| Citrus Huanglongbing(HLB)is one of the major diseases in citrus industry.In this study,newhall navel orange and three red pomelo infected HLB were selected as the research objects,which were treated with Ampicillin(AMP)and Sulfadimethoxine Sodium Salt(SDM)by root irrigation respectively.The purpose was to study the control effect of two chemical agents on HLB and changes in structure and functional genes of rhizosphere bacterium of HLB-infected citrus.To evaluate the relative control of two chemical agents on Candidatus Liberibacter asiaticus(Las)in HLB-infected citrus leaves,and further revealed their effects on rhizobacterial community structure and functional genes.The main conclusions in this study are as follows:(1)The TaqMan q PCR technology was used to monitor the amount of Las dynamically in HLB-infected citrus before and after treatment and determine relative control effect.The results showed that the control effect of chemical agents on newhall navel orange was higher than three red pomelo.(2)With the technology of metagenomics,we analyzed the microbial community structure of rhizosphere soils.The results showed that the structure and relative abundance of rhizobacteria in HLB-infected and HLB-uninfected citrus were different before and after chemical agents treatment.Compared with untreated HLB-uninfected citrus,the relative abundance of Acidobacteria,Actinobacteria and Gemmatimonadetes decreased and Proteobacteria increased in HLB-infected newhall navel orange,while the relative abundance of Acidobacteria in the HLB-infected three red pomelo decreased and Chloroflexi increased.After treatment,the relative abundance of Burkholderiaceae and Bradyrhizobiaceae in the treatment groups increased,while Pseudonocardiaceae Xanthomonasceae decreased.The relative abundance of bacterium in the AMP group was higher than SDM group.The diversity of rhizosphere microbial community structure of HLB-infected three red pomelo was more complicated than that of HLB-infected newhall navel orange.Compared with untreated HLB-infected three red pomelo,the community structure was significantly different in SDM group.In the treatment groups,the relative abundance of Bradyrhizobiaceae,Sphingomonadaceae increased,while Burkholderiaceae,Pseudonocardiaceae and Xanthomonasceae decreased.The characteristic analysis of phosphate-dissolving microorganisms genus showed that it was potentially enriched in the HLB-infected citrus,and the chemical agents affected the relative abundance of Arthrobacter and Pseudomonas.Correlation analysis of soil environmental factors found that environmental factors had little effect on the relative abundance of Proteobacteria and Actinobacteria.Stopping treatment for five months,community structure between treatment groups and HLB-infected citrus were different.The citrus rhizosphere bacteium may have formed a stable environment.When the treatment was stopped for a short time,the community structure and diversity changed little.(3)Using the EggNOG and KEGG database to analyze diversity of rhizosphere bacterial functional genes.Compared with uninfected-HLB newhall navel orange,the relative abundance of functional genes involved in the metabolic function decreased,whlie environmental informationprocessing and cellular processes increased in HLB-infected newhall navel orange.Changes of functional genes of HLB-infected three red pomelo was more complicated than HLB-infected newhall navel orange.Compared with untreated HLB-infected citrus,the content of functional genes for information transmission and the generation of secondary metabolites increased in the newhall navel orange treatment groups,while cell wall/membrane/envelope biogenesis decreased.Compared with untreated HLB-infected three red pomelo,the relative abundance of 10 related functional genes in the SDM group decreased,while only 4 related functional genes decreased in the AMP group.The analysis of the key functional genes of the phosphorus cycle showed that the relative abundance of phoH,phoR,phnB and ppk in the treatment groups changed significantly compared with untreated HLB-infected citrus.Stopping treatment for five months,the composition of functional genes in groups were different and the functional genes diversity were complex.In summary,two chemical agents can effectively inhibit the amount of Las in HLB-infected newhall navel orange.Rhizosphere bacterial community structure and functional genes have changed after treatment and two chemical agents had largest effect on the functional genes of rhizobacteria in HLB-infected three red pomelo than HLB-infected newhall navel orange.The results indicated that the chemical agents had an effect on the life metabolism of citrus rhizosphere microorganisms,especially the function of phosphorus metabolism.This study initially explored the effects of chemical agents on HLB-infected citrus rhizosphere. |
