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.
study suggests gradual breakdown of resistance of Xa21 against
bacterial blight, it also emphasizes the need for careful attention to
Bustamam, M., M. Yunus, H.R. Hifni, M. Baroidan, E.Y. Ardales and R.J.
Nelson, 1996. Population structure of the rice bacterial pathogen, Xanthomonas
oryzae pv. oryzae in Java and Bali, Indonesia. In: Pages 107,
Proc. Of the 3rd Asia-Pacific Conf. on Agricultural Biotechnology:
Issues and Choices, Nov 10-15, Prachuapkhirikhan, Thailand.
DRR, 2002. Directorate of Rice Research Progress Report, 2001, Vol. 2.
Entomology and Pathology, All India Coordinated Rice Improvement Programme
(ICAR), Directorate of Rice Research, Rajendranagar, Hyderabad-500 030,
Goel, R.K., L. Kaur and R.G. Saini, 1998. Effectiveness of different Xa
genes against Xanthomonas oryzae pv. oryzae population causing
bacterial blight of rice in Punjab (India). Rice Genet. Newslet. 15:
Ikeda, R., G.S. Khush and R.E. Tabien, 1990. A new resistance gene to
bacterial blight derived from O. longistaminata. Jpn. J. Breed.
Khush, G.S. and D.S. Brar, 2001. Rice genetics from Mendel to functional
genomics. In: Rice Genetics IV (Khush, G.S; Brar, D.S. and Hardy, B. eds.),
Proc. 4th Intern. Rice Genet. Symp., Oct 22-27, 2000, pp. 3-25,
Intern. Rice Res. Inst., Philippines, Science Publishers, India.
Khush, G.S., E. Bacalangco and T. Ogawa, 1990. A new gene for resistance
to bacterial blight from O. longistaminata. Rice Genetics Newsletter
Khush, G.S., D.J. Mackill and G.S. Sidhu, 1989. Breeding rice for resistance
to bacterial blight. In: Pages 207-217, Proc. International Workshop on
Bacterial Blight of Rice, International Rice Research Institute, Los Banos,
Lavanya, B., V.B. Priyadarisini, S. Leenakmari, S.S. Gnanamanickam and
M. Levy, 1998. Lineage-exclusion resistance breeding: pyramiding of blast
and bacterial blight resistance genes for rice disease management in India.
In: Page 21 Proc. 7th Natl. Rice Biotechnology Network Meeting,
Oct 25-29, 1998, UAS-NCBS, Bangalore, India.
Mew, T.W., 1987. Current status and future prospects of research on bacterial
blight of rice. Ann. Rev. Phytopathol. 25: 359-82.
Ou S.H., 1985. Rice Diseases, 2nd ed. Commonwealth Agric. Bureaux.,
p. 380, Central Sales. Farnham Royal, Slough, U.K.
Reano, R. and J.L. Pham, 1998. Does cross-pollination occur during seed
regeneration at the International Rice Genebank? Intern. Rice Res. Notes
Reddy, J.N., M.R. Baraoidan, M.A. Bernardo, M.L.C. George and R. Sridhar,
1997. Application of marker-assisted selection in rice for bacterial blight
resistance gene, Xa21. Current Science 73: 873-875.
Ronald, P.C., B. Albano, R. Tabaien, L. Abenes, K. Wu, S. McCouch and
S. Tanksley, 1992. Genetic and physical analysis of the rice bacterial
blight resistance locus, Xa21. Mol. Gen. Genet. 236: 113-120.
Sharma, N.P., R.M. Sundaram and R.V. Sonti, 2001. DNA markers in genetic
purity testing and marker assisted selection technology in rice improvement.
In: Pages 32-38, Proc. 8th Natl. Rice Biotechnol. Network Meeting,
Oct 21-25, 2001, Aurangabad, India.
Singh, K., Y. Vakil, J.S. Sidhu, D.S. Brar and H.S. Shaliwal, 2001. Rice
cultivar PR 106, pyramided with three bacterial blight resistance genes,
using marker assisted selection, reaches plant-breeding trials. In: Pages
68a-71, Proc. 8th Natl. Rice Biotechnol. Network Meeting, Oct
21-25, 2001, Aurangabad, India.
Song, W-Y., G-L. Wang, L. Chen, H-S. Kim, Li-Ya Pi, J. Gardner, B. Wang,
T. Holsten, W-X. Zhai, L-H. Zhu, C. Fauquet and P. Ronald, 1995. A receptor
kinase-like protein encoded by the rice disease resistance gene Xa21.
Science 270: 1804-1806.
Sridhar, R., 2002. Use of molecular tools for understanding bacterial
blight pathogen populations of rice and usefulness of disease resistance
genes. Indian Phytopathol. 55: 417-429.
Sridhar, R., J.N. Reddy, U.D. Singh, P.K. Agrawal, C. Sirishia and D.
Mishra, 2001. Molecular characterization of bacterial blight and blast
pathogen populations of India and marker-assisted breeding for bacterial
blight and blast resistant genotypes. In: Pages 42-44. Proc. 8th
Natl. Rice Biotechnol. Network Meeting, Oct 21-25, 2001, Aurangabad, India.
Tu, J., I. Ona, Q. Zhang, T.W. Mew, G.S. Khush and S.K. Datta, 1998. Transgenic
rice variety 'IR72' with Xa21 is resistant to bacterial bight.
Theor. Appl. Genet. 97: 31-36.
Wang, G.L., W.Y. Song, D.L. Ruan, Sideris and P. Ronald, 1996. The cloned
gene, Xa21, confers resistance to multiple Xanthomonas oryzae
pv. oryzae isolates in transgenic plants. MPMI 9: 850-855.
Zhang, S., W-Y. Song, L. Chen, D. Ruan, N. Taylor, P. Ronald, R. Beachy
and C. Fauquet, 1998. Transgenic elite indica rice varieties, resistant
to Xanthomonas oryzae pv. oryzae. Mol. Breed. 4: