Bacterial Priming of Scots Pine Seedlings Under Drought: Biochemical Evidence from Three Genotypes

Drought stress increasingly threatens forest ecosystems, highlighting the need for sustainable and bio-based mitigation strategies for resilient forest management. This study examined the ability of a plant growth-promoting bacterium (PGPB) derived from Scots pine (Pinus sylvestris L.) phyllosphere, previously identified as Priestia megaterium (Prie), to improve drought tolerance in three pine genotypes (8J, 16J, and 519J) through biochemical profiling over a 10-week period.

Under greenhouse conditions, three-year-old pine seedlings were foliar-sprayed with bacterial inoculant at week 0 and week 3, with controlled drought conditions imposed from week 4 onward. Photosynthetic pigments (carotenoids, total chlorophyll a + b) and stress-related secondary metabolites (total phenolic content, total flavonoid content, soluble sugars) were assessed at weeks 2, 6, and 10, representing pre-drought baseline, early stress response, and peak stress conditions, respectively.

Prie inoculation most consistently enhanced total chlorophyll content, with Prie-treated and Prie × drought-treated plants in genotypes 519J and 16J maintaining or exceeding control pigment levels at week 10 despite ongoing drought. Carotenoid responses were genotype-specific, with the most pronounced Prie benefit observed in genotype 519J. Flavonoid accumulation was strongly increased in genotype 8J under combined Prie × drought treatment, reaching the highest values recorded across all conditions, while genotype 16J showed the greatest soluble sugar accumulation under Prie × drought, indicating amplified osmotic adjustment capacity. Genotype 519J exhibited a distinct response pattern, with drought alone driving stronger osmotic responses than bacterial inoculation, suggesting genotype-specific differences in PGPB sensitivity.

Overall, Priestia sp. inoculation showed clear biochemical benefits for pine drought tolerance, particularly for photosynthetic pigment protection and osmotic adjustment, although the magnitude and nature of responses were strongly genotype-dependent. These findings emphasize that effective PGPB-based strategies in forest nurseries must account for genotypic variation in bacterial responsiveness.