xa21 locus was shorter than 3.8 cM.3. Screening 354 RFLP probes identified 158 markers that were polymorphic between Minghui 63 and IRBB21. One hundred and twenty-eight of the 158 polymorphic markers, representing all the 12 chromosomes with the largest interval less than 30 cM, were used for "cleaning" up the genetic background of the selections.4. In the process of improving the resistance of Minghui 63, aXa21 containing recombinant was obtained between Xa21 and AB9 in BC,F1, a subsequent recombinant was obtained between the gene and C189 on the other side in BC2F1, and the genetic background of Minghui 63 genotypes was recovered in BC3F, using 128 RFLP markers mentioned above. Thus, the improved version of Minghui 63 was obtained in BC3F2 and designated as Minghui 63 (Xa21).5. In order to seek an optimum molecular marker for genetic background selection, RAPD, SSR, RAMP, RFLP and AFLP were compared in various ways. AFLP is regarded as the best one because of its highest ratio of polymorphism, its much more polymorphic bands from each pain of primers, and its good repeatability among RAPD, SSR, RAMP and AFLP, as well as the same effect with RFLP for genetic background selection.6. In the process of improving the BB resistance of 6078, one/resistant plant with more than 92.95% of 6078 genotype background was obtained by PCR and AFLP selection in BC,F1, one Xa21 containing recombinant with at least 97.67% of 6078 genotype background was obtained in BC2F, in the same way, and one double recombinant with Xa21 gene was obtained by PCR. Subsequently, the improved version of restorer line 6078 (Xa21) was obtained in BC3F2.7. Compared with the original restorer lines, Minghui 63 and 6078, the BB resistance of the improved version of restorer lines, Minghui 63 (Xa21) and 6078 (Xa21) with five sterile lines widely used in rice hybrid production in China showed no significant difference in seven agronomic important traits between the two original and the two improved lines, indicating that there is little genetic modification to the restorer line to be improved in this marker-assisted backcross breeding program.9.
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