Effect Of Invasive Plant Xanthium Strumarium On Mycorrhizal Fungi And Rhizobium Of Glycine Max Under Different Light Environments

We are increasingly challenged by the risk of invasion by exotic plant species.The influence of invasive plants on symbiotic microorganisms in native plants has become a hot topic in the studies of invasion mechanism.The symbioses of mycorrhizal fungi and rhizobium with plants are very common in legume species,one of the important agricultural plants.However,our knowledge about the impacts of invasive plants on such double symbiotic relationship is very limited.Meanwhile,plant associations with mycorrhizal fungi and rhizobium are highly dependent on the supply of photosynthates.Therefore,light intensity can be an important factor regulating the microbial symbiosis through its effects on photosynthesis.Here,we selected an noxious invasive plant in Northeast China,Xanthium strumarium,and a native legume species,Glycine.Max as study materials.We aimed to explore the impacts of invasive plants on the mycorrhizal fungi and rhizobium in legumes roots,and whether such impacts were altered under shading treatment(42% light transmittance).Results of this study could advance our understanding of invasion mechanism from the perspective of microbial symbiosis.The main results are listed below:(1)Under full sunshine,there was no significant change of mycorrhizal fungi colonization in G.max when mixed with X.strumarium.At the same time,the community composition and structure of mycorrhizal fungi in mixed G.max did not change at the Phylum level.However,at the genus level,the Glomus of mixed G.max decreased significantly.This may indicate that the response of arbuscular mycorrhizal fungi is diverse.Different genera respond differently,and may have a genus-specificity.Under unshaded condition,X.strumarium had significant effects on the number and the biomass of nodules in G.max,but had no significant effect on the single nodules biomass.In addition,we did not find any change of rhizobium community composition,but the nif H copy number of nitrogen fixing gene in G.max increased significantly,and the potential nitrogen contribution by nodule remained unchanged.Under the shading condition,there was no significant change in MYC of mixed G.max,and no significant change in Glomus was found at different classification levels.However,shading treatment significantly reduced NN and NB of G.max,while SNB remained unchanged.Shading had no significant effect on the composition of the rhizobium community,but nif H remained unchanged,which reduced PNC.This shows that G.max nodules respond to invasive species X.strumarium by changing nitrogen fixation activity and nodule numbers(or biomass)rather than by changing the composition of rhizobium.(2)We did not find relationship between the root strategy(indicated by root nitrogen content;phosphatase enzyme activity)and mycorrhizal strategy(indicated by mycorrhizal fungi colonization)in G.max.However,there was a significant negative correlation between the root strategy(root nitrogen content)and the nodule strategy(nif H copy number)in G.max.That is,more active the root itself is,the less dependent of the root on nodules.This may be due to the higher energy consumption of nitrogen fixation by nodules,thus reducing its association with rhizobium when root strategy is strong.In general,G.max was not highly dependent on mycorrhizal fungi,and there was no relationship between mycorrhizal fungi and nodules.This shows that G.max meet its nutrient requirement through its shift along the trade-off between root strategy and nodule strategy in response to the invasion of X.strumarium.(3)As the situation under full sunshine,correlation between root strategy(root nitrogen content;phosphatase enzyme activity)and mycorrhizal strategy(mycorrhizal fungi colonization)was not dound under the shading treatment.However,part of the nodule strategies changed,e.g.,the number of nodule was reduced under shading treatment,while the copies of nif H gene did not change.This would significantly reduce the potential nitrogen contribution to host plant by nodules.Therefore,the nodule strategy responded to invaders quite differently under different light conditions: reducing the number and increasing the activity of nodules under full light;reducing the number and keep it activity of nodules under shading.Our results highlight the important role of the physiological index of nodule activity in response to the invasion of exotic plants,which needs more attention in future studies.