Objective To identify beneficial endophytic bacterial strains with growth-promoting potential in Pinus thunbergii Parl., endophytic bacteria was isolated from the roots, stems, and leaves of P. thunbergii and screened for multiple plant growth-promoting traits. The growth-promoting effects of selected strains were further validated through root irrigation treatment, with the aim of providing a theoretical foundation and microbial resources for the development and application of P. thunbergii endophytic bacteria.

Method Endophytic bacterial strains were obtained from different tissues (roots, stems, and leaves) of P. thunbergii through a tissue fragment culture protocol, following standardized surface sterilization procedures to ensure microbial purity. The isolates were preliminarily identified at the genus level based on 16S rDNA sequence analysis. Subsequently, the growth-promoting characteristics of the strains, including indole acetic acid (IAA) production ability, phosphate solubilization ability, nitrogen fixation ability, and siderophore production ability were determined. Multifunctional endophytic bacteria were selected to perform root irrigation treatment on Arabidopsis thaliana L. seedlings, and physiological indicators such as root length, fresh weight of underground parts, and fresh weight of aboveground parts were measured to further verify the functions of P. thunbergii endophytic bacteria. Finally, 16S rDNA was used for molecular identification of the multifunctional bacterial strains, and a phylogenetic tree was constructed using the Bayesian method to determine the taxonomic status of the strains.

Result A total of 344 endophytic bacterial strains were isolated from P. thunbergii representing 13 genera. The dominant genera were Bacillus (73.5%), Enterobacter (6.1%), and Priestia (4.9%). Based on growth-promoting trait screening, strains B199 and B203-exhibiting multiple functions including IAA production, phosphate solubilization, and nitrogen fixation, were selected for further study. Compared with the control, the root length, fresh weight of underground parts, fresh weight of aboveground parts, dry weight of underground parts, and dry weight of aboveground parts of A. thaliana seedlings treated with root irrigation using B199 and B203 increased by: 67.6%, 78.9%, 77.9%, 80%, 64.8%, and 59.9%, 52.6%, 30.2%, 60%, 18.6%, respectively. Molecular identification revealed that both B199 and B203 belong to Enterobacter asburiae within the Enterobacter genus.

Conclusion P. thunbergii endophytic bacteria exhibit high diversity and possess multiple growth-promoting characteristics. The strains B199 and B203 selected in this study promote the growth of A. thaliana seedlings, enriching microbial resources and providing theoretical support for ecological restoration and sustainable forestry development.