Glutinous rice cultivar "RD6" is well known for its fragrance and high cooking and eating qualities, and is the most popular glutinous cultivar in the north and northeastern regions of Thailand. However, it's susceptible to blast and bacterial blight (BB) diseases. Previously, four blast resistance QTLs on chromosomes 1, 2, 11, and 12, and a single BB resistance gene xa5 pyramided to the background of the RD6 cultivar were tested for a broad spectrum of disease resistance under greenhouse conditions. In the present study, a field experiment was conducted during the rainy seasons of 2015, 2016, 2017, and 2018, across three locations, for performance evaluations of promising lines in terms of disease reaction, agronomical characteristics, grain yield, and quality attributes. The results revealed that the ILs (BC 2 F 5 2-7-5-36, BC 2 F 5 2-7-5-43, BC 2 F 5 2-8-2-25, and BC 2 F 5 6-1/15-2-11) exhibited higher level resistance to leaf blast and neck blast disease. The BC 2 F 5 2-8-2-52 showed resistance to both blast and BB diseases and, like all ILs, exhibited superior yield compared to the original RD6. Furthermore, the agronomic traits and grain qualities were similarly displaced, and were therefore recommended as near-isogenic lines to the RD6. This clearly demonstrated that farm phenotypic selection plays an important role in achieving not only NIL resistance to diseases, but also high yield potential, as well as representing an effective way in which to enhance BB, leaf blast, and neck blast resistance in rice planting in the north and northeastern regions of Thailand.
Rice blast (BL) caused by Magnaporthe oryzae is a fungal disease causing significant yield losses in rice production worldwide. To overcome the breakdown of resistance by the rapid adaptation of pathogens, identifying resistance (R) genes or QTLs in indigenous rice, which harbors the R genes that co-evolved with the local pathogen race, is necessary. In this study, a recombinant inbred line (RIL) population derived from a cross between RD6 and Phaladum (PLD) was used to map quantitative trait loci (QTL) for BL resistance through a QTL-seq approach. A single QTL (qBLchr4) associated with BL resistance at the seedling and maximum tillering stages was mapped on the long arm of chromosome 4. Five genes, LOC_Os04g0616600, LOC_Os04g0617900 (OsGLP4-1), LOC_Os04g0619600 (OsRLCK161), LOC_Os04g0620800 (Pi63), and LOC_Os04g0621500, were considered the candidate genes representing qBLchr4. Subsequently, the Kompetitive Allele-Specific PCR (KASP) markers specific for the SNP variant and position of each gene were designed for validation in the mapping population. These markers showed the high phenotypic variance explained (PVE) values in all testing methods and/or environments, signifying the major effect of qBLchr4. Among these markers, the Pi63-KASP marker explained the highest and most stable phenotypic variation across all testing methods and/or environments, with 84.18%, 80.34%, and 23.43% in the upland short row (USR) method, Sila environment, and Mueang environment, respectively. Therefore, Pi63 was suggested to be the strongest candidate gene. These results represent the potential utility of future BL resistance breeding and/or pyramiding using marker-assisted selection (MAS).
Bacterial leaf blight (BLB) disease caused by Xanthomonas oryzae pv. oryzae (Xoo) is one of the most serious diseases in rice production. Breeding varieties specifically for their resistance to BLB disease is therefore an efficient and cost-effective strategy. However, the resistance gene for BLB can be race and non-race specific, meaning it is often overcome by the pathogen. The identification of new sources of resistance genes for Xoo is crucial in rice breeding programmes. In this study, six rice varieties were assessed using six Xoo isolates in multiple screening conditions. The GGE biplot analysis considers both genotype (G) and genotype environment (GE) interaction effects and demonstrates GE interaction. The first two principal components (PCs) accounted for 95.46% of the total GE variation in the data. Based on lesion length and stability performance, Phaladum was the most ideal genotype against all Xoo isolates in the four screening conditions. The results relayed that Phaladum indigenous rice varieties could be considered as new sources of bacterial leaf blight resistance in Thailand. In the future, the BLB resistance gene in this variety will be identified in regard to mode of inheritance and used as parental line in rice breeding programmes for resistance to BLB.
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