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.

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.

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.

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.

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.