| Vol. 5 |
H. Nakai1, K. Nakamura1, S. Kuwahara1 and M. Saito2
1) Faculty of Agriculture, Shizuoka University, 836 Ohya, Shizuoka, 422 Japan
2) Research Reactor Institute, Kyoto University, Kumatori-cho, Osaka, 590-04 Japan
Significant contribution of induced mutations to enrichment of gene sources of rice for resistance to bacterial blight (BLB) caused by Xanthomonas campestris pv. oryzae has been demonstrated by Nakai et al. (1974, 1975, 1977, 1985). A number of the mutants showing varying levels of resistance to BLB were obtained. One of the mutants, M41, which was induced by thermal neutron irradiation of seed of the BLB susceptible rice variety, Harebare, was selected because of its multiple resistance to four BLB races. This study was undertaken to determine the inheritance of resistance in this mutant.
The M41 mutant was crossed with the mother variety, Harebare, which belongs to the Kinmaze group of differentials for BLB resistance. This group is susceptible to all races of BLB in japan. F1 and F2 progenies and the F3 lines derived from randomly selected F2 plants were grown in experimental rice field derived from randomly selected F2 plants were grown in experimental rice field of Shizuoka University in 1987. Four flag leaves of individual plants at flowering stage were inoculated by clipping the leaf tip with scissors which had been dipped in a suspension of bacterial cells (109- 1010 cells/ml) of the isolates T7174, T7147, T7133 and H75373. These isolates represent bacterial groups (races) I,II, III and IV, respectively (Ezuka and Horino 1974; Horino 1978, 1981). One flag leaf of each plant was inoculated with one race.Three weeks after inoculation, length of the lesion developed on the inoculated flag leaf was measured, as an index of severity of infection by BLB.
Results of inoculation tests with race III are presented in Fig. 1. As seen in the figure, the distribution pattern of disease severities of the F1 plants was similar to that of the P2 (Harebare) with an average lesion length of 120 mm. Distribution of disease severity of the F2 plants was bimodal showing one mode with a small number of plants and the same rang of disease severity as P1 (M41) and the other with a large number of plants and the same or very similar range of disease severity as P2 (Harebare). Similar distributions of disease severities in F2 populations were observed when inoculated with races IV, II and I (Fig. 2). All of F2 segregation patterns for disease severity show a bimodal distribution. When the F2 plants were grouped into resistant and susceptible classes, the data agreed with a 1 resistant: 3 susceptible ratio for each race (Table 1). In geneeral, an F2 plant resistant to a race was also resistant to other three races. and the reactions to the four races were strongly intercorrelated among F2 plants.
Fig. 1. Distributions of lesion length in M41 (P1), Harebare (P2), F1 and F2 plants inoculated with isolate III (standard). M=Mean (shown by arrow pointing down); V=Variance; N=No. of plants.
Fig. 2. Distributions of lesion length in M41 X Harebare F2 plants inoculated with isolate I, II and IV.
M=Mean (shown by arrow pointing down); V=Variance; N=No. of plants
Table 1. Segregation for resistances to BLB isolate I,II, III and IV in the F2 of Harebare X M41
===============================================================================
Isolate No. of plants Chi-square P
============================== (1:3)
Resistant Susceptible Total
===============================================================================
I 120 381 501 0.29 >0.50
II 123 368 491 0.001 >0.90
III 133 394 527 0.02 >0.90
IV 121 387 508 0.38 >0.50
===============================================================================
These results indicate That a single recessive gene governs resistance to the four races of BLB. The F3 lines segregated into 29 resistant, 60 segregating and 30 susceptible, for reaction to the four races, giving a good fit to a 1:2:1 ratio (X2=0.025, P=0.70-0.90). Consequently, it was concluded that BLB resistance of the M41 mutant against the Japanese races is conditioned by a single recessive genet. No rice variety of Japonica type carring a recessive gene for BLB resistance has so far been found (Ogawa, 1987).
It should be noted that there is no Japonica variety which shows resistance to 4 Japanese races of BLB (Uchiyamada et al. 1980). Therefore this is a new gene which originated as a result of mutagenesis. It would be useful for developing BLB resistant varieties in Japan.
References
Ezuka, A. and O. Horino, 1974. Classification of rice varieties and Xanthomonas oryzae strains on the basis of their differential interactions. Bull. Tokai- kinki Nat. Agr. Exp. Sta. 27: 1-19.
Horino, O., 1978. Distributions of pathogenic strains of Xanthomonas oryzae (Uyeda et Ishiyama) Dowson in Japan in 1973 and 1975. Ann. Phytopath. Soc. Japan 44: 297-304.
Horino, O., 1981. Survey of geographical distribution of pathogenic groups of Xanthomonas campestris pv. oryzae from Japan in 1977 and 1979. Ann. Phytopath. Soc. Japan 47: 50-57.
Nakai, H. and M. Goto, 1974. An approach for breeding varieties of rice resistant to bacterial leaf blight with induced mutations. J. Agric. Sci. 84: 167-172.
Nakai, H. and M. Goto, 1975. Studies on mutation breeding of rice for bacterial leaf blight reactions found in M2 generations. SABRAO J. 7: 159-170.
Nakai, H. and M. Goto, 1977. Mutation breeding of rice for bacterial leaf blight resistance. In Induced Mutations against Plant Diseases, pp. 171-186. IAEA, Vienna.
Nakai, H., M. Kobayashi and M. Saito, 1985. Induction and selection of mutations for resistance against bacterial leaf blight in rice. Euphytica 34: 577-585.
Nakai, H., M. Saito, K. Sone, K. Nomura and S. Kuwahara, 1985. Studies on mutation breeding of rice for bacterial leaf blight resistance, 3. Efficiency of thermal neutrons for induction of resistance mutants. Jpn. J. Breed. 35 (Suppl. 1): 188-189. (in Japanese)
Ogawa, T., 1987. Gene symbols for resistance to bacterial leaf blight. RGN 4: 41-43.
Uchiyamada, H., Y. Fujita, K. Kimura, K. Takayanagi and K. Mori, 1980. Survey of characteristics of domestic and alien rice varieties, 3. In Hokuriku-Nogyo- Kenkyu-Shiryo, 7. Hokuriku Nat. Agr. Exp. Sta., Joetsu, Niigata-ken. (in Japanese)
| Vol. 5 |