Molecular mapping of qBK1Z, a major QTL for bakanae disease resistance in rice

Bakanae disease is a fungal disease of rice (Oryza sativa L.) caused by the pathogen Gibberella fujikuroi (also known as Fusarium fujikuroi). Recently the disease incidence has increased in several Asian countries and continues to spread throughout the world. No rice varieties have been developed yet to be completely resistant to this disease. With increasing need to identify various genetic resources to impart resistance to local elite varieties, this study was carried out to identify novel quantitative trait loci (QTLs) from an indica variety Zenith. We performed a QTL mapping using 180 F2:9 recombinant inbred lines (RILs) derived from a cross between the resistant variety, Zenith, and the susceptible variety, Ilpum. A primary QTL study using the genotypes and phenotypes of the RILs indicated that the locus qBK1z conferring bakanae disease resistance from the Zenith was located in a 2.8 Mb region bordered by the two SSR markers, RM1331 and RM3530 on chromosome 1. The log of odds (LOD) score of qBK1z was 13.43, accounting for 30.9% of the total phenotypic variation. A finer localization of qBK1z was delimited at an approximate 730 kb interval in the physical map between Chr01_1435908 (1.43 Mbp) and RM10116 (2.16 Mbp). The development of a rice variety with a higher level of resistance against bakanae disease is a major challenge in many rice growing countries. Introducing qBK1z or pyramiding with other previously identified QTLs could provide effective genetic control of bakanae disease in rice.


Introduction
Bakanae disease, which means foolish seedling in Japanese, was firstly identified in 1828 in Japan [1], 45 and has widely distributed in temperate zone as well as tropical environment and occurring infected rice seeds. The application of fungicides is not functioning well for destroying the spores of 61 this fungal pathogen, and some pathogen showed resistance to the fungicides [13, [16][17][18]. Therefore, 62 the genetic improvement of rice using the QTLs/genes providing the bakane disease resistance would 63 be a more effective way to control bakanae disease.

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Several QTLs associated with bakanae disease resistance have been identified and those can be used 65 for marker-assisted selection in rice breeding as well as for understanding the mechanisms of 66 resistance. Yang  Mb) from the japonica variety Wonseadaesoo. They also found that resistance of gene pyramided 82 lines harboring two QTLs, qBK1 WD and qBK1 was significantly higher than those with only qBK1 WD 83 or qBK1. This finding relates to the problem of varieties with single resistance gene losing its effect  In this study, we aimed to provide a new genetic source, qBK1 z with detailed gene locus information   The inoculation and evaluation of bakanae disease were conducted using a method that described by Beloeil, QC, Canada). Before inoculation, the seeds in the in the tissue-embedding cassette were 109 surface sterilized with hot water (57 ℃) for 13 min, then allowed to drain and cool. Subsequently, the 110 seeds were soaked in the spore suspensions (1 × 10 6 spores/mL) for 3 days for inoculation with gentle 111 shaking four times a day for equilibration. After inoculation, thirty seeds per line were sown in 112 commercial seedling tray, and seedlings were grown in a greenhouse (28 ± 3 ℃ day/23 ± 3 ℃ night,  showed that a significant QTL associated with bakanae disease resistance at the seedling stage was 181 located between the SSR markers, RM1331 and RM3530 on chromosome 1, and it was designated 182 qBK1 z . The LOD score of qBK1 z was 13.43, which accounted for 30.9% of the total phenotypic 183 variation (Table 1).   Table). Finally, seven homozygous recombinants were selected from 197 the F 2:9 lines using 14 markers in the 2.8 Mb region around the SSR markers RM1331 and RM3530 198 ( Fig. 4 and Fig. 5).

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The proportion of healthy plants of the seven homozygous recombinants was evaluated with three classified to Group a were regarded as resistant, and Group b as susceptible (Fig. 4) showed a higher resistance than those with qBK1. Furthermore, the pyramided lines harboring 248 qBK1 WD + qBK1 had a much higher level of resistance than those possessing either qBK1 WD or qBK1.

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These results showed the utility of MAB in gene pyramiding can achieve higher resistance in many 250 bakanae disease prone rice growing areas.

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In this study, we identified a new major QTL qBK1 z conferring bakanae disease resistance from QTL qBK1 z will be a useful material for studying an interaction between the pathogen (F. fujikuroi) 259 and rice host plants.