Bacterial blight of rice caused by Xanthomonas oryzae pv. oryzae is a destructive disease in many rice-growing countries in Asia. As the use of varietal resistance is the only promising control method, the genetics of resistance has been extensively studied mainly in Japan and at IRRI. As a result, 23 resistance genes have been identified to date. Of these, Xa1 and Xa4 are representative resistance genes in Japan and at IRRI, respectively, and they have been widely used in breeding for resistance to the disease. The rice cultivar IR20 has two resistance genes Xa1 and Xa4. IR20 is highly resistant to Japanese races IA, IB and V, and moderately resistant to Japanese races II, III and IV (Ogawa and Khush 1989). Ogawa and Khush (1989) reported that the resistance of IR20 is controlled by a single dominant gene Xa4, which confers moderate resistance to all known Japanese races. In addition, their allele tests with Kogyoku showed that IR20 also has Xa1 and Xa12. Thus, two genes Xa1 and Xa4 confer resistance to race I in IR20. Resistance expression in IR20 to race I was compared with that in Kogyoku in which the resistance to race I is controlled by Xa1 only.

When multiple genes are involved for resistance to a given race of a pathogen, it appears difficult to analyze quantitatively the dosage effect of the genes. As was indicated in a previous paper, however, bacterial pathogens have an advantage over fungi and viruses in the study of resistance of plants, since the bacterial population during the hostparasite interaction can be measured by agar-plate count technique. Therefore, the population levels in inoculated tissues were compared among IR20, Kogyoku and IR26.

Fully-opened leaves of the adult plants of IR20, Kogyoku and IR26 were inoculated with the strains of races I and II by a single needle-pricking method, and the number of the bacterial cells was determined using the agar-plate count technique. In IR20 and Kogyoku, no symptoms appeared after inoculation with race I, while, in IR26, small lesions developed around the point of inoculation after inoculation with the same race. In symptom appearance, no differences were observed between IR20 and Kogyoku. In the degree of the bacterial multiplication, however, marked difference was detected between the two cultivars. The results of population experiments are presented in Fig. 1. The population level of bacterial strain T7174 in the inoculated leaves of IR20 at 12 days after inoculation was 3.6 x 10⁴ cfu per inoculated tissue, and was significantly lower than that in the inoculated leaves of Kogyoku. Upon inoculation with race II (T7147), small lesions appeared on IR20 and IR26, while large typical lesions developed on Kogyoku. In the inoculated leaves of IR20 and IR26, the population levels of T7147 were around 107 cfu per inoculated tissue, while the population level was more than 108 cfu per inoculated tissue in Kogyoku.

The resistance expression was also compared histologically between IR20 and Kogyoku after inoculation with bacterial strains T7174(race I) and T7147(race II). The results showed that translocation of the T7174 was very limited to the area adjacent to the inoculation point, while T7147 multiplied and extended to the surrounding areas through vascular network from the point of inoculation.

The rice cultivars IR26, IR28, IR29 and IR30 are also known to have the resistance gene Xa4. Therefore, the adult plants of the cultivars were inoculated with the representative strains of Japanese races I to V. The results showed that all the cultivars except for IR26 showed high degree of resistance to races I and V. In contrast, all the cultivars showed moderate resistance reaction to races II, III and IV. Based on the reaction to Japanese race I, all the cultivars except IR26 were assumed to carry resistance gene Xa1 in addition to Xa4.

The resistance expression at seedling stage was compared among IR20, IR26, IR28, IR29, IR30 and Kogyoku. At 4-leaf stage, resistance was very stable and no lesions appeared on IR20, IR28, IR29 and IR30, while small lesions developed on Kogyoku and moderate reaction was observed on IR26 at 12 days after inoculation with race I (T7174).

In a previous report, the dosage effect was clearly observed in rice cultivars with resistace genes Xa1 and Xa3 against race I strains (Kaku 1997). Based on the population study, it was found that the two resistance genes have similar reaction to race I strains. In IR20, however, the dosage effect was not so high as that in Java 14 with Xa1 and Xa3, since Xa4 confers moderate resistance and the degree of resistance expression is not so high as that of Xa3.

Among the bacterial blight resistance genes, Xa1-h and Xakg-h (renamed Xa12-h by Ogawa 1987) were found to confer high level of resistance to race I and race V, respectively, at seedling stage (Yamada and Horino 1981). Based on the results of this study, it appears that the high level of resistance conditioned by Xa1-h at seedling stage may be caused by the additive effect of resistance genes Xa1 and Xa4.

Thus, the additive effect of the resistance genes Xa1 and Xa4 was detected in resistance expression at seedling-stage and population experiments. It should be noted that, in this study, the degree of resistance expression was compared among cultivars with Xa1, Xa4 and those with both genes. It would be desirable to compare the resistance expression more precisely through the use of near-isogenic lines.

References

Kaku, H., 1997. The dosage effect of bacterial blight resistance genes Xa1 and Xa3 in rice. RGN 14: 64-67.

Ogawa, T., 1987. Gene symbols for resiatance to bacterial leaf blight. RGN 4: 41-43.

Ogawa, T. and G.S. Khush, 1989. Major genes for resistance to bacterial blight in rice. In: Bacterial blight of rice. IRRI, Los Banos, Philippines, p. 177-192.

Yamada, T. and O. Horino, 1981. Studies on genetics and breeding of resistance to bacterial blight in rice. V. The multiple alleles resistant to the bacterail groups I and V of Xanthomonas campestris pv. oryzae of Japan in the varieties IR28, IR29 and IR30. Jpn. J. Breed. 31: 423-431.