| Southern corn rust, causing by Puccinia polysora Underw, is one of the most importance maize diseases in Huanghuaihai Summer Maize Region of China. The yield loss from this disease can be over 10% for highly susceptible cultivars if the disease occurs in early stage of maize growth. At present, most of the commercial maize cultivars including some widely grown cultivars are susceptible or highly susceptible. The deployment of germplasm resistant to Southern corn rust for breeding purpose is the main measure to solve the production problem in the future. My study is to identify the resistant maize germplasm resources, analyze the genetic relationship among them using SSR markers, and understand the resistant characteristics of maize infected by P. polysora through the cytohistological, physiological, and biochemical methods, as well as the expression of the resistance-related genes. The main outcomes were as follows:1.1136 germplasm lines were evaluated for resistance to Puccinia polysora. by means of artificial inoculation from 2008 to 2010. a few lines were highly resistant. The percentages of the highly resistant, resistant, moderately resistant, susceptible, and highly susceptible lines in the maize germplasm were 2.46%,7.13%,29.93%,36.79%, and 23.68%, respectively. The resistance cultivars in the inbred lines, landraces, foreign landraces and domestic landraces were occupied 42.19%,36.95%,75.76% and 34.60%, respectively.2. Microsatellite markers were adopted to determine heterotic group and genetic relationship of 14 resistant cultivars with unknown details about the originations. Cluster analysis reveals that the resistance inbred lines Liao2204, DoubleM9B-1, Zun90110, Liao2202 and Jiku6 were clustered in group B. The resistance hybred lines 5362 and Jiku12 were clutered in group A. The other seven resistance inbred lines were separated in other groups which were far from A and B group. All of these shows that the germplasm of maize contain some precious resources resistant to the Puccinia polysora Underw, especially the germplasm from foreign landraces containing more than from domestic landraces. The genetic background of the selected resistance inbred lines are narrow.3. Transmission electronic microscopy technique was used to study the cytological changes of the host in the process of infection by the pathogen of P. polysora. The results showed that:(1) In the initial stage of pathogen infection, cytological changes of the resistant and the susceptible were identical. (2) In the early stage of pathogen infection, certain resistant substances like the dark electronic density of black particals and the cell wall appositions were present in the mesophyll cells of the resistant lines. (3) In the mid-period of pathogen infection, there was significant difference between the resistant and the susceptible lines. Some of the cells in the resistant lines died to inhibit the infection and growth of the pathogen. While the mesophyll cells in the susceptible materials produced some instructure substances to provide the nutrition for the growth of pathogen continously. (4) In the later stage of pathogen infection, the mycelia in the mesophyll cells of the resistance were decomposed and died. The mycelia in the mesophyll cells of the susceptible were propagated massively, and then lysised and died for lack of nutrition. As a result, the resistant characteristics of the maize to the pathogen appeared in the early stage and mid-period of pathogen infection. The specific characteristics of the resistant materials were consistant with the field performances, such as hypersensitive cell death of the materials and small area of uredium of P. polysora on the leaves.4. The changes of the activity of some biochemical parameters, such as phenylalanine ammonia lyase (PAL), polyphenol oxidase (PPO), superoxide dismutase (SOD), soluble sugar, soluble protein, and proline, were detected in four maize inbred lines with different levels of resistance to P. polysora. The results showed that:(1) The disease incidence of the susceptible inbred line Huangzao4 was the highest and up to 62%, while the resistant hybred line Qi319 were only 10.5%. (2) Except for proline, other biochemical parameters were significant different among inbred lines, treatments, and times after inoculation with P. polysora. (3) The activities of PAL and PPO were raised obviously after inoculation with P. polysora. The peak and the increase rate of PAL and PPO were earlier and higher in the resistant lines than in the susceptible ones. The activity of SOD was higher in the resistant lines in the early stage after inoculation, but it was higher in the later stage after inoculation in susceptible lines. The content of soluble sugar was higher in the susceptible lines than in the resistant lines, which indicated that Southern corn rust is one of the high sugar diseases. (4) By means of fitting curve estimation with SPSS software, the relationship between the parameters of PAL, PPO, and soluble sugar and the incidence of infected plants one day after inoculation of P. polysora was significant. The characteristics appeared in the process of physical and biochemical were consistent with the cytological changes of the pathogen in each period of infection.5. The method of RT-PCR was applied to analyze the changes of the related factors after defense in the resistant and susceptible inbred lines when infected by P. polysora. The results revealed that after infection by the pathogen, the transcription levels of the defense-related genes, such as Phenylalanine ammonia-lyase(PAL), Pathogenesis related protein-1 (PR-1), Acidic beta-1,3-glucanase(PR-2a) and so on were enhanced significantly. Meanwhile, the expression time of the induced defense genes in the resistant lines was earlier than the susceptible lines, and the induced expression level was higher in the resistant than the susceptible lines. The transcription levels of the resistant genes Maize proteinase inhibitor (MPI) and Lipoxygenase(LOX) related to the JA signal pathway had no changes in both resistant and susceptible lines. |
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