In interaction between rice and Xanthomonas oryzae pv. oryzae, resistance reaction varies from symptomless to browning depending on the resistance genes. Ezuka et al. (1975) showed that the resistance of the rice cultivar Wase Aikoku 3, the representative of Wase Aikoku group, is controlled by a single dominant gene Xaw. The Xaw gene was later designated as Xa3 by Ogawa (1987). We have demonstrated that all the cultivars carrying resistance gene Xa3 showed browning reaction to the incompatible strains of races I, II and III of X. o. pv. oryzae (Kaku and Hori 1977; Kaku and Kimura 1978). We report here the results of genetic analysis of the relationship between the browning reaction and the resistance gene Xa3, and the reaction of the cultivars carrying Xa4b, Xa6 and xa9 which were shown to be allelic to Xa3 with special reference to browning reaction.

For the genetic analysis, rice cultivar Kuntulan was crossed with Kinmaze, and F₂ plants were tested for their resistance to the incompatible race II (strain T7147) of X. o. pv. oryzae and its relation to the browning reaction. The resistance of Kuntulan to races I, II and III was shown to be governed by Xa3, and this was the first cultivar that had been demonstrated to show the browning reaction to the incompatible races. In contrast, Kinmaze is susceptible to all Japanese races of X. o. pv. oryzae, and it does not show browning reaction against the bacterium.

The reaction of the F₂ population is summarized in Table 1 along with the reaction of parental plants. The reaction was evaluated as R, when the disease index is less than 3 using Ezuka and Horino's scale, which was used for genetic analysis of the resistance. All the inoculated plants of the cultivar Kuntulan showed browning reaction when inoculated with the strain T7147. However, browning reaction was not observed in the inoculated plants of Kinmaze. In contrast, all the population of Kinmaze showed typical symptom of yellowing upon inoculation. Of 337 F₂ plants of Kuntulan x Kinmaze, 251 showed resistance to the strain T7147. In all the resistant plants, browning reaction was observed around the point of incubation. The reaction developed approximately 4 days after inoculation, and the brown necrosis continued to increase in size. The remaining 86 plants showed susceptible reaction to the strain T7147, and none of them showed browning reaction. The typical symptoms in the susceptible plants appeared around 8 days after inoculation. F₂ population of Kuntulan x Kinmaze showed a clear-cut segregation ratio of 3R:1S. In addition, all the resistant population showed browning reaction without exception. These results lead to the conclusion that the browning reaction is the pliotropic effect of Xa3.

In addition to the genetic analysis, we observed the resistance reactions of the cultivars carrying the resistance genes Xa4b, Xa6 and xa9 which were shown to be identical to Xa3 (Ogawa et al. 1986). The reaction of the cultivar Semora Mangga with the resistance gene Xa4b to the incompatible Japanese races (races I, II and III) and Philippine races (races 1, 2, 3, 4 and 5) was browning type and was different from that of IR20 which has Xa4 and showed small yellow lesion. Similar results were obtained in the cultivars carrying resistance genes Xa-6 and xa9. The cultivars Zenith with Xa-6 and Sateng with xa-9 also showed browning type of resistance reaction to the same Japanese and Philippine races. Thus, it was also confirmed that Xa4b, Xa6 and xa9 are identical to Xa3 on the basis of browning reaction.

In a previous work, we have demonstrated that the bacterial colonies of incompatible races of X. oryzae pv. oryzae were confined to the brown necrosis region in the leaf tissues of rice cultivars with Xa3 (Kaku and Hori 1977; Kaku and Kimura 1978). Histological study also showed that the process of the browning reaction is similar to that of hypersensitive reaction in potato-Phytophthora infestans system. In addition, atypical symptom of browning reaction appears much more rapidly than the typical symptoms. These observations indicate that the browning reaction is associated with resistance expression controlled by the Xa3 gene. The reaction is significant in genetic aspect as well as in pathological aspect.

References

Ezuka, A., O. Horino, K. Toriyama, H. Shinoda and T. Morinaka, 1975. Inheritance of resistance of rice variety Wase Aikoku 3 to Xanthomonas oryzae. Bull. Tokai-Kinki Natl. Agr. Exp. Sta. 28: 124-130.

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, 1978. Reaction types of rice cultivars to strains of Xanthomonas oryzae. Bull. Chugoku Natl. Agric. Exp. Stn. E13: 17-43.

Ogawa, T., R.E. Tabien, G.A. Busto, G.S. Khush and T.W. Mew, 1986. The relationship of Xa3, Xa-4 and Xa- 4b for resistance to rice bacterial blight. RGN 3: 83-84.

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

Ogawa, T., T. Yamamoto, G.S. Khush and T.W. Mew, 1986. Inheritance of resistance to bacterial blight in Sateng - a reinvestigation. RGN 3: 80-82.