International Rice Research Institute, P. O. Box 933, Manila, Philippines
In 1977, Dr. S. Devadath, Plant Pathologist, Central Rice Research Institute (CRRI) Cuttack, informed the senior author that a strain of Oryza barthii from Mali being maintained at CRRI under the number 12156-1 (Jachuck and Sampath, 1966) is resistant to all the races of bacterial blight in India. He had distributed this strain to several research institutes in India. While visiting the Rice Research Institute Mithapur of Rajendra Agricultural University (RAU), Bihar, India in 1978, the senior author obtained a clone of this resistant strain through the courtesy of Dr. R.C. Chaudhary, then Professor of Plant Breeding at RAU. This clone was multiplied at IRRI, Los Banos and examined for its taxonomic status and for resistance to races of bacterial blight in Philippines. Taxonomically the strain is akin to O. longistaminata, as it is rhizomatous and perennial. It was resistant to all six known races of bacterial blight in the Philippines (Khush et al.1989).
We crossed this strain with IR24, an improved cultivar of O. sativa. IR24 is susceptible to the six races of bacterial blight in Philippines. The F1 was resistant to the six races, thereby showing that the resistance of O. longistaminata was dominant. Since O. longistaminata has many poor agronomic traits, we made four backcrosses with this F1 using IR24 as the recurrent parent. During this program we inoculated the backcross progenies with the six races of bacterial blight and only the resistant plants were used for making successive backcrosses. The fourth backcross progenies were essentially similar to IR24 but were resistant to the six races of bacterial blight in the Philippines. An heterozygous plant of fourth backcross was selfed and the F2 population was inoculated with race 2 of bacterial blight. There were 179 resistant and 77 susceptible plants. These data fit the 3: 1 ratio and show that a single dominant gene fromO. longistaminata confers resistance to race 2 of bacterial blight. Ikeda et al. (1990) also studied the inheritance of resistance in two BC4F3 lines using Philippine races 1, 2, 4 and 6 of bacterial blight and confirmed that the same gene conveys resistance to all these races.
To-date, seventeen genes for resistance to bacterial blight have been identified (Ogawa, 1987; Ogawa et al., 1987; Taura et al., 1987; Noda and Ohuchi 1989; Ogawa et al. 1989). The new gene appears to be distinct from all the known genes. The level of resistance is so high that only a small lesion is formed at the cut end of the leaf. We have designated this gene as Xa-21(t). We are in the process of studying allelic and linkage relationships of this gene with other known genes for resistance.
Ikeda, R., G. S. Khush and R. E. Tabien, 1990. A new resistance gene to bacterial blight derived from O. longistaminata. Jap. Jour. Breed. 40 (Suppl. 1) 280-281.
Jachuck, P. J. and S. Sampath, 1966. Variation pattern in Oryza barthii Cheval. Oryza 3: 49-51.
Khush, G. S., D. J. Mackill and G. S. Sidhu, 1989. Breeding rice for resistance to bacterial blight. In Bacterial blight of rice, p. 207-217. International Rice Research Institute, P. O. Box 933, Manila, Philippines.
Ogawa, T., 1987. Gene symbols for resistance to bacterial blight. RGN 4: 41-43.
Ogawa, T., L. Lin, R. E. Tabien and G. S. Khush, 1987. A new recessive gene for resistance to bacterial blight of rice. RGN 4: 98-100.
Ogawa, T., H. Kaku and T. Yamamoto, 1989. Resistance gene of rice cultivar, Asaminori to bacterial blight of rice. Jpn. J. Breed. 39 (Suppl. 1): 196-197.
Noda, T. and A. Ohuchi, 1989. A new pathogenic race of Xanthomonas campestris pv. oryzae and inheritance of resistance of differential rice variety, Tetep to it. Ann. Phytopath. Soc. Japan 55: 201-207.
Taura, S., T. Ogawa, R. E. Tabien, G. S. Khush, A. Yoshimura and T. Omura, 1987. The specific reaction of Taichung Native 1 to Philippine races of bacterial blight and inheritance of resistance to race 5 (Pxo 112). RGN 4: 101-102.