Improvement of bacterial blight resistance of 'Minghui 63', an elite restorer line of hybrid rice, by molecular marker-assisted selection

'Minghui 63' is a restorer line widely used in hybrid rice production in China. However, this line has become increasingly susceptible to bacterial blight (BB), resulting in a rapid decline of its use in rice production. The objective of this study was to improve the BE resistance of Minghui 63 by introgressing Xa21, a broad-spectrum BE resistance gene, into Minghui 63 by molecular marker-assisted selection (MAS). A polymerase chain reaction (PCR)-based MAS system was established consisting of a marker that is a part of Xa21, a marker located at 0.8 centimorgans (cM) from the Xa21 locus on one side, and a marker at 3.0 cM from the gene on the other side. A total of 128 restriction fragment length polymorphism (RFLP) markers, evenly distributed on the 12 chromosomes, were used to recover the genetic background of Minghui 63, The resulting improved version of Minghui 63, or 'Minghui 63(Xa21)', was exactly the same as the original except for a fragment of less than 3.8 cM in length surrounding the Xa21 locus. Both Minghui 63(Xa21) and its hybrid with 'Zhenshan 97A' referred to as 'Shanyou 63(Xa21)' showed the same spectrum of BE resistance as the donor parent. Field examination of a number of agronomic traits showed that Minghui 63(Xa21) and Shanyou 63(Xa21) were identical to Minghui 63 and Shanyou 63, when there was no disease stress. Under heavily diseased conditions, Minghui 63(Xa21) showed significantly higher grain weight and spikelet fertility than Minghui 63, and Shanyou 63(Xa21) was significantly higher than Shanyou 63 in grains per panicle, grain weight, and yield.