Bacterial blight, caused by Xanthomonas oryzae pv. oryzae, is a seriousdisease of rice worldwide. A new dominant gene for bacterial blightresistance in rice, Xa25(t), was identified from Minghui 63, a restorerline for a number of rice hybrids that are widely cultivated in China. Thisgene conferred resistance to Philippine race 9 (PXO339) of X. oryzae pv. oryzae in both seedling and adult stages. It was mapped to the centromericregion of chromosome 12, 2.5cM from a disease resistance genehomologoussequence, NBS109, and 7.3cM from a restriction fragmentlength polymorphism marker, G1314. The genomic location of this geneis similar to the previously identified blast resistance genes, Pi-ta andPi-ta2.

Rice bacterial blight, caused by Xanthomonas oryzae pv, oryzae (Xoo), is one of the most serious rice diseases worldwide, A rice gene, Xa26, conferring resistance against Xoo at both seedling and adult stages was isolated by map-based cloning strategies from the rice cultivar Minghui 63, Xa26 belongs to a multi-gene family consisting of four members, It encodes a leucine-rich repeat (LRR) receptor kinase-like protein and is constitutively expressed, Sequence analysis revealed that IRBB3 and Zhachanglong lines that are resistant to a broad range of Xoo strains, also carry Xa26, However, significant difference in lesion length was observed among these lines after inoculation with a set of Xoo strains, Moreover, transgenic plants carrying Xa26 showed enhanced resistance compared with the donor line of the gene in both seedling and adult stages, These results suggest that the resistance conferred by Xa26 is influenced by the genetic background.

Degenerated oligonucleotide primerswere used to amplify, clone, and analyze sequence heterogeneity and chromosomaldistribution of 23 PCR fragments corresponding to the reverse transcriptase domain of copia-like retrotransposonsin rice. Of the 23 fragments 22 could be aligned by their deduced amino acid sequences and were dividedinto 6 groups according to the phylogenetic and Southern blot analyses. Amino acid sequence differences among the22 aligned fragments ranged from 1 to 64%. Southern blot analysis of 10 rice accessions including indica, japonicaand common wild rice, using these 23 fragments as probes, showed that copia-like retrotransposons were presentin moderate to high copy numbers in all the rice genome although the exact copy number cannot be determined.The major difference revealed by southern analysis is a differentiation between the four indica varieties as onegroup and the four japonica varieties and the two wild rice accessions as another group. Polymorphisms were alsodetected among the indica and japonica varieties by major bands and repeatable minor bands. Five hybridizationbands were mapped to chromosomes 3, 4, 8, and 9, respectively. All the five bands were inherited in a dominantMendelian fashion and were not allelic with each other, indicating that the same element did not reside on the samelocation in different rice accessions. No transcript of the copia-like reverse transcriptase was detected on northernblot. The results suggest that the sequence heterogeneity and distributional variability of retrotransposons may beone of contributory factors causing genetic diversity in rice.

We used 22 fragments corresponding to thereverse transcriptase domain of copia-like retrotransposonsas representatives to study the organization and distributionof these elements in the rice genome. The loci detected by these22 fragments were assigned to 47 locations in the molecularlinkagemap involving all 12 chromosomes. The distributionalfeatures of copia-like retrotransposons found in the ricegenome indicated that (i) the loci detected were located mainlyin one arm of each chromosome; (ii) one fragment usuallydetected several loci that were mapped to similar locations ofdifferent chromosomes; (iii) retrotransposons sharing highidentity in nucleotide sequences were usually assigned tosimilar locations of the chromosomes; and (iv) concurrencesof multiple loci, detected by different fragments, in similarlocations or stretches of different chromosomes were commonin the rice genome. We also determined that the copy numberof copia-like retrotransposons in rice genome may be as low as'100 per haploid genome. The restricted distribution, alongwith low copy number, suggested that copia-like retrotransposonsin rice were relatively inactive during evolution comparedwith those in other plants. The distributional featuresof the copia-like retrotransposons suggested the existence ofpossible lineages among the rice chromosomes, which in turnsuggested that chromosome duplication and diversificationmay be a mechanism for the origin and evolution of the ricechromosomes. The information provided by fine mapping ofthe retroelements in the genetic linkage map may also beuseful for gene tagging and molecular cloning.

Bacterial blight, caused by Xanthomonas oryzae pv. oryzae, is a seriousdisease of rice worldwide. A new dominant gene for bacterial blightresistance in rice, Xa25(t), was identified from Minghui 63, a restorerline for a number of rice hybrids that are widely cultivated in China. Thisgene conferred resistance to Philippine race 9 (PXO339) of X. oryzae pv.centromericregion of chromosome 12, 2.5cM from a disease resistance genehomologoussequence, NBS109, and 7.3cM from a restriction fragmentlength polymorphism marker, G1314. The genomic location of this geneis similar to the previously identified blast resistance genes, Pi-ta andPi-ta2.