Phenotypic and molecular characterization of bacterial blight resistance of improved modern varieties, IRRI elite lines and new plant types of rice

English. The genetic basis of rice-Xanthomonas oryzae RV oryzae (Xoo)pathosystem follows the gene-for-gene hypothesis where for each gene conditioning resistance or susceptibility in the host, there is a specific gene conditioning virulence or avirulence in the pathogen. Of 26 Xa genes for resistance to bacterial blight (BB)caused by Xoo, the Xa4 gene confers specific resistance to race PX061. Due to the diversity of germplasm use in breeding for various traits, the resistance of 12 strains of Xoo of 322 classical elite lines, 44IR and PSBRc varieties and 84 new plant types (NPTs) of rice was analyzed using phenotypic approaches aided by molecular markers that lagged five known Xa genes. The resistance of NPT donor cultivars was also determined and compared to the NPT lines. PCR markers linked to Xa4, Xa5, Xa13 and Xa21 were used to detect the presence of these genes. Pedigree analysis using the International Crop Information System database were perfor med on all lines to trace the possible source of resistance gene detected. Two hundred and eight classical elite lines were resistant to PX061 indicating the presence of Xa4. PCR primers for Xa4 confirm these results. Xa7 and Xa5 were also detected in 2 and 10 lines, respectively, as indicated by their reaction to Xoo and confirmed by PCR markers. Twenty-seven elite lines were resistant to most of the Xoo races indicating the rpesence of several Xa genes. PCR markers detected gene combinations Xa4/Xa5, Xa4/Xa21, and Xa5/Xa21 in these lines. Pedigree analyses show that these genes have been introgressed from donor, cultivars TKMG, IRIS45-339, IRBB7 and Oryza barthii (now O. longistaminata) for Xa4, Xa5, Xa7, Xa21, respectively. Xa4 was also present in 27 IR and all PSBRc varieties, PSBRc 82, showing highlevel of resistance to most Xoo races has Xa4/Xa5. Previous studies on BB resistance utilized 6 races of Xoo. Using 12 strains of Xoo, an unidentified gene conferring resistan ce to PXO339 was detected in several elite lines singly or in combinations with Xa4.IR5 previously characterized as susceptible to BB, resistance to PXO339. Twenty-one IR varieties were also resistant to PXO339. Phenotypic and molecular analyses of 84 NPT lines and 17 NPT donor varieties detected the presence of Xa4, Xa5, Xa7 or Xa5/Xa7 showed susceptible reaction to all races except PX079 and PXO339. IR 72225-20-3-2-3 and IR 72976-AC1 carry Xa4/Xa5/Xa7/Xa13 and have very short lesions (0.9-4.5 cm) against all Xoo races. Seven NPT donor varieties exhibited adult resistance to BB indicative of the Xa3 gene suggesting that Xa3 may also be present in some of the NPTs. The gene-for-gene relationship of rice-Xoo- pathosystem was demonstrated for lines with single Xa genes . For lines with two or more Xa genes, it was difficult to distinguish the effect of one gene on the host phenotype. Thus, the use of molecular markers aided in the detection of two or more Xa genes in the rice lines used in the study.

English. Conference of the Pest Management Council of the Philippines Cebu City, Philippines 6-9 May 2003