The interactions between Xanthomonas oryzae pv. oryzae (Xoo) and rice are controlled in a gene-for-gene manner. In this study, a 359 bp DNA fragment of the avrXa3 gene containing three nuclear localization signal (NLS) motifs present in all members of the avrBs3/pthA family was used as a probe to screen a genomic library of the JXOIII strain of Xoo. The results demonstrated that diverse members of the family exist in the pathogen genome. The avrBs3/pthA genes occurred at isolated individual portions or in clusters. The positive avr gene clones were transferred into the virulent recipient PXO99A . Pathogenicity tests in near isogenic lines of rice confirmed that four resistance (R) genes ( Xa2 , Xa3 , xa5 and xa8 ) matched the four avr genes ( avrXa2 , avrXa3 , avrxa5 and avrxa8 ) in the genome of Xoo strain JXOIII. The avrBs3/PthA -like gene (1·7 kb) present in cosmid p54, may specifically interact with the Xa3 gene present in IRBB3, and is designated avr/pthA3 . Sequencing indicated that there are only 1·5 copies of the 102 bp repeat unit in avr/pthA3 . Alignment of the twelfth and thirteenth amino acids in the repetitive units encoded by this gene with those in other representatives of the AvrBs3 family revealed a unique repeat arrangement which might contribute to variation in the avirulence genes in Xoo. The parental rice line IR24 was found to contain several R genes for resistance to Xoo bacterial blight.
Brown planthopper(BPH) is one of the most serious and destructive insect pests of rice in most rice growing regions of the world. In this study, two major resistance genes against BPH have been identified in an Oryza rufipogon (Griff.) introgression rice line, RBPH54. Inheritance of the BPH resistance in RBPH54 was studied by screening the resistance in parents, F1, F2 and BC1 generations against BPH biotype 2. A population of BC3F2 lines was developed and SSR markers were employed for the gene mapping, and new markers were designed for fine mapping of the resistance genes, while sequence information of BAC/PAC clones was used to construct physical maps of the genes. The results showed that the BPH resistance in RBPH54 was governed by recessive alleles at two loci, tentatively designated as bph20 (t) and bph21(t). The locus bph20(t) was fine mapped to the short arm of chromosome 6 about 2.7 cM to the upper marker RM435 and 2.5 cM to lower marker RM540 and in a 2.5 cM region flanked by two new SSR markers BYL7 and BYL8 which were developed in the present study. The other BPH resistance locus bph21(t) was initially mapped to a region 7.9 cM to upper marker RM222 and 4.0 cM to lower marker RM244 on the short arm of chromosome 10. For physical mapping, the bph20(t)-linked markers were landed on BAC/PAC clones of the reference cv., Nipponbare, released by the International Rice Genome Sequencing Project. The bph20(t) locus was physically defined to an interval of about 75 kb with clone P0514G1. Identification and location of these two genes in the present study have diversified the BPH resistance gene pool, which give benefit to the development of resistant rice cultivars, and the linkage PCR-based SSR markers for the bph20(t) and bph21(t) genes would help realize the application of the genes in rice breeding through marker-assisted selection.
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