| Stalk rot in maize is one of the most devastating soil-born diseases around the world, and it could cause severe yield loss in the areas with a high prevalence of this disease. In China, stalk rot is often caused by Pythium inflatum and occurs at the late milk stage of the corn, so it is difficult to control this disease by conventional techniques such as the use of fungicide. It has been proved that the most efficient management for controlling stalk rot is to breed resistant cultivars. Thus, in this study we identified and located the resistance genes in inbred lines Qi319 and X178, providing potentially new stalk rot resistant germplasms that can be used in maize breeding. Additionally, we investigated the resistances to stalk rot among various maize germplasm resources, and found many resources highly resistant to stalk rot. Last, we approached the relation between sugar metabolism in maize plant and Pythium stalk rot resistance level.The main results were as follows:1. Two novel Pythium stalk rot resistance genes Rpi QI319-1 and Rpi QI319-2 were found in inbred line Qi319. A completely resistant inbred line Qi319(the donor parent) was crossed with the susceptible line inbred Ye107, and F2(n = 673), and F2:3(n = 662) populations were generated for genetic analysis. The resistance to P. inflatum(P85-67) in Qi319 were found to be controlled by two independently inherited dominant genes(χ2 = 0.6182, P = 0.4317; χ2 = 2.3884, P = 0.3029). The two genes Rpi QI319-1 and Rpi QI319-2 were found to be located on the bin 1.03 and bin 10.02, respectively, using the BSA and marker-assisted selection method. The fine mapping results indicated that marker SSRZ33 was at a genetic distance of 0.2 c M from the Rpi QI319-1 gene on one side, while on the other side of Rpi QI319-1, marker SSRZ47 was 0.5c M away from the resistance gene. Additionally, the Rpi QI319-1 gene was refined into a ~500 kb interval flanked by the markers SSRZ33 and SSRZ47. Rpi QI319-2 was located between the markers umc2069 and bnlg1716, with a genetic distance of 3.6 c M and 4.6 c M, respectively, from the resistance gene. Additionally, the two genes Rpi QI319-1 and Rpi QI319-2 were proved to be new and different from those previously reported genes.2. In inbred line X178, resistance to Pythium stalk rot were controlled by two new genes Rpi X178-1 and Rpi X178-2. F2(n = 736), and F2:3(n = 689) populations derived from a cross between completely resistant inbred line X178 and susceptible line Ye107, were used for genetic analysis. The results suggested that the resistance to P. inflatum(P85-67) in X178 were conferred by two independently inherited dominant genes Rpi X178-1 and Rpi X178-2(χ2 = 0.0232, P = 0.8790; χ2 = 0.1863, P = 0.9110). The two genes Rpi X178-1 and Rpi X178-2 in X178 were then found to be located on the bin1.09 and bin4.08, respectively. Thereafter, fine mapping of the two genes was performed, and Rpi X178-1 was found to be between markers SSRZ8 and IDP2347. Marker SSRZ8 was also found to be at a genetic distance of 0.6 c M from the Rpi X178-1 gene, and marker IDP2347 was 1.1 c M away from the resistance gene. The physical distance between SSRZ8 and IDP2347 was about 700 kb. Rpi X178-2 was found to be between markers bnlg1444 and umc2041, with a distance of 2.4 c M between the two markers. Additionally, the results of the comparison of the fine mapping in our study and those reported previously, showed that the two genes Rpi X178-1 and Rpi X178-2 were new and different from those genes previously reported on chromosome 1 and 4.3. We investigated the resistances to Pythium inflatum stalk rot among 618 maize inbred lines and 249 local varieties(including 52 introduced landraces), and found that most germplasms showed resistance. For 570 resources, reactions to stalk rot were evaluated in 2012 and repeatedly in 2013 year. About 43.51% of the 570 resources showed high-level resistance to stalk rot, while the susceptible rate was only 15.26%. The resistant reactions of 297 resources were evaluated in 2014, and we found that 178 inbred lines and 47 landraces performed high-level resistance, with a resistance rate 95.96%. The flanked markers of gene Rpi QI319-1 was used to investigate the prevalence of the resistence gene among 202 germplasm resources. The results showed that 10 germplasms carrying this gene. These stalk rot-resistant germplasms identified in this study provide useful resistant resources for developing maize cultivars against stalk rot.4. No direct correlation between the content of total sugar and reducing sugar in the basal internodes and the stalk rot resistance was found in this study. The content of total sugar and reducing sugar of the basal nodes 1-5 at different growth stages among 6 inbred lines(3 resistant and 3 susceptible) and 20 hybrids(10 resistant and 10 susceptible) was determined in this study. The sugar metabolism levels of basal 1-3 nodes and 4-5 nodes at different growth stages, with or without inoculation treatment were also analysed. The results indicated that the sugar content of 1-3 nodes and 4-5 nodes had similar changing trends. However, the content of total sugar and reducing sugar between the resistant resources and the susceptible resources showed no significant difference. Therefore, we can draw a conclusion that there was no direct correlation between the content of total sugar or reducing sugar and the stalk rot resistance. |
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