Ecological Adaptation Mechanism Of Ageratina Adenophora And Its Specialist Herbivore Procecidochares Utilis Mediated By Bacillus Cereus

Rhizosphere microbial communities play important roles in facilitating or inhibiting the establishment of exotic species.Since some invasive plants and their natural enemies interact with soil microbial communities such as rhizosphere bacteria,changes triggered by rhizosphere bacteria may alter competitive interactions between exotic and native plants.Ageratina adenophora is a notorious invasive weed in China,and Procecidochares utilis Stone a host specific natural enemy,which is utilizing by many countries to control A adenophora,but the control effect in field is limited.Studies have confirmed that after invasion of A.adenophora,it will affect the structure of rhizosphere soil microbial community and form a positive feedback effect on its own growth,but the underlying mechanism has yet to be explore further;and there are few reports on the ecological adaptability of soil microorganisms to natural plant enemies.Previous investigations found that the content of a beneficial bacterium Bacillus cereus in rhizosphere soil in areas with severe invasion of A.adenophora was significantly higher than that in areas with moderate invasion.Therefore,from the perspective of promoting the growth of beneficial microorganisms in the rhizosphere of plants and their feedback effects,this study intended to reveal the action mechanism of rhizosphere beneficial bacteria in the invasion and expansion of A.adenophora.B.cereus,invasive A.adenophora and its specialist natural enemy were used as experimental objects to study the effects of A.adenophora and its natural enemy on the content of B.cereus and the feedback effect of B.cereus on A.adenophora and its natural enemy.The main results are as follows:(1)Absolute quantitative PCR was used to determine B.cereus in rhizosphere soil of A.adenophora under different treatments.The results showed that B.cereus contents increased with intensification of the invasion process in the rhizosphere of A.adenophora,newly invaded soil contained almost twice as much bacteria as noninvaded soil.When rhizosphere soil was added to the root exudates of A.adenophora,the contents of B.cereus were twice as much as control except for the first day,and certain soil parameters increased significantly,such as NH4+-N and AP were 41%and 27%higher,respectively.After punching and pruning,the content of B.cere us in rhizosphere soil of A.adenophora decreased in different degrees with time.Compared with control,the bacterial content changed significantly on the third day,the content of B.cereus was significantly reduced by 77.8%in the treated by leaves of two hole/leaf,and the content of B.cereus in the four hole/leaf and six hole/leaf treatment were 36.7%and 26.7%lower than those of the control,respectively;2/3 plant cut decreased the contents of B.cereus by 79.2%.The different parasitic strength of Procecidochares utilis Stone can increase the content of B.cereus in the rhizosphere soil.For example,at the beginning of galls,the content of B.cereus in the rhizosphere soil of A.adenophora under light parasitism was significantly higher than that of control.When the adults emerged,the content of B.cereus in the rhizosphere soil of moderate and severe parasitic was 95%and 61%higher than control respectively.The above results indicated that the deepening of the invasion of A.adenophora and the parasitic degree of natural enemies will promote the growth of B.cereus in the rhizosphere,while the mechanical damage effect is opposite.The internal mechanism of this difference needs to be explore further.(2)The feedback effects of B.cereus on A.adenophora and its natural enemy were studied by using the method of small cup seedling,high performance liquid chromatography(HPLC).The results showed that the addition of B.cereus in soil increased the germination rate of A.adenophora by 50%,the root length by 117%,the seedling length by 48%and the fresh weight by 81%,which had no effect on local plants.The concentration of DTD was obviously lower than those of sterile soil in 48,72 and 96 hours.In the experiment of B.cereus on the population life parameters of P.utilis,we didn't find any change of life parameters.These results showed that B.cereus has partially beneficial effects on A.adenophora,which can also effectively promote the degradation of allelochemicals,help to eliminate the self toxicity of A.adenophora.Combined with the effect of A.adenophora on the growth of B.cereus,the hypothesis of beneficial rhizosphere bacteria to promote the expansion of invasive plants can be put forward,and the ecological adaptive mechanism of A.adenophora invasion can be explored.