Screening Of Corn High Endophytic Fungi Strains Against Insect And Primary Investigation Of Insect-Resistant Mechanism

Endophytic fungi,which are ubiquitous in healthy plant tissues or organs,can promote plant growth,enhance host plant resistance to diseases,insects and stress tolerance by a serious of metabolites or signal transductions.In recent years,endophytes,as a new type of microbial resources,have become a research hotspot in the fields of agriculture and forestry,and have been applied in production.Studies on corn endophytes are mainly focused on diseases resistance and stress resistance,as well as growth promotion.To screen the resources corn endophytic fungi for insect-resistant,highly pathogenic Beauveria bassiana YB15,YB8and corn endophytic fungi strains YC1,GX5 against Ostrinia furnacalis,Athetis dissimilis and Rhopalosiphum maidis were screened out,and the colonization rates in corn tissues were determined.In order to reveal the mechanism of insect resistance,the effects of YC1 and GX5on the growth,nutrients and secondary substances of corn,as well as on the growth,development and the main enzyme activities of R.maidis were determined.The results are as follows:1.The highly pathogenicity fungus Beauveria bassiana strains YB15 and YB8 against O.furnacalis and A.dissimilis were screened out,which could cause the corrected mortalities of 96.67%and 100%,the cadaver rates of 73.33%and 86.67%after inoculation with 1×10~9 spores per milliliter at 26℃for 10 days respectively;the seedling emergence rates of the corn were more than 94%,and the colonization rates were 33.33%~61.41%,with the control efficacis against R.maidis were more than 50.00%after seeds impregnation.The high-efficency corn endophytic fungi strains against R.maidis were YC1 and GX5,with the control efficacis against R.maidis were both 74.80%,the emergence rates of corn were100%and 93.85%,and the colonization rates in corn were 33.23%~63.44%respectively,indicating the great application potential.2.Strains YB8,YB15,YC1 and GX5 all promoted the growth and biomass accumulation of corn after seeds impregnation.The fresh weight of roots,stems and leaves increased by 0.04%~18.32%,and the dry weight increased by 0.25%~20.51%.Among them,the dry weight of corn leaves treated with YC1 increased significantly compared with that of the control,and the root length of corn treated with GX5 was significantly higher than that of the control.Strains treatments had effects on nutrient contents,SA,JA signal transduction pathway and secondary substance contents of corn,while the changes of nutrients,JA,SA and secondary metabolites in corn stems of the strain treatment groups were different from those of the control group after feeding on R.maidis,with the significant differention of different strains.The contents of protein,AA and soluble sugar in the roots of corn treated with strain YC1 were higher than that of the control.After feeding on R.maidis,the contents of protein and AA in corn stems of YC1 treatment group increased continuously,AA content increased by 21.98%at 8 h,while the content of protein in control group was basically unchanged and AA content in corn stems of the control group increase at first and then decrease.This indicated that YC1 increased the accumulation of nutrients in corn.The content of JA in corn stems of the YC1 treatment group was significantly higher than that of the control group at 8h after feeding on R.maidis,indicating that YC1 improved the emergency defense of JA signal transduction pathway.The contents of protein and AA in the roots and soluble sugar in the stems of corn treated with GX5 were significantly higher than these of the control groups.The content of protein in the stems of GX5 treatment group continued to increase at 8 h after feeding on R.maidis,indicating that GX5 increased the accumulation of nutrients in corn.The content of SA in roots,stems and leaves of the treatment groups were significantly higher than these of the control group,and the contents of SA in corn stems were still significantly higher after feeding on R.maidis,indicating that GX5 mainly strengthened SA signal transduction pathway and emergency defense of corn.The changes of tannin and TP contents in maize stems in the treatment group were similar to these in the control groups,indicating that the strains had little effect on the secondary substance.3.After feeding on the corns treated with strains YB8,YB15,YC1 and GX5,the the reproduction periods of R.maidis were lower than control significantly;and the net reproductive rate,average generation time,innate capacity for increase as well as finite rate of increase were all lower than the control group,but there was no significant difference.The ratios of R.maidis choice YC1,GX5,YB8 and YB15 treatment corns were 16.88%,19.31%,19.91%and 18.21%,all significantly lower than that of control group(25.69%).The main enzyme systems of R.maidis were affected by strains YC1 and GX5.The activities of sucrase,THL and CarE in YC1 treatment group at 1 h were higher than these in the control group,and the activities of THL and CarE at 8 h were still higher than these in the control groups.The activities of sucrase,THL,CarE and GST in the GX5 treatment groups at1 h and 8 h were all higher than these in the control groups.Indicating that both strains accelerated the sugar consumption of R.maidis.In summary,the highly insect-resistant corn endophytic strains YC1,GX5,YB8 and YB15 were screened in this study and had a high colonization rates in corn,which could promote the growth and dry matter accumulation of corn.YC1 and GX5 increased the accumulation of nutrients in corn and the content of two soluble sugar enzymes in R.maidis.YC1 improved the emergency defense of JA signal transduction pathway,while GX5 mainly acted on SA signal transduction pathway and emergency defense.