Soil Microecological Mechanism Of Brevibacillus Laterosporus BL12 On Control Of Potato Common Scab

Potato is the fourth largest food crop in the world.Potato common scab（PCS）caused by pathogenic Streptomyces has caused serious economic losses worldwide.Previous reports reported that the occurrence of PCS is closely related to soil microbial communities,and studying soil microbial communities is of great significance for the control of PCS.Biocontrol is one of the ideal methods to control PCS.However,there are few studies on the effects of biocontrol agent on soil microbial communities of PCS,and the soil microecological mechanism for controlling PCS by biocontrol agent is still unknown.In this study,a strain Brevibacillus laterosporus BL12,which has an antagonistic effect on the PCS pathogen,Streptomyces bottropensis,was used as the research object.The biocontrol efficiency of BL12 on PCS was tested through a pot experiment（control group was inoculated with pathogen S.bottropensis AMCC400023,experimental group was inoculated with pathogen S.bottropensis AMCC400023 and biocontrol strain BL12）.The tuberosphere soil and rhizosphere soil were collected at the early tuber bulking stage,tuber bulking stage and harvesting stage.The colonization ability of biocontrol strain BL12 and the abundance of pathogen AMCC400023 in the soil were measured using the fluorescent quantitative PCR（q PCR）.The effects of biocontrol strain BL12 on the composition and function of soil bacterial communities were analyzed by high-throughput sequencing of 16S r RNA gene.The main results are as follows:（1）Biocontrol strain B.laterosporus BL12 reduced PCS.The Oxford cup assay was used to verify the antagonistic effect of biocontrol strain BL12 on the pathogen S.bottropensis AMCC400023.The results showed that biocontrol strain BL12 had a strong antagonistic activity against the pathogen AMCC400023,and the average diameter of the inhibition zones was 18.17±0.29 mm.Through morphological,physiological,biochemical,and genetic identification,the biocontrol strain BL12 was identified as B.laterosporus.The results of pot experiment showed that compared with the treatment inoculated with pathogen AMCC400023alone,the inoculation of pathogen AMCC400023 and biocontrol strain BL12 significantly reduced the disease severity and disease index of PCS at the harvesting stage,the disease severity decreased from 3.33 to 2.19,and the disease index decreased from 37.04%to 24.34%.The biocontrol efficiency of strain BL12 was 34.29%.（2）The biocontrol strain BL12 effectively colonized the soil and reduced the abundance of pathogen.Quantitative results of the 16S r RNA gene copy number of biocontrol strain BL12showed that during potato growth,biocontrol strain BL12 effectively colonized the tuberosphere soil(1.93×10⁴ to 7.05×10⁴ copies·ng^-1 DNA)and rhizosphere soil(1.47×10⁴to 3.03×10⁴ copies·ng^-1 DNA).The abundance of pathogen AMCC400023 was detected by quantifying the copy number of the txt AB gene which encodes phytotoxin thaxtomin synthetase.The results showed that compared with treatment inoculated with AMCC400023 alone,the inoculation of pathogen AMCC400023 and biocontrol strain BL12 significantly reduced the copy number of txt AB gene in the DNA of the tuberosphere soil and rhizosphere soil at the harvesting stage,that is,significantly reduced the abundance of pathogen AMCC400023 in the tuberosphere soil and rhizosphere soil at the harvesting stage.（3）The biocontrol strain BL12 significantly affected the composition of soil bacterial communities.The results of dominant species analysis showed that Proteobacteria was the most dominant bacterial phylum in the tuberosphere soil and rhizosphere soil,and Pseudomonas was the dominant genus in the tuberosphere soil and rhizosphere soil.The results of principal coordinates analysis（PCoA）combined with permutational multivariate analysis of variance（PERMANOVA）showed that the inoculation of biocontrol strain BL12 significantly altered composition of soil bacterial communities,which was the largest influence factor on the composition of soil bacterial communities in this study.Analysis of species differences between groups showed that compared with the treatment inoculated with pathogen AMCC400023 alone,the inoculation of pathogen AMCC400023 and biocontrol strain BL12 significantly increased the relative abundance of Actinobacteria and Firmicutes in the tuberosphere soil and Verrucomicrobia in the rhizosphere soil at the harvesting stage,also significantly increased the relative abundance of the dominant bacteria such as Pseudomonas,Devosia and Cellvibrio in the tuberosphere soil and rhizosphere soil at the harvesting stage.（4）Some potential biocontrol bacteria were significantly related to biocontrol strain BL12and PCS.The results of spearman correlation analysis showed that the relative abundance of biocontrol strain BL12 was significantly negatively correlated with the disease index and the relative abundance of pathogen AMCC400023 in the tuberosphere soil and rhizosphere soil at the harvesting stage.The relative abundance of Pseudomonas,Bacillus,Microbacterium and Nocardioides,etc.was significantly positively correlated with the relative abundance of the biocontrol strain BL12,and significantly negatively correlated with the disease index and/or the relative abundance of pathogen AMCC400023.They might be synergistic with the biocontrol strain BL12 to control PCS.（5）Biocontrol strain BL12 played key roles in the networks of soil bacterial communities.Molecular ecological networks of bacterial communities in the tuberosphere soil and rhizosphere soil at the harvesting stage were constructed,and the keytone species in the networks were identified through the topological roles of the nodes.The results showed that the biocontrol strain BL12 was the keystone species in the bacterial community network of the tuberosphere soil.Compared with the treatment inoculated with pathogen alone,the inoculation of pathogen and biocontrol strain BL12 significantly increased the relative abundance of keystone species Nocardioides in the bacterial network of the tuberosphere soil and keystone species Chitinophagaceae,Cytophagales,Chthoniobacter,Pseudomonas and Fibrobacteraceae in the bacterial network of the rhizosphere soil.（6）The biocontrol strain BL12 significantly affected the function of soil bacterial communities.The results of nonmetric multidimensional scaling（NMDS）analysis and analysis of similarities（ANOSIM）based on predictive function showed that biocontrol strain BL12significantly changed the function of soil bacterial communities.Analysis of functional differences between groups showed that metabolic pathways related to antibacterial activity,such as biosynthesis of macrolides,biosynthesis of ansamycin,and biosynthesis of siderophore group nonribosomal peptides were significantly enriched in the soil inoculated with pathogen AMCC400023 and biocontrol strain BL12,potential pathogenic-related metabolic pathways such as nitrogen metabolism,bacterial secretion system,and plant-pathogen interaction were significantly enriched in the soil inoculated with pathogen AMCC400023 alone.The above results show that the biocontrol strain B.laterosporus BL12 can stably colonize the soil,reduce the abundance of pathogen in the soil,significantly affect the composition and function of soil bacterial communities,and reduce PCS.This study has analyzed the potential microecological mechanism for the control of PCS by biocontrol agent,and will provide theoretical basis and scientific guidance for the application of biocontrol agent.