The brown planthopper (BPH), Nilaparvata lugens, is one of the most serious insect pests throughout rice growing areas in Asia. This is especially true in countries mainly culturing Japonica rice cultivars that do not have a gene conferring resistance to BPH, and where outbreaks of BHP are therefore a severe problem. The BPH causes direct damage to crops and indirect damage by acting as a vector for viral diseases. Chemical treatment is the conventional method of controlling pests such as BPH, even though it is expensive and harmful to the environment. Many researchers have reported that host plant resistance is the most effective way of controlling pests including BPH, and thus breeding of insect resistance has taken priority in rice improvement programs. Until now, 13BPH resistance genes, together with several quantitative trait loci (QTLs) controlling BPH resistance, have been reported in two wild relatives and indica cultivars. Diverse sources of BPH resistance have been identified and genetic analysis has revealed 6 dominant [Bph1, 3, 6, 9, 10, and 13(t)] and 7 recessive [bph2, 4, 5, 7, 8, 11(t), and 12(t)] genes controlling BPH resistance. Bph1, bph2, Bph9, and Bph10(t) were assigned to rice chromosome 12. Bph1 and bph2 confer resistance to biotypes 1, 3 and 1, 2 which are widely distributed in Southeast Asia. Many studies aimed at identifying BPH resistance genes have been conducted over the years in order to develop a resistant cultivar; however, a japonica cultivar with a BPH resistance gene has not yet been developed. Thirteen of the BPH resistance genes identified so far are not from japonica rice, but from indica rice. In bioassays, it has been reported that the reaction of early rice seedlings to BPH differed between japonica and indica. Japonica introgression lines with BPH resistance genes exhibited undesirable characteristics such as poor grain quality and lodging-related traits to which the resistance genes seemed to be highly linked. The undesirable linkage drag between a BPH resistance gene and genes conferring agriculturally important characters may be removed by intensive work to select recombinants between the traits, and a molecular marker tightly linked to the target gene could be useful for selecting the desired recombinants。
The use of tightly linked genetic markers for resistance genes offers great scope for improving the efficiency of conventional plant breeding by allowing selection to be based on molecular markers linked to a trait at an early stage of growth, rather than being based on the trait itself. Resistance genes to gall midge and blast have been identified in rice, and linkage between DNA markers and these resistance genes has been analyzed. In this way linkage maps of genes associated with resistance to diseases and pests have been constructed in wheat, barley, and other economically important crops. The current study was conducted to identify Bph1-related DNA markers in rice, and thus to permit the establishment of a marker-assisted breeding program to introgress the BPH-resistance gene into japonica rice cultivars.
