| Plant growth-promoting rhizobacteria are a group of beneficial bacteria that can colonize in plant rhizosphere and promote plant growth directly or indirectly through mechanisms such as increasing the effectiveness of soil nutrients,producing exogenous phytohormones and inhibiting pathogenic microorganisms.As an important biocontrol resource,plant growthpromoting rhizobacteria can play a role in disease-prevention through nutrition and niche competition,production of antimicrobial substances and extracellular hydrolases,induction of plant systemic resistance and so on.Among the antimicrobial substances secreted by plant growth-promoting rhizobacteria,volatile organic compounds are characterized by low boiling points,strong lipophilicity,easy to spread over long distances in complex media such as atmosphere,soil and plant rhizosphere,and are friendly to environment,which have been considered as important factors for plant growth-promoting rhizobacteria to exert soil fungistasis and induce plant systemic resistance in recent years.In this study,plant growth-promoting rhizobacteria cryopreserved in our laboratory were used as the strains to be screened,and five common plant pathogenic fungi(Sclerotium rolfsii,Rhizoctonia solani,Fusarium oxysporum,Fusarium graminearum and Alternaria alternata)were used as the target pathogens.The functional strains with the activity of producing antimicrobial volatiles were screened by two-sealed-base-plate assays.The volatile organic compounds produced by the functional strains were analyzed and identified by gas chromatography-mass spectrometry.The functional strains with good antimicrobial effect and broad antimicrobial spectrum were selected from the initial screening,and their diseaseprevention and growth-promoting functions and colonization characteristics were analyzed.The effects of selected strains on peanut growth and rhizosphere microbial community were studied through pot experiment.The main results were as follows:(1)Twelve strains of bacteria that could significantly inhibit the mycelial growth of pathogenic fungi were screened by two-sealed-base-plate assays.Among them,strain Y316,strain M2-11 and strain M1-18 showed stronger inhibitory effects on Sclerotium rolfsii,and the inhibition rates reached 91.28%,75.14% and 69.17%,respectively;strain M3-4,strain M4-1 and strain M1-18 showed stronger inhibitory effects on Rhizoctonia solani,and the inhibition rates reached 54.02%,43.63% and 36.82%,respectively;strain Y316,strain IV-4and strain M3-4 showed stronger inhibitory effects on Fusarium oxysporum,and the inhibition rates reached 44.95%,32.88% and 29.82%,respectively;strain KC211,strain M1-18 and strain M2-11 showed stronger inhibitory effects on Fusarium graminearum,and the inhibition rates reached 65.97%,50.69% and 49.14%,respectively;strain M4-1,strain M3-4and strain M2-11 showed stronger inhibitory effects on Alternaria alternata,and the inhibition rates reached 82.02%,70.57% and 63.70%,respectively.(2)Twelve functional strains were identified on the basis of cell morphology,colony characteristics,physiological and biochemical characteristics,homology analysis of 16 S r DNA and gyrB sequences.The results showed that strain BYN1 was Paenibacillus polymyxa;strain IV-4 and strain KC211 were Bacillus subtilis;strain JF-8,strain M1-5,strain M2-6 and strain Y316 were Bacillus amyloliquefaciens;strain DY,strain M1-18,strain M2-11,strain M3-4 and strain M4-1 were Bacillus velezensis.(3)The types and relative content of volatile organic compounds produced by twelve functional strains were analyzed by headspace solid-phase microextraction-gas chromatography-mass spectrometry,and the results showed that acetoin and 2,3-Butanediol,[R-(R*,R*)]-were the main components of volatile organic compounds produced by each functional strain.The beneficial components reported in the literature accounted for50.47%～89.63% of the total volatile organic compounds produced by the functional strains.The ecological functions of the beneficial components include: inhibition and killing of pathogenic microorganisms(pathogenic bacteria,fungi,nematodes)and their propagules;reducing the expression level of quorum sensing-related genes in pathogen cells and hindering their colonization in plant rhizosphere;regulating signal transmission between functional strains and plants to promote plant growth;inducing systemic resistance,etc.(4)The disease-prevention and growth-promoting functions and colonization characteristics of five functional strains with good antimicrobial effect and broad antimicrobial spectrum were studied.The results showed that five functional strains were able to produce ACC deaminase,siderphore and fungal cell-wall degrading enzymes such as cellulase,protease,chitinase and xylanase.Among them,strain M1-5 had the strongest ability in producing ACC deaminase,the activity of ACC deaminase was 0.72 μmol α-ketobutryric acid/h·mg protein in its fermentation broth;strain M1-18 had the strongest ability in producing siderphore,the concentration of siderphore was 13.17 mg/L in its fermentation broth;strain M1-5 had the strongest ability in producing cellulase,the activity of cellulase was 18.08 U/m L in its fermentation broth;strain M2-11 had the strongest ability in producing protease,the activity of protease was 15.37 U/m L in its fermentation broth;strain Y316 had the strongest ability in producing chitinase,the activity of chitinase was 16.41 U/m L in its fermentation broth;strain M1-18 had the strongest ability in producing xylanase,the activity of xylanase was 7.25 U/m L in its fermentation broth.The five functional strains also had the ability of phosphorus-solubilization,potassiumsolubilization and producing IAA.Among them,the phosphorus-solubilization ability of all five functional strains met general technical requirements for production strain quality of microbial fertilizer(NY 882-2004),and the relative content of available potassium was increased by 41.33%～138.03% in their fermentation broth.Strain M1-5,strain M1-18,strain M2-11 and strain M4-1 can colonize in the peanut rhizosphere,but strain Y316 can not colonize in the peanut rhizosphere.(5)The effects of four functional strains(Strain M1-5,strain M1-18,strain M2-11 and strain M4-1)on peanut growth,soil chemical properties,soil enzyme activity and rhizosphere microbial community were studied through pot experiment.The results showed that all four functional strains could significantly increase the plant height,length of the first and second pairs of lateral branches and aboveground biomass of peanut plants;inoculation with strain M4-1 could significantly increase the available phosphorus and organic matter content as well as catalase,urease and sucrase activities of peanut rhizosphere soil.Illumina MiSeq sequencing results showed that all four functional strains could changed the peanut rhizosphere microbial diversity and community structure,increasing the relative abundance of beneficial microorganisms and decreasing the relative abundance of pathogenic microorganisms to varying degrees.In the treatment groups inoculated with strain M1-5,strain M1-18,strain M2-11 or strain M4-1,the relative abundance of benefical genera such as Arthrobacter,Chaetomium,Cladosporium,Trichoderma and Aspergillus was higher than control groups;in the treatment groups inoculated with strain M1-5,the relative abundance of plant pathogenic fungi Fusarium and Alternaria was lower than control groups;in the treatment groups inoculated with strain M1-18,the relative abundance of Alternaria was lower than control groups;in the treatment groups inoculated with strain M2-11 and strain M4-1,the relative abundance of Neocosmospora and Fusarium was lower than control groups. |
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