5. Multiple alleles at the Xa-1 and Xa-kg loci for resistance to bacterial leaf blight

The mode of resistance and allelic relationships of genes for the resistance to bacterial leaf blight, caused by Xanthomonas campestris pv. Oryzae (Ishiyama 1922) Dye 1978, were investigated in a Japanese (Kogyoku) and three IRRI varieties (IR28, IR29 and IR30). The IRRI varieties are resistant to bacterial groups I (strain T7141) and V (H75304) at all stages of plant growth, whereas the Japanese variety shows resistance at mature stage only. Varieties resistant at all stages of plant growth are rare. No varieties are known which show resistance to bacterial groups II, III and IV at all stages of growth.

A test of the F\1\ and F\2\ plants from crosses between a Japanese susceptible variety Toyonishiki and the IRRI varieties indicated that the resistance to each of the groups I and V was controlled by a single major gene, respectively, and the reactions at the seedling and mature stages to each group were controlled by the same gene. The F\1\ and F\2\ plants from crosses between Kogyoku and the IRRI varieties were also tested for resistace at both seedling and mature stages. Kogyokyu, susceptible at seeding and resistant at mature stage, is known to have two resistance genes, Xa-1 for resistance to bacterial group I (Sakaguchi 1967) and Xa-kg for resistance to group V (Ogawa et al. 1978). The F\1\ plants of all three crosses were resistant at both seedling and magure stages, and the F\2\ populations segregated into 3 resistant and 1 susceptible types at the seedling but were wholly resistant at the mature stage. This indicates that the genes of the IRRI varieties for the resistances to groups I and V were allelic to, and dominant over Xa-1 and Xa-kg, respectively. They were then symbolized Xa-1^h and Xa-kg^h respectively.

Xa-1 and Xa-kg are closely linked (recombination 2%, Ogawa et al. 1978), and Xa-1 is also linked with Ph for phenol staining (Sakaguchi 1967). These genes are located on chromosome 11. From the segregation data of crosses of IRRI varieties with Toyonishiki, recombination values were estimated as follows: Xa-l^h - Xa-kg^h: 2.0 +/- 0.65; Xa-1^h - Ph : 2.8 +/- 0.77; Xa-kg^h - Ph : 3.7 +/- 0.87.

Varieties having Xa-1^h and Xa-kg^h which are resistant at all stages of plant growth, cannot serve as the initial source of infection, so far as bacterial groups I and V are concerned. Their resitance is also effective against kresek phase of bacterial blight which causes the complete wilting of infected tillers at the seedling stage. The breeder can evaluate the resistance of the materials segregating for these genes in the nursery.

Ogawa, T., T. Morinaka, K. Fujii, and T. Kimura, 1978. Inheritance of resistance of rice varieties Kogyoku and Java 14 to bacterial group V of Xanthomonas oryzae. Ann. Phytopath. Soc. Japan 44: 137-141.

Sakaguchi, S., 1967. Linkage studies on the resistanc to bacterial leaf blight, Xanthomonas oryzae (Uyeda et Ishiyama) Dowson, in rice. Bull. Natl. Inst. Agric. Sci. D 16: 1-18 (in Jap. with Eng. summary).