31. The relationships between Xa-3, Xa-4, and Xa-4b for resistances to rice bacterial blight

T. Ogawa, R.E. Tabien, G.A. Busto, G.S. Khush and T.W. Mew

International Rice Research Institute, PO Box 933, Manila, Philippines


A dominant gene for resistance to bacterial blight (caused by Xanthomonas campestris pv. oryzae) was identified in IR20, IR22, and several other varieties and was designated as Xa-4 (Librojo et al. 1976; Olufowate et al. 1977; Petpisit et al. 1977). Librojo et al. (1976) also analyzed Semora Mangga which showed resistance at adult plant stages but was susceptible at seedling and maximum tillering stages. Its crosses with susceptible variety TN1 were inoculated at flowering stage. The F\1\ progenies were resistant and the F\2\ population segregated into 3 resistant: 1 susceptible. It was concluded that Semora Mangga had one dominant gene for resistance. In order to determine the allelic relationships of Semora Mangga gene with Xa-4, this variety was crossed with IR22. The F\2\ population was inoculated at flowering stage and all the plants were resistant. It was also concluded that Semora Mangga has a different allele of Xa-4 which gives resistance at adult stage only. This allale was designated Xa-4b as compared to Xa-4a which conveys resistance at seedling stage.

All these studies were conducted with Philippine race 1 of bacterial blight. However, in recent years, we have evaluted many resistant varieties will all the four Philippine races. In these tests, we found that the reaction of Semora Mangga was similar to those of Java 14, Chugoku 45, Wase Aikoku 3, Zenith, and Sateng. These varieties have Xa-3 and are resistant to four races at booting and flowering stages. Varieties with Xa-4, such as IR20 are resistant to race 1, moderately resistant to race 4 but are susceptible to races 2 and 3 (Table 1). From the reaction patterns to four races, it can be concluded that Semora Mangga has Xa-3 for resistance. An F\2\ population from the cross, Java 14xSemora Mangga consisting of 331 plant was inoculated with four races. All the plants were resistant. These results also confirm that Semora Mangga has Xa-3 for resistance.

The F\1\ progenies of the cross IR20xJava 14 were resistant to four races. In the F\2\ population of 344 plants, 254 plants were resistant to four races and 90 were resistant to race 1, moderately resistant to race 4, but susceptible to races 2 and 3. However, no susceptible plants appeared in this population. These data show that Xa-3 and Xa-4 are very closely linked. Thus, if a variety with Xa-3 is crossed with Xa-4 tester, such as IR20 or IR22, and the F\2\ population is inoculated with only race 1 at booting stage, no susceptible plants are observed in most cases. Thus, when Librojo et al. (1976) did not observe segregation for susceptibility in the IR22xSemora Mangga cross, they concluded that Semora Mangga had Xa-4b for resistance. Similar results were also obtained by Sidhu et al. (1978; 1979) who carried out genetic analysis of several varieties using race 1 only. Some of the varieties which are resistant at booting stage only were designated as having Xa-4b. However, later on, we inoculated these varieties with four races. On the basis of reaction pattern, most of them were found to have Xa-3 for resistance. The distinction between varieties having Xa-4 and those having Xa-3 can be easily made on the basis of reaction patterns to four Philippine races of bacterial blight.

The question whether Xa-3 and Xa-4 are at the same locus or are tightly linked loci remains to be resolved. In some F\2\ populations between a variety having Xa-3 and another having Xa-4, we have obtained ocassional susceptible plants. However, we plan to grow a large F\2\ population from one such cross to resolve this problem.


Table 1. The reaction of representative rice varieties to four Philippine races of bacterial blight at booting stage

=============================================================================
Variety          Race 1         Race 2          Race 3           Race 4
            =============== =============== =============== =================
            Lesion Reaction Lesion Reaction Lesion Reaction Lesion   Reaction
            Length          Length          Length          Length
             (1)      (2)     (1)    (2)     (1)     (2)     (1)       (2)
=============================================================================
IR24 
(susceptible
   check)     24.7    S      18.5    S       20.6     S       18.4       S
IR20           1.8    R      13.8    S       14.8     S        7.4       MR
Semora Mangga  1.9    R       0.7    R        0.7     R        0.8       R
Wase Aikoku 3  1.8    R       0.7    R        0.6     R        0.6       R
Chugoku 45     2.0    R       1.5    R        1.1     R        0.7       R
Java 14        2.1    R       1.2    R        1.0     R        0.8       R
Zenith         3.4    R       1.2    R        0.5     R        0.5       R
Sateng         6.9    MR      8.7    R        7.8     R        3.4       MR
=============================================================================
(1) Average lesion length in cm. in 15 inocculated leaves (3 leaves from 5 plants) at 14 days after inocculation.

(2) R=resistant; MR=moderately resistant; and S=susceptible.



References

Librojo, V., M.E. Kauffman and G.S. Khush, 1976. Genetic analysis of bacterial blight resistance in four varieites of rice. SABRAO J. 8: 105-110.

Olufowate, J.O., G.S. Khush and H.E. Kauffman, 1977. Inheritance of bacterial blight resistance in Rice. Phytopathology 67: 772-775.

Petpisit, V., G.S. Khush and H.E. Kauffman, 1977. Inheritance to bacterial blight in rice. Crop Sci. 17: 551-554.

Sidhu, G.S., G.S. Khush and T.W. Mew, 1978. Genetic analysis of bacterial blight resistance in seventy-four cultivars of rice, Oryza sativa L. Theo. Appl. Genet. 53: 105-111.

Sidhu, G.S., G.S. Khush and T.W. Mew, 1979. Genetic analysis of resistance to bacterial blight in seventy cultivars of rice, Oryza sativa L., from Indonesia. Crop. Improv. 6: 19-25.