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Resistance-like sequences have been amplified from first strand cDNA and genomic DNA of rice by PCR using oligonucleotide primers designed from sequence motifs conserved between resistance genes of tobacco and Arabidopsis thaliana . 3 PCR clones, designated Osr1, Osr2 and Osr3 which were 98% identical in nucleotide sequence level, have been found to be significantly homologous to known plant resistance genes and all contained the conserved motifs of NBS-LRR type resistance genes, such as P-loop, kinase2a, kinase3a and transmembrane domain. Southern hybridization revealed that rice resistance gene hornologues were organized as a cluster in the genome. RFLP mapping using a DH population derived from an indica/japonka cross (Zhaiyeqing 8/Jingxi 17) and an RFLP linkage map assigned two copies of Osrl and one copy of Osr3 to the distal position of chromosome 12 where a blast resistance QTL has been mapped previously. Northern blot analysis showed that Osrl gene was constitutively transcribed in rice leaves, shoots and roots. Further study concerning isolation of full-length cDNAs would be conducive to elucidating the functions of these genes.
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