Mechanism Of Contribution Of Metarhizium Robertsii Symbioticed With Plants To Persistent Ability

Many fungi have a variety of life styles(saprophytic,parasitic or symbiotic),and it is believed by common reasoning that diverse life styles enable them to persist in natural environment.However,the true contribution of a fungus’ s specific lifestyle to its environmental persistence is less understood.Metarhizium robertsii is widely distributed in the environment with brilliant persistence owing to its multiple lifestyles and strong resistance to stress.The fungus can be either saprophytic,parasitic on insects or symbiotic with plants.As a beneficial symbiotic microorganism promoting plant health and development,M.robertsii can colonize plant roots and rhizosphere soil.As an entomopathogenic fungus,M.robertsii has a broad spectrum of host insects,thus is a good material for studying the contribution of different lifestyles to fungal environmental persistence.Studying and elucidating the relationship between lifestyles and the environmental persistence of M.robertsii could provide theoretical basis for the development of efficient,stable and environmental-friendly fungal insecticides.In this study,the relationship between three lifestyles and the environmental persistence of M.robertsii was studied,and the results of these basic studies were used to establish a culture method for producing high-quality spores of M.robertsii.The main results obtained are summarized as below:1)Compared with saprophytic(on PDA medium)and parasitic(infected Galleria mellonella)lifestyles,the spores produced by symbiotic lifestyle(symbiotic with Arabidopsis thaliana)had more virulence.Further analysis showed that spores produced by symbiotic lifestyle germinated faster on insect wings,which led to earlier formation of appressorium,increased turgor pressure in appressorium,enhanced ability to penetrate insect cuticle,and faster killing of insects.The spores produced by symbiotic lifestyle also contained more lipid droplets and triacylglycerol.Autophagy is crucial to utilize lipid droplets metabolism during conidial germination.The key protein ATG8 in the autophagy pathway had a higher expression level during conidial germination,which enabled the spores produced by the symbiotic lifestyle rapidly utilized lipid droplets and generated more nutrition for spore germination and germ tube elongation when germinated on the insect integument.2)Compared with spores produced by saprophytic and parasitic lifestyles,spores produced by symbiotic lifestyles germinated faster in nutrient-rich,barren or even no-carbon and no-nitrogen environments,and had stronger tolerance to key environmental stress such as UV radiation,high temperature and freezing.3)Symbiosis with M.robertsii induced plants to produce high levels of serotonin.Similar to spores produced by plant symbiosis,spores produced on the common medium supplemented with serotonin also had higher virulence and stress resistance,suggesting that serotonin is an important factor in improving the environmental persistence of M.robertsii that symbiose with plants.4)High-quality M.robertsii spores can be produced by adding serotonin to the culture medium to mimic a symbiotic environment with plants.The spores were more efficient fungal insecticides.The high cost of serotonin made it unsuitable for mass production of spores,but a cheaper serotonin analogue,serotonin hydrochloride,can be used as a substitute for serotonin,since the addition of this compound to culture medium can also contribute to the production of high-quality M.robertsii spores with enhanced virulence and stress resistance,laying a foundation for the production of high efficient fungal insecticides.In conclusion,the contribution of three life styles to the environmental persistence of M.robertsii was investigated.It was found that the spores produced by symbiosis with plants had stronger virulence,stress resistance and environmental persistence,indicating that the symbiotic life style with plants played a more important role in the environmental persistence of M.robertsii.Furthermore,serotonin,a secondary metabolite of plants,was found to be an important factor in enhancing spore environmental persistence.Based on the results of this study,a culture method for producing high-quality M.robertsii spores with enhanced virulence and stress resistance was established,which has good value in practice.