Rice blast (Bl), caused by the fungal pathogen Pyricularia grisea (Sacc.), is one of the most serious diseases in indica as well as japonica rice worldwide. About 20 genes conferring complete resistance to several blast pathogen races and 10 QTLs associated with partial resistance have been identified (McCouch et al. 1994). Nevertheless, the japonica rice cultivars in South Korea are highly susceptible to Bl disease because of their narrow genetic diversity and lack of suitable resistance genes to confer resistance against variable pathogenicity of P.grisea. It is essential to identify new genetic sources with broad-spectrum resistance to diverse isolates of Bl pathogen. Resistance genes for Bl have usually been identified from cultivated rice germplasm. However, one exception, the resistance gene Pi9(t) from the wild rice species O. minuta (Amante-Bordeos et al. 1992), suggests that other wild species of Oryza may also be potential sources of Bl resistance genes. In this study, we report a new source of Bl resistance against Korean isolates from an alien introgression line of rice developed at the International Rice Research Institute (IRRI), Manila, Philippines.

We evaluated 10 Korean japonica rice cultivars, 11 introgression lines derived from the crosses of O. sativa with O. minuta, O. australiensis and O. longistaminata using 15 recommended Bl isolates of Korea. The seedlings at the four-leaf stage were inoculated with the isolates by injection (Shinoda et al.1971). Blast disease was evaluated one week after inoculation following the procedure of Mackill and Bonman (1992). Of the 23 rice genotypes tested, the introgression line IR65482-4-136-2-2 expressed strong and broad-spectrum resistance encompassing most of the isolates (Table 1).

A cross was made between the line IR65482-4-136-2-2 and the Korean japonica rice cultivar Sangjubyeo. The F₁ plants showed resistance to Bl isolates tested indicating the dominant nature of the gene. The 55 F₂ progenies of the cross evaluated against four recommended races of Korea showed 3:1 segregation ratio for inoculations with two KJ races (KJ201 and KJ301) and one KI race (KI409). However, the F₂ progenies inoculated with the race KJ105 showed significant chi² value for 3:1 segregation ratio which might be due to interaction of multiple resistance genes (Table 2). This result indicated that a single locus might be controlling a broad-spectrum resistance to diverse Bl isolates of Korea and could be a useful source of resistance for incorporation into Korean japonica rice cultivars.

We are making allele tests to confirm the nature of resistance gene present in the line IR65482-4-136-2-2 and also mapping the resistance gene with molecular markers.

References

Amante-Bordeos A., L.A. Sitch, R. Nelson, R.D. Dalmacio, N.P. Oliva, H. Aswidinoor and H. Leung. 1992. Transfer of bacterial blight and blast resistance from the tetraploid wild rice Oryza minuta to cultivated rice, Oryza sativa. Theor. Appl. Genet. 84: 345-354.

McCouch S.R., R.J. Nelson, J. Tohme and R.S. Zeigler. 1994. Mapping of blast resistance genes in rice. In: Zeigler RS, Leong SA, Teng PS (eds) Rice blast disease. CAB International, Wallingford, UK, pp 167-186.

Mackill D.J. and J.M. Bonman. 1992. Inheritance of blast resistance in near-isogenic lines of rice. Phytopathology 82: 746-749.

Shinoda H., Toriyama K, Yunoki T, Ezuka A and Sakrai Y. 1971. Studies on the varietal resistance of rice to blast; 6. Linkage relationship of blast resistance genes. In: The Bulletin of the Chugoku National Experiment station.
Series A (Crop Division), No. 20. (Japanese).