(1) International Rice Research Institute, PO Box 933, Manila Philippines
(2) Tropical Agriculture Research Center, Tsukuba, Ibaraki, 305 Japan
The genetic analysis of Zenith was carried out at International Rice Research
Institute (IRRI) using Philippine race 1 of bacterial blight by Sidhu and Khush
(1978). The resistance was found to be governed by a single dominant gene which
was designated Xa-6. Later studies at IRRI and at Tropical Agriculture Research
Center (TARC) showed that the reaction of Zenith to Japanese as well as
Philippine races was similar to those of Chugoku 45 and Java 14 which have Xa-3
for resistance. These three varieties showed similar reactions (resistant) to
Japanese race IA, IB, II, IIIA, and IIIB and Philippine race 1,2,3, and 4.
Moreover, these varieties had brownish areas around the bacterial blight
leisons (Kaku and Kimura, 1978).
We again carried out the genetic analysis of Zenith using Japanese as well as Philippine races of bacterial blight. F\1\ and F\2\ populations of ToyonishikixZenith were inoculated with Japanese race IIIA. The F\1\ was resistant and F\2\ segregated in a ratio of 3 resistant to 1 susceptible. No segregation for susceptibility was observed in the F\2\ population of the cross Chugoku 45xZenith. Similarly, the F\1\ plants of IR 24xZenith were resistant to the four Philippine races at IRRI and F\2\ population segregated in a ratio of 3 resistant : 1 susceptible (Table 1). Moreover, the F\2\ populations from Chugoku 45xZenith and Java 14xZenith did not segregate for susceptibility.
Table 1. Data on the reactiona to bacterial blight of the F\1\ and F\2\
population of hybrids involving variety Zenith at 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. Kinmaze/Zenith R 282 109 1.726 0.1-0.2
2. Toyonishiki/Zenith R 306 92 0.800 0.3-0.5
3. Chugoku 45/Zenith R 398 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/Zenith RRRR 165 47 0.906 0.3-0.5
5. Chugoku 45/Zenith RRRR 230 0
6. Zava 14/Zenith RRRR 277 0
=============================================================================
aR = resistant , S = susceptible
A random sample of F\3\ lines derived from F\2\ plants with known reaction to
race IIIA at TARC were grown and inoculated at IRRI using Philippine race 2.
The F\3\ lines derived from resistant F\2\ plants were either homozygous resistant or were segregating and those derived from susceptible F\2\ plants were susceptible. These data show that Xa-3 and Xa-6 are allelic. Henceforth, the resistance gene of Zenith will be designated as Xa-3.
It was pointed out by Sidhu and Kush (1978) that plants heterozygous for this gene are susceptible at booting stage but resistant at flowering stage. In the present studies, we also observed that heterozygous plants had longer lesions than those of homozygous resistant plants. However, heterozygous resistant and homozygous plants could be separated if scored at 20 days after inoculation. The lesion development in heterozygous plants stops at 20 days after inoculation but in homozygous susceptible plants, the lesion continues to develop beyond 20 days. The lesion length of resistant and susceptible plant overlaps at 14 days after inoculation.
References
Kaku, H. and T. Kimura, 1978. Reaction types of rice cultivars to strains of Xatyhomas oryzae. bull. Chugoku Natl. Agric. Exp. Stn., Ser. E. 13: 17-43.
Sidhu, G.S. and G.S. Khush, 1978. Dominance reversal of a bacterial blight resistance gene in some rice cultivars. Phytopathology 68: 461-463.