Asian Program

Japanese

Data

Former Fellows

Dissertation Abstracts

Philippines

  Blast, caused by P. grisea Sacc., is a serious disease of rice worldwide. The use of host resistance offers the most economical and effective means of conrolling blast disease.
  Despite the importance of rice particularly IRRI-bred varieties in the tropics, little is known regarding the genetic constitution for blast resistance of these varieties. There are several factors for these limited studies and one of which was the lack of suitable differential system for efficient identification of blast resistance genes in the tropics. This problem has been recently addressed with the development of a differential system for blast resistance genes involving Philippine blast isolates. Following this new system which is based on the gene-for-gene relationship between rice and P. grisea Sacc., the presence of blast resistance genes in 42 IRRI-bred rice varieties was estimated. Using 14 Philippine isolates of P. grisea Sacc., the varieties were classified into seven variety groups (VG) based primarily on the presence of Pi20, Pita, and Pik^†. The presence of at least seven genes, Pi20, Pita, Pik^†, Pib, Pik-s, Piz-t, and Pii or Pi3, was estimated in these varieties. Genetic analysis using backcross progenies and allelism tests with DVs were performed to clarify and identify these estimated genes. Seven genes, Pi20, Pita, Pik^†, Pia, Pib, Pik-s, and Piz-t, were identified in the ten varieties representing the seven variety groups (VG). In some varieties, genes that were not estimated by reaction patterns in the previous analysis namely Pia, Pib, Pik-s, and Piz-t, were also identified by genetic analysis. Among the genes identified, Pib and Pik alleles (Pik-s or Pik^†) were considered widely distributed among these varieties. The genotypes of Pib and Pita were also determined using DNA markers. It showed that DNA markers are useful and can be a complementary method to standard genetic analysis for gene identification.
  Such a differential system also made it possible to develop NILs for use as multiline variety with elite rice line IR49830-7-1-2-2 as the recurrent parent (RP). The RP was identified to harbor five resistance genes, Pia, Pib,Pik-s, Pita, and Pi11(t) in its genetic background. This first set of NILs targeting genes Pik, Pi7(t), Pi3, Pi5(t), Pita-2, Piz, Piz-5, Pi9(t),and Pish, is suitable for the rainfed lowland regions of the tropics. These NILs were characterized in relation to resistance to blast isolates from Philippines, agricultural traits including submergence tolerance, and graphical genotypes using DNA markers. Through allelism tests with DVs, each NIL was identified to carry five to six genes for blast resistance in its background which include Pia, Pib, Pik-s, Pita,Pi11(t), and target gene. The NILs are already fixed for agronomic and morphological traits and showed similarities to RP for most of these traits. Except for a NIL of Pi5(t), the submergence tolerance of these NILs was found comparable to RP. NIL IRBL5-M/RL showed lower submergence tolerance. It appears that the chromosome segment with theSub1 locus was exchanged by Pi5(t) segment from donor variety which does not harbor Sub1. Graphical genotype data using 124 SSR markers showed that the NILs were almost similar to RP with respect to genomic structures. The introgression of the target genes in the NILs particularly Pik, Pi5(t), Pita-2, and Pish was also determined using these markers.
  These studies presented new and useful information about the genes for blast resistance of important Indica-type rice varieties bred in IRRI, and the NILs with the genetic background of elite rice line IR49830-7-1-2-2 for use as multiline variety in the rainfed lowland ecosystem. These studies demonstrated the importance of a suitable differential system for the genetic analysis of blast resistance. Because of their relevance towards durability of resistance to blast disease in the tropics, these studies were therefore undertaken.