The Physiological Defferentiation And The Genetic Plymorphism Of Exerohilum Turcicum

The physiological differentiation and the genetic polymorphism of Exerohilum turcicum were studied systematically in terms of host-pathogen interaction with several techniques or methods,involving observation of pathogen culture traits,identification of pathotypes on differential hosts,electrophoresis of isozyme,analysis of amplified fragments length polymorphism(AFLP) and immunoassay techniques.In order to prove the factors of the increasing epidemic of northern corn leaf blight in China in some years,the composition and population variation of physiological races were investigated.204 isolates of the pathogen were isolated in 28 areas of Heilongjiang,Jilin,Liaoning,Hebei,Henan,Shandong,Shanxi, Shaanxi,Sichuan provinces and Beijing and Tianjin and Tangshan regions in 2005 and 2006 for identification of physiological races on differential hosts of corn inbred lines containing single resistance genes such as Ht1,Ht2,Ht3 and HtN.At last nine races were identified,including 0,1,2,3,12,13,23,123 and 23N.Among these physiological race 0 and 1 were predominant with occurrence frequency of 62.25% and 19.12%.The results about the analyzed based on virulence frequency showed that the virulence frequency for the race population against gene Ht1 and HtN were 33.33%and 1.47%.The resistance genes of Ht1 and Ht2 have to some extent lost their function against the pathogen in some areas of China.It was found that the physiological differentiation of the pathogen was complicated,and new races were frequently found.The variations of the physiological races of Exerohilum turcicum in Henan,Sichuan provinces and Northeastern China were higher than in other corn-planting regions.To explore the mechanism of the physiological and molecular biology about the physiological races variation of Exerohilum turcicum,the environmental adaptability of physiological races,the isozyme and DNA polymorphism of the pathogen were studied.By investigating the long-term sub-cultures of isolates, continuous inoculation,and ITS sequences analysis of physiological races of Exerohilum turcicum,the biological characteristics,the pathogenicity and rDNA-ITS sequences of Exerohilum turcicum were relatively stable.The homology of rDNA-ITS sequences of physiological races of Exerohilum turcicum was 96.9%～100%under the stable conditions of host selection pressure.The polymorphism of the biological characteristics and the isozyme of Exerohilum turcicum were studied.The results indicated that there were the significant difference among the different physiological races in the colony morphometries,the growth rate of mycelium,sporulation,spore germination and pigment-productivity of colony.The polymorphism of different physiological races could be well exhibited by the diversified changes of electrophoresis profile of six kinds isozyme including esterase(EST), peroxidase(PO),polyphenoloxidase(PPO),superoxide dismutase(SOD),catalase(CAT),and malate dehydrogenase(MDH).It was concluded that EST and SOD present the obvious polymorphism among the tested isolates.AFLP was conducted to analyze the genetic polymorphism of Exerohilum turcicum.The genetic similarity among the isolates was from 0.61 to 1.0.It was concluded that AFLP analysis could present the genetic polymorphism of Exerohilum turcicum.The clustering results of AFLP had correlation with physiological differentiation.But the genetic similarity of the same physiological races collected from different geographical regions reached 1.0.The physiological differentiation had no obvious correlation with their geological location.Assistant methods for physiological races classification were studied.The specific primers of physiological race of Exerohilum turcicum were screened by using AFLP bands specific.The primers being able to be used as an assistant method for No.0 and No.13 physiological race identification.Based on the genetic polymorphism of Exserohilurn turcicum,antiserum of 1,2,12 and 13 physiological race were prepared by immunological methods.The results showed that there was significant differences in antiserum titer among different physiological races through the titer,cross-reactivity,ELISA test and Western blot method.It had race-specific about the antiserum of the NO.1 and NO.12.Resistance identification of main cultivars in Liaoning province to physiological races of Exserohilum turcicum was completed.There was obvious difference in the physiological races to main cultivars.The highly resistant cultivars was 17.86%,and moderate resistant cultivars was 53.57%.Resistance identification to northern corn leaf blight and pedigree analysis showed that the resistance of cultivars to the different physiological races had correlation with the blood relations.In addition,climatic conditions impacted on the results of the resistance identification.The main conclusions about this paper were as follows:(1) the pathogenicity differentiation and biological polymorphism really existed in the population of the pathogen.(2)The increasing epidemic of northern corn leaf blight in some areas of China or in some years were ascribed to the continuing occurrence of new races of the pathogen,the increasing virulence frequencies of the minor races,and the resistance-losing of the cultivars.(3)The study could extrapolate that one of the important reason for genetic variation of Exerohilum turcicum was the variations of host selection pressure.4)AFLP analysis and immunological methods provided a new method for assistant identification of the physiological race of Exerohilum turcicum and improved the accuracy.(5)In recent years,common corn cultivars in Liaoning province had a strong resistnce agains the predominant races of Exerohilum turcicum.