17

Blast resistance genes in IRRI cultivars of rice have not been studied well due to their complexity, although the blast disease is one of the most serious disease of rice. We first estimated blast resistance genes in each IRRI cultivar by inoculating more than 20 blast isolates with different pathogenicity to known resistance genes (unpublished). One of IRRI cultivars, TR24 was resistant to several blast isolates including BN209, V850196, and BN111, and genes for the isolates were estimated as Pi b, Pi k-s, and a new gene respectively, from reaction pattern of the cultivar. Genetic analysis was conducted to confirm the two known genes, and to identify the new gene.

F₃ lines of C039/IR24, and BC₁F₂ lines of C039/IR24/C039 were inoculated with the isolates. C039 was susceptible to all the three blast isolates. To map the genes, recombinant inbred lines (RILs) of F₇, Asominori/IR24 which were characterized with RFLP markers (Tsunematsu et al. 1996) were also inoculated with the three isolates. RILs were kindly provided by Dr. A. Yoshimura, Kyushu University. Asominori were susceptible to all the isolates. Another blast isolate, B9002, which was incompatible to C039, Asominori, and IR24 was also used for inoculation of F₃ lines, BC₁F₂ lines, and RILs.

The F₃ lines were segregated into 1 homozygously resistant, 2 segregating, and 1 homozygously susceptible, to each isolate, indicating resistance to each isolate was controlled by a single gene in IR24 (Table 1). Segregation in reaction to the three isolates showed that resistance genes to the three isolates were independent of each other. The BC₁F₂ lines showed the ratio of 1 segregating and 1 homozygously susceptible, also indicating one resistance gene for each blast isolate.

Based on results of RILs, the gene for resistance to BN209, one for V850196, and one

for BN111 were mapped on the end of chromosome 2, the end of chromosome 11, and chromosome 12 respectively (Fig. 1). Pi b locus was mapped on the end of chromosome 2 (Shinoda et al. 1971;Kiyosawa 1978;Miyamoto et al. 1996) and Pi k locus on the end of chromosome 11 (Goto et al. 1981), and therefore, our estimation of the genes were confirmed.

The new gene for BN111 was mapped on chromosome 12, where pi ta locus is located (Yu et al. 1991: Ise 1993; Inukai et al. 1994). Reactions of IR24 to blast isolates

The F₃ lines, the BC₁F₂ lines, and the RILs were all resistant to the blast isolate B90002 which is avirulent to Pi a. CO39 and Asominori were supposed to possess Pi a gene, and IR24 was also supposed to possess the gene.

From the results mentioned above, we concluded that IR24 possess at least the four genes for blast resistance, i.e.; Pi a, Pi b, Pi k-s, and Pi 20(t), a new gene for blast resistance on chromosome 12.

Table 1. Segregation of F₃ lines of C039/IR24 in reaction to blast isolates
Blast isolate		No. of F₃ lines		?²⁽1:2:1)	P.
Blast isolate	R-homo.¹	Segregating	S-homo.²	?²⁽1:2:1)	P.
BN209 V850196 BN111	21 19 18	30 24 44	18 13 15	1.435 2.429 1.805	0.30-0.50 0.10-0.30 0.30-0.50

Reaction to IK81-25		Reaction to BNI 11		Total
Reaction to IK81-25	R-homo¹	Segregating	S-homo²	Total
R-homo¹	0	2	22	24
Segregating	0	47	0	47
S-homo²	13	0	0	13
Total	13	47	22	84