The Role of the Phyllosphere Microbiome in Plant Health and Function

The above‐ground surfaces of plants (the phyllosphere) harbour a diverse variety of microorganisms, and this phyllosphere microbiome interacts with the host plant affecting its health and function. Phyllosphere microorganisms, predominantly bacteria and fungi, can act as mutualists promoting plant growth and tolerance of environmental stressors, commensals using the leaf habitat for their own growth and reproduction, or as antagonistic pathogens. Although much of the literature has focused on plant–pathogen interactions and disease mechanisms, we expand this discussion to the structural and functional dynamics of the whole phyllosphere community, rather than single‐species populations. We highlight the particular challenges posed to microorganisms living on the leaf surface, describe the structure of the phyllosphere microbiome, and discuss how these microbial constituents interact with the plant with respect to stress tolerance, growth promotion, nutrient acquisition, and disease resistance. Lastly, we pose new directions for the field made possible by advances in sequencing and computational technologies.