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.

Comparative genomic analyses have revealed extensive colinearity ingene orders in distantly related taxa in mammals and grasses, whichopened new horizons for evolutionary study. The objective of ourstudy was to assess syntenic relationships of quantitative trait loci(QTL) for disease resistance in cereals by using a model system inwhich rice and barley were used as the hosts and the blast fungusPyricularia grisea Sacc. as the pathogen. In total, 12 QTL against threeisolates were identified in rice; two had effects on all three isolates,and the other 10 had effects on only one or two of the three isolates.Twelve QTL for blast resistance were identified in barley; one hadeffect on all three isolates, and the other 11 had effects on only oneor two of the three isolates. The observed isolate specificity led to ahypothesis about the durability of quantitative resistance commonlyobserved in many plant host-pathogen systems. Four pairs of the QTLshowed corresponding map positions between rice and barley, twoof the four QTL pairs had complete conserved isolate specificity, andanother two QTL pairs had partial conserved isolate specificity. Suchcorresponding locations and conserved specificity suggested a commonorigin and conserved functionality of the genes underlying theQTL for quantitative resistance and may have utility in gene discovery,understanding the function of the genomes, and identifying theevolutionary forces that structured the organization of the grassgenomes.

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.

The inheritance of resistance for bacterialblight, caused by Xanthomonas oryzae pv. oryzae (Xoo),was studied in Minghui 63, an elite restorer line for anumber of widely used rice hybrids in China. A newdominant gene against a Chinese Xoo strain JL691 in boththe seedling and adult stages was identified in Minghui 63and designated as Xa26(t). Using a total of 477 highlysusceptible individuals from an F2 population, the Xa26(t)locus was mapped to a region of about 1.68 cM. Thislocus co-segregated with marker R1506 and was 0.21 cMfrom marker RM224 on one side and 1.47 cM frommarker Y6855RA on the other side, in rice chromosome11. A contig map, composed of five non-redundantbacterial artificial chromosome (BAC) clones and spanningapproximately 500 kb in length, was constructed.Analysis of recombination events in the Xa26(t) regionwith the highly susceptible F2 individuals anchored thegene locus to a region covered by three overlapped BACclones. Assay of the lines showing a double crossover inmarker loci flanking Xa26(t), in a population of recombinantinbred lines carrying Xa26(t), further delineatedthe gene to a 20-kb fragment. The Xa26(t) locus is tightlylinked to another bacterial blight resistance gene locus,Xa4.

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.