Study On The Infection Process And Toxicological Mechanism Of Beauveria Bassiana Against Adelphocoris Suturalis

Chemical control,as one of key technique of integrated pest management to cotton mirids,often causes a serial of negative problems such as pest resistance,pesticide residue,even natural enemy killed and environmental pollution.Biological control can help reduce the pressure of chemical control,delay the development of pest resistance and improve environmental friendliness.Beauveria bassiana,a kind of entomogenous fungi,can control lepidopteran,coleopteran,hymenopteran pests effectively,but the research on its control of hemipterous insects is few.The infection process of B.bassiana to Adelphocoris suturalis was observed using scanning and transmission electron microscopy.The conidial production rates and the activities of protease,chitinase,esterase,lipase between two isolates with different virulence to A.suturalis were detected by Nageotte hemocytometry or microplate reader respectively.Chitinase gene Bbchitl was cloned by PCR and its expression level was analyzed by qrt-PCR method.The results revealed the pathogenicity mechanism of B.bassiana to A.suturalis and provided theoretical basis for biological control of cotton mirids..1.Infection process of B.bassian to A.suturalis.At the beginning of inoculation,B.bassiana adhered on the surface of A.suturalis,then began to secrete extracellular mucilage to reinforce the adhesion;The germinated conidia formed germ tube and appressorium.Germ tube and appressorium began to invade the cuticle of A.suturalis at 16 h after inoculation with the aid of extracellular hydrolyzing enzymes and mechanical stress;The hyphae invaded fatbody,muscle tissue and started septation at 48 h after inoculation;The hyphae penetrated out from A.suturalis cuticle and developed into aerialhyphae at 72 h after inoculation.These results showed the condia of B.bassiana completed the infection process within 72 h after inoculation with the combination of mechanical pressure and enzymatic action.2.The role of conidial production rate and extracellular hydrolyzing enzymes in the pathogenicity mechanism of B.bassiana to A.suturalisThe conidial production rates of high virulent Ⅷ strain and low virulent Ⅵ strain were 2.79 × 108conidia/d and 2.9 × 108conidia/d,respectively,which showed no significant difference between the two strains.Compared with the activity of extracellular hydrolyzing enzymes in Ⅶ strain,the activities of protease,esterase and lipase in Ⅷ strain did not increased significantly.The activity of chitinase(3217.75U/mg)in Ⅷ strain,however,was 6.54 times of that in VI strain.These results revealed that conidial production rate might not be involved in the virulence of B.bassiana,but the chitinase was a key factor affecting the virulence of B.bassiana to A.suturalis.3.The role of chitinase in the pathogenicity mechanism of B.bassiana to A.suturalisThe Bbchitl gene in B.bassiana contained a 1 047 bp open reading frame,which encoded 348 amino acid residues.The deduced amino acid sequence showed the common properties of chitinase,such as N-terminal signal peptide,catalytic site,serine/threonine riching region,the chitin binding site,and C-terminal extension region.There was no polymorphism in amino acid sequence of chitinase between high virulent Ⅷ strain and low virulent Ⅵ strain.The transcription level of Bbchitl in Ⅷ strain was approximately 27.61-fold as high as that in VI strain.The transcription level of Bbchitl might play an important role in the virulence of B.bassiana to A.suturalis.