Abstract
The GacS/GacA two-component system is essential for virulence in many plant pathogenic bacteria, and thus represents a promising anti-virulence target. In the present study, we isolated and screened rhizobacteria that were capable of inhibiting the expression of the gacS gene in the phytopathogenic bacterium Pseudomonas syringae pv. tomato (Pto) DC3000. One candidate inhibitor bacterium, BR3 was obtained and identified as a Bacillus sp. strain based on 16s rRNA gene sequence analysis. Besides the gacS gene, the GacA-dependent small RNA genes rsmZ and rsmY were repressed transcriptionally when DC3000 was treated with an extract from strain BR3. Importantly, the extract also influenced bacterial motility, the expression of type three secretion system effector AvrPto, and the plant hypersensitive response triggered by strain DC3000. The results suggested that the extract from strain BR3 might offer an alternative method to control bacterial diseases in plants by targeting the GacS/GacA system.
Highlights
To thrive and survive in complex environmental conditions, bacteria employ versatile signal transduction pathways to rapidly adapt to alterations in their surroundings
Type three secretion system (TTSS)-inducing minimal medium was used for immunoblotting analysis of AvrPto protein (Huynh et al, 1989)
Extracts from 5000 isolates obtained from soil were tested for their abilities to suppress the β-galactosidase activity of the gacS–lacZ transcriptional fusion in the strain pv. tomato (Pto) DC3000
Summary
To thrive and survive in complex environmental conditions, bacteria employ versatile signal transduction pathways to rapidly adapt to alterations in their surroundings One of these pathways is the GacS/GacA two-component system, which detects and responds coordinately to external and internal stimuli, including different physiological state (Kay et al, 2005), metabolic levels (Chavez et al, 2010), and pH (Mondragón et al, 2006), and translates them into appropriate adaptive responses. In Pseudomonas aeruginosa and Legionella pneumophila, the activated GacA exclusively regulates the expression of several small non-coding RNAs (sRNAs) RsmY and RsmZ (Brencic et al, 2009; Sahr et al, 2009) Mutation of all these sRNAs in many bacteria resulted in the same phenotypes as mutants of the GacS/GacA system (Kay et al, 2005; Brencic et al, 2009). In contrast to RetS, LadS in P. aeruginosa activates the function of GacA under high calcium conditions (Broder et al, 2016)
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