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

Scots pine (Pinus sylvestris) 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 Pseudomonas 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 Pseudomonas strains isolated from Scots pine stems using in vitro 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 Pseudomonas bacteria. Through 16S rRNA gene sequencing, the isolates were identified as Pseudomonas putida P57, Pseudomonas lurida P88 and 10-1, and Stenotrophomonas maltophilia P77. All isolates inhibited fungal pathogens Botrytis cinerea and Fusarium sporotrichiella, 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 P. lurida P88 strain. Additionally, the Pseudomonas 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.