Dual-functionality effects of natural mixtures of bacterial volatile compounds on plant growth.

Bacteria emit volatile compounds that modulate plant growth. Previous studies reported the impacts of bacterial volatile compounds on plant growth; however, the results varied depending on bacterial nutrient availability. We investigated whether the effects of plant growth-inhibiting volatiles (PGIVs) and plant growth-promoting volatiles (PGPVs) depended on the perceived dose by evaluating the growth of Arabidopsis thaliana seedlings placed at 7, 14, and 21 cm away from Bacillus amyloliquefaciens GB03 colonies growing in rich medium. A large bacterial colony (500 μL inoculum) inhibited plant growth at 7 cm and promoted growth at 21 cm, whereas a small bacterial colony (100 μL inoculum) induced the reverse outcomes. We identified pyrazine and 2,5-dimethylpyrazine as candidate PGIVs that significantly reduced plant growth at 7 cm distance. PGIV effects were validated by exposing plants to synthetic 2,5-dimethylpyrazine and bacteria emitting PGPVs, which showed that PGIVs overwhelm PGPVs to rapidly increase salicylic acid levels and related gene expression pathways. This is referred to as the defense-growth trade-off. Our results indicate that high PGIV concentrations suppress growth and promote plant immunity, whereas low PGPV concentrations promote growth. This study provides novel insights into the complex effects of bacterial volatile mixtures and fine-tuning of bacteria-plant interactions.