Bakanae or foot rot disease is a prominent disease of rice caused by Gibberella fujikuroi. This disease may infect rice plants from the pre-emergence stage to the mature stage. In recent years, raising rice seedlings in seed boxes for mechanical transplanting has increased the incidence of many seedling diseases; only a few rice varieties have been reported to exhibit resistance to bakanae disease. In this study, we attempted to identify quantitative trait loci (QTLs) conferring bakanae disease resistance from the highly resistant japonica variety Wonseadaesoo. A primary QTL study using the genotypes/phenotypes of the recombinant inbred lines (RILs) indicated that the locus qBK1 WD conferring resistance to bakanae disease from Wonseadaesoo was located in a 1.59 Mb interval delimited on the physical map between chr01_13542347 (13.54 Mb) and chr01_15132528 (15.13 Mb). The log of odds (LOD) score of qBK1 WD was 8.29, accounting for 20.2% of the total phenotypic variation. We further identified a gene pyramiding effect of two QTLs, qBK WD and previously developed qBK1. The mean proportion of healthy plant for 31 F4 RILs that had no resistance genes was 35.3%, which was similar to that of the susceptible check variety Ilpum. The proportion of healthy plants for the lines with only qBK WD or qBK1 was 66.1% and 55.5%, respectively, which was significantly higher than that of the lines without resistance genes and that of Ilpum. The mean proportion of the healthy plant for 15 F4 RILs harboring both qBK WD and qBK1 was 80.2%, which was significantly higher than that of the lines with only qBK WD or qBK1. Introducing qBK WD or pyramiding the QTLs qBK WD and qBK1 could provide effective tools for breeding rice with bakanae disease resistance. To our knowledge, this is the first report on a gene pyramiding effect that provides higher resistance against bakanae disease.

Bakanae disease is a fungal disease of rice (Oryza sativa L.) caused by the pathogen Gibberella fujikuroi (also known as Fusarium fujikuroi). This study was carried out to identify novel quantitative trait loci (QTLs) from an indica variety Zenith. We performed a QTL mapping using 180 F_2:9 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 qBK1^z conferring bakanae disease resistance from the Zenith was located in a 2.8 Mb region bordered by the two RM (Rice Microsatellite) markers, RM1331 and RM3530 on chromosome 1. The log of odds (LOD) score of qBK1^z was 13.43, accounting for 30.9% of the total phenotypic variation. A finer localization of qBK1^z was delimited at an approximate 730 kb interval in the physical map between Chr01_1435908 (1.43 Mbp) and RM10116 (2.16 Mbp). Introducing qBK1^z or pyramiding with other previously identified QTLs could provide effective genetic control of bakanae disease in rice.

Bakanae disease is an important fungal disease caused by Gibberella fujikuroi. Incidence of rice bakanae disease creates serious problems in the foremost rice growing countries, and no rice variety has been found to be completely resistant to this disease. However, breeding rice varieties resistant to bakanae disease may be a cost-saving solution preferable to the application of fungicides. In this study, we aimed to determine the exact position and the candidate gene for qBK1, a major resistant quantitative trait locus (QTLs) for bakanae disease. The genotypes/phenotypes of recombinants selected from backcrossed recombinant inbred lines of two rice varieties, Shingwang (resistant) and Ilpum (susceptible), indicated that the locus qBK1, conferring resistance to bakanae disease in Shingwang, was delimited to a 35-kb interval delimited by InDel 18 (23.637 Mbp) and InDel 19–14 (23.672 Mbp). Sequence analysis of this 35-kb region revealed four candidate genes, LOC_Os01g41770, LOC_Os01g41780, LOC_Os01g41790, and LOC_Os01g41800. There were many non-synonymous SNPs in LOC_Os01g41770 and the transcript of LOC_Os01g41790 was early terminated in Shingwang, whereas there were no differences in both LOC_Os01g41780 and LOC_Os01g41800 sequences between Ilpum and Shingwang. Expression profiling of the four candidate genes showed the up-regulation of LOC_Os01g41770, LOC_Os01g41780, and LOC_Os01g41790 in Ilpum and of LOC_Os01g41800 in Shingwang after inoculation of G. fujikuroi. Utilization of marker-assisted selection (MAS) with a precise molecular marker on qBK1 could provide an effective tool for breeding rice varieties resistant to bakanae disease. To our knowledge, this is the first report on fine mapping and candidate gene approaches for identifying the gene for qBK1.