Fig. 1

Arrival of microbes on the leaf surface and assembly of epiphytic communities. Diverse microbes (1) land on leaves by a number of different routes, including rain (2) and by air movement (3), as there are large fluxes of microbes into the atmosphere from surfaces including soil, leaves and cryptogamic coverings of, for example, rocks; and many fungal spores and bacteria can survive in the boundary layer and may thus be transported worldwide. Microbes may be deposited directly from the air or rained out of the atmosphere. Rain can also be an additional source of microbes that may be splashed from neighbouring soil and plant surfaces (4), meaning that both the neighbouring plant community composition and seasonal factors (5) can affect the diversity present. Once on the leaf, microbes exist in large aggregates (6) whose location and size are determined by the heterogeneous nature of the leaf surface topology and nutrient distribution, although individuals (7) and microcolonies are also present. Aggregates include mixed species communities which may exist as biofilms (8). Leaves harbour a less diverse microbiome than the surrounding environment, partially due to the effects of stresses such as competition (9), high temperatures, U.V. radiation and drying (10), low and heterogeneous water and nutrient availability (11), and the presence of toxins such as plant defence compounds and agricultural antimicrobials (12). The influence of the host is then proposed to further select a specialised epiphytic microbiome