Isolation and Characterization of Multi-Trait Plant Growth-Promoting Endophytic Bacteria from Scots Pine Tissues.

Scots pine () is a globally significant tree species with considerable economic importance in forestry. A major challenge in afforestation, particularly in stressful environments, is growing seedlings with high viability and stress resistance. Recent studies suggest that strains can alleviate stress and promote growth in crops, though limited evidence exists for trees. This study aimed to assess the plant growth-promoting (PGP) properties of strains isolated from Scots pine stems using assays, and to evaluate their potential as bioinoculants through a two-year long field trial. From over sixty bacterial isolates originating from Scots pine stem tissues, only four were selected as being similar to bacteria. Through 16S rRNA gene sequencing, the isolates were identified as P57, P88 and 10-1, and P77. All isolates inhibited fungal pathogens and , and exhibited PGP activities including nitrogen fixation and production of IAA (1.24-17.74 mg/l), ammonia (4.06-12.71 μM/ml), and siderophores, with the highest value of 1.44 ± 0.19 for the P88 strain. Additionally, the strains demonstrated phosphate solubilization capacity. We revealed that bioinoculation with strains P57 and P88 enhanced field germination of seeds by 35-45% and increased aerial biomass of two-year-old seedlings by 80-140%. Both strains adhered to seed surface and colonized roots and stems at levels of 2.4-3.2 log CFU/g fresh tissue up to two years post-inoculation. These findings highlight the potential of these bacterial strains as effective bioinoculants for improving Scots pine seedling growth under natural conditions.