Across diverse ecosystems, bacteria and their host organisms engage in complex relationships having negative, neutral, or positive effects. However, the specific effects of leaf-associated bacterial community functions on plant growth are poorly understood. This study investigated mechanistic relationships between bacterial community function and host plant growth in a carnivorous plant that relies on microbes to break down insect prey via extracellular hydrolytic enzymes. Sterile, freshly opened leaves (pitchers) of purple pitcher plants (Sarracenia purpurea) were inoculated with three functionally distinct bacterial communities to test the effects of bacterial function on plant growth and leaf nutrient content. Bacterial community composition and function were measured using physiological assays, metagenomics, and metatranscriptomics. Distinct bacterial functions affected plant traits, with a bacterial community enriched in decomposition and secondary metabolite production traits leading to leaves with almost double the biomass of control pitchers. Functional differences in bacterial communities, measured by physiological assays, were supported by metatranscriptomic analysis; for example, the bacterial community with the highest chitinase activity had greater relative abundance of transcripts associated with chitinase enzymes. The bacterial community associated with larger pitchers also had increased differential expression of transcripts linked to microbially-produced plant hormones. The direct relationship between bacterial community function and plant growth observed, indicates specific mechanisms for host-associated bacterial functions to support plant health and growth.
