IDENTIFICATION OF A MAJOR QUANTITATIVE TRAIT LOCUS CONFERRING RICE BLAST RESISTANCE USING RECOMBINANT INBRED LINES

Blast disease caused by Pyricularia oryzae is one of the limiting factors for rice production world wide. The use of resistant varieties for managing blast disease is considered as the most eco-friendly approaches. However, their resistances may be broken down within a few years due to the appearance of new virulent blast races in the field. The objective of the present study was to identify the quantitative trait locus (QTL) conferring resistance to blast disease using 126 recombinant inbred (RI) lines originated from a crossing of a durably resistant upland rice genotype (Laka) and a highly susceptible rice accession cultivar (Kencana Bali). The RI population was developed through a single seed descent method from 1997 to 2004. Resistance of the RI lines was evaluated for blast in an endemic area of Sukabumi, West Java, in 2005. Disease intensity of the blast was examined following the standard evaluation system developed by the International Rice Research Institute (IRRI). At the same year the RI lines were analyzed with 134 DNA markers. Results of the study showed that one major QTL was found to be associated with blast resistance, and this QTL was located near RM2136 marker on the long arm of chromosome 11. This QTL explained 87% of the phenotypic variation with 37% additive effect. The map position of this QTL differed from that of a partial resistant gene, Pi34, identified previously on chromosome 11 in the Japanese durably resistant variety, Chubu 32. The QTL, however, was almost at the same position as that of the multiple allele-resistant gene, Pik. Therefore, an allelic test should be conducted to clarify the allelic relationship between QTL identified in this study and the Pik. The RI lines are the permanent segregating population that could be very useful for analysing phenotypic variations of important agronomic traits possibly owned by the RI lines. The major QTL identified in this study could be used as a genetic resource in improvement of rice varieties for blast resistance in Indonesi

Mapping of QTLs Controlling Carbon Isotope Discrimination in the Photosynthetic System using Recombinant Inbred Lines Derived from a Cross between Two Different Rice(Oryza sativa L.) Cultivars

Carbon isotope discrimination (Ċ) occuring in the process of photosynthesis, shows variation among rice (Oryza sativa L.) cultivars. Elucidation of specific traits associated with the extent of this discrimination under irrigated conditions may be useful to improve photosynthetic ability in rice plants. We measured leaf photosynthesis and Ċ in Milyang 23 and Akihikari, and conducted quantitative trait loci (QTL) analysis on Ċ at heading stage using a population of 126 recombinant inbred lines (RILs), derived from a cross between the two cultivars. While the two parental cultivars showed a similar Ċ, the RILs showed a wide variation in Ċ including transgressive segregation. Seven QTLs were detected for Ċ; four on chromosomes 2 (two regions), 7, and 11 were those for Ċ that is increased by the Milyang 23 allele, whereas the other three on chromosomes 1, 2, and 6 were those for Ċ that is increased by the Akihikari allele. These results suggest that 13C in Milyang 23 may be discriminated through a photosynthetic process different from that in Akihikari. Milyang 23 showed a higher stomatal conductance and a higher ratio of intercellular to ambient CO2 concentration (Ci/Ca), while Akihikari showed a higher carboxylation efficiency but a lower Ci/Ca. According to the theory that a higher Ci/Ca leads to a higher Ċ, the QTLs for Ċ that is increased by the Milyang 23 allele might be related to a higher stomatal conductance. However, the theory provided no persuasive factors to explain the QTLs for Ċ that is increased by the Akihikari allele. Plausible factors associated with these QTLs are discussed