31. Susceptibility of IRBB 21 carrying the resistance gene Xa21 to bacterial blight

1) Central Rice Research Institute, Cuttack 753 006, India
2) International Rice Research Institute, Los Banos, Philippines
3) Present address: International Institute of Biotechnology and Toxicology, Padappai - 601301, Kancheepuram District, Tamil Nadu, India

Bacterial blight caused by Xanthomonas oryzae pv. oryzae is one of the important constraints of rice production in both irrigated and rainfed lowland ecologies (Ou 1985, Mew 1987). In the absence of any chemicals effective against bacterial blight, the only way to protect the crop from the disease is the use of resistant varieties which is not only economical but is also an environmental-friendly method of disease control (Khush et al., 1989). Until now, 25 genes for resistance to bacterial blight have been identified (Khush and Brar 2001, Sridhar 2002). The dominant gene Xa21 located in chromosome 11 is a valuable gene for plant breeding as it confers broad spectrum resistance to most isolates of X. oryzae pv. oryzae (Ikeda et al. 1990, Khush et al. 1989, 1990). Thus, Xa21 has been one of the most preferred genes for improving resistance in rice against bacterial blight. Xa21 has been tagged with molecular markers (Ronald et al., 1992), and using marker-assisted breeding (Sharma et al. 2001, Singh et al. 2001, Sridhar et al. 2001) and by transformation (Song et al. 1995, Wang et al. 1996, Tu et al. 1998, Zhang et al. 1998), Xa21 has been introgressed either singly or in combination with other genes. However, ineffectiveness of IRBB 21 carrying Xa21 to resist bacterial blight in a few locations in India (DRR 2002, Lavanya et al. 1998, Goel et al. 1998) and in Indonesia (Bustamam et al. 1996) has been reported.

During a 5-year period (1998 to 2002) of testing at the Central Rice Research Institute, Cuttack, IRBB 21 has been consistently resistant to the disease in both wet and dry seasons in the bacterial blight trap nursery where the effectiveness of different resistance genes under natural disease condition is evaluated. However, at the disease trap nursery in 2002 wet season, isolated plants of IRBB 21 exhibited susceptible reaction to the disease. Seeds from these susceptible plants of IRBB 21 were used for raising three rows of plants at 50 hills per row flanked at either side with three rows of susceptible spreader row cultivars alternated by Karuna and Annadha in the same disease trap nursery. To assess whether or not IRBB 21 has succumbed to the disease, 20 of 150 plants expressing varied degrees of susceptible reaction to bacterial blight were collected at random and detected for the presence of Xa21 using markers synthesized from the kinase domain of the gene (Ronald et al. 1992), (Table 1). The original donor of the gene Oryza longistaminata (12156-1), IRBB 21 and IR 24 (recurrent parent of the near-isogenic line IRBB 21) served as control in the PCR analysis.

Three allelic conditions of the plants susceptible to the disease were detected: homozygotes for resistance allele, homozygotes for susceptible allele and heterozygotes. The detection of individuals of homozygotes for susceptible allele and heterozygotes for the Xa21 locus susceptible to bacterial blight may be attributed to a possible occurrence of out-crossing events, as a set of 20 near-isogenic lines in the background of IR 24 were present in the nursery together with IRBB 21. The probability of outcrossing in rice, which is a self-pollinated crop, particularly when grown along with other genotypes that flower at the same time cannot be ruled out (Reano and Pham 1998). However, detection of susceptibile plants in homozygous condition for the gene revealed occurrence of pathogen isolates compatible to Xa21. Lesion length in these plants varied from 6.5 to 12.2 cm. Although the disease is less severe, this indicates the possible gain of virulence by the pathogen strain in the process of co-evolution. While this

study suggests gradual breakdown of resistance of Xa21 against bacterial blight, it also emphasizes the need for careful attention to seed purity.


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