Al toxicity is a problem for rice grown on acid soils. It is considered as one of the primary causes of low rice productivity on acid upland and lowland acid sulfate soils (IRRI 1978). The use of molecular-marker techniques in quantitative trait locus (QTL) analysis opened new opportunities to work with the tolerance for Al toxicity. So, in this study, a mapping population of 81 recombinant inbred lines (RILs), derived from a cross between a japonica variety Kinmaze and an indica variety DV85 by the single-seed descent methods, was used to detect quantitative trait loci (QTLs) for relative root elongation (RRE, an indicator for Al tolerance) under Al³⁺ stress condition in rice. Seeds of each RILs were placed on the net that floated on a 0.5 mM CaCl₂ solution (pH 4.5) in a 7-liter plastic container. The solution was renewed every 2 d. On the 5^th day the seedlings were exposed to a 0.5 mM CaCl₂ solution (pH 4.5) with 100 micro M AlCl₃ or without Al. The root length was recorded with a ruler before and after the 24h treatment. The RRE of each lines was calculated as follows: RRE(%) = [root elongation (average of 8 replicates) with Al solution/root elongation without Al solution] x 100. The experiments were conducted at 32C/17C day/night under natural light. The distributions of the relative root elongation index of the RILs are continuous, but there is a little difference in relative root elongation between two parents (Fig. 1). A linkage map used for QTL detection comprised 137 markers (1386.2 cM) with an average marker density of 10.1 cM. Using QTL Cartographer software (Basten et al.1999), QTLs were detected by a com-

posite interval mapping method. A LOD score >2.0 was considered significant for QTL detection. Five QTLs controlling Al toxicity tolerance were detected on chromosomes 1, 5, 8, 9 and 11, respectively (Table 1 and Fig. 2). Individual QTL accounted for 8.64% - 18.60% of the phenotypic variance in the RILs population. Direction of additive gene effect coincided with that predicted by phenotypes of the parents at three loci, qRRE5, qRRE8 and qRRE11. At these three loci, the DV85 alleles increased the tolerance to Al toxicity, and at qRRE1 and qRRE9, the Kinmaze alleles increased the tolerance to Al toxicity.

Due to the same marker C1121 ( Table 1, Fig. 2), qRRE8 ( R² = 0.182 ) apparently mapped to the same chromosomal region with the major QTL detected in the doubledhaploid population derived from the CT9993 and IR62266 (Nguyen et al. 2002). One of the major QTLs identified in the population of 171 RILs (F₆) from 2R64 and O. rufipogon (Acc106424) was also located on chromosome 8 flanked by RG28 and RZ650. It was found that the marker C1121 was 27.7 cM from RG333, and the marker RG28 was 21.8cM from RG333. It is likely that these QTLs are located on the same chromosomal region (Nguyen et al. 2003). The putative QTLs for Al tolerance were also detected on chromosome 1, 5, 9 and 11 (Wu et al. 2000, Nguyen et al. 2002, Ma et al. 2002, Nguyen et al. 2003). However, it is unclear whether these

QTLs are at the same position as the QTLs reported here because different sets of DNA markers were used. Further fine mapping of these QTL regions will verify whether these QTLs are common in different rice genetic background.

We gratefully acknowledge the financial support of National Science Fund for Distinguished Young Scholars Abroad.

References

International Rice Research Institute, 1978. Annual report for 1977. IRRI, Manila, Philippines.

Basten. C. J, B.S. Weir and Z.B. Zeng, 1999. QTL Cartographer version 1.13: A reference manual and tutorial for QTL mapping., World Wide Web URL:Http://statgen.ncsu.edu/qtlcart/cartographer.html

Ma J.F., R.F. Shen, Z.Q. Zhao, M. Wissuwa, Y. Takeuchi, T. Ebitani and M. Yano, 2002. Response of rice to Al stress and identification of quantitative trait loci for Al tolerance. Plant Cell Physiol. 43(6): 652-659.

Wu P., C.Y. Liao, B.Hu, K.K.Yi, J.J.Ni and C.He, 2000. QTLs and epistasis for aluminum tolerance in rice (Oryza sativa L.) at different seedling stages. Theor Appl Genet 100: 1295-1303.

Nauyen V. T., B.D.Nguyen, S. Sarkarung, C. Martinez, A.H. Paterson and H.T. Nguyen, 2002. Mapping of genes controlling aluminum tolerance in rice: comparison of different genetic backgrounds. Mol Genet Genomics 267: 772-780.

Nguyen B. D., D. S. Brar, B. C. Bui, T. V. Nguyen, L. N. Pham and H. T. Nguyen, 2003. Identification and mapping of the QTL for aluminum tolerance introgressed from the new source, ORYZA RUFIPOGON Griff., into indica rice (Oryza sativa L.). Theor Appl Genet 106: 583-593.