Surface characteristics and stress tolerance of submerged conidia,blastospores and aerial conidia of biocontrol fungus Cordyceps javanica IF-1106
Cordyceps javanica is an entomopathogenic fungus with biocontrol potential in control of many important pests belonging to Homoptera,Hemiptera,Lepidoptera,Thysanoptera and Coleoptera.Submerged conidia and blastospores can be obtained by liquid fermentation of C.javanica,and aerial conidia can be obtained by solid culture.The differences in morphology,size and surface ultrastructure of the three kinds of spores of C.javanica IF-1106 were compared.It is clear that aerial conidia,submerged conidia and blastospores have different morphology,and the average sizes are 4.45,4.17 and 7.67 μm,respectively.A similar structure,the scar at one end of three kinds of spores,was observed by scanning electron microscope.The surface of the aerial conidia was rough and appeared to be covered with a layer of small rods.The surface of blastospores is smooth with obvious cracks,and sometimes segmented.The surface of the submerged conidia is smooth with a few thin cracks,and some of them have structures like bud scars.Image of transmission electron microscopy showed that the cell wall thickness and outer layer structure of the three kinds of spores were significantly different.The hydrophobicity of three kinds of spores of C.javanica IF-1106 was determined by the test of microbial adhesion to hydrocarbons(MATH)and the results showed the hydrophobicity ranked as aerial conidia＞blastospore＞submerged conidia.When pH ranging from 3.0 to 9.0,the Zeta potential of aerial conidia varied from+2.68 mV to-18.44 mV,that of blastospores ranged from+5.16 mV to-5.51 mV,and that of submerged conidia changed from+0.39 mV to-18.09 mV.The isoelectric points of the three kinds of spores were in acidic range and all spores were negatively charged in neutral condition.The germination rate and stress tolerance of the three kinds of spores were compared.It was found that blastospores germinated fastest but were least stress-tolerant;the aerial conidia were more stress-tolerant but slowest germinated,while germination rate of submerged conidia was close to that of blastospores and the stress-tolerance was the best,showing good potential for utilization and development.These results provide a theoretical basis for the application of C.javanica IF-1106 in production.
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