2023 Volume 73 Issue 5 Pages 450-456
The development of resistant rice (Oryza sativa L.) varieties is a key strategy for the eco-friendly control of brown planthopper (BPH: Nilaparvata lugens Stål). However, BPH outbreaks occur frequently owing to the evolution of virulent strains in the field and the rapid breakdown of monogenic resistance to BPH. Therefore, to enhance BPH resistance and gauge the effectiveness of gene pyramiding against strongly virulent BPH, we developed pyramided lines (PYLs) in the genetic background of ‘IR64’ carrying BPH resistance genes. We developed six IR64-PYLs (BPH3 + BPH17, BPH32 + BPH17, BPH32 + BPH20, BPH3 + BPH17-ptb, BPH20 + BPH3, and BPH17-ptb + BPH32) through marker-assisted selection. To assess the resistance of the IR64-PYLs, we conducted antibiosis test, honeydew test, and modified seedbox screening test (MSST) using strongly virulent BPH populations. The level of BPH resistance increased in all six IR64-PYLs compared to both ‘IR64’ and the corresponding NILs in MSST. Among them, IR64-BPH3 + BPH17 and IR64-BPH32 + BPH17 exhibited the highest resistance to BPH. However, the resistance level of most IR64-PYLs was not significantly higher than that of the corresponding NILs in antibiosis test. Thus, these PYLs could serve as a valuable resource for breeding programs aimed at improving resistance to virulent strains of BPH and enhancing their durability.
The brown planthopper (BPH: Nilaparvata lugens Stål) is a major threat to sustainable rice (Oryza sativa L.) production in Asia. BPH not only produces ‘hopper burn’ symptoms as direct damage due to heavy infestation in paddy fields, but also hampers rice production indirectly through the transmission of viral diseases such as Rice ragged stunt virus and Rice grassy stunt virus diseases (Wei et al. 2018). BPH outbreaks caused severe damage in Japan and Korea in 2005, in China in 2005–2007, in Central Thailand in 2009, and in Indonesia in 2011 and 2014 (Jena et al. 2017, Li et al. 2019). Rice farmers rely on chemical control of BPH, but such measures are not eco-friendly. The use of host-plant resistance offers a lower-cost, environmentally friendly way to mitigate BPH problems in field conditions.
Bred conventionally, the resistant variety ‘IR26’, carrying resistance gene BPH1, was released in 1973 and widely grown in the Philippines, Indonesia, and Vietnam, but within a few years, BPH biotype 2 overcame its resistance (Cohen et al. 1997). In 1976, to enhance resistance to biotype 2, ‘IR36’, containing resistance gene BPH2, was released, but BPH overcame its resistance in 1982 (Alam and Cohen 1998, Khush and Virk 2005). Lines and varieties carrying a single resistance gene became susceptible to current strong BPH populations (Fujii et al. 2021, Myint et al. 2012, Nguyen et al. 2019). Therefore, lines and varieties with a single resistance gene are not durable, because it breaks down quickly with the emergence of new virulent BPH.
In response to severe damage and frequent outbreaks of BPH in several rice-growing regions, pyramiding of multiple resistance genes into elite rice variety has been used to develop durable resistance. Pyramiding of resistance genes or QTLs resulted in stable and long-lasting resistance against BPH (Alam and Cohen 1998). It is also effective against the rapid adaptation by BPH to resistant varieties (Horgan 2018). Pyramided lines (PYLs) offered greater resistance than that of lines harboring single genes against BPH (Qiu et al. 2012). PYLs with BPH6 and BPH12 had higher resistance than lines containing either one alone. The introduction of BPH14 and BPH15 into ‘Minghui 63’, ‘Huahui 938’, and ‘Huang-Hua-Zhan’ increased resistance (Han et al. 2018). PYLs carrying BPH3 and BPH27(t) in the ‘Ningjing 3’ and ‘9311’ background (Liu et al. 2016) and a PYL carrying BPH3, BPH14, and BPH15 in the ‘Hemeizhan’ background had higher resistance than that of NILs containing single genes (Hu et al. 2016). A PYL carrying BPH14, BPH15, and BPH18 in Indica rice ‘93-11’ had the highest resistance (Hu et al. 2013).
The elite Indica Group variety ‘IR64’ has been widely grown in South and Southeast Asia since the 1980s (Khush and Virk 2005) for its good eating quality and high yields. The variety ‘IR64’ carrying BPH1 and BPH37 had moderate resistance to BPH but became susceptible in Indonesia during 1990s (Yang et al. 2019). BPH populations that migrated recently into Japan have already overcome resistance conferred by the single genes BPH1, BPH2, BPH3, BPH17, BPH20, and BPH32 (Fujii et al. 2021, Nguyen et al. 2019). However, these genes might still be able to enhance BPH resistance through gene pyramiding. Therefore, the objective of this study is to develop PYLs and evaluate the pyramiding effect of four resistance genes in the ‘IR64’ genetic background. We characterized these PYLs against two BPH populations (Koshi-2013 and Koshi-2020, which recently migrated from China into Japan) to confirm their effectiveness, even though each single resistance gene has lost its effect.
Five IR64-NILs—IR64-BPH32, IR64-BPH17-ptb, IR64-BPH20, IR64-BPH17, and IR64-BPH3 (Kamal et al. 2023) were used for the development of the PYLs. PYLs for two resistance genes were developed from IR64-NILs descended from the BC3F2 generation (Fig. 1). The F1 plants derived from crosses between NILs were self-pollinated to produce F2 plants. From 96 F2 plants, plants that were homozygous for the two BPH resistance genes were selected by marker-assisted selection (MAS) using SSR markers (Supplemental Table 1). We developed six PYLs—IR64-BPH3 + BPH17, IR64-BPH32 + BPH17, IR64-BPH32 + BPH20, IR64-BPH3 + BPH17-ptb, IR64-BPH20 + BPH3, and IR64-BPH17-ptb + BPH32—carrying four BPH resistance genes.
Breeding scheme for the development of pyramided lines (PYLs) with BPH resistance genes in the genetic background of Indica Group variety ‘IR64’.
We extracted total DNA by the potassium acetate method (Dellaporta et al. 1983). The genotype of each plant was determined by PCR and agarose gel electrophoresis as described by Kamal et al. (2023). In MAS for resistance genes on chromosome 4S, the plants with BPH17, BPH17-ptb, and BPH20 were selected with SSR markers RM8213, MS10, and RM16535. In MAS for genes on chromosome 6S, BPH32 was selected with SSR markers RM508 and RM19341 and BPH3 with RM508 and RM588 (Supplemental Table 1).
Verification of the presence of BPH17We used InDel markers to confirm BPH17 in the NILs and PYLs (He et al. 2020). BPH17 was cloned as a gene cluster within a segment of <50 kb (6.93–6.98 Mbp) on the short arm of chromosome 4 (Liu et al. 2015). We conducted a polymorphism test between ‘IR64’ and the donor parent using eight InDel markers (Supplemental Table 2). Markers I531 and I729 showed polymorphism between the parents. Using these two markers, we confirmed the presence of BPH17.
Brown planthopper populationsBPH populations, Koshi-2013 and Koshi-2020 were collected from Koshi city, Kumamoto Prefecture, Japan, in 2013 and 2020, respectively. Kyushu Okinawa Agricultural Research Center, National Agriculture and Food Research Organization (NARO), Kumamoto, Japan provided both populations, which were subsequently maintained on the susceptible rice variety ‘T65’ (Taichung 65) at 25°C with 16 h/8 h of light/dark at Saga University for the characterization of the resistance of the PYLs.
Characterization of PYLs against BPH populations Antibiosis testWe used the antibiosis test to evaluate adult mortality as described by Myint et al. (2009). Five plants of each PYL, the corresponding NILs, and the parental lines (‘IR64’, ‘Rathu Heenati’, and ‘T65’) were separately grown in 215-mL plastic cups. All seedlings were trimmed to 15 cm height at 4 weeks and placed in a plastic case. Each plant was infested with 5 thin-abdomen brachypterous female BPH. At 5 days after infestation (DAI), adult mortality (i.e., the number of dead adults on each plant) was calculated.
Honeydew testAreas of honeydew were measured by the method of Heinrichs et al. (1985) with modifications. Seedlings ~30 days old were grown in 215-mL plastic cups. Filter paper treated with 0.2% bromocresol green was placed inside a ventilated plastic chamber to absorb the plant honeydew excreted by BPH. After BPH were starved for 1 h, 2 large-abdomen brachypterous females were placed inside the chamber on each plant. Filter papers were collected after 24 h, and honeydew areas were measured in ImageJ v. 1.53a software.
Modified seedbox screening testTo evaluate the resistance of PYLs, we performed the modified seedbox screening test (MSST) (Horgan et al. 2015). Twenty-five seeds of each PYL, the corresponding NILs, ‘Rathu Heenati’, ‘IR64’ and ‘T65’ were sown per row in a plastic tray (23.0 × 30.0 × 2.5 cm) with 2.5 cm between rows. Seven days after sowing (DAS), the plants were thinned to 20 per row and infested with second and third instar nymphs at a density of around 20 nymphs per plant. When all ‘T65’ plants had dried, the damage scores (DSs) of all lines and varieties were classified according to the standard evaluation system established by IRRI (IRRI 2014).
Characterization of PYLs for agronomic traitsThe IR64-PYLs and ‘IR64’ were grown in a paddy field at Saga University in 2022. Seedlings were transplanted at 30 DAS, at one plant per hill, with 20 cm between hills and 25 cm between rows. We recorded days to heading (DTH), culm length (CL), panicle length (PL), flag leaf length (LL), flag leaf width (LW), and panicle number (PN) of six plants in a row as described by Nguyen et al. (2019).
From F3 generations, we selected six PYLs for evaluating BPH resistance: IR64-BPH3 + BPH17, IR64-BPH32 + BPH17, IR64-BPH32 + BPH20, IR64-BPH3 + BPH17-ptb, IR64-BPH20 + BPH3, and IR64-BPH17-ptb + BPH32 (Table 1).
Development of pyramided lines for BPH resistance in Indica Group variety ‘IR64’ genetic background
| PYL | Genea | Donor | Generation of PYLsb |
|---|---|---|---|
| IR64-BPH17-ptb + BPH32 | BPH32 (6) | PTB33 | F3 (BC3F2) |
| BPH17-ptb (4) | PTB33 | ||
| IR64-BPH32 + BPH17 | BPH32 (6) | PTB33 | F3 (BC3F2) |
| BPH17 (4) | Rathu Heenati | ||
| IR64-BPH32 + BPH20 | BPH32 (6) | PTB33 | F3 (BC3F2) |
| BPH20 (4) | IR71033-121-15 | ||
| IR64-BPH3 + BPH17-ptb | BPH3 (6) | Rathu Heenati | F3 (BC3F2) |
| BPH17-ptb (4) | PTB33 | ||
| IR64-BPH3 + BPH17 | BPH3 (6) | Rathu Heenati | F3 (BC3F2) |
| BPH17 (4) | Rathu Heenati | ||
| IR64-BPH20 + BPH3 | BPH3 (6) | Rathu Heenati | F3 (BC3F2) |
| BPH20 (4) | IR71033-121-15 |
a Numbers in parentheses indicate chromosome number.
b Generations of PYLs shown in parentheses are the generations of the NILs used as parents.
Using Koshi-2013, we assessed the resistance of the six IR64-PYLs by antibiosis test using adult mortality at 5 DAI (Table 2). The adult mortality on ‘IR64’ was 48.0% and that on the six IR64-PYLs ranged from 92.0% to 100.0%. However, that on IR64-BPH32 + BPH17, IR64-BPH3 + BPH17-ptb and IR64-BPH17-ptb + BPH32 were significantly higher than that of at least one corresponding IR64-NIL against Koshi-2013.
Adult mortality (%) of Koshi-2013 and Koshi-2020 in antibiosis test at 5 days after infestation on IR64-PYLs
| Entry | Adult mortality (%) (Mean value ± SD) | |
|---|---|---|
| Koshi-2013 | Koshi-2020 | |
| IR64-BPH32 | 72.0 ± 10.9 bc | 70.0 ± 11.5 ab |
| IR64-BPH17-ptb | 68.0 ± 10.9 cd | 65.0 ± 19.1 ab |
| IR64-BPH20 | 80.0 ± 14.1 abc | 65.0 ± 10.0 ab |
| IR64-BPH17 | 92.0 ± 10.9 ab | 80.0 ± 16.3 ab |
| IR64-BPH3 | 88.0 ± 10.9 abc | 70.0 ± 25.8 ab |
| IR64-BPH3 + BPH17 | 100.0 ± 0.0 a | 90.0 ± 11.5 a |
| IR64-BPH32 + BPH17 | 100.0 ± 0.0 a | 95.0 ± 10.0 a |
| IR64-BPH32 + BPH20 | 92.0 ± 10.9 ab | 90.0 ± 11.5 a |
| IR64-BPH3 + BPH17-ptb | 100.0 ± 0.0 a | 80.0 ± 16.3 ab |
| IR64-BPH20 + BPH3 | 96.0 ± 8.9 a | 80.0 ± 16.3 ab |
| IR64-BPH17-ptb + BPH32 | 96.0 ± 8.9 a | 85.0 ± 19.1 a |
| Rathu Heenati | 100.0 ± 0.0 a | 95.0 ± 10.0 a |
| IR64 | 48.0 ± 17.9 d | 45.0 ± 10.0 b |
| Taichung 65 | 4.0 ± 8.9 e | 5.0 ± 10.0 c |
Values with the same letter are not significantly different at P < 0.05 by Tukey–Kramer test.
We also evaluated the adult mortality of Koshi-2020 (Table 2). The adult mortality on ‘IR64’ was 45.0% and that on the six IR64-PYLs ranged from 80.0% to 95.0%. That on IR64-PYLs was non-significantly higher than that on the corresponding IR64-NILs against Koshi-2020.
Honeydew excretion of BPH on IR64-PYLsWe assessed the reaction of the Koshi-2013 feeding rate to the six IR64-PYLs on the basis of honeydew excretion area (Table 3, Supplemental Fig. 1B). The area of honeydew excreted ranged from 2.6 to 6.2 mm2 and was significantly lower than that on ‘IR64’ (26.3 mm2). The area of honeydew on IR64-PYLs was slightly lower than that on the corresponding IR64-NILs, but not significantly different from most of IR64-PYLs. However, that on IR64-BPH3 + BPH17-ptb and IR64-BPH17-ptb + BPH32 were significantly lower than that of at least one corresponding IR64-NIL. We also assessed the feeding rate of Koshi-2020 (Table 3). The area of honeydew on the IR64-PYLs ranged from 6.0 to 9.6 mm2 and was significantly lower than that on ‘IR64’ (28.4 mm2). It was non-significantly lower than on the corresponding IR64-NILs.
Areas of honeydew (mm2) excreted by BPH feeding (Koshi-2013 and Koshi-2020) in antibiosis test at 24 h after infestation on IR64-PYLs
| Entry | Honeydew area (mm2) (Mean value ± SD) | |
|---|---|---|
| Koshi-2013 | Koshi-2020 | |
| IR64-BPH32 | 23.7 ± 16.9 bcd | 19.6 ± 8.6 b |
| IR64-BPH17-ptb | 38.5 ± 19.1 b | 32.6 ± 9.7 b |
| IR64-BPH20 | 20.6 ± 9.3 bcd | 16.1 ± 12.5 b |
| IR64-BPH17 | 5.3 ± 2.1 d | 7.9 ± 2.4 b |
| IR64-BPH3 | 18.4 ± 15.9 bcd | 16.7 ± 2.5 b |
| IR64-BPH3 + BPH17 | 2.6 ± 2.2 d | 6.0 ± 4.7 b |
| IR64-BPH32 + BPH17 | 3.6 ± 3.2 d | 6.6 ± 1.2 b |
| IR64-BPH32 + BPH20 | 3.3 ± 1.3 d | 9.6 ± 2.0 b |
| IR64-BPH3 + BPH17-ptb | 4.2 ± 1.9 d | 9.6 ± 6.9 b |
| IR64-BPH20 + BPH3 | 4.9 ± 4.1 d | 8.8 ± 2.1 b |
| IR64-BPH17-ptb + BPH32 | 6.2 ± 5.6 cd | 8.4 ± 3.5 b |
| Rathu Heenati | 3.2 ± 2.2 d | 7.4 ± 2.4 b |
| IR64 | 26.3 ± 9.6 bc | 28.4 ± 13.6 b |
| Taichung 65 | 79.9 ± 20.3 a | 83.8 ± 24.1 a |
Values with the same letter are not significantly different at P < 0.05 by Tukey–Kramer test.
To characterize resistance of IR64-PYLs, we evaluated the DSs of IR64-PYLs against Koshi-2013 by the MSST (Fig. 2, Supplemental Fig. 1A). The DS of ‘IR64’ was 7.6 and those of the six IR64-PYLs ranged from 2.8 to 5.0. The DSs of most IR64-PYLs were significantly lower than those of the corresponding IR64-NILs, excepting IR64-BPH32 + BPH17. We also evaluated the DSs of IR64-PYLs against Koshi-2020 (Fig. 3). The DS of ‘IR64’ was 8.2 and those of the six PYLs ranged from 4.2 to 5.5. The DSs of most of IR64-PYLs were significantly lower than those of the corresponding NILs. However, the DSs of IR64-BPH3 + BPH17 and IR64-BPH32 + BPH17 were significantly lower than those of one corresponding IR64-NIL but not the other NIL.
Damage scores of IR64-PYLs by Koshi-2013 in modified seedbox screening test. Values with the same letter are not significantly different at P < 0.05 by Tukey–Kramer test.
Damage scores of IR64-PYLs by Koshi-2020 in modified seedbox screening test. Values with the same letter are not significantly different at P < 0.05 by Tukey–Kramer test.
IR64-BPH3 + BPH17 and IR64-BPH32 + BPH17 had higher resistance than the other PYLs against current strongly virulent BPH populations. We confirmed the presence of BPH17 from ‘Rathu Heenati’ in the two PYLs by PCR amplification of two InDel markers (Supplemental Figs. 2, 3). Both PYLs showed the same DNA band as ‘Rathu Heenati’.
Agronomic traits of the IR64-PYLsIn general, the agronomic traits of the IR64-PYLs were similar to those of ‘IR64’. PL, LL, and PN did not differ significantly (Table 4). DTH, CL, and LW were also similar to those of ‘IR64’, with the exception of DTH of IR64-BPH3 + BPH17 and IR64-BPH32 + BPH17, and CL and LW of both IR64-BPH32 + BPH20 and IR64-BPH20 + BPH3.
Agronomic traits of near-isogenic lines and pyramided lines carrying BPH resistance genes
| Line | Agronomic trait (mean ± SD) | |||||
|---|---|---|---|---|---|---|
| DTH | CL (cm) | PL (cm) | LL (cm) | LW (cm) | PN | |
| IR64-BPH32 | 101.0 ± 1.3 | 89.3 ± 5.3** | 24.8 ± 1.2* | 24.2 ± 2.3 | 1.5 ± 0.1** | 16.1 ± 2.7 |
| IR64-BPH17-ptb | 101.6 ± 0.9 | 92.6 ± 2.2 | 26.1 ± 1.4 | 25.7 ± 2.1 | 1.6 ± 0.1 | 15.3 ± 1.7 |
| IR64-BPH20 | 96.4 ± 1.4*** | 88.5 ± 4.3** | 26.8 ± 0.7 | 24.8 ± 2.6 | 1.6 ± 0.1 | 15.7 ± 2.1 |
| IR64-BPH17 | 98.2 ± 1.3*** | 91.1 ± 3.0 | 26.6 ± 1.2 | 24.0 ± 1.7 | 1.5 ± 0.1 | 15.4 ± 1.9 |
| IR64-BPH3 | 100.9 ± 2.0 | 95.4 ± 1.8 | 26.2 ± 1.2 | 27.0 ± 2.5 | 1.6 ± 0.1 | 16.8 ± 1.9 |
| IR64-BPH3 + BPH17 | 99.4 ± 1.3** | 92.4 ± 3.8 | 26.5 ± 0.9 | 25.3 ± 2.3 | 1.6 ± 0.1 | 16.7 ± 2.1 |
| IR64-BPH32 + BPH17 | 99.5 ± 2.0** | 92.1 ± 5.5 | 27.0 ± 1.2 | 27.1 ± 2.6 | 1.6 ± 0.1 | 15.2 ± 1.8 |
| IR64-BPH32 + BPH20 | 101.2 ± 2.5 | 89.8 ± 5.7** | 25.2 ± 1.9 | 24.6 ± 3.4 | 1.4 ± 0.1*** | 16.7 ± 2.8 |
| IR64-BPH3 + BPH17-ptb | 102.3 ± 0.5 | 91.6 ± 3.7 | 27.0 ± 1.1 | 27.7 ± 3.2 | 1.6 ± 0.1 | 15.7 ± 2.6 |
| IR64-BPH20 + BPH3 | 101.3 ± 1.5 | 89.9 ± 6.1* | 26.9 ± 1.4 | 25.1 ± 3.9 | 1.5 ± 0.2* | 16.3 ± 2.3 |
| IR64-BPH17-ptb + BPH32 | 101.9 ± 0.8 | 92.2 ± 4.3 | 26.9 ± 1.4 | 27.3 ± 2.2 | 1.6 ± 0.1 | 15.3 ± 1.9 |
| IR64 | 101.0 ± 1.5 | 93.7 ± 3.1 | 26.1 ± 1.2 | 25.4 ± 3.5 | 1.6 ± 0.1 | 16.6 ± 2.8 |
DTH, days to heading; CL, culm length; PL, panicle length; LL, flag leaf length; LW, flag leaf width; PN, panicle number; * P < 0.05, ** P < 0.01, *** P < 0.001 (Dunnett’s multiple comparison tests against ‘IR64’).
Several BPH-resistant varieties and lines have been developed through the introgression of resistance genes for the eco-friendly management of BPH. But most older BPH-resistant varieties have only single resistance genes, which BPH overcomes rapidly, within a few years of release (Alam and Cohen 1998, Cohen et al. 1997). Therefore, it is crucial to develop varieties with multiple BPH resistance genes for effective control of BPH. To confirm the effect and interaction of each gene, NILs and PYLs with BPH resistance genes have been developed in the genetic backgrounds of susceptible variety (Han et al. 2018, Hu et al. 2013, Liu et al. 2016, Nguyen et al. 2019). For example, Nguyen et al. (2019) characterized resistance level of NILs and PYLs in the susceptible ‘T65’ background to understand effects of each gene. On the other hand, Kamal et al. (2023) developed NILs in the genetic background of resistance variety ‘IR64’ for the estimation of useful combination of BPH resistance genes against BPH population with strong virulence. Here, to confirm effectiveness of resistance genes those has lost its effect, we developed PYLs with four resistance genes in the elite variety ‘IR64’ genetic background, which already has BPH1 and BPH37. This allowed the efficient development of PYLs with four resistance genes in ‘IR64’ background.
Several studies have shown that pyramiding of multiple resistance genes can increase resistance to BPH in rice. PYLs carrying BPH14 and BPH15 had higher resistance than the corresponding NILs (Hu et al. 2012), and a PYL carrying BPH3 and BPH27(t) had significantly higher resistance (Liu et al. 2016). Pyramiding of BPH6 and BPH12, as well as of BPH25 and BPH26, also resulted in higher resistance than in corresponding NILs (Myint et al. 2012, Qiu et al. 2012). Here, IR64-PYLs had higher resistance than the corresponding NILs (Table 2, Figs. 2, 3), and BPH on these PYLs excreted less honeydew than on the corresponding NILs, suggesting that the IR64-PYLs inhibit the sucking of phloem sap from the plant (Table 3, Supplemental Fig. 1B). IR64-BPH32 + BPH20, IR64-BPH3 + BPH17-ptb, IR64-BPH20 + BPH3, and IR64-BPH17-ptb + BPH32 showed additive effects of gene pyramiding, with increased resistance to both Koshi-2013 and Koshi-2020 (Figs. 2, 3, Supplemental Fig. 1A). The resistance of IR64-BPH3 + BPH17 and IR64-BPH32 + BPH17 was close to that of ‘Rathu Heenati’ against both populations because of the strong effect of BPH17. However, the pyramiding effects were masked by BPH17, which is a cluster of three genes (OsLecRK1–3), which might work together to provide broad-spectrum and durable resistance to BPH (Liu et al. 2015). The results suggest that these combinations of genes in PYLs control strongly virulent BPH populations such as Koshi-2013 and Koshi-2020.
Overall, the virulence of Koshi-2020 was slightly higher than that of Koshi-2013 in antibiosis test and MSST. Nguyen et al. (2019) reported that Koshi-2013 was more virulent than Hadano-1966. Testing the long-term virulence of BPH populations to several varieties. Fujii et al. (2021) found that it became stronger year by year. The resistance of ‘Rathu Heenati’ and ‘Balamawee’ was generally the strongest, but it was slightly decreased against a 2019 population (Fujii et al. 2021). Here, the resistance of ‘Rathu Heenati’ and PYLs to Koshi-2020 was slightly lower than that to Koshi-2013. These facts suggest that the virulence of BPH increased from 2013 to 2020. We developed six PYLs in ‘IR64’ with a total of four resistance genes. These lines might be useful in monitoring strongly virulent migratory BPH stains and might also increase the durability of host-plant resistance to BPH through the development of multiline cultivars.
MMK and DF designed the study. MMK, CDN, and DF developed the plant materials. SS-M reared the insects. SZ supported the research and wrote the manuscript. MMK and DF performed the experiments and wrote the manuscript.
We thank the Bangabandhu Science and Technology Fellowship Trust, Bangladesh, for the doctoral fellowship granted to MMK. This work was supported by JSPS KAKENHI grants 18KK0169 and 21K05527. We also thank the staff of the Insect Pest Management Research Group, NARO, for rearing and providing the insect populations. This research was part of a dissertation submitted by the first author in partial fulfilment of a Ph.D. degree. All authors have provided their consent.
