Pathogen Of Maize Seedling Blight In Hexi Corrodor And Effects Of Its Mycotoxins On Maize Roots

As being a yield reducing factor,maize seedling blight(MSB)has been a worldwide problem.The reported pathogens of MSB include Fusarium spp.,Pythium spp.,Rhizoctonia spp.and others.Hexi Corridor is the biggest area of maize seed production in China,where 60%of 900 thousand tons of hybrid seed are produced.MSB was found sporadicly in Hexi Corridor in 2000 but has become a common disease with 10%to 40%incidence in recently years,long-term consecutive mono-croppings favores inoculum increase and pathogen diversity,which may explain the increasing incidence and spread of MSB in recent years.This study aimed to identify the main pathogens of MSB in Hexi Corridor,characteristics the pathogen and evaluate the effffects of mycotoxins on maize growth and the function of mycotoxins during infection.The main results were as followes.1)Pathogens of maize seedling blight in Hexi CorridorTo determine the pathogen of MSB,symptomatic maize plants were collected from Hexi Corridor.A total of 49 isolates obtained from diseased seedlings,36 of them were identified as Fusarium spp.,results of pathogenicity assays showed that F.graminearum was the the most virulent species associated with MSB.Mixtures inoculation of Fusarium isolates from different species showed additative effects on the disease severity.2)Genotype and chemotype of F.graminearum strains from Hexi CorridorPolymerase chain reaction assay showed all 23 F.graminearum isolates were belonged to the 15-acetyldeoxynivalenol(15-ADON)genotype.Both deoxynivalenol(DON)and 15-acetyl-deoxynivalenol(15-ADON)in either rice culture medium or maize seedling roots were detected by high performance liquid chromatography and mass spectrometry(HPLC-MS),so the chemotype of all isolates was 15-ADON.In maize seedling roots,15-ADON dominated at 6 days post inoculation(dpi)and DON increased sharply at 12 dpi.3)Effects of DON and 15-ADON on seed germination and root developmentBoth DON and 15-ADON affected seed germination and root development,with 15-ADON being more destructive than DON.Under scanning electron microscopy(SEM)and transmission electron microscopy(TEM),root hair formation and development were delayed by DON,but completely inhibited by 15-ADON.15-ADON caused cell shrinkage,loose cellular structure and widened intercellular spaces;it also destroyed organelles and caused plasmolysis,and eventually ruptured cell membranes causing cell death.DON did not affect cell morphology and arrangement,but altered the morphology of organelles,forming concentric membranous bodies and a large amount of irregular lipid droplets.Thus,both mycotoxins contributed to symptom expression of maize seedling blight,but 15-ADON was more destructive than DON.4)Effects of F.graminearum infection on genes related to resistance of maize genotypePec,Xy1 and Lip of F.graminearum,related to degrade cell wall of host,reached much higher expression levels in susceptible inbreds compared to resistant inbreds;Tri5 is the key gene during the mycotoxins synthesis,which showed opposite trend compared to the other genes of F.graminearum.The upregulation of genes involved in SA,JA synthesis and host defense response occurred earlier and reached much higher expression levels in resistant inbreds compared to susceptible inbreds.Genes related to host detoxification were upregulated in different time points without any regulation.5)Effects of DON and 15-ADON on genes related to resistance of maize genotypeMost tested genes were more sensitive to 15-ADON.The lowest concentration(0.1 ?g/ml)of 15-DON induced upregulation of PR5,PR10 and genes involved SA,JA synthesis,the upregulation levels were higher than those induced by DON.DON of 0.1 ?g/ml induced a higher expression of POX and Chitinase than 15-ADON.GTF is a gene involved in covalent binding of DON and glucose,while CS,GST and MRP involved in covalent binding of DON and glutathione,in this study GTF showed a higher expression in DON treatment than 15-ADON,however,CS,GST and MRP showed a higher expression in 15-ADON treatments than DON.ICS and PAL are key genes of SA synthesis in different pathway,both genes upregulated in DON and 15-ADON treatment.ICS showed highest expression level in susceptible inbred,but PAL showed in resistant inbred.Genes related to host defense response showed earlier and higher expression in susceptible inbreds than in resistant inbreds.6)Translation elongation factor 1-a gene ofF.graminearum strains from seven provinces were used for phylogenetic analysis.Strains isolated from Hexi Corridor showed a far phylogenetic relation with strains isolated from wheat root,maize root and ear in Henan province and north-west of China.A closer phylogenetic relation was found between F.graminearum strains from Hexi corridor and Hebei,Jilin and Henan when compared to the phylogenetic relation with strains from Sichuan province.A correlation was fund between population genetic distance and geographic distribution.