73 Australian Journal of Crop Science     2(2):73-82 (2008)  ISSN: 1835-2707 Dynamic QTL analysis for rice blast resistance under natural infection conditions Yibo Li, Changjun Wu, Yongzhong Xing, Huilan Chen and Yuqing He * National Key Laboratory of Crop Genetic Improvement, National Center of Plant Gene Research (Wuhan) and College of Life Science and Technology, Huazhong Agricultural University, Wuhan 430070, China *Corresponding author email: yqhe@mail.hzau.edu.cn Abstract Dynamic analyses of rice blast resistance for the assessment of genetic and environmental effects were characterized employing a recombinant inbred lines (RILs) population. The study was conducted at three different developmental stages of rice using natural infection tests in two years. The number of main-effect quantitative trait loci (QTL), epistatic QTL and their environmental interactions greatly differed across various measuring stages. Two major QTL ( r11a and r11b) both on chromosome 11 could be detected at all stages, whereas most QTL were identified only at one or two stages in the population. It was suggested that the unstable activities of most QTL identified for blast resistance may well be due to effects of major QTL, epistatic effects between different loci, the developmental status of rice, and the environments in which they were grown. Comparison of QTL analysis conducted under the conditions of natural infection and artificial inoculation was performed and drew a new conclusion that QTL analysis of plant resistance based on natural infection would have more advantages than that based on artificial inoculation. Keywords: Rice blast resistance, Dynamic analysis, Main-effect QTL, Epistatic interactions, QTL-by- environment interactions Introduction Rice blast, with its genetic instability (Reddy and Bonman, 1987), is still one of the most destructive diseases of rice in both tropical and temperate countries, despite great efforts toward its control. Many studies (Wang et al., 1994; Nagato and Yoshimura, 1998; Ahn et al., 2000; Wu et al., 2005; Li et al., 2007), indicated that the genetic control of blast resistance is complex and involves both major and minor genes with complementary or additive effects (He et al., 1989), as well as their environment interactions (Bonman, 1992). Nevertheless, gene or QTL pyramiding remains to be a promising method to provide broad-spectrum and durable rice blast resistance (Tabien et al., 2002). Up to now, selection for resistance has been performed under natural infection conditions. What is more, environmental conditions, such as temperature and moisture, greatly affect the epidemics of rice blast diseases and hamper the breeding programs for this trait. However, almost all the genetic studies including rice blast resistance was based on the artificial inoculation, and always focused on plants at a specific or a final growth stage except our former work (Li et al., 2007). Such studies could not fully capture the real gene action during the growth of plant. Moreover, breeders want to know whether the results from artificial inoculation are consistent with those obtained under natural infection conditions (Lübberstedt et al., 1999). Therefore, dynamic mapping and gene expression studies of plants at