Non-pathogenic free-living bacteria that inhabit the phyllosphere and rhizosphere of plants are ubiquitous, but their ecological importance has been understudied.  Based largely on results with selected crop species, some strains of rhizobacteria appear to act as mutualists, and are often grouped based on their ability to induce systemic resistance to disease in plants, to promote plant growth, or both.  Such effects are mediated through changes in host plant quality, which may affect the interactions of plants with other ecological factors.  We examined the impacts of the association of Arabidopsis thaliana with three species of rhizobacteria, known to induce systemic resistance to disease and enhance growth, on plant resistance to the generalist caterpillar, Spodoptera exigua.  We grew A. thaliana plants in sterile soil in the greenhouse and inoculated the rhizosphere with either a strain of Pseudomonas fluorescens (89B61) a strain of Serratia marcescens (90-166) or one of two strains of Bacillus pumilus (SE34, T4).  We examined the effects of the rhizobacterial treatments on plant growth and seed production, as well as on growth of young S. exigua caterpillars fed tissue from treated plants. 

Results/Conclusions

Across two experimental trials, S. exigua caterpillars generally consumed more tissue and grew faster on “bacterized” plants regardless of the strain identity than on untreated plants that were grown in sterile soil.  Furthermore, association with these rhizobacterial strains generally failed to improve growth and seed production in the absence of damage.  Although these rhizobacteria were not pathogenic, per se, their presence reduced plant resistance to generalist herbivore feeding, which could lead to higher rates of herbivore damage in the field.  This pattern is similar to reductions in insect resistance observed on plants infected with foliar bacterial pathogens that induce systemic acquired resistance.  The importance of such negative indirect effects likely varies with the identity of the rhizobacterial and herbivore communities associating with plants at any given time in the field, and must be considered in light of the potentially beneficial effects that rhizobacteria can have on plant growth.