40. Identification of a fourth blast resistance gene in the Vietnamese indica cultivar
        Tetep  
T. Inukai1 , D. L. Viet1 , T. Imbe2 , R. S. Zeigler2 , T. Kinoshita1 and R. J.Nelson2

1) Faculty of Agriculture, Hokkaido University, Sapporo 060. Japan
2) International Rice Research Institute, P.O.Box 933, Manila, Philippines

     The Vietnamese indica cultivar Tetep is known to be broadly resistant to blast. It has shown resistant reactions across many test sites over several years in the International Rice Blast Nursery (IRBN) (Ahn 1994). Several studies have been conducted on the genetics of blast resistance in Tetep, and it has been shown that at least three complete resistance genes are present in this cultivar (Kiyosawa 1973). Using mutational analysis, Kiyosawa (1973) estimated that one of the resistance genes in Tetep was Pi-kh. Using the C039 near-isogenic lines (NILs) carrying resistance genes derived from Tetep, Mackill and Bonman (1992) and Inukai et al.(1994) recently showed that Tetep had Pi-1, Pi-ta and an unnamed resistance gene. We report here that Tetep carries one additional resistance gene, bringing the total number of identified resistance genes in Tetep to four.
    The lines C103TTP (carrying Pi-1), CIOITTP-1 (carrying Pi-ta) and C105TTP-4L23 (carrying Pi-ta and an unnamed resistance gene, hereafter refered to as Pi(t)) were among the C039 NILs developed using Tetep as the donor parent (Mackill and Bonman 1992; Inukai et al. 1994). Tetep showed resistance to certain Philippine isolates, such as V85094, to which these C039 NILs showed susceptibility (Table 1). This indicated that Tetep carried resistance gene(s) additional to those identified in the C039 NILs. To determine the number of resistance genes in Tetep effective against V85094, it was inoculated to an F2 population of a cross between C039 and Tetep. The result showed that Tetep has a single gene conditioning resistance to V85094 (Table 2). Therefore it was concluded that Tetep carried at least four resistance genes.
    For two of the four resistance genes detected in Tetep, Pi(t) in C105TTP-4L23 and the resistance gene detected here using the isolate V85094, allelic relationships with previously-identified genes were not known. As a first step toward determining this information, a line was developed carrying only the unknown gene in C105TTP-4L23. A cross between C039 and C105TTP-4L23 was made, and the 108 F3 lines derived from the cross were divided into two sets. One set was inoculated with isolate 43 (incompatible

Table 1. Reaction of Tetep and C039 NILs carrying resistance genes derived from Tetep to
            Philippine isolates
 

Lines Test isolates
Pi gene V85016 TV84028 V8905 V85094 B8401 PO83-C5-5 VK605 V86046
Tetep R R R R R R R R
C103TTP Pi-1 S S S S S S S S
CIOITTP-1 Pi-ta S S S S S S S S
C105TTP-4L23 Pi-ta, Pi(t) S S S S S S S S
C039 S S S S S S S S

 

Table 2. Reaction of F2 plants to isolate V85094 in a cross

C039/Tetep
 

Parental lines 
and F2		 No. of plants observed 
for each class		 Total Expected ratio Probability
R  S 20
C039 0 20 20
Tetep 
F2 C039/Tetep		 20 
188 		 20  235 3:1 0.05-0.10

to only Pi-la), while the other set was inoculated with isolate V86010 (incompatible to only Pi(t)). Clear segregation of two independent loci was observed in the F3 population (Table 3). Two F3 lines homozygous for Pi(t) only were selected as candidate for NIL carrying Pi(t). These lines will be used to identify P;(t) and added to the set of C039 NILs as one of the differentials. Genetic analysis of the resistance gene detected using isolate V85094 is now under way. It is necessary to confirm the allelism relationships between known resistance loci and the two unnamed resistance genes in Tetep.

Table 3. Observed frequencies of genotypes on Pi-ta and Pi(t) in fj population of a cross
            C039/C105TTP-4L23
 

Cross Gene

pair

 Genotypea Total Probability
AABB AABb AAbb AaBB AaBb Aabb aaBB aaBb aabb
C039 (aabb) 4 20 9 18 26 10 2 15 4 108 0.10-0.25
/C105TTP-4L23
(AABB) Pi-ta-P(t) (6.75)b (13.5) (6.75) (13.5) (27.0) (13.5) (6.75) (13.5) (6.75)

a.A and a represent genotype of Pi-ta. B and b represent genotype of Pi(t).
b. Expected value estimated by theoretical ratio 1:2:1:2:4:2:1:2:1.

References

Ahn, S. W., 1994. International collaboration on breeding for resistance to rice blast. In Rice Blast Disease,
        CAB International, Wallingford, pp. 137-153.
D. J. Mackill and J. M. Bonman. 1992. Inheritance of blast resistance in near-isogenic lines of rice.
        Phytopathology. 82:746-749.
Inukai, T., R. J. Nelson, R. S. Zeigler, S. Sarkarung, D. J. Mackill, J. M. Bonnan, I. Takamure
        and T. Kinoshita, 1994. Allelism of blast resistance genes in near-isogenic lines of rice.
        Phytopathology 84: 1278-1283.
Kiyosawa, S., 1973. Gene analysis of indica type varieties. Basic Studies on Breeding Measures to Counter
        Explosive Outbreaks of Blast Epidemics in Resistant Rice Varieties. Ministry of Agriculture, Forestry
        and Fisheries Council, Tokyo. Japan 47-61. (in Japanese)