Plants are colonized by a variety of microorganisms, including bacteria, fungi and yeasts. In the above-ground part of plants, known as the phyllosphere, bacteria are the most abundant inhabitants. The importance of the phyllosphere microbiota for resistance against phytopathogenic bacteria or fungi is not yet well understood. Members of the genus <i>Sphingomonas</i> constitute a significant fraction of up to 20% of the phyllosphere population of healthy plants making them prime candidates to investigate in how far indigenous phyllosphere bacteria have the ability to protect the plant against pathogens. Recently, we have shown that various <i>Sphingomonas</i> strains such as <i>Sphingomonas</i> sp. Fr1 protect Arabidopsis against the bacterial pathogen <i>Pseudomonas syringae</i> DC3000 in a gnotobiotic model system. A number of complementary experiments will be presented including a bacterial forward genetic in planta screen and plant transcriptome analyses aiming at elucidating the mode of action of indigenous phyllosphere bacteria in plant protection. Vorholt JA (2012) Microbial life in the phyllosphere. Nature Rev. Microbiol. 10:828-840.
