The Preliminary Research On The Host Selection Mechanism Of Infection By Sheath Blight Fungus

Rice sheath blight is one of the three major diseases of rice,for a wide range of host and great destruction,Sheath blight has a significant effect on yield,especially the important food crops rice,corn and soybeans without highly resistant crop cultivars.Rhizoctonia solani AG1 I A,the pathogen of sheath blight,was also the first reported genome of the pathogen Rhizoctonia genus,and it may serve as a model for studying the pathogenic mechanisms,but the mechanisms of host selection is still unclear.The purpose of this study is to analyze the differences between sheath blight fungi isolated from different hosts,including rice,maize and soybean,and provide information and clues to proven the molecular pathogenic mechanisms of sheath blight induced byRhizoctonia solani.This paper consists of three parts:the first part is to explore the biological characteristics and the virulence between the Rhizoctonia solani isolates from different hosts from the cellular level providing theoretical basis for the comprehensive control of sheath blight for a variety of crops.The second part is to explore the sequence differences and expression levels of the key pathogenic genes from the molecular level,and to identify the pathogenic ability of the effectors providing basis for the function research of the genes.The third part is the transcriptome sequencing analysis of R.solani AG1 IA isolates cultivated on the special host,non-special host and without the host.The purpose is to explore selective expression of genes in the strains in order to adapt to different environment and different host.The results are as follows:First,the study of the biological characteristics and the virulence levels of R.solani AG1IA from different hosts.There was no obvious difference in the mycelia morphology and the time of sclerotia formation among isolates RiceIA、MaizeIA、SoybeanIA,but the mycelial growth rate of MaizeIA is slightly lower than RicelA and SoybeanIA.The nucleus number of RiceIA was more than MaizelA and SoybeanIA.All the three strains could produce the appressorium and infection mat,which had no visible difference in the morphology and the formation time,on the rice,maize and soybean leaves.And these three strains had somatic incompatibility reactions which might reflect their genetic differences.All these findings revealed that the differences indeed exist among R.solani AG1IA isolates from different hosts.The second is pathogenicity determination of strains on the vitro leaves.The three isolates all could lead the rice,maize and soybean leaves to the sheath blight,and have been proved to be R.solani AG1 IA by ITS identification.The calculation of disease spot sizes on the leaves showed that all these three isolates induced the largest one on their own special host.The third is the cloning and the pathogenic verification of the effector genes.The sequencing alignment of AG1IA₀7795 showed the consistency was 75.45%.RiceIA and MaizeIA had the same base sequences,but SoybeanIA had 5 SNPs and two insertions with 55bp/56bp.The sequencing alignment of AG1IA₀9161 showed the consistency was 83.25%.Comparing with the reference.RiceIA had one SNP,MaizeIA had 3 SNPs,but SoybeanIA had 3 SNPs and two insertions with 69bp/56bp.The expressed proteins were inoculated into rice,maize and soybean leaves,and these two secreted effectors from RiceIA and MaizeIA caused cell death phenotypes,but not with SoybeanIA.We speculate that the change of the effector pathogencity may relate to their gene sequence in order to survive.The forth is the transcriptome analysis of strains cultured under different hosts.From the data,we found that SoybeanIA under different processing conditions had the most SNP about 60,000,MaizeIA with 40,000 SNPs and RiceIA with 30,000 SNPs.But the number of SNPs in RiceIA after interacting with the maize leaves rise to 40,000.8 samples had produced 6 types of alternative splicing events（AS）,IntronRetention events occurred most frequently and AltFirstExon/AltLastExon events occurred at least.AG1IA₀9161 in the sample T6 have two ASs,IntronRetention and Alt5splice,but in the control sample T7 have only IntronRetention events.And AG1IA₀9161 of sample T2 and T4 had no AS.This suggested that the insertion of AG1IA₀9161 in SoybeanIA might be the IntronRetention.There was no AS in AG1IA₀7795 of the 8 samples.Comparing the number of DEG between treatment samples and control samples,the up-regulated genes were generally more than the down-regulated genes.But we found the secondary metabolite biosynthesis enzymes genes which were related to pathogencity,AG1IA₀5539 and AG1IA₀5360,express lower in treatment samples.The DEG of T1 and T2 samples are mainly enriched in metabolic pathways related to the energy synthesis and transformation.