17. A new gene for blast resistance in rice cultivar, IR24

T. IMBE. S. OBA, M.J.T. YANORIA and H. TSUNEMATSU

International Rice Research Institute. P.O. 933, 1099 Manila, Philippines

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each IRRI cultivar by inoculating more than 20 blast isolates with different pathogenicity to known resistance genes (unpublished).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 CO39/IR24, and BC1F2 lines of C039/IR24/C039 were inoculated with the isolates. CO39 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 CO39, 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 were different from those of pi ta cultivars, and therefore, the gene was not identical to pi ta. So, F3 lines from a cross of IR24 and C101PKT which possesses Pi ta gene, were tested with blast isolates, BN111 and IK81-25. The latter isolate was avirulent to Pi ta. The F3 lines showed co-segregation to the two isolates except for two F3 lines (Table 2), indicating the gene to BN111 was closely linked to Pi ta locus. The gene was designated as Pi 20(t).

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.
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Table 1. Segregation of F3 lines of CO39/IR24 in reaction to blast isolates
Blast isolate
No. of F3 lines
χ2(1:2:1)
P.
R-homo.1
Segregating
S-homo.2
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

1. R-homo: homozygously resistant.
2. S-homo: homozygously susceptible.

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References

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Inukai, T., R.J. Nelson. R.S. Zeigler, S. Sarkarung, D.J. Mackill, M.J. Bonman, I. Takamure and T. Kinoshita. 1994. Allelism of blast resistance genes in near-isogenic lines of rice. Phytopathology 84: 1278-1283. 

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