Development Of YSBR1/Lemont Chromosome Segment Substitution Lines And Fine Mapping Of The Major SB Resistance QTL

Sheath blight (SB) disease is one of the most serious diseases worldwide, and has become the first serious disease in some parts of Southern China. The resistance of rice to SB disease is a typical quantitative trait, controlled by poly genes, namely quantitative trait locus (QTL). YSBR1, developed by our lab, is a new rice line showing stable and high resistance to SB. Through an F2 clonal population (F2CP) derived from the cross between YSBR1 and Lemont (a susceptible cultivar), some SB resistance QTLs were detected. In the present study, we developed a series of chromosome segment substitution lines (CSSLs) using Lemont as recurrent parent and YSBR1 as donor parent, and validated SB resistance QTLs using these lines. The main results were listed as follows:1. In order to confirm and further map the major SB resistance QTLs from YSBR1, we developed 104 CSSLs in Lemont genetic background. Among them,69 CSSLs were from (BC6) advanced generation and 35 from BC3 generation. The YSBR1 chromosome segment mixed together in these CSSLs covered YSBR1 genome of around 97.32%.22 advanced CSSLs were inoculated in field and validated 6 SB resistance QTLs, qSB2-1YSBR1, qSB2-2YSBR1, qSB7-1YSBR1, qSB7-2YSRR1, qSB12YSBR1 and qSB5LE. The resistant allele(s) under qSB5LE was from Lemont, and the remaining resistant alleles were all from YSBR1.2. qSB12YSBR1 was considered of a major SB resistance QTL, which could explain the total phenotypic variation of more than 10% estimated in both greenhouse and field. In adult stage, the qSB12 was found to reduce SB rating scores of 1.2 in field and of 1.57 in greenhouse. For determining the precise location of qSB12YBR1, we developed a few new lines, which named sub-CSSLs because these lines have cross-over in the region covered qSB12YSBR1. By using sub-CSSLs and their phenotypes, we narrowed the qSB12YSBR1 down to a 1.2Mb region of markers RZ19.9 and RZ21.1.3. A few new CSSLs were developed which contain the donor fragment covered most of the region of qSB2-2YSBR1. By using these lines, we found that qSB2-2YSBRI was able to reduce SB rating scores of more than 2.0 in seedling stage in greenhouse, and reduce SB rating scores of 1.19 in field. After combining the phenotypes and genotypes of CSSLs and the QTL mapping results of F2CP population, we reasoned that qSB2-2YSBR1 is most probably located in a 2.3Mb region of markers RM13672 and RM525.4. To accelerate the application of SB resistant allele(s) of YSBRl on Japonica rice breeding of Jiangsu province, we decided to develop a new CSSLs’population by using a widely planted japonica variety TJ394 as acceptor and YSBR1 as donor. The backcrosses have been advanced to BC3 generation. A total of 220 polymorphic markers between TJ394 and YSBR1 were developed, which evenly distributed on rice 12 chromosomes and laid a solid ground for further developing of CSSLs covered the whole genome of YSBR1.