Focus on Bacterial Blight of Rice Bacterial blight of rice, caused by Xanthomonas or-pw. aryzap (37), was first reported in Japan over a century

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During the 1995 wet season, harvested rice seed was collected from farmers' fields at different locations in Iloilo, Philippines. Bacterial isolations from crushed seed yielded 428 isolates. The isolates were characterized by BOX-polymerase chain reaction fingerprinting of total genomic DNA and represented 151 fingerprint types (FPT). Most FPTs were found on a single occasion, although matching fingerprints for isolates from different samples also were found. Identifications were made by cellular fatty acid methyl ester analysis and additional use of Biolog GN/GP MicroPlates and API 20E/50CHE systems. The predominant bacteria were Enterobacteriaceae (25%), Bacillus spp. (22%), and Pseu-domonas spp. (14%). Other bacteria regularly present were identified as Xanthomonas spp., Cellulomonas flavigena, and Clavibacter michiganense. Of the total number of isolated bacteria, 4% exhibited in vitro antifungal activity against Rhizoctonia solani or Pyricularia grisea. Two percent of isolates were pathogens identified as Burkholderia glumae and Burkholderia gladioli. Five percent of isolates induced sheath necrosis on only 50 to 90% of inoculated plants and were related to Bacillus pumilus, Paenibacillus spp., Pseudomonas spp., and Pantoea spp.

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Four genes of rice, Oryza sativa L., conditioning resistance to the bacterial blight pathogen Xanthomonas oryzae pv. oryzae (X. o. pv. oryzae), were tagged by restriction fragment length polymorphism (RFLP) and random amplified polymorphic DNA (RAPD) markers. No recombinants were observed between xa-5 and RFLP marker loci RZ390, RG556 or RG207 on chromosome 5. Xa-3 and Xa-4 were linked to RFLP locus XNpbl81 at the top of chromosome 11, at distances of 2.3 cM and 1.7 cM, respectively. The nearest marker to Xa-lO, also located on chromosome 11, was the RAPD locus 0072o00 at a distance of 5.3 cM. From this study, the conventional map [19,28] and two RFLP linkage maps of chromosome 11 [14,26] were partially integrated. Using the RFLP and RAPD markers linked to the resistance genes, we selected rice lines homozygous for pairs of resistance genes, Xa-4 + xa-5 and Xa-4 + Xa-lO. Lines carrying Xa-4 + xa-5 and Xa-4 + Xa-lO were evaluated for reaction to eight strains of the bacterial blight pathogen, representing eight pathotypes and three genetic lineages. As expected, the lines carrying pairs &genes were resistant to more of the isolates than their single-gene parental lines. Lines carrying Xa-4 + xa-5 were more resistant to isolates of race 4 than were either of the parental lines ('quantitative complementation'). No such effects were seen for Xa-4 + Xa-lO. Thus, combinations of resistance genes provide broader spectra of resistance through both ordinary gene action expected and quantitative complementation.