Spontaneous Gac Mutants of Pseudomonas Biological Control Strains: Cheaters or Mutualists?

Abstract

Bacteria rely on a range of extracellular metabolites to suppress competitors, gain access to resources, and exploit plant or animal hosts. The GacS/GacA two-component regulatory system positively controls the expression of many of these beneficial external products in pseudomonad bacteria. Natural populations often contain variants with defective Gac systems that do not produce most external products. These mutants benefit from a decreased metabolic load but do not appear to displace the wild type in nature. How could natural selection maintain the wild type in the presence of a mutant with enhanced growth? One hypothesis is that Gac mutants are "cheaters" that do not contribute to the public good, favored within groups but selected against between groups, as groups containing more mutants lose access to ecologically important external products. An alternative hypothesis is that Gac mutants have a mutualistic interaction with the wild type, so that each variant benefits by the presence of the other. In the biocontrol bacterium Pseudomonas chlororaphis strain 30-84, Gac mutants do not produce phenazines, which suppress competitor growth and are critical for biofilm formation. Here, we test the predictions of these alternative hypotheses by quantifying interactions between the wild type and the phenazine-and biofilm-deficient Gac mutant within growing biofilms. We find evidence that the wild type and Gac mutants interact mutualistically in the biofilm context, whereas a phenazine-defective structural mutant does not. Our results suggest that the persistence of alternative Gac phenotypes may be due to the stabilizing role of local mutualistic interactions.

Secondary Metabolism and Interspecific Competition Affect Accumulation of Spontaneous Mutants in the GacS-GacA Regulatory System in Pseudomonas protegens

“…For example, the motility of strain Pf-5 is influenced by the GacS-GacA system (16), and the predominance of Gac Ϫ mutants at the advancing edge of swarming colonies of P. protegens Pf-5 has been attributed to their hyperflagellation (17). Biofilm formation is also influenced by the GacS-GacA system (16), and Driscoll et al (23) reported evidence for a mutualism between Gac Ϫ mutant and wild-type cells of P. chlororaphis 30-84 in laboratory biofilms. Duffy and Défago (24) proposed that the osmotic potential of the medium influenced the proportion of Gac Ϫ mutants in cultures of P. protegens CHA0, recognizing that GacS-GacA influences the response of bacterial cells to environmental stress (20,27).…”

“…Given the broad influence of the GacS-GacA system on bacterial physiology, it is striking that spontaneous gacS and gacA mutants (i.e., Gac Ϫ mutants) emerge and accumulate in cultures of Pseudomonas spp. (17,(21)(22)(23)(24)(25)(26)(27). Colonies of these mutants can be distinguished from those of the wild type visually by their expanded colony size, flattened appearance, increased fluorescence under UV light, and lack of extracellular protease production (21,24).…”

The path to re-evolve cooperation is constrained inPseudomonas aeruginosa

“…We believe that the insights gained from our study have implications for other public good traits for two reasons. For one thing, it was shown that public goods traits can easily be lost through SNPs in their key regulators [21,22,[34][35][36]. However, regulators are usually highly specialized proteins encoded by a single gene, such that mutational targets allowing reversion or compensation are likely limited [9,37,38].…”

Beyond the Black Queen Hypothesis

“…Indeed, every function has a constitutive cost, considering both genomic content and metabolism (Giovannoni et al, 2005, Kreft andBonhoeffer, 2005;Lahti et al, 2009;Driscoll et al, 2011) but this cost is usually offset by the fitness benefits the function provides (Giovannoni et al, 2014). If a function loses its beneficial effects it will eventually be purged to reduce energy costs.…”