16. Chromosomal location of Xa-2 1

Ryoichi IKEDA, Rodante E. TABIEN and Gurdev S. KHUSH

International Rice Research Institute, P.O. Box 933, 1099 Manila, Philippines

A dominant gene for resistance to bacterial blight (BB) from Oryza longistaminata was transferred into IR24 by backcrossing and was designated as Xa-21 by Khush et al. (1990). This gene conveys resistance to all the six known races of BB in the Philippines. Ikeda et al. (1990) reported that this gene might be linked to Xa-3 and Xa-4 on chromosome 11. Another dominant gene for resistance to BB, Xa-10, is also located on chromosome 11 at a map distance of 27 from Xa-4 (Yoshimura et al. 1984). Xa-10 confers resistance to races 2 and 5 of BB but not to other races. We investigated the linkage relation between Xa-10 and Xa-21. Two lines (GSK76 and GSK80) with Xa-21 were crossed with IR-BB1O which has Xa-10. The two F2 populations were inoculated with four races, 1, 2, 4, and 6 at booting stage by the clipping inoculation method (Kauffman et al. 1973). The lesion length of the uppermost three leaves of each plant was measured for each isolate at 18 days after inoculation.

Almost half of both the F2 populations showed resistance to all four races. The rest were resistant to race 2 but susceptible to races 1, 4 and 6 (Table 1). Only one plant in one F2 Population was susceptible to all four races, indicating that Xa-21 and Xa-10 are tightly linked.

In order to delimit the locus of Xa-21 on the linkage map, we crossed it with d-27, z-2 and la which are located at map positions 28, 33 and 55 of linkage group 11 (RGN 7; 16-50, 1990). The resulting F2 populations were inoculated with race 6. The segregation of each marker gene agreed with 1 : 3 ratio. The recombination values between different gene pairs were calculated from the F2 data and are shown in Table 2. As Figure 1 shows Xa-21 is located between z-2 and la but is closer to z-2.

An isogenic line with Xa-21 has been produced by making five backcrosses to IR24. Following the nomenclature for isogenic lines (Ogawa et al. 1988) we have designated this line as IR-BB21.


Table 1. Reaction of parents and F2 populations to races 1, 2, 4 and 6 of BB
===============================================================================
Parent             Reaction1)   to Races 1, 2,  4  and  6           X2  
  or              ========================================  ===================
F2 population              RRRR  SRSS  SSSS Total            12:3:1      1:1
===============================================================================
GSK80 (Xa-21)              10     0     0     10
GSK76 (Xa-21)              10     0     0     10
IR-BB10 (Xa-10)             0    19     0     19
F2(GSK80 X IR-BB10)       109   101     0    210             124.5***  0.305
F2(GSK76 X IR-BB10)       115   101     1    217             115.1     0.907
===============================================================================
1) R=resistant; S=susceptible

     
Table 2. Data on linkage relations between Xa-21 and the markers of linkage
group 11
==============================================================================
        Reaction to BB(race 6)           Goodness  of  fit
Marker =======================   =============================================
       Res.  Sus.  Total         Item          Ratio          d.f.  X2
==============================================================================
+       126    12    137         Total         9:3:3:1          3  159.9***
                                 BB(Xa-21)       3:1            1    3.5*
z-2       1    47     48         z-2             1:3            1    0.1
                                 Independence                   1  155.4***
Total   127    59    186         (R.C.V.                 6.9+ 1.93)
==============================================================================
+       152    12    164         Total         9:3:3:1          3  137.3***
                                 BB              3:1            1    0.5
d-27      7    47     54         d-27            1:3            1    0.01
                                 Independence                   1  136.8***
Total   159    59    218         (R.C.V.                 9.0+2.05)
==============================================================================
+       106    26    132         Total         9:3:3:1          3   38.6***
                                 BB              3:1            1    3.2
la       23    31     54         la              1:3            1    1.6
                                 Independence                   1   33.8***
Total   129    57    186         (R.C.V.                28.0+4.00)
==============================================================================
*,***: significant at 5 and 0.1% level, respectively. R.C.V.: Recombination value.

References

Ikeda, R., G. S. Khush, R. E. Tabien and T. Ogawa, 1990. A new resistance gene to bacterial blight from Oryza longistaminata. Jpn. J. Breed. 40 (Suppl. 1): 280-281 (in Japanese).

Kauffman, H. E., A. P. K. Reddy, S. P. Y. Hsien and S. D. Merca, 1973.An improved technique for evaluating resistance of rice varieties to Xanthomonas oryza. Plant Disease Reporter 57(6): 537-541.

Khush, G. S., E. Bacalangco and T. Ogawa, 1990. A new gene for resistance to bacterial blight from O. longistaminata. RGN 7: 121-122.

Ogawa, T., T. Yamamoto, G. S. Khush, T. W. Mew and H. Kaku, 1988. Near-isogenic line as international differentials for resistance to bacterial blight of rice. RGN 5: 106-107.

Yoshimura, A., T. W. Mew, G. S. Khush and T. Omura, 1984. Inheritance of resistance to bacterial blight in rice cultivar, Cas 209. Phytopath. 73: 1409-1412.