Background: The development of resistant cultivars has been the most effective and economical strategy to control bacterial leaf blight (BB) disease of rice caused by Xanthomonas oryzae pv. oryzae (Xoo). Molecular markers have made it possible to identify and pyramid valuable genes of agronomic importance in resistance rice breeding. In this study, three resistance genes (Xa4 + xa5 + Xa21) were transferred from an indica donor (IRBB57), using a marker-assisted backcrossing (MAB) breeding strategy, into a BB-susceptible elite japonica rice cultivar, Mangeumbyeo, which is high yielding with good grain quality. Results: Our analysis led to the development of three elite advanced backcross breeding lines (ABL) with three resistance genes by foreground and phenotypic selection in a japonica genetic background without linkage drag. The background genome recovery of the ABL expressed more than 92.1% using genome-wide SSR marker analysis. The pathogenicity assays of three resistance-gene-derived ABL were conducted under glasshouse conditions with the 18 isolates of Xoo prevalent in Korea. The ABL exhibited very small lesion lengths, indicating a hypersensitive reaction to all 18 isolates of Xoo, with agronomic and grain quality traits similar to those of the recurrent parent. Pyramiding the resistance genes Xa4, xa5 and Xa21 provided a higher resistance to Xoo than the introduction of the individual resistance genes. Additionally, the combination of two dominant and one recessive BB resistance gene did not express any negative effect on agronomic traits in the ABL. Conclusions: The strategy of simultaneous foreground and phenotypic selection to introduce multiple R genes is very useful to reduce the cost and the time required for the isolation of desirable recombinants with target resistance genes in rice. The resistance-gene-derived ABL have practical breeding value without a yield penalty by providing broad-spectrum resistance against most of the existing isolates of BB in South Korea and will have a high impact on the yield stability and sustainability of rice productivity.

Jung-Pil Suh, Yong-Jae Won, Eok-Keun Ahn, Jeong-Heui Lee, Woon-Goo Ha, Myeong-Ki Kim, Young-Chan Cho, Eung-Gi Jeong, Bo-Kyeong Kim. PBB 2014;2:158-66. http://dx.doi.org/

The Rural Development Administration (RDA) of the Republic of Korea, in collaboration with International Rice Research Institute (IRRI), developed six temperate japonica rice varieties—MS11, Japonica 1, 2, 6, 7, and Cordillera 4—which were officially approved for release in tropical environments. These varieties offer improved eating quality, enhanced lodging resistance, and increased market value. Although initial evaluations indicated that the varieties were resistant to moderately resistant to major biotic stresses, recent field trials revealed a gradual increase in susceptibility over time. To address this, we conducted comprehensive evaluations of these varieties against rice blast under both greenhouse and field conditions and assessed their responses to bacterial leaf blight (BLB), rice tungro spherical virus (RTSV), and brown planthopper (BPH) under controlled environments. Additionally, whole-genome sequencing was employed to confirm the presence of known resistance alleles. Our findings revealed variable resistance profiles across the six varieties. Japonica 1 exhibited the most stable resistance to blast, supported by the presence of the Pi5 allele. Japonica 7 showed strong resistance to key BLB isolates and moderate resistance to a broader range of Xoo races, supported by the resistant Xa25/OsSWEET13 haplotype. In addition, Japonica 7, along with Japonica 6, carried the tsv1 gene for RTSV resistance. However, none of the six varieties possessed other major resistance genes for BPH. These results highlight the urgent need to introgress durable resistance genes into tropical japonica rice to enhance resilience and broaden the spectrum of biotic stress resistance—critical traits for sustainable rice production in tropical environments.

Bacterial grain rot (BGR), caused by Burkholderia glumae, is a major constraint in rice production, particularly under high temperature and humidity. However, effective molecular markers for BGR resistance remain to be developed. Thus, we aimed to identify quantitative trait loci (QTLs) associated with BGR resistance and develop molecular markers to facilitate marker-assisted selection (MAS). Kele (xa5, resistant to BGR) and IS592BB (Xa3, Xa21, susceptible to BGR), we constructed Kele/IS592BB recombinant inbred lines (KIRILs) to map QTLs associated with resistance to BGR. Based on the identified QTLs, we developed CAPS and dCAPS markers, which were subsequently validated in the Kele/IS592BB//IS592BB backcrossing inbred lines. Five QTLs were detected: qRBG(RDA)1 [logarithm of odds (LOD): 16.4, phenotypic variation explained (PVE): 37.0%] and qRBG(RDS)1 (LOD: 19.1, PVE: 44.4%) on chromosome 1; qRBG(RDS)3 (LOD: 4.2, PVE: 8.2%) on chromosome 3; and qRBG(RDA)6 (LOD: 3.5, PVE: 6.3%) and qRBG(RDS)6 (LOD: 4.7, PVE: 12.3%) on chromosome 6. Based on the flanking sequences of these loci, two cleaved amplified polymorphic sequences (CAPS) and four derived CAPS (dCAPS) markers were developed. Marker-assisted selection using these markers resulted in BGR-resistant lines with less than 30% infected grain ratio and lesion area. Among the selected lines, KIBIL84 (Kele/IS592BB//IS592BB backcross inbred line) exhibited strong resistance to BGR and harbored a pyramided gene combination (Xa3+xa5+Xa21) for resistance to bacterial leaf blight (BLB). Under BLB pressure, KIBIL84 showed reduced lesion length (1.2 cm) and lesion area (5.2%). In addition, it demonstrated improved aboveground growth under field conditions compared with its parental lines, suggesting a positive relationship with yield potential. These findings indicate that the developed markers are effective for screening BGR resistance and that KIBIL84 is a promising breeding material for improving resistance to multiple bacterial diseases in rice.

Purpose Rice (Oryza sativa L.) production is threatened by bacterial leaf blight (BLB), particularly from the virulent K3a strain. Reliableand efficient phenotyping is essential for breeding resistant cultivars, but traditional manual methods are time-consuming and prone to subjectivity. Methods This study assessed the accuracy of smartphone-based image analysis for measuring lesion length (LL) and lesion ratio (LR) in BLB-infectedrice. A total of 102 recombinant inbred lines (RILs) derived from Chamdongjin and Sindongjin were evaluated using both manual methods and imageanalysis. Images were captured in the field using a smartphone (12MP, 4000×3000 pixels), without any auxiliary equipment. The images were analyzedusing ImageJ and Python-based tools and compared with manual measurements. Results Image-based data showed strong correlation with manual results(r = 0.84–1.00), and principal component analysis (PCA) explained 94.14% of the variance, effectively distinguishing resistant and susceptible lines.Statistically significant differences were observed between groups (p < 0.01), confirming reliability. Images taken at 3x magnification showed highaccuracy (r = 0.88–0.92), while those at 1x magnification showed reduced accuracy (r = 0.76–0.79), likely due to the inclusion of surrounding plants—highlighting the importance of proper field design and image framing. Conclusion This study suggests that smartphone-acquired images, even withoutauxiliary equipment, may serve as a feasible and sufficiently accurate source for image-based phenotyping. This accessible approach has the potential tosupport rapid, low-cost data collection in field settings, which could contribute to breeding and disease resistance research.

The acceleration of global warming poses a serious threat to food security, particularly affecting rice production. High temperatures during the grain-filling period deteriorate the grain quality and appearance of brown and milled rice. This study investigated meteorological changes in Korea in 2023 and 2024 and analyzed key factors influencing rice grain appearance quality. The grain appearance quality of brown rice was influenced by whole grain and immature grain levels, whereas milled rice quality was determined by perfect grain, chalky grain, and broken grain levels. In 2024, the number of days with average temperatures exceeding 30 °C increased by 12 compared to 2023, and the number of days with maximum temperatures above 35 °C increased by 8. These climatic changes significantly affected the grain appearance quality of Dongjin2, Boramchan, Sugwang, and Hwanggeumnuri. However, Asemi6 and Nampyeong showed consistent grain appearance-related traits across 2023 and 2024, identifying stable expression despite climatic changes The negative impact of high temperatures during the grain-filling period resulted in reduced 1000-grain weight and grain quality, weakening market competitiveness and decreasing farmer income. These findings highlight the increasing need for research to address the effects of climate change on rice production. This study provides a systematic analysis of the relationship between temperature variations and rice grain appearance quality. Also, we identified rice cultivars that exhibited stable grain appearance-related traits despite environmental variations. These selected resources are expected to serve as valuable parental materials for breeding high-yield cultivars with stable trait expression under future climate change.