Powdery mildew of wheat caused by Blumeria graminis DC f.sp.tritici Em.Marchal,is one of the most devastating foliar diseases of common wheat（Triticum aestivum L.）worldwide including China.The use of resistant cultivars provides the most cost effective,environmentally sound,and consistently used method to reduce yield losses.Molecular markers closely linked to resistance gene can be used to marker-assisted selection（MAS）and pyramiding different resistance genes into a single genotype/cultivar for broad-spectrum resistance in advanced wheat breeding programs.Elucidating the molecular basis of the broad-spectrum plant disease resistance offers further possibilities for durable resistance.In this study,the common wheat line N0308 carrying the resistance gene PmG25 introgressed from wild emmer accession G25（Triticum turgidum var.dicoccoides）was used to identify and map the gene（s）at early stage of resistance using genetic analysis and molecular markers,and get the closely linked markers for marker-assisted selection（MAS）.A SSH（suppression subtractive hybridization）cDNA library was constructed from wheat leaves after inoculation with Bgt isolate E09 at early stage.Qualified Expressed sequence tags（ESTs）and expression pattern of disease resistance related genes were analyzed by semi-quantitative RT-PCR.The main results were obtained as follows:1.The common wheat（Triticum aestivum L.）germplasm line N0308 possesses genetic resistance to powdery mildew introgressed from the wild emmer accession G25.Phenotypic evaluation of F2 population and F3 families derived from the cross N0308/Shaanyou 225 indicated that resistance to the ’Guanzhong 4’ isolate of powdery mildew was controlled by a singled dominant gene in N0308.PmG25 was linked with 11 SSR markers（Xgpw1082,Xgpw3191,Xfcp1,Xfcp393.Xfcp394,Xgpw7425,Xwmc75,Xgwm408,Xwmc810,Xbarc232 and Xbarc142）and two EST-STS（BF482522 and BF202652）markers on the long arm of chromosome 5B.Among them four markers（Xfcpl,Xgpw7425,Xbarc232 and Xbarcl42）were inherited co-dominant,and nine（Xfcp393,Xfcp394,Xgpw3191,Xgpw1082,Xwmc810,Xgwm408,Xwmc75,BF482522 and BF202652）were complete dominant.Xfcp1/Xfcp393 and Xgpw3191 were flanking markers and tightly linked to powdery mildew resistance gene PmG25 at genetic distances of 1.3 and 3.3 cM,respectively.The PmG25 locus was located on the chromosome bin 5BL-14-0.75-0.76 in the test with a set of deletion lines.Based on the origin and chromosomal location it is suggested that the resistance gene introgressed from wild emmer G25 may be allelic or closely linked to Pm36.2.SSH-cDNA library was constructed from wheat germplasm N0308,resistant to powdery mildew,inoculated by Bgt isolate E09 at two leaves stage.About 350 clones were chosen randomly from SSH-cDNA library.A total of 175 positive clones from the library were subjected to sequencing,and 90 expressed sequence tags（ESTs）were obtained after removing repeated and redundancy,and submitted to GenBank.Accession numbers of GenBank is from JZ124067 to JZ124156 and dbEST-Id from 77682937 to 77683026.BlastX results in nr-protein database revealed that 47 ESTs were highly homologous with known proteins,involved in defense and stress response（22%）,transcription（17%）,and energy（13%）,metabolism（11%）,signal transduction（9%）,protein synthesis and storage protein（6%）,transporter（6%）,cell growth and division（6%）and immune system（4%）.BlastNr results showed that 35 and 25 ESTs had high identities with known unigene and function-unknown ESTs,respectively.Fourteen ESTs were both in the nucleic acid and protein databases,including 20 ESTs associated with powdery mildew resistance.Among them,5 were responsible for signal transduction,2 for hypersensitive necrosis reaction system,and 13 for systemic acquired resistance system.This analysis provides valuable information on potential players in the defense mechanism and can help focusing future research on particular genes and their contribution to plant resistance.3.Comparing the EST sequences among the SSH-cDNA libraries,gene expression pattern of seven ESTs in resistance reaction of powdery mildew were analyzed by using semi-quantitative reverse transcription-polymerase chain reaction（RT-PCR）.They were violaxanthin de-epoxidase（VDE）,sulfatase,gag-pol-polyprotein（GPP）,pathogenesis-related protein 17（PR-17）,betacarbonic anhydrase 2（β-CAs2）,thioredoxin h-like protein（Trxh）and Coronatine-insensitive（COI）.Expression of five genes（sulfatase,pathogenesis-related protein 17,betacarbonic anhydrase 2,thioredoxin h-like protein and Coronatine-insensitive）transcripts were induced and up-regulated to their highest levels at 72 hours after Bgt infection,while that of two genes（violaxanthin de-epoxidase and gag-pol-polyprotein）were expressed at the highest level at 12 and 18 hpi,respectively.These genes were highly induced at an early stage of infection,suggesting they are transcriptionally activated for the host defense response.