1)International Rice Research Institute, PO Box 933, Manila Philippines
2) Tropical Agriculture Research Center, Tsukuba, Ibaraki, 305 Japan
Genetic analysis of Sateng was first carried out by Singh et al. (1983) at
International Rice Research Institute (IRRI) with Philippine race 1 of
bacterial blight. The segregating populations were inoculated at maximum
tillering stage. It was concluded that Sateng has one recessive gene for
resistance which was designated xa-9.
Our recent investigations showed that the reaction pattern of Sateng to Japanese and Philippine races was very similar to that of Chugoku 45, Java 14 and Zenith. The latter three varieties have Xa-3 for resistance. We, therefore, reinvestigated the genetics of resistance in Sateng and used Philippine as well as Japanese races of bacterial blight for genetic studies. Sateng was crossed with Toyonishiki and IR 24 which are susceptible to bacterial blight. The hybrid progenies were inoculated at late booting or early flowering stages. The F\1\ plants were resistant and F\2\ populations when inoculated with a Japanese race segregated in a ratio of 3 resistant to 1 susceptible (Tabe 1). When the F\2\ populations were inoculated with four Philippine races at IRRI, segregation ratio was 3 resistant (reaction pattern RRRR to 4 races): 1 susceptible (reaction pattern SSSS to four races). These data are presented in Table 1 and Fig. 1.
Table 1. Data on the reactiona to bacterial blight races of F\1\ and F\2\
populations at booting to flowering stages
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Crosses Reaction of F\1\ Reaction of F\2\ to X2 P
to race IIIA race IIIA 3:1
=====================
R(NO.) S(NO.)
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(at TARC)
1. Toyonishiki/Sateng R 299 98 0.021 0.8-0.9
2. Chugoku 45/Sateng R 356 0
3. Sateng/Zenith R 302 0
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Reaction of F\1\ Reaction of F\2\
(at IRRI) to races 1,2,3,4 to races 1,2,3,4
================ ================
RRRR(No.) SSSS(No.)
4. IR24/Sateng RRRR 139 58 2.073 0.1-0.2
5. Toyonishiki/Sateng RRRR 172 51 0.539 0.3-0.5
6. Chugoku 45/Sateng RRRR 253 0
7. Java 14/Sateng RRRR 339 0
8. Sateng/Zenith RRRR 355 0
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aR = resistant , S = susceptibleCrosses 1-3 were inoculated with race IIIA of Japan.
Crosses 4-6 were inoculated with races 1,2,3, and 4 of Phlippines.
Fig. 1. Frequency distribution of lesion length (18 days after inoculation) in
the F\2\ population between Toyonishiki (P\2\) and Sateng (P\1\).
The F\2\ populations from the crosses Chugoku 45XSateng and Java 14XSateng did
not segregate for susceptibility when inoculated with Japanese races at TARC
and Philippine races at IRRI (Table 1). Forty F\3\ lines from the crosses of
Sateng with Java 14 and Zenith were inoculated and were homozygous resistant.
These results show that like Java 14 and Chugoku 45, Sateng also has Xa-3 for
resistance. Gene symbold (xa-9) is, therefore, redundant.
The earlier identification as Sateng having a recessive gene for resistance was in error which was caused because the F\1\ and F\2\ populations were inoculated at maximum tillering stage rather than booting or flowering stages. When plants heterozygous for Xa-3 are inoculated at maximum tillering stage, they show susceptible reaction. Since the F\1\ and F\2\ populations of TNIxSateng studied by Singh et al. (1983) were inoculated at maximum tillering stage, the F\1\ showed susceptible reaction and F\2\ gave a segregation ratio of 1 resistant to 3 susceptible (heterozygous plants were classified as susceptible). It was, therefore, concluded that Sateng has a recessive gene which was designated as xa-9. Thus, the error was caused because the hybrid populations were inoculated at a wrong stage of plant growth. This type of dominance reversal because of the growth stage at which hybrid populations are inoculated was discussed by Dihu and Khush (1978).
References
Sidhu, G.S. and G.S. Khush, 1978. Dominance reversal of a bacterial blight resistance gene in some rice cultivars. Phytopathology 68: 461-463.
Singh, R.J., G.S. Khush and T.W. Mew, 1983. A new gene for resistance to bacterial blight in rice. Crop Sci. 23: 558-560.