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.

Rice blast disease, caused by the fungalpathogen Pyricularia grisea Sacc., is one of the mostdevastating crop diseases worldwide. Previous studieshave shown that the dominant blast resistance genePi-2(t) confers resistance to a broad spectrum ofpathogenic strains. Using a population of 292 recombinantinbred lines combined with bioinformatic analysis,we mapped Pi-2(t) between the SSR (simple-sequencerepeat) marker SSR140 and the RFLP (restrictionfragment length polymorphism) marker JSH12, 0.9 cMfrom both SSR140 and JSH12. A physical map consistingof six overlapping BAC (bacterial artificialchromosome) clones was anchored to the region containingthe Pi-2(t) locus. By analyzing recombinationevents in this region, the Pi-2(t) locus was localized to aDNA fragment of 118 kb in length. The detailed geneticand physical maps of the Pi-2(t) locus will facilitateboth molecular isolation of the gene and marker-assistedtransfer of the gene in breeding programs.

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.

Bacterial blight and fungal blast diseases ofrice, caused by Xanthomonas oryzae pv. oryzae andPyricularia grisea Sacc., respectively, are two of the mostdevastating diseases in rice worldwide. To study thedefense responses to infection with each of thesepathogens, expression profiling of 12 defense-responsivegenes was performed using near-isogenic rice lines thatare resistant or susceptible to bacterial blight and fungalblast, respectively, and rice cultivars that are resistant orsusceptible to both pathogens. All 12 genes showedconstitutive expression, but expression levels increasedin response to infection. Based on their expression patternsin 12 host-pathogen combinations, these genescould be classified into three types, pathogen non-specific(6), pathogen specific but race non-specific (4) andrace specific (2). Most of the 12 genes were only responsiveduring incompatible interactions. These resultssuggest that bacterial blight and fungal blast resistancesshare common pathway(s), but are also regulated bydifferent defense pathways in rice. Activation of thecorresponding R gene is the key step that initiates theaction of these genes in defense responses. The chromosomallocations and pathogen specificities of seven ofthe 12 genes were consistent with those of previouslyidentified quantitative trait loci for rice disease resistance,which indicates that some of the 12 genes studiedmay have a phenotypic impact on disease resistance inrice.

Bacterial blight caused by Xanthomonas oryzaepv oryzae is a devastating disease in rice worldwide.The resistance gene Xa4 has been widely used in breedingprograms and played an important role in protectingrice from this disease. Using 642 highly susceptible individualsand a random sample of 255 individuals from anF2 population developed from a cross between IRBB4and IR24, the Xa4 gene was genetically mapped to aregion less than 1 cM. A contig map was constructed forthe Xa4 region consisting of six non-redundant bacterialartificial chromosome (BAC) clones and spanning approximately500 kb in length. Analysis of recombinationevents in the Xa4 region located the gene locus to oneBAC, 3H8. Assay of the recombinants using the subclonesof 3H8 in combination with sequence analysisfurther narrowed the Xa4 locus down to a 47-kb fragment.