Mechanism Of The Effect Of Rhizosphere Bacillus And Arbuscular Mycorrhizal Fungi On Growth And Defense Of Ageratina Adenophora

The invasive plant can form a positive symbiotic relationship with soil microorganisms to promote its invasion and expansion.The invasion of Ageratina adenophora posed a severe threat to the development of agriculture,forestry and livestock industries,and is considered one of the most harmful invasive weeds in China.Bacillus and arbuscular mycorrhizal（AM）fungi are the species of microorganisms accumulated in the rhizosphere soil of invasive plants.The strong growth and defense ability of invasive plants may be closely related to the species of AM fungi and Bacillus accumulated in rhizosphere soil.Different AM fungi and Bacillus may play different roles in the growth and defense of invasive plants,and the synergistic effect among different AM fungi and Bacillus further facilitate the invasion of exotic plants.However,previous studies only focus on the feedback study of the dominant AM fungi and Bacillus in the rhizosphere of plants,lacking comparative research on the effects of different bacteria on the growth and defense ability of invasive plants.We carried out the experiments based on the scientific question of“the selective aggregation of Bacillus and AM fungi in the rhizosphere of A.adenophora and their synergistic effects enhance the invasion ability of A.adenophora”.Firstly,the effects of the rhizosphere Bacillus community on the competitive growth of A.adenophora were investigated,and the Bacillus community was isolated and identified.The Bacillus with high abundance and rich function was selected for pot experiment to verify its effects on the growth and resistance of herbivores of A.adenophora.Secondly,the effects of the dominant AM fungi in the rhizosphere aggregation of A.adenophora on its growth and response to the feeding of the generalist herbivore Aphis gossypii.Then,Bacillus and AM fungi with the strong growth promoting effect and Bacillus and AM fungi with the strong resistance of herbivores,were selected to explore whether synergistic effects can be formed and its influence on the competitive growth and response to parasitism of specialist herbivore Procecidochares utilis.Main results are as follows:1.Community structure and function of rhizosphere Bacillus in A.adenophora and its feedback effectPot experiment was conducted to explore the effects of rhizosphere Bacillus communities of invasive plant A.adenophora and native plant Rabdosia amethystoides on the competitive growth of the two plants,the community structure of Bacillus in rhizosphere soil and the growth promoting function of Bacillus strains in rhizosphere soil were compared between A.adenophora and R.amethystoides,and testing the effects of the dominant abundance and strong functions on the growth and resistance of herbivore of A.adenophora.The results showed that Bacillus idriensis,B.toyonensis and B.cereus were accumulated in the rhizosphere soil of A.adenophora after Bacillus inoculation of A.adenophora in a pot experiment,which significantly increased the nitrate nitrogen（NO₃^--N）content in the soil and the total carbon and nitrogen concentrations in A.adenophora in the mixed treatment.Inoculated with Bacillus in the rhizosphere soil of A.adenophora treatment the biomass of A.adenophora in monoculture and mixture treatments were 17.63 and 16.42 times that was uninoculated treatment,and the corrected index of relative competition intensity of A.adenophora-inoculated Bacillus reached double that of the uninoculated treatment.16 taxa and 122 strains of Bacillus were isolated from the rhizosphere soil of A.adenophora,8taxa and 88 strains of Bacillus were isolated from the rhizosphere soil of R.amethystoides.The dominant Bacillus that accumulated in the rhizosphere soil of A.adenophora were B.idriensis,B.toyonensis and B.cereus.The number and ability of Bacillus strains that have growth promoting functions such as inorganic phosphorus,organic phosphorus,potassium solubilizing and nitrogen fixation in the rhizosphere soil of A.adenophora were higher than R.amethystoides.Except for B.toyonensis,inoculation with the dominant and rich functions of Bacillus strains significantly increased the contents of total carbon,nitrogen,phosphorus and potassium,improved the root growth parameters,increased the plant height and leaf area,and thus promoted the growth of A.adenophora,among which the B.idriensis had the best effect on promoting the growth of A.adenophora.In response to the feeding effects of A.gossypii,inoculation of B.idriensis and B.thuringiensis significantly increased the root development,nutrient contents,and increased the biomass of A.adenophora,thus enhancing the tolerance to A.gossypii.It can also increase the proline content,antioxidant enzyme activity,jasmonic acid,salicylic acid,flavonoid and tannin acid contents,thus enhancing the chemical defense of A.adenophora against the A.gossypii,inhibiting the growth and reproduction of A.gossypii,and B.thuringiensis has the strongest ability to improve the insect resistance of A.adenophora.2.Effects of AM fungi on the A.adenophora growth and response to feeding of A.gossypiiA.adenophora and its dominant AM fungi Septoglomus constrictum and Claroideoglomus etunicatum were used to investigate the effect of different dominant AM fungi on the growth of A.adenophora and their response to the feeding of the generalist herbivore A.gossypii.The results showed that the inoculation with S.constrictum,C.etunicatum,S.constrictum and C.etunicatum significantly increased the root growth parameters,total carbon,nitrogen,phosphorus and potassium contents,plant height and leaf area of A.adenophora.The biomass of A.adenophora inoculated with S.constrictum,C.etunicatum,S.constrictum and C.etunicatum was significantly increased by 81.203%,47.831%and60.093%.Among them,the AM fungal colonization rate inoculated with S.constrictum was the highest,and it has a strong promoting effect on the growth of A.adenophora.In response to the feeding effect of A.gossypii,inoculation of S.constrictum and C.etunicatum significantly increased the biomass,nutrient contents and root growth parameters of A.adenophora,thus improving the tolerance of A.adenophora to the feeding of A.gossypii.Furthermore,inoculation of two AM fungi also significantly increased the proline content,antioxidant enzyme activity,jasmonic acid and salicylic acid content,total phenol and flavonoid content,thereby improving the chemical defense of A.adenophora,and the survival rate of A.gossypii and density of nymphs were significantly inhibited.Among them,the AM fungal colonization inoculated with C.etunicatum was the highest,and it has the strong resistance of herbivore of A.adenophora.3.Effect of AM fungi and Bacillus and their synergistic effects on the competitive growth of A.adenophora and its response to the parasitism of P.utilisThis study investigated the effect of AM fungi,Bacillus and the synergistic effects on the competitive growth of A.adenophora and its response to the parasitism of specialist herbivore P.utilis,through previous studies that showed the strong growth promoting effect of AM fungi S.constrictum,and Bacillus B.idriensis,as well as the strong resistance of herbivore effect of AM fungi C.etunicatum and Bacillus B.thuringiensis.The results showed that the nitrogen and phosphorus contents of A.adenophora inoculated with S.constrictum in the mixture treatment increased by 14.72%and 26.01%and biomass increased by112.07%,while the nitrogen and phosphorus contents of R.amethystoides decreased by 5.42%and 4.47%and the biomass decreased by 59.52%.The results indicated that S.constrictum provides a competitive advantage to A.adenophora by accumulating higher N and P contents.Inoculation with B.idriensis significantly promoted the growth of the two plants both in monoculture and mixture treatments by activating available soil nutrients.Co-inoculation with S.constrictum and B.idriensis significantly increased the AM colonization rate and Bacillus density significantly higher than that of inoculated with S.constrictum or B.idriensis,indicating that the S.constrictum and B.idriensis could form a synergistic effect and further strengthen the competitive ability of A.adenophora.In response to the parasitism effect of P.utilis,inoculation with B.thuringiensis significantly increased the chemical defense of A.adenophora and thus promoting the growth of A.adenophora after the parasitism of P.utilis.Inoculation with C.etunicatum not only improved the tolerance of A.adenophora by increasing the biomass and nutrient content,but also improved the chemical defense of A.adenophora by increasing the antioxidant enzyme activity,defense hormones,total phenol and tannic content.The synergistic effect of C.etunicatum and B.thuringiensis could further strengthen the tolerance and chemical defense of A.adenophora under the parasitism of P.utilis,and inhibiting the growth of P.utilis,thus improving the direct and indirect defense ability of A.adenophora against the specialist herbivore.In conclusion,different functional Bacillus and AM fungi can be selectively accumulated during the invasion process of A.adenophora,which can increase the growth and defense ability of A.adenophora.Both Bacillus and AM fungi with enhanced growth promoting ability and stronger resistance of herbivore ability could form a synergistic effect,which further improved the competitive advantage of A.adenophora over a native plant and its resistance to the parasitism of specialist herbivore P.utilis,thus enhancing the rapid expansion of A.adenophora in the invasion area.By studying the role of Bacillus and AM fungi in the invasion process of A.adenophora,this study laid a theoretical foundation for the prevention and control of A.adenophora.