Xa23

Annotated Information

Introduction

Rice bacterial blight (BB), caused by Xanthomonas oryzae pv. oryzae (Xoo), is one of the most destructive bacterial diseases of rice in the world．A present，planting resistant cultivars is the most economic and effective measure for the control and prevention of BB．A new resistance gene to bacterical blight of Xa23 from O. rufipogon was identified based on the resistance spectrum evaluated, comparison of resistance type, inheritance analysis by Zhang Qi and others. Xa23 is conditioned by a new complete domiance overall resistance gene with the widest spectrum resistance, and highest efficiency of resistance transfer among all the known genes identified so far. The resistance gene is located on rice chromosome 11 (5.3cM to OSR06, 27.7cM to RM244).

Function

Amongst all the known resistance genes to BB, Xa23, a single completely dominant resistance gene effective at all growth stages, identified from wild rice species of Oryza rufipogon emerged as the most promising one. The Xa23 gene was found to be highly resistant to all the 20 strains of BB consisting of 10 Philippine races (P1eP10), 7 Chinese pathotypes (C1eC7) and 3 Japanese races (T1eT3) at the maximum tillering stage. It was indicated that Xa23 gene possessed a broad resistance spectrum among all known R genes. In 2006, Blast X analysis showed that the ORF6 is totally different from all the previous cloned plant disease resistance genes. The Xa23 gene represents a new type of plant disease resistance genes.

Mutation

Thirty-two bacterial blight resistance genes have been identified from cultivated rice and wild rice species. Six of these resistance gene , Xa1, xa5, xa13, Xa21, Xa26 and Xa27 have been cloned. It is essential to identify and clone new dominant genes with broader resistance spectrum genes for application in hybrid rice breeding. BB resistance gene Xa23, with wide spectrum resistance, complete dominance and all growth stage resistance, has been identified from O. rufipogon and it’s near-isogenic line CBB23 has been developed. The near-isogenic line (NIL) with a new gene Xa23 for resistance to bacterial blight was developed and designated as CBB23 in 1993-1998. A susceptible indica variety JG30 with improved plant type was used as a recurrent parent. The progenies derived from the cross JG30/H4 harboring the gene Xa23 were developed by back-cross until BC5, and selfing for 4 times. Evaluation of the resistance gene Xa23 with specific strain P6 and selection of plant type successively were carried out till the line showing stable resistance and agronomic characters similar to the recurrent parent. The NIL is also valuable in rice breeding.

Expression

Xa23, is a novel resistance genes to BB, and a reasonable utilization and deployment of Xa23 gene for efficient control of BB disease in hybrid rice production was recommended. It is expected that overtime the new virulent Xoo races will certainly overcome the Xa23 gene once it is widely used in breeding programs. In order to delay the loss of another broad spectrum resistance gene such as Xa23, it is highly encouraged to use different sources of resistance genes for BB in varietal development and effective deployment strategies. Chen et al. (2000) and Zhai et al. (2001) developed a BB resistant restorer line, Minghui63-Xa21 by transgenic means. Therefore, BB resistant Shanyou63 produced from the two resistant restorer lines, Minghui63-Xa23 and Minghui63-Xa21 could be rotated for hybrid rice production. Another strategy to achieve enhanced and durable resistance against plant diseases is to pyramid Xa23 with other resistant genes thereby increasing resistance level and spectrum as reported in all previous gene-pyramiding programs on BB resistance (Huang et al., 1997;Sanchez et al., 2000; Singh et al., 2001; Yoshimura et al., 1995). Alternatively, the mixture of the varieties in the same genetic background with different resistance genes could be co-cultivated (Ayliffe et al., 2008; Zhu et al., 2000). BB resistant genes Xa23 and Xa21 as reported above have been derived from wild rice and are now being considered as of great value in hybrid rice breeding programs among all reported genes (Ni o-Liu et al., 2006). Careful utilization of the broad spectrum BB resistant genes is urgently required to efficiently control BB and to delay the speed of development of new virulent Xoo strains by the way of mixed planting of the hybrid rice combinations derived from the same genetic background restorer lines with Xa23 and Xa21, respectively. Simultaneous search for new resistant sources for BB from other wild rice varieties needs to be carried out for diversifying varieties possessing different sets of BB resistant genes.

Evolution

The gene Xa23 not only had good inheritance of resistance to BB, but also showed high level imported effect.

Knowledge Extension

Prevalence of BB disease in China is jeopardizing the expansion of the popular hybrids as a result of their susceptibility. The three different restorer lines i.e. Minghui63, YR293 and Y1671 have been widely utilized in hybrid rice production in China. Incorporation of a broad spectrum resistance gene into these restorer lines will greatly improve BB resistance of hybrid rice in China. Careful analysis of the literature showed CBB23(B) with Xa23 gene to be an effective donor against all races collected from China, Philippines and Japan (Zhang, 2005). In 2011, Yongli Zhou and others incorporated a broad spectrum BB resistance Xa23 gene into the three commercially popular restorer lines using MAS-based backcrossing and stringent phenotypic selections within a short period. Minghui63-Xa23, YR293-Xa23, Y1671-Xa23 and their hybrid combinations, Shanyou63-Xa23, Fengyou293-Xa23 and Zhongyou1671-Xa23 exhibited a significant yield advantage over their original counterparts with the same level and spectrum of resistance as that of the resistant donor, CBB23(B). Their study clearly indicated that Xa23 gene is valuable for the improvement of BB resistance in hybrid rice production. Improvement of restorer lines for BB resistance through MAS-based backcross schemes using Xa23 gene with broad spectrum resistance raised certain key issues in rice breeding for increasing production.

Labs working on this gene

• Institute of Crop Sciences/ National Key Facility for Crop Gene Resources and Genetic Improvement, Chinese Academy of Agricultural Sciences, 12 South Zhong-Guan-Cun Street, Beijing 100081, PR China
• Institute of Germplasm Resources, GASS, Nanning,530007, China
• Rice Research Institute, Guangxi Academy of Agricultural Sciences, Guangxi Nanning 530007, China
• Institute of Genetics, Academy Sinica, Beijing, 100101, China
• Food Crops Institute, Hubei Academy of Agricultural Sciences, Wuhan 430064, China
• International Rice Research Institute, DAPO Box 7777, Metro Manila, Philippines

References

• Zhang Qi, Zhao Bingyu, Zhao Kaijum, et al. (2000) Identifying and mapping a new gene Xa23 for resistance to bacte-rial blight (Xanthomonas oryzae pv. oryzae) from O. rufipogon. Acta agronomica sinica 26(5): 536-542
• Yongli Zhou, Veronica NE Uzokwe, Conghe Zhang, et al. (2011) Improvement of bacterial blight resistance of hybrid rice in China using the Xa23 gene derived from wild rice (Oryza rufipogon). Crop Protection 30: 637-644q
• Wang Chunlian, Qihua Xiong, Pan Haijun, et al. (2005)EST-Markers flanking the rice bacterial blight resistance Xa23 gene and their application in marker-assisted selection. Scientia Agricultural Sinica 38(10): 1996-2001
• Qin Gang, Li Daoyuan, Li Yangrui, et al. (2008) Imported effects of resistance gene Xa23 of bacterial blight of rice. Southwest China Journal of Agricultural Sciences 21(1): 71-74
• Pan Haijun, Wang Chunlian, Zhao Kaijun, Zhang Qi, et al. (2003)Molecular mapping by PCR-based markers and marker-assisted selection of Xa23, a bacterial blight resistance gene in rice. Acta agronomica sinica 29(4): 501-507
• Zhang Qi, Wang Chunlian, Zhao Kaijun, et al. (2002)Development of near-isogenic line CBB23 with a new resistance gene to bacterial blight in rice and its application. Chinese J Rice Sci 16(3): 206-210
• Pei Qingli, Wang Chunlian, Liu Piqing, et al. (2011)Marker-assisted selection for pyramiding disease and insect resistance genes in rice. Chinese J Rice Sci 25(2): 119-129