A straightforward and reliable method for bacterial <i>in planta</i> transcriptomics: application to the <i>Dickeya dadantii</i>/<i>Arabidopsis thaliana</i> pathosystem

Summary Transcriptome analysis of bacterial pathogens is a powerful approach to identify and study the expression patterns of genes during host infection. However, analysis of the early stages of bacterial virulence at the genome scale is lacking with respect to understanding of plant–pathogen interactions and diseases, especially during foliar infection. This is mainly due to both the low ratio of bacterial cells to plant material at the beginning of infection, and the high contamination by chloroplastic material. Here we describe a reliable and straightforward method for bacterial cell purification from infected leaf tissues, effective even if only a small amount of bacteria is present relative to plant material. The efficiency of this method for transcriptomic analysis was validated by analysing the expression profiles of the phytopathogenic enterobacterium Dickeya dadantii , a soft rot disease‐causing agent, during the first hours of infection of the model host plant Arabidopsis thaliana . Transcriptome profiles of epiphytic bacteria and bacteria colonizing host tissues were compared, allowing identification of approximately 100 differentially expressed genes. Requiring no specific equipment, cost‐friendly and easily transferable to other pathosystems, this method should be of great interest for many other plant–bacteria interaction studies.