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Development of Japonica Rice “Iksan478” carrying Xa7 resistant gene to Bacterial Blight

Mun Sik Shin1, Myung Kyu Oh2, Tae Hwan Noh1, Ki Young Kim1, Bo Kyeong Kim1, Jae Kweon Ko1 and Jae Kil Lee1

1 Honam Agricultural Research Institute, Email
National Institute of Crop Science, Email


In Korea, the first bacterial blight (BB) resistant japonica variety (Seomjin) of rice (Oryzae sativa L.) was released in 1982. So far, 53 additional resistant varieties have been developed and released. The possibility of their resistance breaking down is likely because they have only one or the other resistant genes Xa1 or Xa3. As a countermeasure, research into gene diversity in BB resistance began in the early 1990s. Iksan478 was developed by backcrossing DV85 four times to the recurrent parent Suwon345. This is the first japonica resistant variety carrying Xa7gene, a broad spectrum gene for BB resistance, in the Korean japonica varieties.

Media summary

A new rice variety -“Iksan478”- with the Xa7 gene to bacterial blight, developed by the breeding team of Honam Agricultural Research Institute, will reduce the incidence of bacterial blight in rice in southern Korea.

Key Words

Rice, Japonica, Iksan478, Bacterial blight, Xa7 gene.


Bacterial blight (BB) caused by Xanthomonas oryzae pv. oryzae (Xoo) is one of the most destructive diseases of rice (Oryzae sativa L.) throughout the world. In Korea it mainly occurs in southern parts and causes deteriorated grain quality and crop yield reduction. Some chemicals have been developed to control this disease, but none of them have been fully effective in Xoo outbreaks. Therefore, the most effective approach to combat BB is the use of resistant varieties. So far, roughly 29 resistance genes have been identified (Kinoshita 1995; Khush and Angeles 1999; Lee et al. 2003; Shin et al. 1995; Zang et al. 1998). Some of these have been incorporated into modern rice varieties and used for development of near-isogenic lines (Ogawa et al. 1991; Shin et al. 1998; Shin et al. 2000). In Korea, the breeding programs for BB resistance in japonica varieties began in 1975 and 1986, resulting in Seomjin carrying Xa1 gene and Hwayeong carrying Xa3 gene, respectively. Seomjin was released in 1982 and Hwayeong was released in 1993. 53 resistant varieties have so far been developed and released all of which carry one or the other BB resistance gene Xa1 or Xa3 respectively. The possibility of BB outbreaks is likely because of the lack of more genetic diversity for resistance within the breeding population. Here we report the development and characterization of a rice variety, Iksan478, carrying the Xa7, broad spectrum BB resistance gene.


Development of Iksan478

Study of the genetic diversity of BB resistance in rice began in the early 1990s. Rice varieties DV85, used as donor parent has the Xa7 resistant gene (Sidhu et al. 1978), and Suwon345, used as recurrent parent, is susceptible to BB. The isolates of Xoo used for the experiment were HB9101(race K1), HB9102(race K2), HB9103(race K3), and HB01009(race K3a). Suwon345/DV85 was made, and F1 plants were crossed with susceptible parent Suwon345. The selection of the plants carrying the resistant gene was started by inoculation using race K1. From the second backcross, at least six plants were crossed with recurrent parent. This process was repeated until the BC4F2 generation.

Screening for BB resistance

In the BC4F3 generation, each line was inoculated with race K1 and the morphology was compared with that of the recurrent parentand the uniformity of plant type was examined. From BC4F4 to BC4F7, each line was inoculated with three races, K1, K2, and K3. From BC4F8 to BC4F9, each line was inoculated with four races, K1, K2, K3, and K3a. In the BC4F9 generation, one breeding line was given the name as “Iksan 478”. BB pathogens were inoculated at maximum tillering stage of the progenes in each generation from BC1F1 to BC4F9. Inoculation was conducted by the leaf clipping method. The concentration of inoculum was about 108-109 cells/ml. The inoculum was grown for 72h in nutrient broth at 30oC. A disease score of inoculated plants was measured visually 3 weeks after inoculation and plants were classified into resistant (symptomless to brown necrosis) and susceptible (typical BB symptom).


DV85 was crossed with the susceptible variety, Suwon345 in the summer 1992. F1 plants were grown in the greenhouse in winter of 1992/1993 and backcrossed with Suwon345 as the recurrent parent. From BC1F1, plants showing resistant reaction to BB and being similar to heading date of Suwon345 were selected, and used as parents in the backcrossing processes. 20 BC4F1 plants in 1995, 80 plants in BC4F2 population in 1996, and 22 lines in BC4F3 in 1997 were selected, respectively. In BC4F4, each line was examined for uniformity to BB resistance, heading date, plant height, and plant type. Five lines were selected. After preliminary and replicated yield trials in BC4F5 to BC4F9, one line was selected, “Iksan478”. This variety is the first japonica resistant variety carrying Xa7 bred in Korea. It shows resistant reactions to all races of Xoo, K1, K2, K3, and K3a found in Korea.

Table 1. Reaction of Iksan478 to Korean races of bacterial blight.



Reaction to race























R : Resistant S : Susceptible


Iksan478 is being used as a new resistant donor with Xa1 or Xa3 resistant varieties. Also, to increase genetic diversity of Xoo resistance in japonica varieties, we are developing many crossing populations using Xa4, xa5, xa8, Xa11, Xa14, and Xa21 resistant genes.


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