The GacS/GacA two-component system strongly regulates antimicrobial competition mechanisms of Pseudomonas fluorescens MFE01 strain

Pseudomonas fluorescens MFE01 is an environmental bacterium characterized by an hyperactive type 6 secretion system (T6SS) and a strong emission of volatile organic compounds (VOCs). In a previous study, a transposition mutant, 3H5, exhibited an inactive T6SS and altered VOC emission. In 3H5, the interruption of trpE gene by the transposon was insufficient to explain these phenotypes. To determine the actual impact of this insertion, a comparative transcriptomic analysis was performed on the two-component system GacS/GacA, known to influence numerous phenotypes in Pseudomonas. The results demonstrated that the gacS gene is less expressed in 3H5 than in MFE01. Phenotypic analysis of a gacS deletion mutant, Delta gacS, confirmed many similarities between Delta gacS and 3H5. Indeed, Delta gacS exhibited an inactive T6SS and an altered VOC emission profile. In-depth analysis of volatilomes and phenotypes correlated with the decrease in gacS transcription, highlighting that the emission of 1-undecene is under the strict control of GacS/GacA. This study confirms that 1-undecene is not the sole volatile molecule responsible for MFE01's inhibition of Legionella. Moreover, MFE01 has antimicrobial activity against the phytopathogenic oomycetes Phytophthora infestans via hydrogen cyanide (HCN) emission, which is also controlled by GacS. In MFE01, GacS/GacA is also a partial positive regulator of other VOC emission, whose reduced emission in 3H5 coincides with the decrease in gacS transcription. IMPORTANCE Our model strain Pseudomonas fluorescens MFE01 uses an active type VI secretion system (T6SS) and volatile compounds (VCs) to outcompete other microorganisms in the natural environment. By investigating the cellular mechanism regulating the production of these weapons, we identified the two-component system GacS/GacA. Indeed, GacS cellular membrane sensor plays a crucial role in regulating T6SS activity and VC emission. Among the latter, 1-undecene and hydrogen cyanide are strong aerial inhibitors of the Legionella human pathogen and the Phytophtora infestans major plant pest, respectively. The aim is to improve the understanding of the regulation of these volatile molecule emission and the critical role of a global regulator in both plant and human health.