| Food safety and quality,as significant public health issues,have always been closely related to human health.While citrus,as one of humankind’s essential fruit resources,has been the central pillar of the global fruit-growing industry for a long time.However,the global citrus industry is suffering from the adverse impact brought by Citrus Huanglongbing(HLB)now.Conventional methods such as massive application of chemical pesticides cannot fundamentally and effectively control HLB anymore,instead,it will lead to soil ecological degradation,environmental pollution,and pesticide residues,and further endangers human health.Therefore,exploring safer and more effective new HLB control methods is urgent.Plant symbiotic microorganisms can significantly affect the health and growth of plants and will play an important role in plant disease prevention.Arbuscular mycorrhizal(AM)fungi,as critical symbiotic fungi of plants,have been found to promote the development and growth of many economic crops in recent years.AM fungi also have great potential to be developed as an effective biological control method for plant diseases because they not only can also improve the resistance to abiotic stress such as droughty but also enhance the defense capability of different kinds of biology,including bacteria,viruses and nematodes.At present,there is still a lack of correlational studies of the symbiosis and community distribution between AM fungi and citrus.Whether AM fungi can play a role in improving the defense effect of citrus against HLB is still unresolved.In order to solve the above problems,this study will systematically analyze the distribution and community characteristics of AM fungi in citrus rhizospheric soil of different representative orchards,then move to carry out the isolation and identification of representative strains,and finally analyze their effects on citrus plant growths and HLB resistance.This study first conducted research concerning the AM fungi distribution and community characteristics in different representative citrus orchard rhizospheric soils.Through high-throughput sequencing method,rhizospheric soils of healthy fruit trees and HLB diseased trees were collected from 9 representative citrus orchards in Ganzhou,Jiangxi province,and Fuchuan,Guangxi Province,respectively,to analyze and compare AM fungal community characteristics.In this study,a total of 3 442 622 sequences of Glomeromycota were detected from 85 citrus rhizospheric soil samples.In comparison with Genbank and MaarjAM databases and by constructing NJ phylogenetic tree,the above sequences were annotated to 99 virtual species(VTs)of 4 orders,8 families,and 15 genera of Glomeromycota.The analysis found that Rhizophagus,Glomus and Sclocystis were the main groups in citrus rhizospheric soil,and Glomeraceae(67 VTs)and Glomus(35 VTs)were the dominant family and genus.The effect of HLB infection on the community structure of AM fungi in citrus rhizospheric soils was assessed by principal coordinate analysis(pCoA)based on VTs.The results showed that infection with HLB significantly affected the structural composition of AM fungi communities in citrus rhizospheric soils but did not affect abundance and diversity.Further analysis also showed that Rhizophagus and Glomus were the dominant genera in healthy and HLB-infected citrus rhizospheric soils,respectively.In addition,81 and 94 VTs were detected in healthy and HLB-infected citrus rhizospheric soils,respectively,of which 10 VTs were significantly enriched in HLB-infected citrus rhizospheric soils and 17 VTs were significantly enriched in healthy citrus rhizospheric soils.Representative AM fungi isolates were further isolated and identified by single spore and monosporulation method.A total of 1480 spores were selected for monosporulation culture,81 indigenous AM fungi isolates were isolated from the citrus rhizosphere,and morphological description and molecular biology methods were used to analyze the isolates of indigenous AM fungi in the citrus rhizosphere.The isolates were classified into 27 species,9 genera,and 4 families,including seven isolates of Acaulospora and four of Claroideoglomus,two of Funneliformis,one of Paraglomus,one of Pervetustus,one of Redeckera,six of Rhizophagus,one of Septoglomus,and three to be determined,among which three AM fungi isolates are suspected new species and six newly recorded species in China.In this study,some indigenous AM fungi isolated from citrus rhizosphere were used to study their symbiosis with citrus plants and their effects on plant growth and HLB resistance.This study analyzed the infection ability and growth-promoting effect of the above different AM fungi isolates of C.junos Sieb.ex Tanaka root based on the greenhouse experiment model.Furthermore,Citurs sinensis(L.)Osbeck was inoculated with HLB pathogens by the method of grafting diseased branches,to analyze the content of HLB bacteria in the leaves in different growth stages,and thereby investigate the effect of the symbiosis of different AM fungal isolates on the anti-HLB efficacy of plants.The results showed that different AM fungal isolates had different infection abilities and promoting effects on C.junos and had significant effects on the growth,nutrient absorption,and HLB content of C.junos seedlings and C.sinensis Among them,the symbiosis of Acaulospora sattaca and F.mosseae significantly promoted the growth,nutrient absorption and root activity of C.junos seedlings and R.clarus and C.etunicatum promoted the growth of C.sinensis,while C.lamellosum and Acaulospora.sp.symbiosis could significantly inhibit the growth of C.junos seedlings and Acaulospora kentinensis inhibit the growth of C.sinensis.On the other hand,the relative content of HLB bacteria in the leaves of the symbiotic plants of Redeckera fulvum showed a decreasing trend at the three stages of 60d,120d and 180d after the grafting of HLB diseased branches,which decreased by 25%,74%and 18%respectively compared with the control group.and the symbiotic plants of A.sattaca showed significantly higher HLB content than the control group in different periods.The above results indicated that different AM fungal isolates had significant effects on the growth of C.junos seedlings and C.sinensis,nutrient absorption and defense effect on HLB bacteria showed different effects.Screening suitable AM fungal isolates could effectively enhance symbiotic citrus plants’ growth and HLB resistance.In summary,through systematical analysis of the typical citrus orchard soil of AM fungi community,the study preliminary revealed the citrus plant rhizosphere and community structure of AM fungi diversity characteristics,and established citrus rhizosphere of AM fungi repository.Additionally,it also demonstrated that AM fungi of the citrus plant greatly influence the growth and defense ability of HLB,and a candidate isolate was obtained which could promote the growth of citrus and have a specific defense effect against HLB.The results of this study can enhance the understanding of AM fungi in citrus rhizosphere symbiosis and lay a solid foundation for the subsequent application of AM fungi in HLB biological control and ecological planting of citrus. |
PDF Full Text Request