29. Identification of a new gene for resistance to bacterial blight in a somaclonal mutant HX-3 (indica)
  D.Y. GAO1, Z.G. XU2, Z.Y. CHEN3, L.H. SUN1, Q.M. SUN2, F. LU3, B.S. HU2, Y.F. LIU3 and L.H. TANG4

1)Institute of Agricultural Genetics and Physiology, Jiangsu Academy of Agricultural Sciences, Nanjing 210014, China
2)Department of Plant Protection, Nanjing Agricultural University, Nanjing 210095, China
3)Institute of Plant Protection, JAAS, Nanjing 210014, China
4)Institute of Food Crops, JAAS, Nanjing 210014, China

Bacterial blight is one of the most serious bacterial disease of rice. In China, the damage caused by bacterial blight was prevalent especially in Central, East and South China. Shanyou 63, a hybrid rice cultivar widely planted in China (area covered 8.3 x 106 ha in 1991), is highly

susceptible to bacterial blight and often suffered serious losses when infected by this disease. This condition prompted us to establish a technical system for screening rice somaclonal mutants with resistance to bacterial blight in vitro. Using the mature embryo of a susceptible variety Minghui 63 and a restorer line of Shanyou 63 as an explant, we obtained a resistance somaclonal mutant HX-3. Utilizing HX-3 as a restorer line, we obtained a hybrid rice combination Teyoukang No. 3, which has already been released and recommended in China. We found a new gene for resistance to bacterial blight in HX-3.

From 1992 to 1999, the resistance in R1 to R9 generations of HX-3 was identified. The results showed that the resistance of HX-3 to bacterial blight was stable and heritable. When HX-3 was crossed with three susceptible rice varieties Minghui 63, Yuetai A and Longtepu A, the F1 progenies of these three crosses were all resistant to bacterial blight. The segregation ratio of resistant and susceptible plants in F2 of the three crosses were 510:145, 241:88 and 324:94, respectively. The backcrossing progenies segregated in a ratio of 1R:1S (Table 1). This indicated that the resistance of HX-3 to bacterial blight was controlled by a single dominant gene.

From 1999 to 2000, 22 bacterial blight strains collected from China, Philippines and Japan were used for testing the resistance spectrum of HX-3 and 13 testers containing different major dominant resistant genes. Testing results showed that HX-3 had a broad spectrum resistance, and the resistance reactions of HX-3 to 22 strains was different from those of the 13 testers (Table 2). Our results suggest that the dominant resistant gene involved in somaclonal mutant HX-3 is quite different from other dominant resistant genes so far identified.

To identify the dominant resistant gene in HX-3, allelic tests were carried out by crossing HX-3 with IRBB4, IRBB7 and IRBB21. These testers have similar reactions to HX-3. F2 population of each crossing showed resistant and susceptible segregation (Table 1), which indicated that the resistance gene in HX-3 is different from Xa4, Xa7 and Xa21.

Two new dominant resistant genes, Xa22(t) and Xa23(t), have recently been identified from a Yunnan rice variety ZCL (Lin et al. 1996) and O. rufipogon (Zhang et al. 1998), respectively. ZCL was resistant to Pxo86 and susceptible to ZJ173. WBB1(Xa-23(t)) showed resistance to all 9 Philippines races of bacterial bloght, but HX-3 showed resistance to ZJ173 and susceptibility to Pxo86 and Pxo124. This suggests that the resistance reaction of HX-3 was different from ZCl and WBB1. All these results proved that there was a new dominant resistant gene in HX-3. We have designated the new gene as Xa25.


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