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

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