Among the biotic stressors of rice, blast (BL), bacterial blight (BB), white backed planthopper (WBPH), and brown planthopper (BPH) are the four major pathogens affecting rainfed lowland rice; they are caused by Magnaporthe grisea, Xanthomonas oryzae pv. oryzae, Sogatella furcifera, and Nilaparvata lugens, respectively. These destructive pests greatly reduce rice yield throughout the rainfed lowlands in Asia and also in the northeast (NE) of Thailand   . The NE region accounts for over half of Thailand’s total rice production area  and is well known as a major producer of premium Jasmine rice “Khao Dawk Mali 105” (KDML105). Approximately 65% of the area is dominated by KDML105, which is well adapted to the low-yielding environment of rainfed lowland conditions  . However, this strain is highly susceptible to BL, BB, WBPH, and BPH. In 2018, BL and BPH outbreaks destroyed close to 80% of the rice crop in the rainfed lowland areas of the lower part of the NE  . Therefore, the development of resistant varieties is considered to be the most effective and economical means of maintaining yield stability by controlling the major rice pests in the frequent outbreak areas   .
To date, the various locations on the rice chromosomes of BL, BB, WBPH, and BPH resistance genes have been reported. More than one-hundred major resistance genes and over 350 quantitative trait loci (QTLs) for BL have been identified in the rice genome   and, among them, 30 genes have already been cloned  . About 44 major genes conferring resistance to BB have been identified in cultivated rice, wild relatives, and mutation-induced lines    . At least 34 major genes have been identified for resistance against planthoppers from wild and cultivated rice germplasm  . Taken together, this signifies that recent advances in rice genomic research have enabled the rapid identification of various rice pest resistance genes/QTLs and provided DNA markers for marker-assisted selection (MAS). MAS has been used to transfer and pyramid genes/QTLs associated with biotic stressors into KDML105. Numerous promising lines and varieties that maintain the cooking quality and fragrance of KDML105 have been developed   . These improved varieties have been released in the target area in NE Thailand  . However, varieties containing a major resistance gene induce the rapid evolution of new biotypes of pathogens   . The continuous evolution of pathogenic biotypes causes the breakdown of resistance in many improved varieties. Therefore, novel resistance genes are still highly in demand for continued rice improvement and more analyses of resistance genes are needed to combat destructive pests going forward.
The present study uses two elite rice lines, IR71033-121-15 (IR71033) and IR57514-PMI-5-B-1-2 (IR57514), from the International Rice Research Institute. The introgression line IR71033, derived from an interspecific cross between IR31917-45-3-2 and a wild species Oryza minuta, has been intensively used as a donor for biotic stress resistance studies     . The elite line IR57514, derived from a three-way cross of IR43581-57-3-3-6/KDML105// IR21836-90-3, is well adapted to the low-yielding rainfed lowland environment in the NE of Thailand   . IR71033 is resistant to BL, BB, WBPH, and BPH and IR57514 is resistant to BL and BB. These two rice lines were selected as donors for biotic resistance in breeding programs for improving rice cultivation in rainfed lowland area in NE. To identify biotic stress resistance factors in both rice lines, two recombinant inbred lines (RILs) were developed and used as mapping populations.
In the present study, next-generation sequencing (NGS) technology was used to discover single nucleotide polymorphism (SNP). NGS has reduced both the cost and the time required to generate sequence data. For SNP identification, NGS methods were combined with restriction enzyme digest to reduce genome complexity and to enable cost efficiency  . These data were then used to generate SNP-based linkage maps. The objective of this study was to identify loci associated with BL, BB, WBPH, and BPH resistance by QTL analysis using SNP marker sets. The tightly-linked SNP markers will facilitate MAS, and the investigated rice lines can serve as genetic resources for BL, BB, WBPH, and BPH resistance studies to strengthen the resilience of rice varieties and improve breeding programs, especially in rainfed lowland regions.
2. Materials and Methods
2.1. Plant Materials and Mapping Populations
Two RIL populations derived from crosses of KDML105 × IR71033-121-15 (286 lines) and KDML105 × IR57514-PMI-5-B-1-2 (288 lines) were used as the mapping population. KDML105 is susceptible to BL, BB, WBPH, and BPH. IR71033 is resistant to BL, BB, WBPH, and BPH. IR57514 is resistant to BL and BB. The cross KDML105 × IR71033 (hereafter called “KD-IR71033”) was developed at the Ubon Ratchathani Rice Research Center (rice lines were designated as UBN04128), while the cross KDML105 × IR57514-PMI-5-B-1-2 (hereafter called “KD-IR57514”) was developed at the Phitsanulok Rice Research Center (rice lines were designated as PSL99093). Rice cultivars, TN1, KDML105, KTH17, RD10, PSL2, Rathu Heenati, PTB33, HY71, SPR1, RD7, IRBB5, and IRBB7, were used as resistant and susceptible controls for resistance evaluations (Table 1).
2.2. Phenotypic Evaluations
2.2.1. Field BL Nursery Evaluation
The upland BL nurseries were set up in the wet season at three rice research centers: Ubon Ratchathani Rice Research Center (UBN-RRC) in 2016 and 2018, Sakon Nakhon Rice Research Center (SKN-RRC) in 2016 and 2018, and Phrae Rice Research Center (PRE-RRC) in 2016 (Table 2, Figure 1). Seeds for 286 and 288 RILs from KD-IR71033 and KD-IR57514, respectively, were planted in single-row plots, 60 cm in length, in BL nursery beds. Every five entries were separated by a susceptible check (KDML105 or KTH17) and resistance check (SPR1 or HY71). KDML105 was planted alongside the nursery bed in spreader rows to enhance the spore transmission. Plots and spreader rows were inoculated with a local natural mix of strains. Standard Evaluation System (SES) ratings for BL  were used when all of the susceptible control rows died.
Table 1. Details of rice varieties used as resistant and susceptible controls for the phenotypic evaluations.
Table 2. Phenotypic trait and resistance evaluations for the two RILs populations, KD-IR71033 and KD-IR57514 at Rice Research Centers in Thailand.
DS based on damage score, LL based on lesion leaf length.
Figure 1. A map of trait evaluation sites () and sample collection sites of the Xoo isolates (p) and planthopper populations (n) across north and northeast (NE) Thailand. Light gray areas represent the rainfed lowlands in the NE region.
2.2.2. Bioassay for BB Resistance
Xoo isolates collected from the Ubon Ratchathani (UBN), Sakon Nakhon (SKN), Udon Thani (UDN), and Phrae (PRE) provinces from 2016 to 2018 were used for BB resistance evaluation at the UBN-RRC, SKN-RRC, and RRE-RRC in the wet season (Figure 1). The isolate was grown in nutrient agar media for 72 hours at 28˚C. The bacterial cells were suspended in sterile water and adjusted to ~108 CFU/ml. Xoo isolates were assayed for a resistance reaction in RIL populations, susceptible (RD10, PSL2) and resistance (RD7, IRBB5, IRBB7) controls. BB inoculation was performed in a greenhouse using the leaf-clipping method  . Forty-five days after sowing, two to three fully expanded leaves of each RIL plant were inoculated. Resistance reactions were recorded based on the mean of the percent of the leaf area that appeared to be diseased  and the lesion length  of an individual plant 12 - 14 days after inoculation. The percentage of affected leaf area was used for SKN2016, UDN2016, and UBN2017, while leaf lesion length was used for UBN2016, SKN2018, and UDN2018 (Table 2). A damage score between 0 and 3 and a lesion length shorter than 5 cm were regarded as resistant to BB.
2.2.3. Bioassay for Planthopper Resistance
RILs from the KD-IR71033 population were evaluated for WBPH resistance at the seedling stage and BPH resistance at the tilling stage. A WBPH population was collected from a rice field in the Phitsanulok (PSL) province in 2017 (Figure 1). The WBPH population was maintained on TN1 in a greenhouse at the Phitsanulok Rice Research Center (PSL-RRC). The seedbox screening test  was conducted to evaluate WBPH resistance in the test lines. Ten pre-germinated seeds of each test entry, susceptible (TN1) and resistance (PTB33) controls were sown in seedboxes (60 cm × 40 cm × 10 cm) containing well-puddled soil in the 12-cm rows. Seven days after sowing, the seedlings were infested with second and third instar nymphs of WBPH at ten nymphs per seedling. When all seedlings of TN1 were almost dead, the SES scoring  was used to assign a resistance score to each genotype. A BPH population from a single colony was collected in the outbreak field from the UBN province in 2015 (Figure 1) and was grown on a susceptible variety of TN1 in a temperature-controlled rearing room (25˚C ± 2˚C) at the UBN-RRC. The screening method, which was modified from Jairin et al.  , was conducted in a greenhouse at the UBN-RRC. The seeds of each RIL progeny, a susceptible cultivar (TN1), and a resistant cultivar (Rathu Heenati) were separately sown (10 × 20 cm) in 7 × 24 m2 seedling plots. Twenty days after sowing, the seedlings were infested with 3rd-4th instar nymphs of BPH at ten nymphs per seedling. Then, the insects fed, mated, laid eggs, and hatched freely. We evaluated the severity scores of the test lines according to SES  until TN1 and the susceptible recurrent parents died.
2.3. Genotypic Data
2.3.1. DNA Extraction
Samples included the parental and the two mapping populations, 286 RILs from KD-IR71033 and 288 RILs from KD-IR57514. Genomic DNA was extracted from the young leaves of each sample using the standard cetyltrimethylammonium bromide (CTAB) method  and quantified with a Nanodrop-1000 spectrophotometer (Thermo Fisher Scientific).
2.3.2. SNP Genotyping
A genotyping by sequencing (GBS) protocol using the ApeKI enzyme was applied to prepare the reduced representation libraries for sequencing. The forward adapters contained 9-bp unique barcodes in addition to the 21-bp Ion Forward adapter. The ApeKI restriction site was used to enable multiplex sequencing of the libraries. Genomic DNA digestion and adapter ligation were performed as described in Mascher et al.  . DNA fragments of 250 - 300 bp were selected using E-Gel™ SizeSelect™ Agarose Gels (Invitrogen). The libraries were sequenced on the Ion S5™ XL Sequencer (Thermo Fisher Scientific) with a loaded Ion 540™ Chip according to the manufacturer’s protocol. We multiplexed between 24 samples per run. All sequencing data generated in the present study were analyzed using Ion Torrent™ Suite Software Alignment Plugin V5.2.2 (Thermo Fisher Scientific) with the Nipponbare genome  as the alignment reference. The variants were called using the Torrent Variant Caller (Thermo Fisher Scientific). Filtered vcf files were imported into TASSEL 5  . The default parameters of TASSEL were used to detect informative SNPs. SNPs were named according to their scaffold and base pair position within the Nipponbare genome.
2.4. Linkage Map Construction
The SNP markers with over 30% missing data in the mapping population were excluded from analysis. Also excluded were SNPs that did not show polymorphisms in the parents but were polymorphic in the mapping populations. Segregation distortion of the individual markers was calculated using the Chi-square test on a 1:1 basis. Since segregation distortion has very little effect on marker order and map length  , the SNPs showing segregation distortion with χ2 < 10 were included in the linkage analysis. The remaining SNP markers were used to construct genetic linkage maps in QTL IciMapping 4.1  using the parameters set for the RIL population type. Marker groupings were manually anchored by their physical marker locations on the chromosomes. The filtered markers were then used to construct linkage maps with the MAP function in QTL IciMapping 4.1. The map distance (centimorgans, cM) was converted by recombination fractions using the Kosambi function  .
2.5. QTL Analysis
QTL maps of the BL, BB, WBPH, and BPH resistance phenotypes from the two RIL populations were generated using MapQTL 6.0  . Chisquare goodness-of-fit was employed to fit the segregation pattern of the genotypic and phenotypic ratios. The multiple QTL model (MQM) mapping method was utilized to detect significant associations between phenotypic traits and marker data sets. Significant LOD thresholds were determined using the permutation test at 1000 permutations per trait. The phenotypic variance explained (PVE) by a single QTL was estimated based on the population variance found within the progeny by a maximum likelihood estimation; those with a PVE greater than 15% were considered to be major QTLs.
3.1. Phenotypic Evaluation
3.1.1. BL Resistance
Overall, 286 and 288 RILs from the KD-IR71033 and KD-IR57514 populations, respectively, were screened for BL resistance in the uniform BL nursery with natural infection at UBN-RRC, SKN-RRC, and PRE-RRC. IR71033 and IR57514 were resistant to BL, while KDML105 was highly susceptible in all of the nurseries. This indicated that IR71033 and IR57514 might be carrying broad field resistance against BL. The frequency distributions of the BL damage score for both RIL populations according to location and year are shown in Figure 2. The segregation of resistant and susceptible individuals in the KD-IR71033 population fitted a ratio of 1:3 (UBN2016 χ2 = 1.45, P = 0.23; UBN2018 χ2 = 3.53, P = 0.06; SKN2016 χ2 = 3.08, P = 0.08; PRE2016 χ2 = 1.29, P = 0.26), while in the KD-IR57514 population fitted a ratio of 3:1 (SKN2018 χ2 = 3.17, P = 0.07; UBN2018 χ2 = 3.68, P = 0.06).
3.1.2. BB Resistance
Two scoring methods were used to quantify BB resistance. In 2017 at UBN-RRC, an SES-based damage score was used to evaluate the resistance of the rice lines, while in 2016 and 2018 at UBN-RRC and SKN-RRC, the lesion length on the fully expanded leaves was used (Table 2). IR71033 and IR57514 were resistant to BB, while KDML105 was highly susceptible to all strains in all locations. The frequency distributions of BB damage scores and the lesion lengths for both RIL populations according to the location are presented in Figure 2. The segregation of resistant and susceptible individuals in the RIL populations showed a good fit to the expected ratio of 1:3 (KD-IR71033 population: UBN2016 χ2 = 0.27, P = 0.60; UBN2017 χ2 = 0.04, P = 0.84; SKN2018 χ2 = 0.72, P = 0.39; UDN2016 χ2 = 2.14, P = 0.14; UDN2018 χ2 = 0.44, P = 0.50, KD-IR57514 population: UDN2018 χ2 = 0.17, P = 0.68; SKN2018 χ2 < 0.01, P = 0.97). The segregation pattern indicated that BB resistance derived from IR71033 and IR57514 was controlled by a single recessive gene.
3.1.3. Planthopper Resistance
The donor parent plant IR71033 displayed moderate resistance to WBPH and BPH, while KDML105 was completely susceptible to both insect pests. The level of resistance against BPH of RIL plants could not detected at seedling stage using standard seedbox screening test. Therefore, we decided to screen all RILs at the tillering stage for BPH resistance. The distribution of the damage scores for the progeny of KD-IR71033 was skewed towards susceptibility (Figure 2). The segregation of resistant to susceptible plants was in agreement to 1:3 segregation for WBPH at the seedling stage (χ2 = 0.56, P = 0.45) and BPH at the vegetative stage (χ2 < 0.01, P = 0.97). The segregation pattern indicated that planthopper resistance derived from the donor IR71033 was controlled by a single recessive gene.
3.2. Construction of Linkage Maps
3.2.1. KD-IR71033 Mapping Population
A total of 1897 polymorphic SNPs were used to construct the linkage map for KD-IR71033 and spanned a total genetic distance of 1899.7 cM with linkage groups ranging from 69.9 cM (chromosome 12) to 204.2 cM (chromosome 11). The number of markers mapped to each chromosome varied from 108 (chromosome 9) to 223 (chromosome 1). An average of one SNP per cM region was detected across the genome with three large gaps on chromosomes 9, 10, and 11 (Figure 3).
3.2.2. KD-IR57514 Mapping Population
We used QTL IciMapping 4.1 to construct the linkage map for KD-IR57514 utilizing a total of 2237 polymorphic SNPs that spanned a total genetic distance of 2370.4 cM on all of the 12 chromosomes. The number of markers mapped to each chromosome varied from 64 (chromosome 7) to 322 (chromosome 4). The genetic length of the smallest linkage group was 120.3 cM on chromosome 9, and the largest was 311.3 cM on chromosome 2 (Figure 3).
Figure 2. Frequency distribution of biotic stress resistance traits in the RIL population derived from the crosses of KDML105 × IR71033-121-15 and KDML105 × IR57514-PMI-5-B-1-2.
Figure 3. GBS-based genetic linkage maps of the two RIL populations from the crosses of KDML105 × IR71033-121-15 and KDML105 × IR57514-PMI-5-B-1-2. Chromosome numbers are indicated at the top of each map. Black triangles and square dots indicate the peak positions of the QTLs.
3.3. QTL Analysis
3.3.1. QTL Mapping of BL Resistance Loci
The linkage maps for KD-IR71033 and KD-IR57514 and the phenotypic trait evaluation data were used to map the QTLs associated with BL resistance. MQM mapping analysis detected QTLs for BL resistance in the same genomic regions for all of the three environments (UBN-RRC, SKN-RRC, and PRE-RRC) from 2016 to 2018. At UBN-RRC, QTL-mapping analysis revealed three QTLs associated with BL resistance on chromosomes 9, 11, and 12 of KD-IR71033 (in 2016 and 2018) and KD-IR57514 (in 2018) mapping populations (Table 3). The percent of PVE for BL from these QTL ranged from 7.4% to 59.3% (Table 3). The major QTL from KD-IR71033 was located on chromosome 12 and flanked by SNP markers S12_8036165 and S12_10973592 in 2016 and S12_10254600 and S12_10973592 in 2018 with PVEs of 31.9% and 46.3%, respectively. At SKN-RRC, the only major QTL was detected on chromosome 12 between S12_10060136 and S12_14061647 in 2016 for KD-IR71033 with a PVE of 19.7%. In 2018, three QTLs were detected on chromosomes 9, 11, and 12 from KD-IR57514 at SKN-RRC with 6.1%, 7.3%, and 36.7% PVE, respectively. From PRE-RRC in 2016, we identified three QTLs on chromosomes 9, 11, and 12 in KD-IR71033 with 19.3%, 7.3%, and 47.4% PVE, respectively. Fewer QTLs were observed for BL at SKN-RRC than at other nurseries. All QTLs showed positive additive effect values, indicating that the alleles that confer resistance to BL are those from IR71033 and IR57514 (Table 3) and the major QTLs (qBL1-12UBN16, qBL1-12UBN18, qBL1-12SKN16, qBL1-12PRE16, qBL2-12UBN18, qBL2-12SKN18) were mapped in the same Pi-ta region on chromosome 12.
3.3.2. QTL Mapping of BB Resistance Locus
The MQM mapping analysis detected major QTLs for BB resistance on chromosome 11 of both KD-IR71033 (qBB1-11UDN16, qBB1-11UBN16, qBB1-11UBN17, qBB1-11SKN16) and KD-IR57514 (qBB1-11UDN18, qBB1-11SKN18) with LODs ranging from 8.5 to 52.2 that explained 13.5% to 57.3% of the trait variations (Table 3). The QTL detected from KD-IR71033 was flanked with SNP markers S11_28162887 and S11_28977529 and from KD-IR57514 with S11_27446665 and S11_28704532. The QTLs detected from KD-IR71033 explained 43.9%, 54.3%, 40.7%, and 13.5% of the PVE of qBB1-11UDN16, qBB1-11UBN16, qBB1-11UBN17, and qBB1-11SKN16, respectively. The QTL detected from KD-IR57514 in 2018 explained 28.2% and 41.7% of the PVE of qBB1-11UDN18 and qBB1-11SKN18, respectively (Table 3). The alleles that confer resistance to BB were from IR71033 and IR57514. The major QTL region was shared with the cluster region of Xa4 and Xa26.
3.3.3. QTL Mapping of Planthopper Resistance Locus
WBPH and BPH resistance evaluations were conducted in a greenhouse at PSL-RRC and UBN-RRC in 2018 and 2016, respectively. IR71033 was moderately resistant to the planthoppers, while IR57514 and KDML105 were completely susceptible. Therefore, only the KD-IR71033 mapping population was used to screen and map WBPH and BPH resistance loci. From QTL mapping, two QTLs (qWBPH1-6PSL18 and qBPH1-6UBN16) associated with WBPH and BPH resistance, respectively, were identified. The major QTL for WBPH and BPH were located between the SNP markers S6_841740 and S6_1394987 on the short arm of chromosome 6 and explained 19.7% and 61.8% of the phenotypic variation, respectively (Table 3 and Figure 3). The major QTL region was found to contain the published BPH-resistance gene Bph32.
Table 3. Putative QTLs associated with BL, BB, WBPH, and BPH resistance from the two RILs populations, KD-IR71033 and KD-IR57514.
PVE: percent phenotypic variation; Add: additive effect; * The numbers “1” and “2” before the hyphen indicates the RIL populations of KD-IR71033 and KD-IR57514, respectively.
3.3.4. Number of Predicted Genes Anchored within Targeted QTLs
For BL resistance, the QTLs qBL1-9 and qBL2-9 on chromosome 9 from IR71033 and IR57514 were detected in the regions between SNP markers S9_8172820 and S9_9911937. There are 93 annotated genes within this region in the Nipponbare genome including three BL resistance genes (Pi3, Pi5, and Pi15). The qBL1-11 and qBL2-11 regions between the S11_25260851 and S11_27584310 markers on chromosome 11 contain 85 predicted genes, which include the Pik and Pi44 genes. The major QTLs (qBL1-12UBN16, qBL1-12UBN18, qBL1-12SKN16, qBL1-12PRE16, qBL2-12UBN18, qBL2-12SKN18) were in the Pi-ta region. The 718.9-kb QTL region contains 44 predicted genes.
For BB resistance, the qBB1-11 and qBB2-11 regions contain 85 predicted genes including the Xa3/Xa26, Xa4, Xa40, and xa44 genes. The QTLs from IR71033 qWBPH1-6 and qBPH1-6 associated with WBPH and BPH resistance, respectively, were detected in the Bph32 genomic region between S6_1045301 and S6_1394987 on chromosome 6. This 349.7-kb region of the Nipponbare genome contains 39 predicted genes.
The BL, BB, WBPH, and BPH pathogens are the most devastating for the rice plants in rainfed lowland of NE Thailand   . KDML105 and its induced-mutation varieties (RD15 and RD6) have been wildly grown in this rainfed lowland area, which occupies approximately 5.7 million hectares  . All varieties have the same genetic background and are highly susceptible to BL, BB, WBPH, and BPH due to their lack of resistance genes. The most efficient strategy to manage harmful pests is enhancing the host plant’s resistance to these pathogenic organisms  . Therefore, breeding toward durable resistance against a broad spectrum of these pests is the most practical approach for preventing crop loss  . A large number of genes/QTLs for BL, BB, WBPH, and BPH resistance have been identified and used in rice breeding programs over the past two decades     . Recently, GBS has greatly improved the discovery of QTLs. GBS is a rapid and cost-effective means for identifying SNPs to generate high-density linkage maps  . In the present study, we successfully employed the GBS method to detect SNPs linked to novel pest resistance QTLs in the RIL populations. Most of the major resistance QTLs detected in this study is co-located in the same genomic regions of resistance genes identified in previous studies        .
The introgression lines derived from O. minuta have served as a source for biotic stress resistance genes     . Several QTLs for BL and BPH resistance have been identified from IR71033. Five QTLs on chromosomes 6, 7, 9 and 11 for BL resistance were identified in a previous study  . The major QTLs on chromosomes 9 and 11 were located in the Pi2/9  and Pik/Pikm  regions, respectively. Minor QTLs for BL resistance were detected in the present study in the same regions on chromosomes 9 and 11 from all three environments investigated. Furthermore, we detected a major QTL for BL resistance on chromosome 12 that explained 19.7% - 47.4% of the phenotypic variation across all of the BL nurseries. The major QTLs are co-located in the Pi-ta region located in a linkage block near the centromere, which contains three recently-described genes, Pi-ta, Pi-ta2, and Ptr   . The Pi-ta resistance complex likely contributes to the resistance of IR71033, though this requires further study.
IR71033 showed moderate to high resistance against planthoppers    . We mapped the QTLs associated with WBPH and BPH resistance in the same genomic region on the short arm of chromosome 6. A major QTL for BPH resistance in IR71033 has been reported on chromosome 6  . Later, two major QTLs for BPH resistance in IR71033 were discovered on chromosomes 4 and 12  . Furthermore, a moderately resistant QTL from IR71033-62-24, another introgression line derived from O. minuta, was mapped onto chromosome 6  . The present study confirms the location of a major QTL for BPH resistance in the Bph32 region on chromosome 6  . It was reassuring to find that different QTLs from the same donor can be detected by multiple, independent studies   . The detection of different QTLs might be influenced by factors including the nature of the phenotypic tests and environments, a complex of pathogen or insect biotypes, genetic background, population types, and size or number of markers   .
IR57514 is widely and well adapted in the low-yielding environment of the rainfed lowland throughout Asia  . It has been extensively used in experiments and breeding programs for abiotic tolerance in these areas    . This rice line was identified as a submergence-tolerant variety that carries the Sub1 locus  . QTLs for submergence and drought tolerance-related traits have been detected in IR57514 (Bureerat et al., unpublished data). In addition to abiotic traits, IR57514 also carries resistance genes against BL and BB. QTLs for BL resistance have been detected in the same chromosomal locations of IR71033. In the present study, a novel gene conferring resistance against BB from IR57514 and IR71033 was identified simultaneously in the same genomic regions from three environments and across three years of evaluation experiments. Two QTLs (qBB1-11, qBB2-11) were detected in the distal region of chromosome 11 in a previous study  . Several BB resistance genes were reported in this region (approximately 300 Mb), including Xa4, xa44, Xa40, and Xa3/Xa26     .
Our results suggest that IR71033 and IR57514 could potentially provide multiple important genes for biotic stress resistance including against BL, BB, WBPH, and BPH in multiple environments. Previous studies suggested that multiple genes control durable field resistance to rice pathogens   . Therefore, the use of field resistance is one of the most promising measures for pest control   . QTLs for field resistance identified in this study will be useful for breeding programs aimed to protect rice from diseases and insect pests toward ensuring stable rice production through the MAS. Pyramiding these QTLs may address the problem of resistance breakdown leading to the development of multiple broad-spectrum, durable biotic stress resistant cultivars, particularly in the rainfed lowland areas.
In conclusion, in this study, major QTLs for BL, BB, WBPH, and BPH resistance were detected from two RIL populations, KD-IR71033 and KD-IR57514, from distinct environments at multiple time points. Major BB and BL resistance QTLs occupy the genomic regions on chromosomes 11 and 12 respectively, while WBPH and BPH resistance QTLs are identified on chromosome 6. Almost all of the QTLs detected in this study were co-located in the same regions as previously reported resistance genes: Pi5 (chromosome 9), Pi44 (chromosome 11), Pi-ta (chromosome 12), xa44 (chromosome 11), and Bph32 (chromosome 6). The RILs and markers flanking the target regions should be valuable for improving durable BL, BB, WBPH, and BPH resistance in rice through marker-assisted breeding. Taken together, these efforts should reduce the impact of biotic pathogens on local farmers worldwide.
This research was supported by a research grant from the Rice Department, Thailand (Project No. 705381). We are thankful to the International Rice Research Institute for providing seeds for the elite rice lines used in this study. We thank our colleagues at the UBN-RRC, SKN-RRC, PRE-RRC, PSL-RRC, and UDN-RRC for their assistance with both laboratory and field work.
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