6. Chromosomal location of Xa\4\ gene

A dominant gene for resistance to bacterial blight of rice was identified by Petpisit et al. (1977) and it was designated Xa\4\. This gene gives resistance to bacterial blight at all stages of plant growth and has been widely used in the breedling program (Khush 1981). In fact all the bacterial blight resistant IR varieties are homozygous for Xa\4\.

In order to determine the chromosomal location of Xa\4\, we crossed IR29 (Xa\4\ Xa\4\) with 11 of the 12 primary trisomics which were susceptible to bacterial blight. The F\1\ progenies (trisomic as well as disomic) of all the trisomics except triplo 7 were resistant. The disomic F\1\ plants amongst the progeny of triplo 7 x IR29 were resistant as expected. However, the trisomic F\1\ plants of this cross were moderately susceptible. We interpreted these results to be due to dosage effects of Xa\4\ gene. If Xa\4\ is located on chromosome 7, triplo 7 plants would be Xa\4\ ++ , and one dose of Xa\4\ would not be enough to convey resistance. To verify this hypothesis we tested an F\2\ population from the triplo 7 F\1\ plant. Out of 575 F\2\ plants, 183 were trisomic and 392 were disomic. Amongst the trisomic fraction 50 plants were resistant and 133 were susceptible or moderately susceptible. This agrees with 2:7 ratio (X²:O.141) expected on the basis of dosage effects of a dominant gene (Khush et al. 1984). In the disomic fraction of this cross 301 plants were resistant and 91 were susceptible. These data do not agree with the 5:4 expected ratio. The deviation may be the result of misclassification of some plants.

The F\2\ or backcross populations from the ten other trisomic heterozygotes segregated in a normal 3:1 or 1:1 ratios. These results indicate that Xa\4\ is located on chromosome 7.

Khush, G. S., 1981. Breeding rice for multiple disease and insect resistance. P. 220-237. In Rice Improvement in China and other Asian countries. International Rice Research Institute, Los Banos, Philippines.

-----, R. J. Singh, S. C. Sur, and A. L. Librojo, 1984. Primary trisomics of rice: origin, morphology, cytology, and use in linkage mapping. Genetics 107:141-163.

Petpisit, V., G. S. Khush, and H. E. Kauffman, 1977. Inheritance of resistance to bacterial blight in rice. Crop Sci. 17: 551-554.