Predator-Prey Chemical Warfare Determines the Expression of Biocontrol Genes by Rhizosphere-Associated Pseudomonas fluorescens

Abstract

Soil bacteria are heavily consumed by protozoan predators, and many bacteria have evolved defense strategies such as the production of toxic exometabolites. However, the production of toxins is energetically costly and therefore is likely to be adjusted according to the predation risk to balance the costs and benefits of predator defense. We investigated the response of the biocontrol bacterium Pseudomonas fluorescens CHA0 to a common predator, the free-living amoeba Acanthamoeba castellanii. We monitored the effect of the exposure to predator cues or direct contact with the predators on the expression of the phlA, prnA, hcnA, and pltA genes, which are involved in the synthesis of the toxins, 2,4-diacetylphloroglucinol (DAPG), pyrrolnitrin, hydrogen cyanide, and pyoluteorin, respectively. Predator chemical cues led to 2.2-, 2.0-, and 1.2-fold increases in prnA, phlA, and hcnA expression, respectively, and to a 25% increase in bacterial toxicity. The upregulation of the tested genes was related to the antiprotozoan toxicity of the corresponding toxins. Pyrrolnitrin and DAPG had the highest toxicity, suggesting that bacteria secrete a predator-specific toxin cocktail. The response of the bacteria was elicited by supernatants of amoeba cultures, indicating that water-soluble chemical compounds were responsible for induction of the bacterial defense response. In contrast, direct contact of bacteria with living amoebae reduced the expression of the four bacterial toxin genes by up to 50%, suggesting that protozoa can repress bacterial toxicity. The results indicate that predator-prey interactions are a determinant of toxin production by rhizosphere P. fluorescens and may have an impact on its biocontrol potential.

Pseudomonas brassicacearum Strain DF41 Kills Caenorhabditis elegans through Biofilm-Dependent and Biofilm-Independent Mechanisms

“…Sclerosin, which exhibited almost no toxicity toward C. elegans, was unchanged with respect to gene expression in DF41 grown with the nematodes. These findings are similar to those reported by Jousset and coworkers, wherein cell-free supernatants of the amoeba Acanthamoeba castellanii were found to increase the expression of phlA (diacetylphloroglucinol [DAPG]), prnA (pyrrolnitrin), and hcnA in P. fluorescens CHA0 (22). Notably, there was a correlation between the level of expression and the toxicity of the encoded product, with the greatest increase in gene expression observed for DAPG, the most toxic metabolite tested (22).…”

“…We discovered that DF41 is capable of killing C. elegans through two distinct modes; the first involves exposure to toxic metabolites, while the second entails biofilm formation on the nematode surface. It has been reported that bacteria are capable of responding to predator cues through altered expression of toxin-encoding genes (7,22,23). To see if the same holds true for DF41, we cocultured bacteria with C. elegans and monitored the expression of genes associated with exometabolite production.…”

A bacterial symbiont is converted from an inedible producer of beneficial molecules into food by a single mutation in the gacA gene

“…In one recent study on predator-prey chemical warfare involving P. fluorescens as the prey and another amoeba (Acanthamoeba castellanii) as the predator, pyrrolnitrin was toxic, and supernatants of amoeba cultures strongly induced the production of the molecule (25). Farmer resistance and nonfarmer sensitivity would indicate that the farmers adapted to harboring a potentially lethal symbiont.…”

Impairment of O-antigen production confers resistance to grazing in a model amoeba–cyanobacterium predator–prey system

“…A type VI secretion system confers cytotoxicity of Vibrio cholera to D. discoideum (23) through the translocation of an actin cross-linking protein (24). The soil bacterium Pseudomonas fluorescens induces the expression of genes involved in the production of certain extracellular toxins in response to chemical cues from A. castellani (25).…”