Plant identity drives the expression of biocontrol factors in a rhizosphere bacterium across a plant diversity gradient

Summary Plant performance is influenced by root‐associated bacteria that provide important services to the host plant, such as pathogen suppression. Suppression of pathogens is known to be context‐dependent and to vary between plant species, yet the significance of plant identity in shaping rhizosphere bacterial functioning in multi‐species communities is largely unknown. We questioned whether the activity of a rhizosphere bacterium in producing biocontrol compounds varies with plant identity in a plant diversity gradient. We set up a gnotobiotic microcosm experiment with the model rhizosphere bacterium Pseudomonas protegen s CHA 0, an important biocontrol agent, and investigated the effects of plant identity and diversity on its production of biocontrol compounds. Using GFP ‐based reporter fusions, we assessed gene expression linked to the production of the biocontrol compounds 2,4‐diacetylphloroglucinol, pyrrolnitrin and hydrogen cyanide. The expression of genes coding for biocontrol compounds was driven to a large extent by plant identity and this effect persisted along the plant species richness gradient for all tested genes. Notably, the effect of certain plant identities varied between the three tested genes, indicating a selective impact of plant species on bacterial gene expression. However, some plant species, such as Lolium perenne , consistently stimulated bacterial gene expression irrespective of the diversity of the plant community. Our results indicate that the presence of certain plant species within a community disproportionately impacts biocontrol traits expressed by rhizosphere bacteria, providing new insight into the patterns driving plant health and productivity.