Transcriptome Profiling of the Endophyte Burkholderia phytofirrnans PsIN Indicates Sensing of the Plant Environment and Drought Stress

It is widely accepted that bacterial endophytes actively colonize plants, interact with their host, and frequently show beneficial effects on plant growth and health. However, the mechanisms of plant-endophyte communication and bacterial adaption to the plant environment are still poorly understood. Here, whole-transcriptome sequencing of B. phytofirmans PsJN colonizing potato (Solanum tuberosum L.) plants was used to analyze in planta gene activity and the response of strain PsJN to plant stress. The transcriptome of PsJN colonizing in vitro potato plants showed a broad array of functionalities encoded in the genome of strain PsJN. Transcripts upregulated in response to plant drought stress were mainly involved in transcriptional regulation, cellular homeostasis, and the detoxification of reactive oxygen species, indicating an oxidative stress response in PsJN. Genes with modulated expression included genes for extracytoplasmatic function (ECF) group IV sigma factors. These cell surface signaling elements allow bacteria to sense changing environmental conditions and to adjust their metabolism accordingly. TaqMan quantitative PCR (TaqMan-qPCR) was performed to identify ECF sigma factors in PsJN that were activated in response to plant stress. Six ECF sigma factor genes were expressed in PsJN colonizing potato plants. The expression of one ECF sigma factor was upregulated whereas that of another one was downregulated in a plant genotype-specific manner when the plants were stressed. Collectively, our study results indicate that endophytic B. phytofirmans PsJN cells are active inside plants. Moreover, the activity of strain PsJN is affected by plant drought stress; it senses plant stress signals and adjusts its gene expression accordingly. IMPORTANCE In recent years, plant growth-promoting endophytes have received steadily growing interest as an inexpensive alternative to resource-consuming agrochemicals in sustainable agriculture. Even though promising effects are recurrently observed under controlled conditions, these are rarely reproducible in the field or show undesirably strong variations. Obviously, a better understanding of endophyte activities in plants and the influence of plant physiology on these activities is needed to develop more-successful application strategies. So far, research has focused mainly on analyzing the plant response to bacterial inoculants. This prompted us to study the gene expression of the endophyte Burkholderia phytofirmans PsJN in potato plants. We found that endophytic PsJN cells express a wide array of genes and pathways, pointing to high metabolic activity inside plants. Moreover, the strain senses changes in the plant physiology due to plant stress and adjusts its gene expression pattern to cope with and adapt to the altered conditions.