Over 22 bacterial blight (BB) resistance genes have been identified from cultivated rice and its wild relatives (Khush and Angeles 1999). The recessive gene xa8 was originally identified from a rice line PI 231129 (Sidhu et al., 1978). It showed resistance to five out of six races of the pathogen Xanthomonas oryzae pv oryzae (Xoo) in the Philippines (Khush, 1992). The gene was transferred into IR24 through repeated backcrossing resulting in the production of a near-isogenic line IRBB8 carrying xa8 in IR24 genetic background (Ogawa et al., 1991). The xa8 gene is of special interest to breeding programs in India as it confers moderate to high level of resistance to most of the prevalent Xoo isolates in the Punjab province of India (unpublished results). So far, the chromosomal location of xa8 is not known. Identification of linked markers would be particularly useful in tracking the incorporation of the recessive gene into advanced breeding lines. As a first step, we here report mapping of xa8 using sequence tagged microsatellite (STMS) markers.

A set of 105 F₅ recombinant inbred lines (RILs), generated by single seed descent from a cross of PR106 with IRBB8, was used as the mapping population. A total of 179 STMS markers covering 12 linkage groups were used for polymorphism survey of the parental lines, PR 106 and IRBB8 (Temnykh et al., 2000). The parental lines PR 106 and IRBB8 have a common parent (IR8) in their pedigrees, and hence the overall level of polymorphism between the parental lines was relatively low (27.9%) except for chromosomes 7 and 11 which showed 61.5% and 53.8% polymorphism, respectively (Table 1). Interestingly, STMS markers on a 33-cM region of chromosomes 7 and a 46-cM region of chromosome 11 were all polymorphic, suggesting that these two regions might have been introgressed from the donor line PI231129.

All 105 F₅ RILs together with the parents were analyzed using polymorphic STMS markers on chromosomes 7 and 11. DNA was isolated from six plants in each RIL using the CTAB method (Saghai-Maroof et al., 1984). PCR reactions were performed as described by Chen et al. (1997). The PCR products were separated on 4% polyacrylamide denaturing gels and marker bands were visualized using the silver staining protocol (Panaud et al., 1996). Banding pattern of each line was recorded as PR 106 type or IRBB8 type or heterozygous.

The parents and 105 RILs were inoculated with four Philippine races (PXO 86, PXO 112, PXO 340 and PXO 341) and four races from Punjab (designated I, II, III, IV) (G. L. Raina - personal communication). Six plants in each RIL (67-day after seeding) were inoculated with each of the strains. Lesion length was recorded on three leaves of each plant 14 days after inoculation. Average lesion length for each plant was calculated and the RILs were classified as IRBB8 type or PR106 type. RILs that showed variation in disease score within a line were

classified as segregating. The RILs showing lesion length more than 10 cm against PXO 340 and PXO 341 were classified as susceptible. IRBB8 showed partial resistance to PXO 86 and PXO 112, whereas PR106 showed susceptible reaction to all the four Philippine races (Table 2). IRBB8 showed partial resistance to races I, II, and IV and complete resistance to race III in Punjab (Table 2).

Segregation analysis was conducted by inoculating the RILs with Punjab race III. Out of a total 105 RILs, 51 showed IRBB8 type lesion length, 47 were PR 106 type and 7 were segregating. The segregation pattern was consistent with the expected segregation ratio for a single recessive gene in F₅ ( chi² = 0.86). Linkage analysis using Mapmaker (Mapmaker/Exp. V3.0.) placed xa8 19.9 cM from microsatellite marker RM214 on chromosome 7 (Fig. 1). No polymorphic markers have yet been found on the other side of xa8. For effective marker assisted selection, more closely linked markers flanking both sides of xa8 are needed.

Help provided by Dr G. L. Raina, PAU, RRS Kapurthala, for screening the RILs against the Punjab strains is gratefully acknowledged.

References

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