J Intl Cooper Agric Dev 2020; 18: 18–28 Original
Shuto Yamada 1), Yusuke Kurokawa 1), Keisuke Nagai 1), Rosalyn B Angeles-Shim 1, 5), Hideshi Yasui²), Naruto Furuya³), Atsushi Yoshimura²), Kazuyuki Doi 4 ), Motoyuki Ashikari 1 ⁄ , and Hidehiko Sunohara 1 ⁄ 6 ⁄ 1)Bioscience and Biotechnology Center, Nagoya University, Chikusa-ku Furo-cho, Nagoya, 464-8601 Japan 2)Plant Breeding Laboratory, Faculty of Agriculture, Kyushu University, Nishi-ku Motooka, Fukuoka, 812-8581 Japan 3)Laboratory of Plant Pathology, Faculty of Agriculture, Kyushu University, Nishi-ku Motooka, Fukuoka, 812-8581 Japan 4)Graduate School of Bioagricultural Sciences, Nagoya University, Chikusa-ku Furo-cho, Nagoya, 464-8601 Japan 5)Current address: Department of Plant and Soil Science, Texas Tech University, Lubbock, Texas, USA 79416 6)Current address: Faculty of Advanced Science and Technology, Kumamoto University, Chuo-ku Kurokami, Kumamoto, 860-8555 Japan Received: September 12, 2019  Accepted: December 30, 2019 . . . Abstract. The yield-related gene, WFP increases grain number by increasing the primary branch number per panicle in rice. In the present study, WFP was introgressed from ST-12 to IRBB60, a pyramiding line having the genes Xa4 , xa5 , xa13 and Xa21 for BLB resistance in the IR24 genetic background. The pyramided lines PL-1, PL-4 and PL-5 that were included in the initial BC 2 F 2 selections based on their improved PBN were selected for further agronomic evaluation and generation advance up to BC 2 F 3 . All three lines recorded significantly higher PBN and GN due WFP introgression, although the loss of at least two BLB genes during the breeding process resulted in the variable response of the lines to the different BLB races. Among the lines, only PL-5 showed a significantly higher estimate of actual yield measured in terms of panicle weight per square meter compared to IRBB60. PL-5 also exhibited resistance to five out of the six BLB races used for resistance screening. Despite the negative effects in grain size, the improved yield estimates, as well as the validated BLB resistance of PL-5 makes it a suitable candidate for cultivar adoption under the tropical rice ecosystem of Southeast Asia. Key words : Rice, Wealthy Farmer's Panicle ( WFP ), bacterial leaf blight (BLB), pyramiding, high-yielding . . .
Introduction
By 2050, the current world population is expected to reach 9.7 billion (https://www.un.org/development/desa/ en/). An increase of this magnitude poses a very serious threat to the current state of global food security (http:// www.fao.org/home/en/). To feed 9 billion people, global cereal production alone needs to increase by 38% in the Corresponding author: Hidehiko Sunohara, e-mail: hsunohara@ kumamoto-u.ac.jp next 34 years 1 ). Rice, wheat and maize constitute approximately 50% of the calorie source of the human diet 2 ). Among these three, rice serves as a staple for more than half of the world's population. Although rice is cultivated in more than 100 countries across the globe, more than 90% of rice is still produced and consumed in Asia (GRiSP, http:// www.grisp.net/main/summary). In recent years, a drastic increase in demands for rice has also been reported in Africa 3） (GRiSP,  http://www.grisp.net/main/summary). Given the importance of rice as a staple food, increasing rice production would play a critical role in securing world food supply. Marker-assisted selection (MAS) is powerful tool for an efficient breeding. Since MAS makes it possible to distinguish plants into which target genes/QTLs have been introgressed during nursery, field area and efforts for individual selection of breeding reduce. Also, target genes/ QTLs can be introgressed more reliably than visually selection. MAS has been successfully used in breeding programs to improve agronomic traits, as well as biotic and abiotic stress resistance in rice. In the past decades, MAS have significantly shortened the time requirement while increasing the precision and efficacy by which target genes/QTLs are transferred across different genetic backgrounds 4 , 5 ). To date, several rice cultivars have been improved through MAS and these include the improved varieties developed under the Wonder Rice Initiative for Food Security and Health (WISH) project (http:// motoashikari-lab.com/wp/wp-content/uploads/2018/11/ WISH catalogue lite4.pdf). To effectively utilize MAS in breeding programs, genes/QTLs underlying important traits need to be mapped and identified. In rice, many genes/QTLs that are related to the sink ability have been identified including GN1a/ WFP/IPA17,8),       NAL1/SPIKE/GPS'9,10,11), OsCKX26), APO1/SCM212,13,14), qSW215), GW216), GW6a17) and GS318,19). Conversely, several genes/QTLs that are related to the source or translocation ability of rice has also been reported 20 , 21 , 22 ) that includes AMY2A 23 ) , BAM2 23 ) and GWD124). Wealthy  Farmer's  Panicle  (WFP)/Ideal  Plant Architecture 1 ( IPA1 ) is a yield-related gene that encodes the plant-specific transcription factor OsSPL147,8). OsSPL14 contains a microRNA-targeted sequence in an exon that is targeted and cleaved by OsmiR156. OsSPL14 functions in the regulation of primary branch number and is therefore associated with grain number. The WFP allele from ST-12 has been reported to increase grain number per panicle in rice 7 ). Advanced breeding lines of the japonica rice cultivar Nipponbare and several other indica cultivars having the WFP allele from ST-12 showed at least 28% increase in grain yield compared to the original varieties 7 , 25 ). Bacterial leaf blight (BLB) caused by Xanthomonas oryzae pv. oryzae ( Xoo ) is one of the most serious diseases of rice throughout the world. In some regions in Asia, BLB can cause yield losses of up to 50% 26 . The most effective approach to combat BLB is the use of resistant cultivars 27 ). To date, many resistant genes against BLB have been identified. Huang et al. focused on four BLB resistance genes/QTLs namely Xa4 , xa5 , xa13 and Xa21 , and pyramided all four resistance genes in the IR24 genetic background to generate IRBB60 28 ). IRBB60 has resistance to six races of Xoo from the Philippines 28 ). Currently, the BLB genes xa5, xa13 and Xa21 have been identified and characterized 29, 30, 31) In this study, we aimed to pyramid WFP from ST-12 to IRBB60, a rice cultivar having four genes for BLB resistance. The resulting pyramided lines were evaluated for yield and yield components, as well as for their resistance to BLB races from Vietnam, Myanmar and India. Results of the preliminary assessment of the agronomic performance of advanced backcrosses lines identified PL-5, a pyramiding line with higher yield compared and comparative BLB resistance relative to the recipient parent IRBB60.
Materials and Methods
Marker-assisted introgression of WFP in the pyramiding line IRBB60 IRBB60, IR24, ST-12 and three pyramiding lines (PL-1, PL-4 and PL-5) at the BC 2 F 4 generation were used in the study. IRBB60 is a pyramiding line having the four bacterial leaf blight (BLB) resistant genes, Xa4 , xa5, xa13 and Xa21 in the genetic background of IR24²8), whereas ST-12 is the donor of the yield-related gene, WFP 1 ). Phenotype and marker-assisted backcrossing of the initial hybrids generated from crosses between IRBB60 and ST-12 was carried out up to the BC 2 F 4 generation (Fig. 1). An SSR marker (forward primer: primer: GCGGTAACAACCAACCAACC， reverse was AAAGCAGGACACAGTCACACAGG) used to monitor the introgression of WFP in the IRBB60 background. In 2016, six BC 2 F 3 lines (PL-1, PL-2, PL- 4, PL-5, PL-6 and PL-7) were selected for preliminary evaluation of yield and yield components in the field. Based on this preliminary investigation, three pyramiding lines namely PL-1、PL-4 and PL-5 were selected and advanced for further performance evaluation in the field. All test materials were raised in the greenhouse for four weeks before transplanting them in a rice paddy at the Togo Field Center for Research and Education of Nagoya University in Aichi, Japan (35°06' north, 137°04' east) in 2017. The seedlings were transplanted separately in 3 rows (1.2 m) at a density of 22.2 plants/m² and at a plant spacing of 30cm x 15cm. All materials were grown in three replications in randomized blocks and under low fertilizer conditions i.e. 300 kg K-P-N (16-12-14)/ha only before transplanting. Evaluation of the pyramiding lines for agronomic traits Before sampling panicles from all materials, data on the days to heading (DH), plant height (PH) and panicle length (PL) of five plants per block were recorded from IRBB60 x ST-12 
  BC 2 F 2 ： Visual selection of 6 lines based on high number of primary branches 
 
 BC 2 F 3 ： Pre-evaluation of six lines and selection of 3 candidate pyramided lines & BC 2 F 4 : Agronomic evaluation and genotyping by whole genome sequencing of the 3 candidate pyramided lines Fig. 1. Process of developing the pyramiding lines. X mark in a circle means selfing. The introgression of WFP from ST-12 was confirmed by MAS in BC 1 F 1 , BC 2 F 1 and BC 2 F 3 . plant stands in the paddy. At 40-45 days after heading, all panicles from five plants from each block were sampled. After sampling, panicle weight per plant (PW), panicle number per plant (PN), primary branch number per main panicle (PBN) and grain number per main panicle (GN) were measured. To estimate the actual yield of each pyramiding line in the field, panicle weight per plant was measured and converted to panicle weight per square meter (PWm²). Only plants within rows were sampled to avoid border effects. All panicles were dried for a week in an incubator set at 42°C. To determine fertility ratio (FR), the three largest panicles including the main panicle were chosen from each plant. FR was calculated by dividing the number of filled spikelets by the total number of spikelets, multiplied by 100. The grain weight and number of filled spikelets of the three largest panicles were measured and converted to 1000-grains weight per main three panicles (TGW). Grain shape based on grain length (GL), width (GW) and thickness (GT) was determined from ten randomly selected spikelets in each plant. For the statistical analyses, plants showing maximum and minimum PW in each plot were excluded.
Evaluation of the pyramided lines for bacterial leaf
blight resistance Six races of Xanthomonas oryzae pv. oryzae (Xoo) causing bacterial leaf blight (BLB) from Vietnam (HAU01043, HAU0209, HAU02021-2 and HAU02024- 6) 32) , Myanmar (HKM13) 33) and India (IND2-3) were used to evaluate the resistance of the experimental materials to the pathogen. At booting stage, the plants were inoculated with Xoo using the leaf clipping method 34 ). A preliminary test was conducted using IR24 to confirm the pathogenicity and virulence of the six BLB races. Immediately after that, all the experimental materials including IR24 as the susceptible control and IRBB60 as the resistant control, were screened against the pathogen. After two weeks from inoculation, lesion lengths were measured, and the resistance/susceptibility of each material was evaluated. Classification of the response of materials as resistant, moderate, susceptible or partially resistant was carried out based on a comparison to the response of the susceptible reference IR24 and the resistant reference IRBB60 using Tukey's HSD (Table 1). Screening for BLB resistance including maintenance and culture of the pathogen was
**; only in the case that evaluation value is between the values of the references. carried out in Laboratory of Plant Pathology, Faculty of Agriculture, Kyushu University. Genotyping-by-sequencing analysis of the pyramiding lines Genotyping-by-sequencing (GBS) was conducted for the three pyramiding lines at the BC 2 F 4 generation using the Illumina Miseq sequencing system and Illumina Miseq Reagent Kit v3 (150 cycles) 35 , 36 ). Three plants from each of the pyramiding line, as well as IRBB60 and ST-12 were genotyped by GBS for whole genome. After sequencing, the reads were trimmed, and genotypes were called and filtered using TASSEL-GBS 37 ). SNPs with minor allele frequency (MAF) of >0.02, as well as lines with >0.2 lost genotype data and with >0.125 degree of heterozygous sites were removed from genotyping. Finally, imputation and error correction were carried out with FSFHap in TASSEL 4.0 38)
Statistical analysis
T-test and Tukey's HSD were performed using the free statistical software R (https://www.r-project.org/). A significant difference in Tukey's HSD was set at p<0.05. R was also used for the randomization of blocks in the field design.
Result
Phenotypic evaluation of the three pyramiding lines for yield components The pyramiding lines PL-1, PL-2, PL-4, PL-5, PL-6 and PL-7 in the genetic background of IRBB60 were selected visually from a BC 2 F 2 population for their high primary branching number. The introgression of WFP from ST-12 in these six lines at the BC 7 F 3 generation was confirmed by MAS. Evaluation for agronomic and vield-related traits showed that except for PL-6, all the pyramiding lines had significantly more PBN than IRBB60 (Table 2). PL-6 and PL-7 recorded significant lower PN (panicle number per plant) than IRBB60. The reduction of PN is the negative effect for actual yield, hence both PL-6 and PL-7 were removed from the candidate lines. Similarly, PL-2 was re- moved from the candidate lines because of comprehensive gross morphology. Based on the overall agronomic per- formance and gross morphology of the pyramiding lines, we selected PL-1, PL-4 and PL-5 for further generation advance and agronomic evaluation (Fig. 2). At the BC 2 F 4 generation, PL-1, PL-4 and PL-5 were evaluated for yield components including DH, PH, PL, PN, GN, PBN, FR, TGW, GL, GW and GT (Table 3). Compared to IRBB60, PL-1 recorded a significant 4-day delay in a heading, whereas PL-5 was significantly early by 4 days. Heading date in PL-4 was equivalent to that of IRBB60. PN decreased by 0.33 to 1.33 in all pyramiding lines compared to IRBB60. GN (grain number per main panicle) and PBN (primary branch number per main panicle) of the three pyramiding lines were significantly higher than those of IRBB60, suggesting the positive effects of WFP introgression from ST-12. The PBN of ST-12 was twice higher than that of IRBB60 and IR24. FRs (fertility ratio per three main panicles) of IRBB60, IR24 and PL-5 showed high values, while those of ST-12, PL-1 and PL-4 were low, below 77%. Among the three pyramiding lines, PL-5 showed the highest FR (92.56 ± 0.40%). The FRs of PL-1 and PL-4 were 76.84 ± 2.37% and 72.64 ± 0.61% respectively, which were equivalent to that of ST-12. In PWm², PL-5 recorded a significantly higher yield (845.50 ± 57.20 g/m²) than IRBB60 (782.42 ± 47.90 g/m²), with the rate of increase in PL-5 reaching up to 8.1% of IRBB60. ST-12, the donor of WFP , had signifi- cantly lower PWm² (712.4 ± 43.5 g/m²) than IRBB60 (Fig. 3). The panicle weight of PL-1 and PL-4 were statistically equivalent to that of IRBB60.
Numbers represent average ± standard deviation. PH=plant height; PL=panicle length; PN=panicle number per plant; GN=grain number per main panicle; PBN=primary branch number per main panicle. *; different letters indicate significant differences at p<0.05 by Tukey's HSD. Fig. 2.  Gross morphology of the plants and panicles of the experimental materials. A-F: Gross plant morphology at the ripening stage, G-L: Gross panicle morphology at the ripening stage. A and G: IRBB60, B and H: IR24, C and I: ST-12, D and J: PL-1, E and K: PL-4 and F and L: PL-5. Bar = 20cm (A–F), 5cm (G–L).
Numbers represent average ± standard error. DH=days to heading; PH=plant height; PL=panicle length; PN=panicle number per plant; GN=grain number per main panicle; PBN=primary branch number per main panicle; FR=fertility ratio per three largest panicles in a plant; TGW=1000-grain weight per three largest panicles in a plant; GL=grain length; GW=grain width; GT=grain thickness. *, **; indicate significant difference from IRBB60 at p<0.05 and p<0.01 by t-test, respectively. Based on these results, the high yield of PL-5 can be attributed to the increase in GN and PBN. In spite of PL-5 having 49.7% higher GN than IRBB60, PWm² of PL-5 was only 8.1% higher. As a major cause of this phenomenon, we focused on grain size. The grain size is expected to affect TGW (1000-grains weight per main three panicles). Compared to the TGW of IRBB60 (26.45 ± 0.08 g), the TGW of PL-5 (20.83 ± 0.11 g) was significantly lower (Table 3). Similarly, PL-5 recorded smaller grains in terms of GL (8.60 ± 0.01 mm), GW (2.22 ± 0.01 mm) and GT (1.79 ± 0.01 mm) compared to those of IRBB60 (9.46 ± 0.02 mm, 2.41 ± 0.01 mm, 1.96 ± 0.01 mm) (Table 3). The smaller grains of PL-5 negatively affected its yield.
Evaluation of the pyramiding line for resistance to
bacterial leaf blight
The response of the experimental materials when challenged with BLB races from Vietnam (HAU01043, HAU0209, HAU02021-2 and HAU02024-6), Myanmar (HKM13) and India (IND2-3) are shown in Fig. 4. IRBB60 showed strong resistance to the six BLB races, with lesions lengths ranging from 0 to 1.2 cm. In contrast, Fig. 3. Panicle weight per square meter (PWm²) of the experimental materials. * and ** indicate significant difference from IRBB60 at p<0.05 and p<0.01, re- spectively, by t-test. Error bars show standard error. IR24 and ST-12 were susceptible to the six BLB races, with lesions lengths ranging from 13.3 to 25.2 cm in IR24 and from 10.5 to 26.8 cm in ST-12. The pyramiding lines PL-1, PL-4 and PL-5 showed differential responses to the six BLB races. PL-1 exhibited resistance to the five BLB races from Vietnam (HAU0209, HAU02021-2 and HAU02024-6), Myanmar (HKM13) and India (IND2-3), and moderate to HAU01043. PL-4 exhibited resistance to HAU0209, moderate to IND2-3, partially resistance to HKM13 and susceptibility to HAU01043, HAU02021- 2 and HAU02024-6. PL-5 exhibited resistance to the five BLB races from Vietnam (HAU01043, HAU0209, HAU02021-2 and HAU02024-6) and India (IND2-3), and partially resistance to HKM13 from Myanmar. Confirmation of introgression of targeted genes with genotyping-by-sequencing GBS analysis validated the introduction of the WFP allele from ST-12 and the segregation of the four BLB resistant genes of IRBB60 in each of the pyramiding lines at BC 2 F 3 (Table 4). All pyramiding lines carry the Xa21 allele from IRBB60 and the xa13 allele from ST-12. PL-1 and PL-4 have the Xa4 allele from IRBB60 but not PL-5 which has the Xa4 from ST-12. The xa5 allele from IRBB60 was introgressed in the homozygous form in PL-5 but occurs in a heterozygous form in PL-4. PL-1 has the ST-12 allele for xa5.
Discussion
The pyramiding lines (BC 2 F 4 ) reported in the present study were developed from the cross between ST-12 (used as the donor of WFP ) and IRBB60 (used as the recipient parent with BLB resistance). Introgression of WFP from ST-12 in the PLs was confirmed by MAS and GBS (Fig. l, Table 4). As a result, we developed PL-5, a pyramiding line with higher yield compared to the recipient parent, IRBB60 (Fig. 3). The high yield of PL-5 can be attributed to the increase in both GN and PBN (Table 2, Table 3) due to the introgression of WFP from ST-12. Miura et al. reported that the introgression of WFP from ST-12 can significantly increase both GN and PBN in lines having the genetic background of the japonica rice cultivar Nippon- bare 7 ). Similarly, introgression of WFP has been reported to increase GN and PBN of several rice cultivars with indica genetic backgrounds 25 ). In the present study, the positive effects of WFP from ST-12 was also observed in the genetic background of IRBB60, suggesting that WFP expression can increase GN and PBN in both japonica and indica rice backgrounds, and that WFP from ST-12 is a useful genetic resource that can be used to significantly improve the yield of rice. PL-5 recorded 49.73% higher GN and 71.33% higher PBN compared to IRBB60. Despite the significant increase in GN and PBN, PL-5 recorded a remarkably low increase
**; expected genotypes. Fig. 4.  The response of the experimental materials to the six races of Xoo. A: Lesion length on leaves of the experimental materials. Different letters indicate significance at p<0.05 by Tukey's HSD. Error bars show standard error. B: Le- sions in leaves infected by the six BLB races. The reaction of the experimental materials to the BLB races is shown in two leaves each. The order of BLB races in each panel is HAU01043, HAU0209, HAU02021-2, HAU02024-6, HKM13 and IND2-3 from left to right. The inoculation site corresponds to the top of each panel. Bars = 5cm. of 8.06% in yield (PWm²) compared to IRBB60. A similar phenomenon was observed in PL-1 and PL-4. Rice yield has four important components namely PN, GN, FR and TGW. In PL-5, the main negative factor was TGW, which was 21.25% lower than IRBB60. Based on the results of the current study, an improvement in TGW can further increase rice yield. In PL-1, the main negative factors were PN and FR which were 12.74% and 19.03% lower than IRBB60, respectively. In PL-4, the negative factors were PN, FR and TGW which were 9.58%, 23.46% and 7.75% lower than IRBB60, respectively. The low TGW of PL-5 may be unrelated to the function of WFP. PL-5 has smaller grains, giving it lower TGW than IRBB60. The donor parent ST-12 do not have small grains, and WFP/IPA1 from ST-12 has not been reported to be involved in the regulation of grain size. We hypothesize that the genetic background of PL-5 which resulted from random recombination between the ST-12 and IRBB60 genome contributed to its smaller grains. Miura et al. suggested that WFP from ST-12 is involved in the regulation of shoot branching in the vegetative stage7). It was also suggested that WFP/IPA1 from the japonica cultivar Aikawa 1 and from the japonica cultivar Shaoniejing reduces tiller number 7 , 8 ) . While the presence of WFP/IPA1 allele may increase GN, it may have nega- tive effects on PN. In the present study, an increase in GN due to WFP/IPA1 introgression resulted in a concomitant decrease in PN. To suppress any reduction in PN due to WFP introgression, it is necessary to modify factor(s) re- lated to controlling PN specifically in the vegetative stage. It is considered that the low FR of PL-1 and PL-4 oc- curred as a result of the inability of the source to keep up with the improved sink ability of both lines as conferred by WFP. In PL-5, a remarkable reduction in grain size has been confirmed that allowed the source to cope with the increase in sink. In the context of sink-source relation- ship, WFP is a sink ability-related gene. Improvement of the sink is necessary to improve yield, but this requires a concomitant improvement in the source 39 ). Efficient breeding for high yield in rice requires elucidation of the genetic factors regulating the ability of the source to generate photosynthetic assimilates that will fill the sink. Improving the photosynthetic ability of plant tissues with source functions may be the key to breaking through the yield potential of the current high-yield varieties 40 ). The present study aimed to pyramid WFP with four BLB resistant genes already present in the IRBB60 background. The three selected pyramiding lines however, had WFP and at least two BLB resistant genes each (Fig. 4, Table 4). The incomplete maintenance of all four BLB resistance genes in the pyramiding line accounts for the variability in the response of each line to each of the BLB races. While the BLB resistant genes are not linked, xa13 is linked to WFP at approximately 1.5 Mb distance downstream. In the process of developing these pyramiding lines, genotyping was carried out to monitor the introgression of only WFP. To ensure that all four BLB resistant genes are maintained in the pyramided line, these genes should also be moni- tored in the backcrossed and selfed generations by MAS. Despite losing at least two BLB resistant genes, PL-5 remains resistant to four BLB races from Vietnam and a race from India, whereas PL-1 is resistant to three races from Vietnam and races from Myanmar and India. These results indicate the both PL-1 and PL-5 can be cultivated in areas with a reported incidence of the specific BLB in- fection. Overall, the significantly improved yield of PL-5 in combination with its resistance to five known BLB races from Vietnam and India makes this pyramiding line a vi- able candidate for high yield line with adoption in tropical rice ecosystems in Southeast Asia. This research was funded by the Japan International Cooperation Agency (JICA) as part of the Wonder Rice Initiative for Food Security and Health (WISH) Project, and the Canon Foundation. 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山田　修土り, 黒川　裕介り, 永井　啓祐り,  Rosalyn B Angeles-Shim 1, 51, 安井　秀 21,成人ﾗﾝ、吉村　淳ﾗﾝ，　土井　一行ｳ，芦苅　基行ﾘ，春原　英彦ﾘ,6古座1)名古屋大学生物機能開発利用研究センター 2)九州大学農学部植物育種学研究室3)九州大学農学部植物病理学研究室4)名古屋大学院生命農学研究科5)現所属: Department of Plant and Soil Science, Texas Tech University, Lubbock, Texas, USA 79416 6)現所属:熊本大学先端科学研究部（理）要旨イネ収量関連遺伝子WFPは、一次核梗数増加による着粒数増加をもたらす。IR24に白葉枯病抵抗性遺伝子群（Xa4、xa5、 xa13、Xa21)を集積したIRBB60へ、ST-12が持つWFPの導入を行った。IRBB60を反復親とした戻し交雑後代BC 2F 2とBC 2F3世代で、DNAマーカーの利用と一次枝梗数を含む収量影質の評価から、3ビラミディング系統(PL-1、PL-4、PL-5)を選抜した。BC₂F₄において、これらはWFPによる一次枝複数と溶粒数の増加を示したが、4白葉枯病抵抗性遺伝子の集称が不完全で、白葉枯病レース群への反応の差が見られた。しかし、PL-5はIRBB60よりも収量増加が見込まれ、今回供試した白葉枯病レース(ペトナム、ミャンマー、インド由来6レース)の内5レースに対して抵抗性を示したことから、PL-5は東南アジアの熱帯地域での展開を見込んだ候補系統となり得ると考えられた。キーワード:イネ、WFP遺伝子、白葉枯病抵抗性、ピラミディング、高収量性���