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TWO NEW GENES FOR RESISTANCE TO BACTERIAL BLIGHT (XATHOMONAS ORYZAE PV. ORYZAE) WERE IDENTIFIED FROM WILD RICE
TWO NEW GENES FOR RESISTANCE TO BACTERIAL BLIGHT (XATHOMONAS ORYZAE PV. ORYZAE) WERE IDENTIFIED FROM WILD RICE QI ZHANGl, B.Y. ZHAOl, C.L. WANGl, Y.L. ZHOUl, H LEUNG2 linstitute of Crop Breeding and Cultivation, CAAS, Beijing-100081, China. 2lnternational Rice Research Institute, Philippines. Background and Objectives The gene Xa4 for resistance to bacterial blight (BB) has been widely adopted in the development of hybrid and conventional indica rice in China, which encompass more than 90% of national rice area. Such narrow based genotype is prone to the genetic vulnerability of cultivars to BB, since a strong selection on the pathogen population would increase the frequency of Chinese pathotype V, which is virulent to Xa4 gene [1]. Thus, new sources of resistance must be identified continuously. Twenty one resistance accessions screened from 971 wild oryzae (14 species) were re-evaluated with strains from 7 Chinese pathotypes and 2 Philippine races. An accession, WBB1 from O. rufipogon showed highly resistance to all strains used and another acc. WBB2 (O. rtjf'pogon) was highly resistant to some strains including the Phillipine race 6. The objectives of this study are to: (1) Compare the resistance reaction between O. rufipogon and 6 testers (with different major dominant resistance genes). (2) Determine the genetic basis of the resistance. (3) Tag the resistance gene using AFLP and RGA markers for use in marker aided selection. Materials and Method 1. The resistance of O. rufipogon acc.s WBB1 and WBB2 was introduced into susceptible cultivars by interspecific hybridization. The Fl plants of JG30 / WBB1 was anther-cultured. 2. To test the alielic relationship between Xa21 and the BB resistance gene(s) from WBB 1 and WBB2, sequence tagged sites (STS) primers (Ul 5'-CGA TCG GTA TAA CAG CAA AAC-3, l1 5-ATA GCA ACT GAT TGC TTG G-3) specific to the Xa21 locus were used to detect polymorphism between IRBB21 and the two materials [2]. 3.To tag the new resistance gene using AFLP and Resistance Gene Analogue (RGA) markers. Results and Conclusion 1. WBB1 was highly resistant to 9 races from seedling to tillering stages.' IRBB21(Xa21) showed resistance to all 9 races only at tillering stages. WBB2 showed resistance to all 9 races except race 1 and race 4. The other 5 testers showed no resistance to race 6. 2. In three sets of F2 and BC1 populations derived from WBB1 crossed with susceptible parents and IRBB21, the resistance segregated in a ratio of 3R:1S, 1R:1S, and 13R:3S, respectively, indicating one dominant gene confers the overall resistance to race 6. The gene is non-allelic to Xa21 , The segregation in four sets F2 and BC1 populations derived from WBB2 fit a ratio of 3R,.1S and 1R..1S respectively, which suggested the resistance is conditioned by one dominant gene. 3. Since only Xa21 confers resistance to race 6 in all known dominant genes, it is necessary to test allelism between Xa21 and the new gene(s). STS analysis showed distinct polymorphism between IRBB21 and WBB1, WBB2, suggesting the two genes are different from Xa21 . 4. By screening 122 AFLP primers and 19 RGA primers, eight DNA markers flanking the WBB2 resistance gene were identified, which can be used in marker aided selection. Conclusion: The resistance of WBB1 and WBB2 was conditioned by single dominant gene respectively, the two genes distinguished from Xa21 based on the resistance spectrum, resistance type, genetic basis and STS analysis. References 1. ZHANG Ql, C.L. WANG, Y. L ZHOU, J. E. LEACH. Proceedings: The International Program on Rice Biotechnology, Rice Pests and Pathogens, pp 238. 2. Wang G. L. et al., 1996. MPMI 850-855. This project is supported by the Rockefeller Foundation