Dynamic Changes Of Phenolic Acids And Their Effects On Microbial Community In Panax Quinquefolium L. Soil

American ginseng（Panax quinquefolius L.）is a perennial medicinal plant of genus Panax,which has the functions of enhancing immunity,anti-oxidation and anti-cancer.The continuous cropping obstacles restricted the planting of American ginseng,which caused great economic losses.However,there were insufficient research on the causes of continuous cropping obstacles of American ginseng,and the effects of phenolic acids secreted by roots on rhizosphere microbial communities have not been thoroughly studied.In order to explore the interaction between phenolic acids and microbial community,rhizosphere soils of healthy and diseased American ginseng in different planting years were sampled,the contents of phenolic acids in rhizosphere soil were determined by high performance liquid chromatography（HPLC）,the microbiome composition were determined by high-throughput sequencing.The dynamic changes of phenolic acids and their effects on microbial communities in rhizosphere soil were analyzed,in addition,the main pathogens of root rot were isolated and identified by tissue isolation and Koch′s postulates.The abundances of main pathogens in rhizosphere soil of diseased and healthy American ginseng in different cropping year were determined by q PCR.The allelopathic effects of exogenous phenolic acids on seed germination and mycelium growth of pathogen were studied to further explore the interactions between phenolic acids and microorganisms.Finally,we revealed the main causes of continuous cropping obstacles of American ginseng.The main results were as follows:（1）The dynamic changes of phenolic acids contents,soil physical and chemical properties and enzyme activities in rhizosphere soil of diseased and healthy American ginseng in different cropping years.The results showed that the contents of phenolic acids in the rhizosphere soil of American ginseng gradually decreased with the increase of cropping year.The relative amount of p-coumaric acid was highest among the 12 phenolic acids in the rhizosphere soil.The content of p-coumaric acid was decreased gradually along with the cropping years.Compared with healthy,the contents of phenolic acids in rhizosphere soil of diseased American ginseng was significantly lower in 1 year and 2 year,while there was no significant difference between the diseased and healthy rhizosphere soil in 3year and 4 year.In addition,with the increase of cropping years,the soil physical and chemical properties and enzyme activities in rhizosphere soil showed a decreasing trend.（2）The dynamic changes of microbial community composition in rhizosphere soil of diseased and healthy American ginseng in continuous cropping year.The microbial communities in rhizosphere soil of diseased and healthy American ginseng in different cropping years were analyzed through High-throughput sequencing and bioinformatics methods.The results showed that the composition and structure of microbial communities were altered by cropping and root rot of American ginseng.With the increase of cropping year,the diversity of bacteria in the rhizosphere soil of healthy American ginseng decreased,whereas the diversity of fungi increased.The diversity of microbial community in rhizosphere soil of diseased American ginseng decreased gradually along with the cropping years.In addition,with the increase of cropping year,the relative abundance of Trichoderma in the rhizosphere soil of American ginseng decreased,while the abundance of Ilyonectria increased.The relative abundance of Fusarium increased first and then decreased gradually along with the cropping years.Compared with the healthy plant,the relative abundance of Trichoderma,Bacillus and Burkholderia in the rhizosphere soil of diseased American ginseng were lower,whereas the relative abundance of Fusarium and Ilyonectria were higher.（3）Isolated and identified of the root rot pathogen of American ginseng.The results showed that 55 fungi were isolated from the rotten roots of American ginseng,four of which were proved to be the pathogens.Combined with morphological characteristics and ITS/TEF1-ɑsequences analysis,the pathogens were identified as Fusarium solani（XYS-1）,Fusarium oxysporum（XYS-2）,Fusarium proliferatum（XYS-33）and Alternaria alternata（XYS-44）.With the increase of cropping years,the abundance of F.solani in the rhizosphere soil gradually increased,the abundance of F.oxysporum and A.alternata first increased and then decreased,while the abundance of I.destructans first decreased and then increased.Compared with the healthy plant,the abundances of pathogens in the rhizosphere soil of diseased American ginseng were lower.（4）Effects of exogenous phenolic acid on germination of American ginseng seeds and abundance of root rot pathogens.The result of Person correlation showed that the abundance of F.solani had significantly negative correlations with the content of sinapic acid,p-coumaric acid and total phenolic acids.Compared with control,the mycelial growth of F.solani,F.oxysporum and A.alternata was promoted by total phenolic acid at the concentration of 10-40mg·L^-1（the concentration of phenolic acids in 3-4 years）.Meanwhile,the mycelial of F.solani,F.oxysporum and A.alternata were promoted by the p-coumaric acid at the concentration of 5-15mg·L^-1.With the increase of phenolic acid concentration,the promoting effects of phenolic acid on the mycelial growth were weakened,and gradually showed suppression effect.Compared with control,the germination of American ginseng seeds was promoted at the medium concentration of total phenolic acids,while inhibited by p-coumaric acid at all concentrations.In addition,compared with the control group,the abundance of root rot pathogens were higher at lower concentration of phenolic acid,and higher at middle and high concentration.In conclusion,phenolic acids,which were secreted by American ginseng root,significantly promoted the growth of root rot pathogen,such as F.solani,F.oxysporum and A.alternata.The allelopathic effects affected the microbial community in the rhizosphere soil of American ginseng,then the stability of the root microbial ecosystem was destructed and the root rot disease increased.This study is important for the development of targeted measure for the continuous cropping obstacles of American ginseng.