34. Location of a new gene for resistance to bacterial blight found in Yunnan local variety Haomei

Agronomy Department, Central China Agricultural University, Wuchang, Hubei 430070, China

The Yunnan variety Haomei (Oryza sativa L.) is resistant to 9 strains of bacterial blight (Xanthomonas campestris pv. oryzae), e.g., Jiangling 691, Os75, 76-27, Ks-6-6 (from China), Pxo61, Pxo86, Pxo79, Pxo71 and Pxo211 (from Philippines) but susceptible to the strain Pxo99 from Philippines. The resistance of Haomei to Chinese strain.Jiangling 691 was found to be governed by a new dominant gene, identified and designated tentatively as Xa-i (Lo, L. J., pers. comm.). In order to determine the chromosomal location of this gene, we crossed Haomei with 26 gene marker stocks, e.g., dp, lg, d-10 eg, lax, pn, g, gh-1, nl-2, d-1, nl-1, dp-2, Dn-1, la, sp, z-1, gh-2, tri, bc-1, dl, ch-1, ch-2, rk-2 fl, pgl, and Bh (cf. RGN 1, p. 4-15), kindly supplied by Dr. G. S. Khush of IRRI and Dr. N. Iwata of Kyushu University.

The parents, F1 hybrids and F2 populations were inoculated using bacterial blight strain, Jiangling 691 from Hubei Province, China. The linkage between Xa-i and nl-1 (neck leaf) was detected. However, Xa-i segregated independently of other marker genes used.

In the F2 population of 259 plants from the cross between M27 (carrying marker gene nl-1 and susceptible to Jiangling 691) and Haomei, the mean lesion length in plants with nl-1 phenotype was 6.67 cm while that of the nl-1+ plants was 8.73 cm. The Mann-Whitney U test for the difference in mean lesion length gave t/s=2.802 which was significant at the 1% level (cf. Sokal and Rohlf 1981, p. 433-435).

The F2 plants were classified as 158 resistant-normal, 41 susceptible-normal, 36 resistant-neck leaf, and 24 susceptible-neck leaf. Both resistant: susceptible and normal: neck leaf ratios showed a good fitness to 3: 1. The chi-square value for random combination of these phenotypes was 9.21, being significant at the 5% level. This suggests that Xa-i and nl-1+ are linked in coupling phase and are located on Nishimura's chromosome 2 (linkage group VI+IX).

The recombination value between Xa-i and nl-1 was found to be 37.3 + -3.9% by maximum likelihood estimation. However, gene Xa-i segregated indepently of the other three marker genes gh-1, nl-2 and d-1 which are located on the same chromosome. Probably, these three markers are distant from Xa-i.

Haomei is a Yunnan local variety with glabrous leaf and hull and glutinous endosperm. It has large panicles with a mean of 239 grains, a large grain size of 35.2 g per 1000, and medium tillering. Haomei belongs to Japonica, but its F1 fertilities with Indicas are high. It is a useful parent in breeding.

Sato, S., K. Muraoka and Y. Sano, 1982. Reconstruction of a linkage group corresponding to Nishimura's second chromosome in rice, Oryza sativa L. Jpn. J. Breed. 32(3): 232-238.

Sokal, R. R. and J. F. Rohlf, 1981. Biometry (2nd ed.). Freeman, San Francisco, 859 pp.