Non-pathogenic leaf-colonising bacteria elicit plant immune responses in a colonisation density-dependent manner

Leaves are colonised by a complex mix of microbes, termed the leaf microbiota. Even though the leaf microbiota is increasingly recognised as an integral part of plant life and health, our understanding of its interactions with the plant host is still limited. Here, mature, axenically grown Arabidopsis thaliana plants were spray-inoculated with different densities of the non-pathogenic bacterium Williamsia sp. Leaf354. High bacterial titers caused disease phenotypes and led to severe transcriptional reprogramming with a strong focus on plant defence. Overall design: Examination of aboveground Arabidopsis thaliana tissue treated with different bacterial densities of the non-pathogenic bacterium Williamsia sp. Leaf354 in comparison to mock treated plants. Plants were grown in the 'Litterbox' system. Plants were 6-weeks-old at time of inoculation and were sampled 96 h post inoculation.