Identification of quantitative trait loci for cadmium tolerance and accumulation in wheat

Quantitative trait loci (QTL) for Cadmium (Cd) tolerance and accumulation in wheat (Triticum aestivum L.) were identified, using 103 recombinant inbred lines (RILs) derived from a cross of Ch × Sh at germination and seedling stages. The traits of germination, growth and physiology were measured. Cd tolerance indexes (TI) were calculated for plants under Cd stress relative to control conditions. Cd concentrations in both root and shoot were determined and the amount of Cd accumulation and translocation calculated. The phenotypic variation of the above traits showed a continuous distribution pattern among the RILs. Twenty-six QTLs were detected, (16 of which were designated for the traits under the control and Cd stress, 8 for Cd tolerance and 2 for root Cd accumulation). These 26 QTLs individually could explain 7.97–60.16% of the phenotypic variation. Fourteen QTLs were positive (with the additive effects coming from Ch) while the remaining 12 QTLs were negative (with the additive effects contributed by Sh). No QTL were detected in the same region on the chromosomes of wheat. The results indicated that genetic mechanisms controlling the traits of Cd tolerance were independent from each other. Therefore, in this study, the properties of Cd tolerance and accumulation showed to be independent traits in wheat.     

Mapping of quantitative trait loci for salt tolerance at the seedling stage in rice

Salt tolerance was evaluated at the young seedling stage of rice (Oryza sativa L.) using recombinant inbred lines (MG RILs) from a cross between Milyang 23 (japonica/indica) and Gihobyeo (japonica). 22 of 164 MG RILs were classified as tolerant with visual scores of 3.5-5.0 in 0.7% NaCl. Interval mapping of QTLs related to salt tolerance was conducted on the basis of the visual scores at the young seedling stage. Two QTLs, qST1 and qST3, conferring salt tolerance, were detected on chromosome 1 and 3, respectively, and the total phenotypic variance explained by the two QTLs was 36.9% in the MG RIL population. qST1 was the major QTL explaining 27.8% of the total phenotypic variation. qST1 was flanked by Est12-RZ569A, and qST3 was flanked by RG179-RZ596. The detection of new QTLs associated with salt tolerance will provide important information for the functional analysis of rice salt tolerance.     

Quantitative trait loci for aluminum resistance in wheat

Quantitative trait loci (QTL) for wheat resistance to aluminum (Al) toxicity were analyzed using simple sequence repeats (SSRs) in a population of 192 F₆ recombinant inbred lines (RILs) derived from a cross between an Al-resistant cultivar, Atlas 66 and an Al-sensitive cultivar, Chisholm. Wheat reaction to Al was measured by relative root growth and root response to hematoxylin stain in nutrient-solution culture. After screening 1,028 SSR markers for polymorphisms between the parents and bulks, we identified two QTLs for Al resistance in Atlas 66. One major QTL was mapped on chromosome 4D that co-segregated with the Al-activated malate transporter gene (ALMT1). Another minor QTL was located on chromosome 3BL. Together, these two QTLs accounted for about 57% of the phenotypic variation in hematoxylin staining score and 50% of the variation in net root growth (NRG). Expression of the minor QTL on 3BL was suppressed by the major QTL on 4DL. The two QTLs for Al resistance in Atlas 66 were also verified in an additional RIL population derived from Atlas 66/Century. Several SSR markers closely linked to the QTLs were identified and have potential to be used for marker-assisted selection (MAS) to improve Al-resistance of wheat cultivars in breeding programs.     

Mapping quantitative trait loci associated with selenate tolerance in Arabidopsis thaliana

Selenium is essential for many organisms, but is toxic at higher levels. To investigate the genetic basis of selenate tolerance in Arabidopsis thaliana, quantitative trait loci (QTL) associated with selenate tolerance in accessions Landsberg erecta and Columbia were mapped using recombinant inbred lines (RILs). The selenate tolerance index (TI(D10) = root growth + 30 microm selenate/root growth control x 100%) was fourfold higher for parental line Col-4 (59%) than for parent Ler-0 (15%). Among the 96 F8 RILs, TI(D10) ranged from 11 to 75% (mean 37%). Using composite interval mapping, three QTL were found on chromosomes 1, 3 and 5, which together explained 24% of variation in TI(D10) and 32% of the phenotypic variation for the difference in root length +/- Se (RL(D10)). Highly significant epistatic interactions between the QTL and markers on chromosome 2 explained additional variation for both traits. Potential candidate genes for Se tolerance in each of the QTL regions are discussed. These results offer insight into the genetic basis of selenate tolerance, and may be useful for identification of selenate-tolerance genes.     

Identification and mapping of the QTL for aluminum tolerance introgressed from the new source, <Emphasis Type="SmallCaps">ORYZA RUFIPOGON</Emphasis> Griff., into indica rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

This study was conducted to identify and map the quantitative trait locus (QTL) controlling Al tolerance in rice using molecular markers. A population of 171 F(6) recombinant inbred lines (RILs) derived from the cross of Oryza sativa (IR64), the Al susceptible parent, and Oryza rufipogon, the Al tolerant parent, was evaluated for Al tolerance using a nutrient solution with and without 40 ppm of active Al(+3). A genetic map, consisting of 151 molecular markers covering 1,755 cM with an average distance of 11.6 cM between loci, was constructed. Nine QTLs were dentified including one for root length under non-stress conditions (CRL), three for root length under Al stress (SRL) and five for relative root length (RRL). O. rufipogon contributed favorable alleles for each of the five QTLs for RRL, which is a primary parameter for Al tolerance, and individually they explained 9.0-24.9% of the phenotypic variation. Epistatic analysis revealed that CRL was conditioned by an epistatic effect, whereas SRL and RRL were controlled by additive effects. Comparative genetic analysis showed that QTLs for RRL, which mapped on chromosomes 1 and 9, appear to be consistent among different rice populations. Interestingly, a major QTL for RRL, which explained 24.9% of the phenotypic variation, was found on chromosome 3 of rice, which is conserved across cereal species. These results indicate the possibilities to use marker-assisted selection and pyramiding QTLs for enhancing Al tolerance in rice. Positional cloning of such QTLs introgressed from O. rufipogon will provide a better understanding of the Al tolerance mechanism in rice and the evolutionary genetics of plant adaptation to acid-soil conditions across cereal species.     

QTLs mapping of morphological traits related to salt tolerance in <Emphasis Type="Italic">Medicago truncatula</Emphasis>

To understand the complex inheritance of tolerance to salt stress in Medicago truncatula, quantitative trait loci (QTLs) analysis was performed using a set of recombinant inbred lines (RILs) derived from a cross between the tolerant line Jemalong A17 and susceptible line F83005.5. The RILs and parental lines were grown in individual pots filled with sterilized sand in a greenhouse under 0 and 50 mM NaCl. Plants were harvested after a period of 60 days. Fourteen quantitative traits related to aerial and root growths were measured. Broad-sense heritability of measured traits ranged from 0.21 to 0.83 and from 0.05 to 0.62 in control and in salt-stressed conditions, respectively. Established correlations between measured traits are dependent on treatment effect. We identified and mapped 10 QTLs in control conditions and 19 in salt stress. No major QTL was identified indicating that tolerance to salt stress is governed by several genes with low effects. The QTLs detected under control and under salt-stressed conditions almost did not share the same map locations suggesting that the loci that are not stable across treatments reflect adaptation to this constraint. The maximum of QTLs was observed on the chromosome 8. The usefulness of these QTLs, identified in greenhouse conditions, for marker-assisted selection should therefore be evaluated under field conditions, and validated in other genetic backgrounds.     

Mapping QTLs for submergence tolerance in rice using a population fixed for <Emphasis Type="Italic">SUB1A</Emphasis> tolerant allele

Submergence is a widespread problem of rice production, especially in low-lying areas in South and Southeast Asia. Despite the success of Sub1 mega varieties, repeated instances of prolonged and severe flooding in stress-prone areas suggests that the SUB1 gene is no longer sufficient in those regions and requires improved varieties with increased tolerance. A study was conducted to identify quantitative trait loci (QTLs) associated with submergence tolerance using 115 F₇ recombinant inbred lines (RILs) derived from the cross of Ciherang-Sub1, a popular Indonesian cultivar carrying the SUB1 gene that has relatively higher tolerance to submergence compared to the performance of most other Sub1 lines and the submergence and stagnant flooding tolerant IR10F365. As the tolerant allele at SUB1A on chromosome 9 was fixed in this mapping population, additional QTLs responsible for submergence tolerance were expected to be revealed. Genotyping with an Infinium 6K SNP chip resulted in 469 polymorphic markers that were then used for QTL mapping. Phenotyping was performed under complete submergence with two replicates. A major QTL for submergence derived from Ciherang-Sub1, named qSUB8.1, was detected on chromosome 8 with a LOD score of 10.3 and phenotypic variance of 27.5%. Additionally, a smaller QTL, also derived from Ciherang-Sub1, was detected on chromosome 2 with a LOD score of 3.5 and phenotypic variance of 12.7%. There was no digenic interaction detected between these QTLs suggesting their independent action. The QTLs detected in this study can be used in marker-assisted selection to further improve the tolerance of other Sub1 varieties.     

Mapping of QTL for downy mildew resistance in maize

Quantitative trait loci (QTLs) of maize involved in mediating resistance to Peronosclerospora sorghi, the causative agent of sorghum downy mildew (SDM), were detected in a population of recombinant inbred lines (RILs) derived from the Zea mays L. cross between resistant (G62) and susceptible (G58) inbred lines. Field tests of 94 RILs were conducted over two growing seasons using artificial inoculation. Heritability of the disease reaction was high (around 70%). The mapping population of the RILs was also scored for restriction fragment length polymorphic (RFLP) markers. One hundred and six polymorphic RFLP markers were assigned to ten chromosomes covering 1648 cM. Three QTLs were detected that significantly affected resistance to SDM combined across seasons. Two of these mapped quite close together on chromosome 1, while the third one was on chromosome 9. The percentage of phenotypic variance explained by each QTL ranged from 12.4% to 23.8%. Collectively, the three QTLs identified in this study explained 53.6% of the phenotypic variation in susceptibility to the infection. The three resistant QTLs appeared to have additive effects. Increased susceptibility was contributed by the alleles of the susceptible parent. The detection of more than one QTL supports the hypothesis that several qualitative and quantitative genes control resistance to P. sorghi.     

Boron toxicity in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.). I. Quantitative trait locus (QTL) analysis of tolerance to boron toxicity

Boron toxicity tolerance of rice plants was studied. Modern japonica subspecies such as Koshihikari, Nipponbare, and Sasanishiki were tolerant, whereas indica subspecies such as Kasalath and IR36 were intolerant to excessive application of boron (B), even though their shoot B contents under B toxicity were not significantly different. Recombinant inbred lines (RILs) of japonica Nekken-1 and indica IR36 were used for quantitative trait locus (QTL) analysis to identify the gene responsible for B toxicity tolerance. A major QTL that could explain 45% of the phenotypic variation was detected in chromosome 4. The QTL was confirmed using a population derived from a recombinant inbred line which is heterogenic at the QTL region. The QTL was also confirmed in other chromosome segment substitution lines (CSSLs).     

Quantitative trait loci controlling aluminium tolerance in two accessions of Arabidopsis thaliana (Landsberg erecta and Cape Verde Islands)

Quantitative trait loci (QTL) analysis of aluminium (Al) tolerance was performed using Ler/Cvi recombinant inbred (RI) lines of Arabidopsis thaliana. Relative root length (RRL) (root length with 4 µm Al/root length with no Al at pH 5.0) on day 5 was used as the Al tolerance index for QTL analysis. Al tolerance judged by RRL was well correlated to tolerance judged by other indexes, including accumulation of callose, reactive oxygen species in the root apex and growth performance on acid soil containing a large amount of exchangeable Al. Using data sets with an h_b² of 0.91, two QTLs were detected at the top of chromosome 1 and bottom of chromosome 3. These QTLs explained 40 and 16% of the phenotypic variation of Al tolerance, respectively, and the positive effect of the Cvi allele. The QTL on chromosome 1 overlapped with a major QTL in another recombinant inbred population, and is possibly related to malate excretion. A complete pair-wise search revealed 11 sets of epistatic interacting loci pairs, which accounted for the transgressive segregation among the RI population. Several epistatic interactions shared the same chromosomal region, indicating the possible involvement of regulatory proteins in Al tolerance in Arabidopsis.     

Mapping QTLs for phosphorus deficiency tolerance in rice (Oryza sativa L.)

 The amplified fragment length polymorphism (AFLP) technique combined with selective genotyping was used to map quantitative trait loci (QTLs) associated with tolerance for phosphorus (P) deficiency in rice. P deficiency tolerant cultivar IR20 was crossed to IR55178-3B-9-3 (sensitive to P-deficiency) and 285 recombinant inbred lines (RILs) were produced by single-seed descent. The RILs were phenotyped for the trait by growing them in P-sufficient (10.0 mg/l) and P-deficient (0.5 mg/l) nutrient solution and determining their relative tillering ability at 28 days after seeding, and relative shoot dry weight and relative root dry weight at 42 days after seeding. Forty two of each of the extreme RILs (sensitive and tolerant) and the parents were subjected to AFLP analysis. A map consisting of 217 AFLP markers was constructed. Its length was 1371.8 cM with an average interval size of 7.62 cM. To assign linkage groups to chromosomes, 30 AFLP and 26 RFLP markers distributed over the 12 chromosomes were employed as anchor markers. Based on the constructed map, a major QTL for P-deficiency tolerance, designated PHO, was located on chromosome 12 and confirmed by RFLP markers RG9 and RG241 on the same chromosome. Several minor QTLs were mapped on chromosomes 1, 6, and 9. Received: 21 April 1998 / Accepted: 9 June 1998     

QTL mapping of physiological traits associated with salt tolerance in Medicago truncatula Recombinant Inbred Lines

In this study, QTL mapping of physiological traits in the model Legume (Medicago truncatula) was performed using a set of RILs derived from LR5. Twelve parameters associated with Na+ and K+ content in leaves, stems and roots were measured. Broad-sense heritability of these traits was ranged from 0.15 to 0.83 in control and from 0.14 to 0.61 in salt stress. Variation among RILs was dependent on line, treatment and line by treatment effect. We mapped 6 QTLs in control, 2 in salt stress and 5 for sensitivity index. No major QTL was identified indicating that tolerance to salt stress is governed by several genes with low effects. Detected QTL for leaf, stem and root traits did not share the same map locations, suggesting that genes controlling transport of Na+ and K+ may be different. The maximum of QTL was observed on chromosome 1, no QTL was detected on chromosomes 5 and 6.     

Mapping of QTLs associated with cold tolerance during the vegetative stage in rice

Low-temperature stress is an important factor affecting the growth and development of rice (Oryza sativa L.) in temperate and high-elevation areas. Cold stress may cause various seedling injuries, delayed heading and yield reduction due to spikelet sterility. In this study, 181 microsatellite marker loci were used to identify quantitative trait loci (QTLs) associated with cold tolerance at the vegetative stage in 191 recombinant inbred lines (RILs) derived from a cross of a cold-tolerant temperate japonica cultivar (M-202) with a cold-sensitive indica cultivar (IR50). Different temperature regimes were applied in growth chambers on 191 RILs. The temperature regimes imposed in the growth chamber simulated cold-stress injuries at the seedling and late vegetative stages. In this study a major QTL was identified on chromosome 12, designated as qCTS12a, that was closely associated with cold-induced necrosis and wilting tolerance, and accounted for 41% of the phenotypic variation. A number of QTLs with smaller effects were also detected on eight rice chromosomes.     

Locating QTLs controlling several adult root traits in an elite Chinese hybrid rice

This study aimed to elucidate the genetics of the adult root system in elite Chinese hybrid rice. Several adult root traits in a recombinant inbred line (RIL) population of Xieyou 9308 and two backcross F₁ (BCF₁) populations derived from the RILs were phenotyped under hydroponic culture at heading stage for quantitative trait locus (QTL) mapping and other statistical analysis. There a total of eight QTLs detected for the root traits. Among of them, a pleiotropic QTL was repeatedly flanked by RM180 and RM5436 on the short arm of chromosome 7 for multiple traits across RILs and its BCF₁ populations, accounting for 6.88% to 25.26% of the phenotypic variances. Only additive/dominant QTLs were detected for the root traits. These results can serve as a foundation for facilitating future cloning and molecular breeding.     

Quantitative trait loci for aluminum resistance in Chinese wheat landrace FSW

Aluminum (Al) toxicity is a major constraint for wheat production in acid soils worldwide. Chinese landrace FSW demonstrates a high level of Al resistance. A population of recombinant inbred lines (RILs) was developed from a cross between FSW and an Al-sensitive Chinese line, ND35, using single seed descent, to map quantitative trait loci (QTLs) for Al resistance. Wheat reaction to Al stress was measured by net root growth (NRG) in a nutrient solution culture containing Al(3+) and hematoxylin staining score (HSS) of root after Al stress. After 1,437 simple sequence repeats (SSRs) were screened using bulk segregant analysis, three QTLs were identified to control Al resistance in FSW. One major QTL (Qalt.pser-4DL) was mapped on chromosome 4DL that co-segregated with Xups4, a marker for the promoter of the Al-activated malate transporter (ALMT1) gene. The other two QTLs (Qalt.pser-3BL, Qalt.pser-2A) were located on chromosomes 3BL and 2A, respectively. Together, the three QTLs accounted for up to 81.9% of the phenotypic variation for HSS and 78.3% of the variation for NRG. The physical positions of flanking markers for Qalt.pser-4DL and Qalt.pser-3BL were determined by analyzing these markers in corresponding nulli-tetrasomic, ditelosomic, and 3BL deletion lines of Chinese Spring. Qalt.pser-3BL is a novel QTL with a major effect on Al resistance discovered in this study. The two major QTLs on 4DL and 3BL demonstrated an additive effect. The SSR markers closely linked to the QTLs have potential to be used for marker-assisted selection (MAS) to improve Al resistance of wheat cultivars in breeding programs.     

Natural allelic variation identifies new genes in the Arabidopsis circadian system

We have analysed the circadian rhythm of Arabidopsis thaliana leaf movements in the accession Cvi from the Cape Verde Islands, and in the commonly used laboratory strains Columbia (Col) and Landsberg (erecta) (Ler), which originated in Northern Europe. The parental lines have similar rhythmic periods, but the progeny of crosses among them reveal extensive variation for this trait. An analysis of 48 Ler/Cvi recombinant inbred lines (RILs) and a further 30 Ler/Col RILs allowed us to locate four putative quantitative trait loci (QTLs) that control the period of the circadian clock. Near-isogenic lines (NILs) that contain a QTL in a small, defined chromo- somal region allowed us to confirm the phenotypic effect and to map the positions of three period QTLs, designated ESPRESSO, NON TROPPO and RALENTANDO. Quantitative trait loci at the locations of RALENTANDO and of a fourth QTL, ANDANTE, were identified in both Ler/Cvi and Ler/Col RIL populations. Some QTLs for circadian period are closely linked to loci that control flowering time, including FLC. We show that flc mutations shorten the circadian period such that the known allelic variation in the MADS-box gene FLC can account for the ANDANTE QTL. The QTLs ESPRESSO and RALENTANDO identify new genes that regulate the Arabidopsis circadian system in nature, one of which may be the flowering-time gene GIGANTEA.     

Identification of QTLs with main, epistatic and QTL?×?environment interaction effects for salt tolerance in rice seedlings under different salinity conditions

Salt tolerance of rice (Oryza sativa L.) at the seedling stage is one of the major determinants of its stable establishment in saline soil. One population of recombinant inbred lines (RILs, F (2:9)) derived from a cross between the salt-tolerant variety Jiucaiqing and the salt-sensitive variety IR26 was used to determine the genetic mechanism of four salt tolerance indices, seedling height (SH), dry shoot weight (DSW), dry root weight (DRW) and Na/K ratios (Na/K) in roots after 10 days in three salt concentrations (0.0, 0.5 and 0.7 % NaCl). The main effect QTLs (M-QTLs) and epistatic QTLs (E-QTLs) were detected by QTL IciMapping program using single environment phenotypic values. Eleven M-QTLs and 11 E-QTLs were identified for the salt tolerance indices. There were six M-QTLs and two E-QTLs identified for SH, three M-QTLs and five E-QTLs identified for DSW, two M-QTLs and one E-QTL identified for DRW, and three E-QTLs identified for Na/K. The phenotypic variation explained by each M-QTL and E-QTL ranged from 7.8 to 23.9 % and 13.3 to 73.7 %, respectively. The QTL-by-environment interactions were detected by QTLNetwork program in the joint analyses of multi-environment phenotypic values. Six M-QTLs and five E-QTLs were identified. The phenotypic variation explained by each QTL and QTL × environment interaction ranged from 0.95 to 6.90 % and 0.02 to 0.50 %, respectively. By comparing chromosomal positions of these M-QTLs with those previously identified, five M-QTLs qSH1.3, qSH12.1, qSH12.2, qDSW12.1 and qDRW11 might represent novel salt tolerance genes. Five selected RILs with high salt tolerance had six to eight positive alleles of the M-QTLs, indicating that pyramiding by marker-assisted selection (MAS) of M-QTLs can be applied in rice salt tolerance breeding programs.     

Detection of Fusarium head blight resistance QTL in a wheat population using bulked segregant analysis

A population of 218 recombinant inbred lines (RILs) was developed from the cross of two wheat (Triticum aestivum L.) cultivars, 'Ning 894037' and 'Alondra'. Ning 894037 has resistance to Fusarium head blight (FHB) and Alondra is moderately susceptible. Response of the RILs and their parental lines to FHB infection was evaluated with point inoculation in four experiments both in greenhouse and in field conditions. Distribution of disease severity in the population is continuous, indicating quantitative inheritance of resistance to FHB. Bulked segregant analysis and QTL mapping based on simple sequence repeat (SSR) markers revealed three chromosome regions that are responsible for FHB resistance. A chromosome region on 3BS accounted for 42.5% of the phenotypic variation for FHB resistance. Additional QTLs were located on chromosomes 2D and 6B. These three QTLs jointly accounted for 51.6% of the phenotypic variation. SSR markers linked to the QTLs influencing resistance to FHB have potential for use in breeding programs.     

The genetic control of tolerance to aluminum toxicity in the ‘Essex’ by ‘Forrest’ recombinant inbred line population

Aluminum (Al) toxicity to plant roots is a major problem of acidic soils. The main chemical reaction involved is Al hydrolysis. Application of lime or nitrate fertilizers to raise soil pH reduces Al toxicity but not as economically as a plant genotypes with natural tolerance against this stress. Ammonium fertilization of crops and assimilation of ammonium (even that derived from dinitrogen) are particularly acidifying of the root zone. The aims of the present study were to find genotypes of soybean tolerant to aluminum stress and identify QTL underlying that trait. Used were recombinant inbred lines (RILs) derived from the cross of ‘Essex’ by ‘Forrest’. RILs were grown in a greenhouse for 3 weeks and then transferred to hydroponics in a growth chamber. Root lengths (RL) were measured before and 72 h after Al treatment. RL before and after Al treatment were measured and used to calculate root tolerance index (RTI) and relative mean growth (RMG). RILs 1, 85, 40 and 83 had significant (P < 0.005) tolerance to Al stress judged by RL after Al, RTI and RMG. Eleven minor but significant marker–trait associations (P < 0.05) were detected using one-way ANOVA but only two major loci were significant in composite interval maps (LOD >3.0). The QTL on linkage group F (chromosome 13) was in the interval Satt160–Satt252 with a peak at 24 cM (peak LOD was 3.3). The QTL underlay 31% of trait variation and the Essex allele provided an additional 1.61 cm of root growth over 72 h in the presence of Al. The QTL on linkage group C2 (probably chromosome 4) was in the interval from Satt202 to Satt371 with a peak at 3.2 cM (peak LOD was 14.7). The QTL underlay 34% of trait variation or 1.81 cm of growth over 72 h in the presence of Al. Both loci encompassed genes implicated in citrate metabolism, a method of aluminum detoxification known to vary among soybean cultivars. Two major loci and at least nine minor loci were inferred to underlie tolerance to Al. RILs and markers may be used to select alleles that increase tolerance to soybean against Al stress.     

Validation and high-resolution mapping of a major quantitative trait locus for alkaline salt tolerance in soybean using residual heterozygous line

Alkaline soil restricts soybean plant growth and yield. In our previous study, a major alkaline salt tolerance quantitative trait locus (QTL) was identified in soybean on chromosome 17. In this study, the residual heterozygous line (RHL46), which was selected from a population of F6 recombinant inbred lines (RILs) derived from a cross between an alkaline salt-sensitive soybean cultivar Jackson and a tolerant wild soybean accession JWS156-1, was used for validation and high-resolution mapping of the QTL. In a large segregating population (n = 1,109), which was produced by self-pollinating heterozygotes of RHL46, segregation of alkaline salt tolerance showed a continuous distribution, and the tolerant plants were predominant. Linkage mapping analysis revealed a major QTL with a large dominant effect for alkaline salt tolerance, and the highest LOD score was detected between the single sequence repeat (SSR) markers GM17-12.2 and Satt447. Furthermore, 10 fixed recombinant lines carrying chromosome fragments of different lengths in the QTL region were selected from the RHL46 progeny. Phenotype evaluation and SSR marker analysis of the recombinant lines narrowed down the QTL to a 3.33-cM interval region between the markers GM17-11.6 and Satt447 with a physical map length of approximately 771 kb. High-resolution mapping of the alkaline salt tolerance QTL will be useful not only for marker-assisted selection in soybean breeding programs but also for map-based cloning of the alkaline salt tolerance gene in order to understand alkaline salt tolerance in soybean and other plant species.