Soil bacteria elevate essential oil accumulation and emissions in sweet basil

Plant growth-promoting rhizobacteria ameliorate environmental conditions for plants by facilitating nutrient uptake and mitigating disease susceptibility. While volatile chemicals from certain soil microbes are sufficient to elicit growth and defense responses in Arabidopsis, whether such volatile signals can induce essential oil accumulation and chemical emissions has yet to be reported. Here, we provide biochemical evidence that the plant growth-promoting soil bacterium Bacillus subtilis GB03 releases volatile chemicals that elevate fresh weight essential oil accumulation and emissions along with plant size in the terpene-rich herb sweet basil (Ocimum basilicum). The two major essential oil components from sweet basil, alpha-terpineol and eugenol, increased ca. 2- and 10-fold, respectively, in plants exposed to GB03 volatiles or with root inoculation as compared to water controls. On a fresh and dry weight basis, shoot and root biomass increases of ca. 2-fold were observed with GB03 volatile exposure or GB03 media inoculation as compared with controls. In testing the efficacy of GB03 volatiles to trigger plant growth and secondary compound production, a physical partition separating roots from bacterial media was provided to preclude nonvolatile microbial elicitors from contributing to GB03-stimulated basil responses. These results demonstrate that volatile bacterial elicitors can concomitantly increase essential oil production and biomass in an herbaceous species rich in commercially valued essential oils.