Identification of novel quantitative trait loci associated with brown planthopper resistance in the rice landrace Salkathi

The brown planthopper (BPH) is a potent pest of rice in Asia and Southeast Asia. Host resistance has been found to be the most suitable alternative to manage the insect. But varietal resistance has been found to be short-lived. There has been a constant search for alternate resistance genes. We developed an F₈ recombinant inbred population for the BPH resistance gene in Salkathi, an indica landrace from Odisha, India. Phenotyping of RILs against the BPH population at Cuttack, Odisha showed continuous skewed variation with four peaks at 2.1–3.0, 4.1–5.0, 6.1–7.0 and 8.1–9.0 SES score, suggesting the involvement of quantitative loci for resistance to BPH in Salkathi. Mapping showed the presence of two QTLs on the short arm of chromosome 4. One QTL, with phenotype variance of 37.02% is located between the markers RM551 and RM335. The other QTL, with phenotype variance of 7.1% is located between markers RM335 and RM5633. The two QTLs have been designated as qBph4.3 and qBph4.4. QBph4.3 seems to be a novel QTL associated with BPH resistance. We have successfully transferred qBph4.3 and qBph4.4 into two elite rice cultivars, Pusa 44 and Samba Mahsuri. Fine mapping of the identified QTLs may lead to a successful transfer of QTLs into other elite germplasm backgrounds.     

Mapping quantitative trait loci for tolerance to phosphorus-deficiency at the seedling stage in barley

Phosphorus (P) deficiency in soil is a major factor that limits barley yield production. Increasing the tolerance to P-deficiency of barley is one of the most cost-effective solutions. Quantitative trait loci (QTLs) controlling P acquisition, P utilization efficiency and biomass at the seedling stage were identified using a population of recombinant inbred lines (RILs) subjected to two P concentrations (low P (LP), 25 µM and normal P (NP), 250 µM). The population was derived from a cross between Baudin and CN4027, which is a Hordeum spontaneum accession. In two hydroponic trials conducted in 2014 and 2016, seventeen QTLs were detected on chromosomes 2H, 3H, 4H and 5H at the two P concentrations. Eight of these QTLs influenced P acquisition efficiency (PAE). Phenotypic variation explained by a particular PAE-related QTL ranged from 13.3 to 39.9%. One QTL designated as Qspue.sau-3H.01 was related to P utilization efficiency (PUE); the phenotypic variation explained by this QTL was 12.5% (NP concentration) and 13.1% (LP concentration), respectively. Strong associations were observed between biomass and P efficiency-related traits in our study. Two QTL clusters controlling biomass, PAE- and PUE-related traits simultaneously were stably identified in the intervals bPb3263664–bPb3931069 and bPb3264570–bPb4786261 on chromosome 3H at both P concentrations in both trials. The QTLs related to PAE, PUE and biomass are important for the P-tolerant phenotype and may offer valuable clues for fine mapping and map-based cloning of barley.     

Identification of quantitative trait loci for seedling cold tolerance using RILs derived from a cross between japonica and tropical japonica rice cultivars

Cold tolerance at the seedling stage of rice is an important phenotypic trait that causes normal plant growth and stable rice production in temperate regions as well as tropical high-lands in Asia and Africa. In order to find quantitative trait loci (QTLs)/genes associated with cold tolerance, we constructed a linkage map using 153 recombinant inbred lines (RILs) derived from a cross between a cold-tolerant temperate japonica cultivar, Geumobyeo, and a cold-sensitive tropical japonica breeding line, IR66160-121-4-4-2. The RILs were phenotyped for cold tolerance or sensitivity based on the degrees of cold tolerance as cold tolerance indices at the seedling stage. The seedlings for cold-tolerance/-sensitive traits were scored on the 7th day of the recovery period at 25°C after cold treatment at 10°C. Two QTLs (qCTS4a and qCTS4b) associated with cold tolerance at the seedling stage were identified on the long and short arms of chromosome 4 with an LOD score of 2.89 and 2.75, respectively, using composite interval mapping. The QTLs were flanked by simple sequence repeat (SSR) markers RM3648-RM2799 and RM3375a-RM558 that explained 8.3 and 7.8% of the total phenotypic variation, respectively. Seven of the selected RILs expressed cold tolerance at both the seedling and reproductive stages. The SSR markers associated with the QTLs will be useful for tracking favorable QTLs/genes into cold-sensitive elite cultivars and may have potential for pyramiding different QTLs for the improvement of cold tolerance in rice.     

Quantitative trait loci for resistance to Fusarium head blight in the Chinese wheat landrace Huangfangzhu

The Chinese wheat landrace Huangfangzhu (HFZ) has a high level of resistance to Fusarium head blight (FHB). To identify chromosomal regions that are responsible for FHB resistance in HFZ, F₈ recombinant inbred lines (RIL) were developed from a cross between HFZ and Wheaton, a U.S. hard spring wheat. FHB was evaluated by single floret inoculation in both greenhouse and field environments. Two quantitative trait loci (QTL) with major effects were identified. One QTL was located on the short arm of chromosome 3B, and explained 35.4% of the phenotypic variation; the other QTL was assigned to 7AL and explained 18.0% of the phenotypic variation for FHB response. In addition, three minor QTL were detected on chromosomes 1AS, 1B and 5AS by single marker regression. HFZ contributed all favorable alleles. The RIL with HFZ alleles at the QTL on 3BS and 7AL displayed significantly lower percentages of infected spikelets than RIL without these alleles in both greenhouse and field environments. HFZ combined several alleles from germplasm reported previously and is a promising alternative source for improving wheat FHB resistance.     

Molecular mapping of quantitative trait loci for plant growth, yield and yield related traits across three diverse locations in a doubled haploid rice population

A doubled haploid (DH) population of 125lines derived from IR64 × Azucena, an indica – japonica cross were grown in three different locations in India during the wet season of 1995. The parents of mapping population had diverse phenotypic values for the eleven traits observed. The DH lines exhibited considerable amount of variation for all the traits. Transgressive segregants were observed. Interval analysis with threshold LOD > 3.00 detected a total of thirty four quantitative trait loci (QTL) for eleven traits across three locations. The maximum number of twenty QTL were detected at Punjab location of North India. A total of seven QTL were identified for panicle length followed by six QTL for plant height. Eight QTL were identified on three chromosomes which were common across locations. A maximum of seven QTL were identified for panicle length with the peak LOD score of 6.01 and variance of 26.80%. The major QTL for plant height was located on Chromosome 1 with peak LOD score of 16.06 flanked by RZ730-RZ801 markers. Plant height had the maximum number of common QTL across environment at the same marker interval. One QTL was identified for grain yield per plant and four QTL for thousand grain weight. Clustering of QTL for different traits at the same marker intervals was observed for plant height, panicle exsertion, panicle number, panicle length and biomass production. This suggests that pleiotropism and or tight linkage of different traits could be the plausible reason for the congruence of several QTL. Common QTL identified across locations and environment provide an excellent opportunity for selecting stable chromosomal regions contributing to yield and yield components to develop QTL introgressed lines that can be deployed in rice breeding program. This revised version was published online in July 2006 with corrections to the Cover Date.     

Identification of quantitative trait loci for resistance to bending-type lodging in rice (Oryza sativa L.)

Bending-type lodging is one of the most important factors affecting the yield and grain quality of rice. This study identified quantitative trait loci (QTLs) for physical strength of the upper culms, and evaluated QTL effects on lodging resistance. In 2010 and 2011, QTLs for breaking strength, length, and diameter of the top three internodes were identified by analyzing chromosomal segment substitution lines (CSSLs) developed from ‘Koshihikari’ and ‘Kasalath’. The QTL analysis indicated that ‘Kasalath’ had two types of QTLs: one to strengthen specific internodes and one to simultaneously improve the physical strengths of plural internodes or the top three internodes. A QTL for breaking strengths of the top three internodes (bsuc11) was detected on chromosome 11 in both years. This QTL did not overlap with that for internode length. To evaluate the effects of bsuc11 on lodging resistance, this study selected three CSSLs with bsuc11 and analyzed the breaking strengths of the top three internodes after heading and the pushing resistance of the lower part. Internodes of ‘Koshihikari’ showed decreased breaking strengths after grain filling, while those of CSSLs with bsuc11 did not show this decrease in breaking strength. The pushing resistance of the lower part at the fully ripe stage was the same in ‘Koshihikari’ and CSSLs with bsuc11. These results suggested that bsuc11 could be a target to improve the physical strength of the upper culms to resist bending-type lodging, and that the physical strengths of upper and lower parts are controlled by different genetic factors in rice.     

Identification of quantitative trait loci for drought tolerance at seedling stage by screening a large number of introgression lines in maize

The maize genome hosts tremendous phenotypic and molecular diversity. Introgression lines (ILs), developed by continuous backcrossing to recurrent parents, could provide a unique genetic stock for quantitative trait locus (QTL) mapping. Using maize lines from six heterotic groups of different ecological zones, we developed >500 BC₂F₂ IL sets by crossing 11 inbred lines (as recurrent parents) with >200 local maize inbred lines (as donor parents). Of them, 34 IL sets were selected as a subset for drought tolerance screening and a total of 417 ILs survived under severe water stress at seedling stage. One set of 32 surviving ILs, derived from Chang7-2/DHuang212, was used for QTL mapping with simple sequence repeat markers covering the whole genome, with seven QTL detected. Furthermore, investigating all surviving ILs, we identified two common regions in bin 3.04, corresponding to marker intervals bnlg1904–umc1772 and umc1223–bnlg1957, respectively, which shared high genetic variation in three IL sets. Our results indicated that selective genotyping can be used to identify genetic loci for complex traits. The ILs, highly selected for drought tolerance in this study, provide a unique set of materials for both genomic studies and development of enhanced germplasm resources.     

Mapping quantitative trait loci for salt tolerance at germination and the seedling stage in barley (Hordeum vulgare L.)

Quantitative trait loci (QTLs) controlling salt tolerance at germination and the seedling stage in barley (Hordeum vulgare L.) were identified by interval mapping analysis using marker information from two doubled haploid (DH) populations derived from the crosses, Steptoe/Morex and Harrington/TR306. Interval mapping analysis revealed that the QTLs for salt tolerance at germination in the DH lines of Steptoe/Morex were located on chromosomes 4 (4H), 6(6H), and 7(5H), and in the DH lines of Harrington/TR306 on chromosomes 5(1H) and 7(5H). In both DH populations, the most effective QTLs were found at different loci on chromosome 7(5H). Genetic linkage between salt tolerance at germination and abscisic acid (ABA) response was found from QTL mapping. The QTLs for the most effective ABA response at germination were located very close to those for salt tolerance on chromosome 7 (5H) in both crosses. The QTLs for salt tolerance at the seedling stage were located on chromosomes 2(2H), 5(1H), 6(6H), and 7(5H) in the DH lines of Steptoe/Morex, and on chromosome 7(5H) in the DH lines of Harrington/TR 306. Their positions were different from those of QTLs controlling salt tolerance at germination, indicating that salt tolerance at germination and at the seedling stage were controlled by different loci. This revised version was published online in July 2006 with corrections to the Cover Date.     

Carbon isotope discrimination: potential for screening salinity tolerance in rice at the seedling stage using hydroponics

Rice is a moderately salt‐sensitive crop species and soil salinity is the single most widespread soil toxicity problem lacing rice production. The quantification of salinity resistance poses serious problems in the field because of climatic factors and field heterogeneity. In the present study. rice germplasm obtained from the International Rice Research Institute (IRRI). Philippines, was screened in a naturally lit (11 h daylight) glasshouse‐based hydroponics unit at two salinity levels (4 and 6 dS/m), Phenotypic performance based on survival of tolerant, moderately tolerant and susceptible isogenic lines along with tolerant and susceptible parents was evaluated after 10 and 13 days of salt stress. Plants were harvested after second scoring and carbon isotope discrimination in the leaves (A) was measured. δ¹ ranged from 19.5 to 22.9%, A highly significant negative correlation (r =?0.95. P < 0.001) between δ and visual scoring was observed. Data indicated the potential of using δ as a physiological indicator for salinity tolerance in rice seedlings grown in hydroponics.     

Potential for effective marker-assisted selection of three quantitative trait loci conferring adult plant resistance to powdery mildew in elite wheat breeding populations

Three quantitative trait loci (QTL) associated with adult plant resistance (APR) to powdery mildew (Blumeria graminis) in wheat (Triticum aestivum) cultivar ‘Massey’ were mapped in a previous study. The three QTL were located on chromosomes 2A, 2B and 1B, and explained 50% of the total phenotypic variation. A 293 recombinant inbred line (RIL) breeding population (UJ) derived from the cross of ‘USG 3209’, a derivative of ‘Massey’, and ‘Jaypee’ was used to evaluate the potential effectiveness of marker‐assisted selection (MAS) for APR. Powdery mildew severities of the 293 UJ RILs were evaluated in 2002 (F_{5 : 6}) and 2003 (F_{6 : 7}) under natural disease pressure in the field. The 293 RILs were also evaluated for disease severity in a 2004 (F_{7 : 8}) greenhouse experiment using a composite of five different isolates of B. graminis. Selection of RILs possessing the QTL on chromosome 2A, and to a lesser extent, the one on chromosome 1B was effective in identifying powdery mildew resistance in both greenhouse and field experiments. Overall, selecting RILs with QTL on chromosomes 2A and 2B was most successful in identifying highly resistant RILs, which had mean mildew severities of 4.4% and 3.2% in 2002 and 2003 field experiments, respectively. Breeders implementing MAS programs for APR to powdery mildew via selection of RILs containing the two QTL on chromosomes 2A and 2B likely will obtain RILs having high levels of resistance in the field, however combining all three QTL may ensure greater durability.     

Molecular mapping of quantitative trait loci for field resistance to Fusarium head blight in a European winter wheat population

Fusarium head blight (FHB), one of the most destructive diseases of wheat in many parts of the world, can reduce the grain quality due to mycotoxin contamination up to rejection for usage as food or feed. Objective of this study was to map quantitative trait loci (QTL) associated with FHB resistance in the winter wheat population ‘G16‐92’ (resistant)/‘Hussar’. In all, 136 recombinant inbred lines were evaluated in field trials in 2001 and 2002 after spray inoculation with a Fusarium culmorum suspension. The area under disease progress curve was calculated based on the visually scored FHB symptoms. For means across all environments two FHB resistance QTL located on chromosomes 1A, and 2BL were identified. The individual QTL explained 9.7% and 14.1% of the phenotypic variance and together 26.7% of the genetic variance. The resistance QTL on 1A coincided with a QTL for plant height in contrast to the resistance QTL on 2BL that appeared to be independently inherited from morphological characteristics like plant height and ear compactness. Therefore, especially the QTL on 2BL could be of great interest for breeding towards FHB resistance.     

Identification of quantitative trait loci for cold tolerance during the germination and seedling stages in rice (Oryza sativa L.)

Low temperature is a serious abiotic stress affecting rice production in subtropical and temperate areas. In this study, cold tolerance of rice at the germination and seedling stages were evaluated using one recombinant inbred line (RIL) population derived from a cross between Daguandao (japonica), with highly cold-tolerant at the seedling stage, and IR28 (indica), with more cold-tolerant at the germination stage, and the quantitative trait loci (QTL) mapping was conducted using the multiple interval mapping (MIM) approach. Continuous segregation in low temperature germinability (LTG) and cold tolerance at the seedling stage (CTS) were observed among the RIL populations. Most RILs were moderately susceptible or tolerant at the germination stage, but were susceptible at the seedling stage. No significant relationship was found in cold tolerance between the germination and seedling stages. A total of seven QTLs were identified with limit of detection (LOD) >3.0 on chromosomes 3, 8, 11 and 12, and the amount of variation (R ²) explained by each QTL ranged from 5.5 to 22.4%. The rice LTG might be regulated by two minor QTLs, with the CTS controlled by one major QTL [qCTS8.1 (LOD = 16.1, R ² = 22.4%)] and several minor loci. Among these loci, one simultaneously controls LTG (qLTG11.1) and CTS (qCTS11.1). Several cold-tolerance-related QTLs identified in previous studies were found to be near the QTLs detected here, and three QTLs are novel alleles. The alleles from Daguandao at six QTLs increased cold tolerance and could be good sources of genes for cold tolerance. In addition, only one digenic interaction was detected for CTS, with a R ² value of 6.4%. Those major or minor QTLs could be used to significantly improve cold tolerance by marker-assisted selection (MAS) in rice.     

Identification of quantitative trait loci (QTLs) for the characters of vascular bundles in peduncle related to indica-japonica differentiation in rice (Oryza sativa L.)

The number of vascular bundles in peduncle and the ratio of vascular bundles to primary rachis branches (V/R ratio)distinguishable between indica andjaponica, are the traits associated with the processes of differentiation between indica and japonica inrice (Oryza sativa L.). In this paper a doubled-haploid population derived from the F₁ hybrid of a cross between anindica cultivar and a japonicacultivar was used to map quantitative trait loci(QTLs) controlling numbers of vascular bundles in peduncle, primary rachis branches and the V/R ratio. For vascular bundles, three QTLs were detected and they collectively explained 58.8% of the total variation. Among them, the QTLqVB-8 with the largest effect,located on chromosome 8, individually accounted for 31.1% of the total variation. Two QTLs controlling primary rachis branches, located on chromosome 8and 10 respectively, were identified and they individually explained 10.5% and18.0% of the total variation respectively. Three QTLs for the V/R ratio, mapped on chromosome 1, 2 and 8, respectively,jointly explained 61.3% of the total variation. Of the three QTLs, the QTL qV/R-1 with the largest additive effect,explained 25.3% of the total variation,was located on chromosome 1 and found to be closely linked to the gene sh-2, a major gene underlying grain-shattering ability. In addition, four and two pairs of significant epistatic QTLs were detected for vascular bundles and the V/R ratio,respectively, but none for rachis branches. Our results suggested that the numbers of vascular bundles and primary rachis branches were independently controlled by different polygenic systems, but the two polygenic systems shared a fraction of quantitative trait loci. The present study also demonstrated that the chromosome region carrying the QTL qV/R-1 for the V/R ratio and the gene sh-2 might play an important role in the processes ofindica-japonica differentiation in rice (Oryza sativa L.). This revised version was published online in August 2006 with corrections to the Cover Date.     

Genetic mapping of quantitative trait loci (QTL) for resistance to septoria tritici blotch in a winter wheat cultivar Liwilla

Septoria tritici blotch (STB), caused by Mycosphaerella graminicola (anamorph Septoria tritici, syn. Zymoseptoria tritici), is present in most wheat-growing areas worldwide. Resistance breeding appears to be the most sensible approach to disease control. An attempt was made to identify loci associated with resistance to STB in a resistant winter wheat cultivar Liwilla. In the study we used a set of 74 doubled-haploid lines generated from anthers of F₁ hybrids between the resistant cultivar Liwilla and susceptible cultivar Begra. Four monopycnidiospore isolates of M. graminicola with diverse pathogenicity were used in tests on seedlings under controlled growth conditions and on adult plants under polytunnel conditions over a six year period. In both environments, the percentage leaf area covered by necrosis and covered by pycnidia were measured; time to heading and plant height were also recorded for the polytunnel experiments. Seven isolate-specific quantitative trait loci (QTLs) were associated with STB resistance: QStb.ihar-3A.2, QStb.ihar-6A, QStb.ihar-7A.2, QStb.ihar-1B, QStb.ihar-2B.2, QStb.ihar-3B, and QStb.ihar-5D. QTL on chromosome 5D and 7A represent novel STB resistance loci. The phenotypic variance explained by individual QTLs ranged from 9.5 % to 50.3 %. Three QTLs detected on chromosomes 3A, 7A and 1B showed major effects and were detected consistently in different environments. The locations of QStb.ihar-3A.2 and QStb.ihar-1B coincide with the resistance genes Stb6 and Stb11, respectively. Locus QStb.ihar-3B and a QTL for time to heading mapped to the same location, but are most likely not associated. Most of the mapped QTLs explain the resistance associated with both low necrosis and low pycnidia coverage.     

Specific resistance to leaf rust expressed at the seedling stage in cultivars grown in France from 1983 to 2007

Host resistance is the most economical means to reduce yield losses caused by wheat leaf rust. Knowledge of the effective specific resistance genes is a prerequisite for analysis of the non-specific components of resistance, assumed to be more durable than specific resistance. Lr genes were inferred from seedling response phenotype of 275 wheat cultivars and 21 standard isolates of Puccinia triticina. Enough cultivars were selected for analysis so that findings would account for at least two-thirds of the French agricultural land dedicated to wheat from 1983 to 2007. In this paper, genes Lr13, Lr37, Lr10, Lr14a, Lr3, Lr26, Lr1, Lr24, Lr20 are postulated, alone or in combinations, in, respectively, 67%, 45%, 34%, 20%, 8%, 7%, 6%, 1%, and 1% of the cultivars. Forty five phenotypic arrays were found, the most frequent being (Lr10, Lr13, Lr37) and (Lr13) in 45 and 37 cultivars, respectively. Over the period, the combinations became increasingly complex. Isolates with virulence corresponding to most of the Lr gene combinations were identified in the pathogen population, except for combinations involving Lr24 and some unidentified genes. These findings will help breeders and extension service staff (Arvalis) in diversifying sources of resistance to wheat leaf rust. This information is also crucial for research programs aiming, on the one hand, to identify sources of quantitative resistance, and, on the other hand, to quantify selection pressure exerted on pathogen populations.     

Tagging quantitative trait loci for dormancy, tuber shape, regularity of tuber shape, eye depth and flesh colour in diploid potato originated from six Solanum species

Potato breeding aims at breeding diversified cultivars not only suitable for different purposes, but also resistant to diseases, such as late blight caused by Phytophthora infestans , which is a major constraint in potato production. Resistance to P. infestans has been previously introduced into the diploid hybrid population 98-21 from Solanum verrucosum and Solanum microdontum . In the present study, we assessed the segregation of tuber dormancy, tuber shape, regularity of tuber shape, eye depth and flesh colour in this population. Quantitative Trait Loci (QTLs) affecting these important quality traits were tagged using the genetic map developed for this population to locate QTLs for late blight resistance. The most prominent QTL for dormancy was detected on chromosome II and explained 7.1% of the variance. The most important QTLs for tuber eye depth, flesh colour, shape and shape regularity were identified on chromosomes X ( R ² = 14.7%), IV ( R ² = 5.8%), II ( R ² = 8.0%) and III ( R ² = 10.4%) respectively. All traits were also affected by minor QTLs. The obtained results improve our understanding of the inheritance of traits relevant for variety development in potato.     

Fine mapping of qSS‐9, a major and stable quantitative trait locus,for seed storability in rice (Oryza sativa L.)

Seed storability in rice (Oryza sativa L.) is an important agronomic trait. We previously showed a quantitative trait locus of seed storability, qSS‐9, on chromosome 9 in a backcross population of ‘Koshihikari’ (japonica) / ‘Kasalath’ (indica) // ‘Koshihikari’. In this study, fine mapping of the chromosomal location of qSS‐9 was performed. Effect of ‘Kasalath’ allele of qSS‐9 was validated using a chromosome segment substitution line, SL36, which harboured the target quantitative trait loci (QTL) from ‘Kasalath’ in the genetic background of ‘Nipponbare’ under different ageing treatments in different environments. Subsequently, an F₂ population from a cross between ‘Nipponbare’ and SL36 was used for fine mapping of qSS‐9. Simultaneously, four subnear isogenic lines (sub‐NILs) that represented different recombination breakpoints across the qSS‐9 region were developed from F₃ progeny. Finally, the qSS‐9 locus was located between the Indel markers Y10 and Y13, which delimit a region of 147 kb in the ‘Nipponbare’ genome. These results provide a springboard for map‐based cloning of qSS‐9 and possibilities for breeding rice varieties with strong seed storability.     

Quantitative trait loci associated with seed set under high temperature stress at the flowering stage in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

High temperature stress (HTS), an increasingly important problem in rice production, significantly reduces rice yield by reducing seed set percentage (SSP). Breeding rice varieties with tolerance to HTS at the flowering stage is therefore essential for maintaining rice production as the climate continues to warm. In this study, two quantitative trait loci (QTL) underlying tolerance to HTS were identified using the recombinant inbred lines (RILs) derived from a cross between the HTS-tolerant rice cultivar 996 and the sensitive cultivar 4628. SSP was used as the heat-tolerance indicator for the lines, which were subjected to HTS at the flowering stage in both field and growth chamber experiments. Two major QTL that affected SSP in both conditions were detected in the interval between RM5687 and RM471 on chromosome 4, and between RM6132 and RM6100 on chromosome 10. The QTL located on chromosome 4 explained 21.3% in field and 25.8% in growth chamber of the total phenotypic variation in SSP, and increased the SSP of plants subjected to HTS by 9.1% in field and by 9.3% in growth chamber. The second QTL located on chromosome 10 explained 11.5% in field and 11.6% in growth chamber of the total phenotypic variation in SSP, and increased the SSP of plants subjected to HTS by 7.2% in field and 7.0% in growth chamber. The positive additive effects of the two QTL were derived from the 996 alleles. The two major QTL identified in this study could be useful for further fine mapping and cloning of these genes and for molecular marker-aided breeding of heat-tolerant rice cultivars.