Table 3. Reaction of near-isogenic lines and recurrent parent (CO 39) to five isolates from the collection of the IRRI Plant Pathology Dept.
International Rice Research Institute, PO Box 933, Manila Philippines
Systematic genetic studies on resistance to rice blast (Pyricularia oryzae
Cav.) have been carried out in Japan, where at least 13 major genes at eight
loci have been identified (Kiyosawa 1981). Studies have also been conducted in
the tropics, but progress has been slow. The presence of several resistance
genes against tropical blast isolates in most rices studied makes it difficult
to conduct allelism tests (Mackill et al. 1985; Yu et al. 1987). Furthermore,
it is difficult to determine the relation between genes identified in Japan and
thos identified in the tropics because of the difference in blast isolates
(Kiyosawa et al. 1986). Indica isogenic lines with single resistance genes
would make systematic genetic studies easier, and would also allow study of the
virulence structure of P. oryzae populations.
A program to backcross blast resistance genes into a susceptible background was initiated at IRRI. The Indica cultivar, CO 39, was chosen as a recurrent parent because of its high susceptibility at almost all tropical blast isolates and its very early duration (<100 days seed to seed). Four donor parents and five blast isolates were used (Table 1). Each was given an abbreviation used in the designation of each isogenic line to identify the cultivar-isolate combination. Each backcross F\1\ population was sown in trays and spray-inoculated after 21 days with a suspension containing 50,000 conidia/ml. After inoculation, the plants were incubated for 24 h in a dew chamber, then placed in a humid room in a glasshouse. Six days after inoculation, the plants were scored. Resistant plants were selected for the next cycle of crossing. One row of CO 39 in each tray served as an infection check. CO 39 was used as the female parent in all backcrosses, and only resistant plants in the backcross F\1\ were kept for the next cycle of crossing. Previous studies indicated that Tetep had two genes for resistance to isolates IK81-3 (101) and 43 (105) (Mackill et al. 1985), so up to six separate lines were maintained for each.
Twently lines were isolated after the six backcross (Table 2). Of these, 17 showed a good fit to a single gene model in the BC\6\F\2\. One (C105TTP-1) had fewer susceptible types and two (C104PKT and C103TTP) had fewer resistant types than expected. BC\6\F\4\ lines homozygous for the resistant genes were isolated. Seed from these lines were screened against a diverse sample of isolates from the collection at IRRI. Based on reaction pattern, six groups were identified (Table 3). Studies are underway to determine the number of resistance genes and their linkage relationships.
Table 1. Donor parents and isolates used to develop near-isogenic lines with
blast resistance genes
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Donor parent Abbreviation Isolate Abbreviation
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Tetap TTP IK81-3 101
Pai-ken-tao PKT IK81-25 102
5173 A51 PO6-6 103
LAC 23 LAC PO3-82-51 104
43 105
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Table 3. Reaction of near-isogenic lines and recurrent parent (CO 39)
to five isolates from the collection of the IRRI Plant
Pathology Dept.
References
Bonman, J.M., T.I. Vergel de Dios and M.M. Khin, 1986. Physiologic specialization of Pyricularia oryzae in the Philippines. Plant Dis. 70: 767- 769.
Kiyosawa, S. 1981. Gene analysis for blast resistance. Oryza 18: 196-203.
Kiyosawa, S., D.J. Mackill, J.M. Bonman, Y. Tanaka and Z.Z. Ling, 1986. An attempt of classification of world's rice varieites based on reaction pattern to blast fungus strains. Bull. Natl. Inst. Agrobiol. Resour., Japan 2: 13-39.
Mackill, D.J., J.M. Bonman, H.S. Suh and R. Srilingam, 1985. Genes for resistance to Philippine isolates of the rice blast pathogen. RGN 2: 80-81.
Yu, Z.H., D.J. Mackill and J.M. Bonman, 1987. Inheritance of resistance to blast in some traditional and improved rice cultivars. Phytopathology 77: 323- 326.