Volatile organic compounds (VOCs), characterized by low molecular weight and high vapor pressure, are
produced by all organisms as part of normal metabolism, and play important roles in communication
within and between organisms. We examined the effects of VOCs released by three species of plant
growth-promoting rhizobacteria (Pseudomonas fluorescens, Bacillus subtilis, Azospirillum brasilense) on
growth parameters and composition of essential oils (EO) in the aromatic plant Mentha piperita
(peppermint). The bacteria and plants were grown on the same Petri dish, but were separated by
a physical barrier such that the plants were exposed only to VOCs but not to solutes from the bacteria.
Growth parameters of plants exposed to VOCs of P. fluorescens or B. subtilis were significantly higher than
those of controls or A. brasilense-treated plants. Production of EOs (monoterpenes) was increased 2-fold
in P. fluorescens-treated plants. Two major EOs, (þ)pulegone and ()menthone, showed increased
biosynthesis in P. fluorescens-treated plants. Menthol in A. brasilense-treated plants was the only major
EO that showed a significant decrease. These findings suggest that VOCs of rhizobacteria, besides
inducing biosynthesis of secondary metabolites, affect pathway flux or specific steps of monoterpene
metabolism. Bacterial VOCs are a rich source for new natural compounds that may increase crop
productivity and EO yield of this economically important plant species.