Secondary metabolite production facilitates establishment of rhizobacteria by reducing both protozoan predation and the competitive effects of indigenous bacteria

1. Rhizosphere bacteria live in close contact to plant roots feeding on root exudates and rhizodeposits. By producing toxic exoproducts rhizobacteria may inhibit plant pathogens thereby functioning as biocontrol agents and increasing plant fitness. However, the evolutionary basis why rhizobacteria protect plants is little understood. To persist toxin production needs to improve the competitiveness of the bacteria themselves. 2. We investigated the importance of secondary metabolite production for the establishment of the model soil biocontrol bacterium Pseudomonas fluorescens CHA0 in the rhizosphere of rice. We compared the performance of this toxin-producing strain and its isogenic gacS deficient mutant defective in secondary metabolite production. The bacteria were added to the rhizosphere of rice, where they had to compete with the indigenous flora for resources and to resist predation by the protist Acanthamoeba castellanii. 3. Secondary metabolite production strongly enhanced the establishment of the inoculated bacteria by improving competitive strength and predator resistance. The fitness gain due to attenuation of predation exceeded that due to competition by a factor of 2-3, confirming the importance of grazing resistance for rhizosphere bacteria. 4. Biocontrol properties of Plant Growth Promoting Bacteria such as P. fluorescens therefore gain a new dimension. Toxicity primary plays a role in the interaction with competitors and especially predators, and not in the protection of the host plant. Thus, establishment and efficiency of biocontrol bacteria may be improved by fostering predator defence via toxin production.