19. The dosage effect of bacterial blight resistance genes Xa1 and Xa3 in rice

H. KAKU

National Institute of Agrobiological Resources, Kannondai 2-1-2, Tsukuba, 305 Japan

The resistance in rice to bacterial leaf blight caused by Xanthomonas oryzae pv. oryzae has been extensively studied mainly in Japan and at IRRI. As a result, more than 21 resistance genes have been identified. Resistance to Japanese race I is controlled by two major resistance genes in certain varieties. Ogawa et al. (1978) reported that the resistance of the differential variety Java 14 is governed by the two dominant genes Xa1 and Xa3. Kaku and Kimura (1977,1987, 1992) have shown that resistance reaction in the host-parasite interaction varied from symptomless to small yellow lesion or browning depending on the resistance genes. At adult plant stage, Xa1 controls the symptomless type of resistance reaction, while Xa3 imports browning type of resistance reaction. In this study, resistance expression controlled by the two genes was compared to that controlled by either Xa1 or Xa3. Bacterial pathogens have an advantage over fungi and viruses in the study of resistance of plants, since the bacterial population during the host-parasite interaction can be measured by agar-plate count technique. Therefore, the inoculated tissues were collected, and the number of the bacterial cells were determined using the technique to compare the resistance expression.

Fully opened leaves of the adult plants of cv. Java 14 were inoculated with the bacterial strains of races I, II, III and IV by a single needle-pricking method. No symptoms or minute brown spots appeared around the point of inoculation after the leaves were inoculated with the strains of race I, while browning reaction developed after inoculation with races II and III strains (Fig. 1). The compatible strains of race IV induced typical symptom of yellowing.

For the population study, leaves were inoculated by a single needle-pricking method, and the number of the bacterial cells in the inoculated tissues was determined using agar-plate count technique described above. In this experiment, bacterial suspension was adjusted to the concentration of 5 x 108 colony-forming unit (cfu) per ml spectrophotometrically. Following inoculation, the population of the race I strains in the inoculated tissues was markedly lower than that of race II and race III strains. At 12 days after inoculation, the population level of the race I strains was between 103-104 cfu per inoculated tissue, while that of the race II or race III strains was around 106 cfu (Fig. 2). The population level of race I strain was as high as 106 cfu per inoculated tissue, when the population dosage was measured at 12 days after inoculation in Kogyoku and Chugoku 45 where the resistance is controlled by Xa1 and Xa3, respectively. The compatible race IV strains multiplied to the population level of between 108-109 cfu per inoculated tissue.

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The resistance expression was compared histologically among the Java 14 and Xoo strain combinations. The translocation of the race I strains was very limited to the area adjacent to the inoculation point, while the strains of race II and race III moved to some extent from the point of inoculation. In the latter, however, the bacterial colonies were localized to the brown necrotic zone. The strains of race IV multiplied and traslocated extensively from the point of inoculation, and browning in host tissues was not observed except for at late stage of infection.

In addition, the resistance expression in Java 14 to race I was very stable even at the seedling stage. Atypical symptom of browning appeared after inoculation with race I strains, while typical symptom of yellowing with slight browning developed after inoculation with race II and race III strains. Ezuka et al. (1974) and Ezuka and Horino (1976) showed that the resistance of Wase Aikoku group varieties is unstable at seedling stage. Later, the varieties were grouped into Wase Aikoku group and Java group on the basis of their reaction to race V (Yamamoto et al. 1977). The representative cultivars of the Java 14 group have the resistance genes Xa1 and Xa3. Therefore, the stable resistance in Java 14 appears to be conditioned by the additive effect of the two genes. The cultivars belonging to Java group e.g. Jamica, Amareriyo, Himekei 16 and Zenith G713 showed reaction type similar to that of Java 14.

Thus, dosage effect was clearly observed in rice cultivars with resistance genes Xa1 and Xa3 against race I strains. Based on the basis of population study, it was further concluded that the two resistance genes have additive action against race I strains.

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References

Ezuka, A., Y. Watanabe and O. Horino, 1974. Difference in resistance expression to Xanthomonas oryzae between seedlings and adults of Wase Aikoku group rice varieties (1). Bull. Tokai-Kinki Natl. Agr. Exp. Sta. 27:20-25.

Ezuka, A. and O. Horino, 1976. Difference in resistance expression to Xanthomonas between seedlings and adults of Wase Aikoku group rice varieties (2). Ibid. 29: 76-79.

Kaku, H. and M. Hori, 1977. Browning reaction in rice plant tissues induced by Xanthomonas oryzae. Ann. Phytopath. Soc. Japan 43: 487-490.

Kaku, H. and T. Kimura, 1987. Difference in resistance expression of rice to Xanthomonas campestris pv. oryzae as controlled by resistance gene (1). Resistance expression controlled by resistance gene Xa-1. Ann. Phytopath. Soc. Japan 53: 14-20.

Kaku, H., 1992. Infection types in rice-Xanthomonas campestris pv. oryzae interaction. JARQ 27: 81-87. 

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.

Yamamoto, T., H.R. Hifni, M. Machmud, T. Nishizawa and D.M. Tantera, 1977. Variation in pathogenicity of Xanthomonas oryzae (Uyeda et Ishiyama) Dowson and resistance of rice varieties to the pathogen. Contr. Centr. Res. Inst. Agric. Bogor (Indonesia) 28: 1-22.