Changes Of Key Microbial Communities In Rhizosphere Of Continuous Casuarina Equisetifolia And Their Interaction With Root Exudates

Casuarina equisetifolia is an important tree species for coastal sandland shelter forests and economic timber forests in China.It plays an important role in preventing wave erosion,maintaining water and soil,and restoring coastal ecosystems.However,Casuarina equisetifolia has a problem of continuous planting.After successive generations of planting in the same field,the biomass and net productivity of the Casuarina equisetifolia forest decreased,plant growth and development were poor,and the survival rate of seedlings was significantly reduced.Continuous planting obstacles have caused casuarina yield and quality to decline from generation to generation,which severely restricted the ecological and economic benefits of coastal shelter forests in China.Therefore,revealing the mechanism and process of casuarina continuous cropping obstacles has become an urgent problem.In this study,the rhizosphere soil of Casuarina equisetifolia(First rotation plantation FCP,second rotation plantation SCP,and third rotation plantation TCP)of different serial generations was used as experimental materials.A series of molecular biological methods were used to comprehensively explore the rhizosphere of Casuarina equisetifolia continuous cropping obstacles Biological mechanism.The main conclusions are as follows:(1)The results of PLFA showed that with the increase of successive generations,the total biomarker content decreased significantly,the total content of fungi showed an increasing trend,and the total content of bacteria showed a downward trend,and the fungal / bacterial ratio varied with the number of successive cultivation Increase and rise.Highthroughput sequencing results showed that the structure and diversity of rhizosphere soil fungal communities in Casuarina equisetifolia significantly changed after continuous planting,and there were significant differences in the structure of FCP,SCP,and TCP rhizosphere soil fungal communities.With the increase of serial generations,the Chao1 index and ACE index of rhizosphere soil fungal communities decreased from generation to generation,and there were significant differences.In addition,at the genus level,the relative abundance of Fusarium,Penicillium,Thelephora,and Hortaea significantly increased,while Trichoderma,Polyporus,Tolypocladium,Archaeorhizomyces Relative abundance is significantly reduced.Among them,Fusarium is considered to be one of the main plant pathogens in many plantations.(2)A strain of Casuarina equisetifolia(Fusarium oxysporum)was isolated and screened by PDA plate;a strain of beneficial bacteria(Bacillus amyloliquefaciens)that had an antagonistic effect on Fusarium oxysporum was isolated and screened by LB plate.Meanwhile,by q RT-PCR absolute quantitative analysis,it was found that as the number of successive plantings increased,the content of pathogenic bacteria in the rhizosphere soil increased significantly,while the content of beneficial bacteria decreased significantly.It is speculated that the main reasons for the continuous failure of Casuarina equisetifolia are imbalanced soil microbial community structure and increased fungal content,especially the large increase in the pathogenic bacterium Fusarium oxysporum,and the decrease in the content of the beneficial bacterium Bacillus amyloliquefaciens,resulting in From disease suppression to disease.(3)HPLC was used to determine the types and contents of phenolic acids in rhizosphere soil of Casuarina equisetifolia.Five phenolic acids were detected: Vanillic acid,Ferulic acid,Gallic acid,p-Hydroxycinnamic acid and Salicylic acid.The contents of Vanillic acid,Gallic acid,pHydroxycinnamic acid and Salicylic acid all increased with the number of successive plantings,and the content of Ferulic acid in FCP,SCP,and TCP was low,and there was no significant change.(4)Based on the phenolic acid content of casuarina rhizosphere soil determined by HPLC,the average ratio of 5 phenolic acids in casuarina rhizosphere soil was obtained(Vanillic acid: Ferulic acid: Gallic acid: pHydroxycinnamic acid: Salicylic acid = 32: 6: 19: 5: 8).The effects of phenolic acids in the root exudates of Casuarina equisetifolia on the growth of key microorganisms in rhizosphere soil were simulated by simulating the proportion of phenolic acids in the soil.It was found that mixed phenolic acids at most concentrations could promote the growth of the pathogenic Fusarium oxysporum,and at the same time inhibit the growth of the beneficial bacterium Bacillus amyloliquefaciens.It can be seen that phenolic acids in the root exudates of Casuarina equisetifolia can selectively promote or inhibit rhizosphere microorganisms.To sum up,the continuous planting obstacle of Casuarina equisetifolia was mediated by phenolic acids in root exudates,which led to an imbalance in the microbial community structure of the rhizosphere,the content of beneficial bacteria decreased significantly,and the content of pathogenic bacteria increased significantly.In turn,the imbalance of soil micro-ecological structure leads to poor growth and development of the above-ground part of the plant,resulting in continuous planting obstacles.In summary,the main reasons for the formation of casuarina continuous cropping obstacles are the following two reasons:(1)Casuarina equisetifolia root exudates(Vanillic acid,Ferulic acid,Gallic acid,pHydroxycinnamic acid and Salicylic acid)inhibit some beneficial bacteria(Bacillus amyloliquefaciens),and the number of beneficial bacteria with antagonistic effects decreases significantly as the number of successive cultivation increases;2)Casuarina equisetifolia root exudates can promote pathogenic microorganisms(Fusarium oxysporum).After continuous planting,a large number of pathogenic bacteria broke out,and the rhizosphere soil microbial community became unbalanced,which caused continuous obstacles to Casuarina equisetifolia continuous planting.