Soil bacterial diffusible and volatile organic compounds inhibit Phytophthora capsici and promote plant growth.

Biotic interactions through diffusible and volatile organic compounds (VOCs) are frequent in nature. Soil bacteria are well-known producers of a wide range of volatile compounds (both organic and inorganic) with various biologically relevant activities. Since the last decade, they have been identified as natural biocontrol agents. Volatiles are airborne chemicals, which when released by bacteria, can trigger plant responses such as defence and growth promotion. In this study, we tested whether diffusible and volatile organic compounds (VOCs) produced by soil bacterial isolates exert anti-oomycete and plant growth-promoting effects. We also investigated the effects of inoculation with VOC-producing bacteria on the growth and development of Capsicum annuum and Arabidopsis thaliana seedlings. Our results demonstrate that organic VOCs emitted by bacterial antagonists negatively influence mycelial growth of the soil-borne phytopathogenic oomycete Phytophthora capsici by 35% in vitro. The bacteria showed plant growth promoting effects by stimulating biomass production, primary root growth and root hair development. Additionally, we provide evidence to suggest that these activities were deployed by the emission of either diffusible organic compounds or VOCs. Bacterial VOC profiles were obtained through solid phase microextraction (SPME) and analysis by gas chromatography coupled with mass spectrometry (GC-MS). This elucidated the main volatiles emitted by the isolates, which covered a wide range of aldehydes, alcohols, esters, carboxylic acids, and ketones. Collectively, twenty-five VOCs were identified to be produced by three bacteria; some being species-specific. Our data show that bacterial volatiles inhibits P. capsici in vitro and modulate both plant growth promotion and root system development. These results confirm the significance of soil bacteria and highlights that ways of harnessing them to improve plant growth, and as a biocontrol agent for soil-borne oomycetes through their volatile emissions deserve further investigation.