Aerosol-microbe-plant interactions in the phyllosphere.

The phyllosphere, the foliar surface environment, is a dynamic and diverse ecosystem which is still not represented in current Earth system models (ESMs) but it is increasingly accepted that it plays a critical role on ecosystem function. The microbiome settled on the phyllosphere has received very poor attention and the plant-microbe interactions are still not mechanistically well understood even though it represents one of the largest biological surfaces on earth with a global estimate of 10²⁶ cells. It is now being acknowledged that foliar microbiomes go beyond pathogens and the composition of the phyllosphere is directly linked with the physiology and final phenotype of their hosts. Plant foliar surfaces are directly exposed to different atmospheric processes such as dry and wet deposition of organic aerosols. Aerosol deposition today is recognized as a major global change driver with a big potential to affect land and ocean biogeochemical cycles. Carbonaceous aerosols represent a rapid supply of nutrients (C, N, P, S), to different components of ecosystems such as plants, phytoplankton, or the diverse environmental microbiomes such as the microbial communities residing on leaf surfaces. Therefore, aerosol deposition on plant surfaces should have the potential to impact plant phenotypic responses indirectly through changes in the phyllosphere microbiome structure. A preliminary experiment performed in EMSL with poplar trees showed that deposition of organic aerosols (pollen) produced a significant change on the plant foliar microbiome and metabolome; the chemical phenotype of an organism. There is thus a clear necessity of a deeper understanding of the atmosphere-biosphere interface and the roles of the phyllosphere on plant phenotypes. We still have an incomplete understanding of how carbonaceous aerosol deposition impacts the foliar microbiomes and, subsequently, what are the implications for the plant phenotype.