| Beneficial microbes can be used as biocontrol agents and their role of in disease control and plant growth promotion is closely linked to shifts in rhizosphere microbial community structure.In particular,introduction of a biocontrol agent can increase the population of indigenous beneficial microbes(which promote plant growth and prevent disease occurrence)while reducing the population of indigenous harmful microbes(which inhibit plant growth and induce disease occurrence)in the crop rhizosphere.Our previous studies indicate that a multifunctional biocontrol agent,Streptomyces pactum Act12,has prominent effects at controlling soil-borne decreases and promoting plant growth,while it also plays a role in altering the rhizosphere microbial community structure.Despite these,it remains unclear how the biocontrol agent shifts the distribution of particular dominant microbial populations in the rhizosphere and whether such a population change in turn contributes to the biocontrol agent's role in disease control and plant growth promotion.In this study,the morphology,physiology,biochemistry and molecular biology of Pseudomonas koreensis were identified.The pot experiment was conducted to study the effect and mechanism of P.koreensis GS on tomato,in order to find out whether the change of P.koreensis GS in rhizosphere was directly related to the disease prevention and growth promotion of beneficial bacteria.At the same time,the transcriptome method was used to further explore the Act12 regulation.molecular mechanisms of section P.koreensis GS quantity and control efficiency(1)The taxonomic status of strains P.koreensis GS were determined through the genome.The morphology and physiological characteristics of P.koreensis GS were further studied.(2)After administration of P.koreensis GS,tomato plants exhibit more vigorous growth with more chlorophyll and improved shoot biomass.P.koreensis GS enhanced activities of phenylalanine ammonia lyase,polyphenol oxidase,peroxidase and catalase in tomato leaves.It was revealed that tomato leaves following P.koreensis GS interaction that are likely to be related to plant-PGPR interactions,plant stress-resistance,pathogen-resistance,growth and development and basic and energy metabolism in transcriptomic analyses.Meanwhile,the biosynthesis of secondary metabolites related to plant growth was regulated,including proline,suberine,and flavonoid.Additionally,plant hormones including abscisic acid and salicylic acid were discovered to involve in plant-PGPR interactions.(3)We found that the ability of strain GS to colonize the rhizosphere and compete with other microbes was improved through multiple pathways.First,S.pactum Act12 treatment(e.g.,using cell-free filtrate)enhanced expression of genes associated with chemotaxis and flagella assembly in P.koreensis GS,thereby promoting bacterial motility and adhesion to solid-phase surfaces during the early stage of biofilm formation.An increase in expression of alginate biosynthesis genes also confirmed that S.pactum Act12 treatment was conducive to biofilm formation and root colonization by P.koreensis GS.Second,S.pactum Act12 treatment upregulated biosynthetic and catabolic processes in P.koreensis GS,such as a considerable increase in its ability to metabolize sugars and organic acids;thus,in addition to facilitating strain GS to find nutrient sources(chemotaxis),S.pactum Act12 treatment also improved the ability of strain GS to use energy-providing substances in root exudates.Moreover,S.pactum Act12 treatment enhanced the response of P.koreensis GS to oxidative stress.Upregulation of genes involved in oxidative phosphorylation was also likely to improve the ability of strain GS to deal with other external stresses.In conclusion,introduction of S.pactum Act12 enhances rhizosphere colonization and competition by P.koreensis GS via promoting its biofilm formation,nutrient use,and environmental adaptability,thus increasing the P.koreensis GS population in the crop rhizosphere. |
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