Cloning Of Rice Xanthomnas Oryzae Resistant Candidate Gene--Xa4

Plant disease resistance is often controlled by Mendelian genes and follows a gene-for-gene relationship in many plant species and their pathogens. To date, many resistance gene(R gene) have been cloned. R genes from different species appear to share sequence similarities in certain domains of their gene products such as leucine-rich repeats(LRR) and nucleotide-binding sites (NBS).Bacterial blight (BB), caused by Xanthomonas oryzae pv, oryzae(Xoo)t has been one of the most serious diseases in rice, affecting production in irrigated and rain-fed lowland ecosystems throughout Asia, northern Australia, mainland Africa, the southern part of the United States, and Latin America. The genetic basis of host resistance to bacterial blight has been studied in depth, and many resistance genes have been identified from cultivated rice and wild rice species. Two of these resistance genes, Xa21 and Xal, have been cloned. In this article, we found that a resistance sequence, RS13, was co-segregative with Xa4. RS13 was used to identify BAC clones from bacterial artificial chromosome library. Four and twelve clones from different BAC library, IR64 and IRBB56, have been identified. Two clones have been sequenced and analyzed. Here are the main results.1. Rice resistance to bacterial blight gene, Xa4, was mapped between 56M22F and 26D24R in chromosome 11. The genetic distance between them was 1.1 cM and the physical distance was 90kb. A resistance gene homologue, RS13, was co-segregative with it.2. Four clones were identified from BAC library of DR64. It was found that one clone named 14E19 coved with the other 3 clones. Later, twelve clones were identified from another BAC library. These clones spanned 420kb's physical distance. There were some Xa4candidates in 106P13 and 14E19.3. 14E19 was sequenced by shotgun and a finish sequence map was obtained. Four highly homologous NBS-LRR sequence were found. We named them as Xa4A to Xa4D. Some other genes and repeat sequences are found too. These sequences were the characters of the genome.4. We presumed that Xa4B had the full function of Xa4 because it had a longer LRR than the other copies'. Transformation vector contain Xa4B was constructed and transferred Xa4B to Taipei309, a rice japonic cultivar, by Agrobacterum tumefaciens. The analysis about mRNA showed that the Xa4B might be Xa4 gene.6. Twelve homologous with Xa4B's sequences were found in 106P13, a clone coming from IRBB56's BAG library. The partial sequence of 106P13 was sequenced.7. A new repeat sequence that contained a 90bp repeat unit was found in 106P13. And the repeat sequence was found comprehensively in rice genome, not in the other species'.