Isolation and identification of antagonizing and growth-promoting actinobacteria colonized in plant roots and rhizosphere

Abstract:

[Objective] The aim of this study was to enrich species resources of actinobacteria colonized in plant roots and rhizosphere through isolating, screening and identifying and to provide resources for the development of biocontrol agents via examining the effects of these actinobacteria on antagonizing plant pathogens and promoting plant growth. [Methods] Actinobacteria were isolated from roots and rhizosphere of dominant herbaceous plants grown in three habitats (saline-sodic, wet, and polluted land) using the dilution plate method. The antibacterial strains were screened by plate confrontation test. Then Salkowski colorimetry, Chrome Azurol S plate assay and nitrogen free culture were used to further detect their effect on promoting plant growth. The isolated strains were examined by the combined morphological, physiological and biochemical characteristics and 16S rRNA gene sequence analysis for taxonomical identification. [Results] A total of 283 isolates of actinobacteria colonized in plant roots and rhizosphere, belonging to genera Streptomyces, Nocardia and Micromonospora, were obtained. Among them, 77% belonged to 10 groups of the genus Streptomyces. Seven strains showed strong antagonizing activity and plant growth-promoting effects. Strain H6-1 gave the most significant antagonizing effect among the seven strains, and its fermentation broths inhibition ratio to pathogens Fusarium oxysporum, Fusarium, Botrytis cinerea, Rhizoctonia solani, Macrophoma kawatsukai, and Colletotrichum orbiculare were 32.3%, 42.6%, 48%, 72.2%, 58.1% and 60.5%, respectively. Strain D11-4 showed the strongest plant growth-promoting effect, which was found to be capable of producing IAA (22.3 mg/L), iron carrier (halo diameter 25.2 mm) and fixing nitrogen from air. The seven species of the actinobacteria were identified as Streptomyces angustmyceticus H4-6, Streptomyces rochei S2-2, Streptomyces globosus H6-1, Streptomyces iakyrus GD8-4, Streptomyces bottropensis GH8-6, Streptomyces paradoxus H8-2, and Streptomyces coralus D11-4. [Conclusion] The three habitats had abundant species of actinobacteria colonized in plant roots and rhizosphere, and the seven selected actinobacteria isolates were proven to possess biocontrol potential, which is worth of further research and development.