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 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
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:
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: