Towards a better understanding of the molecular basis of the symbiosis between plants and their root endophytes

Over millions of years, plants have evolved in close proximity to other organisms in their environment, including foliar and soil microbes. This co-existence shaped highly specialized ecosystems in which plants find their niches and flourish. Numerous studies, conducted over recent years, demonstrate that plant−microbe interactions are important to the structure, function, and health of plant communities and that symbiotic fungi contribute to the adaptation of plants to environmental stresses. However, our current understanding of how plants select beneficial symbionts among prevalent microbes and how the mutual plant-microbe interaction is established remains enigmatic. In the presented study, we investigated transcriptional responses in roots of Arabidopsis thaliana that were infected with the beneficial root endophyte fungus Serendipita indica compared to non-infected control plants. The study suggested an important role for calcium signaling in the plant-fungus interaction in the early and later phases of the interaction. A detailed reverse genetics analysis highlighted the crucial role of the Ca2+ sensor CBL7 in conferring fungus-triggered growth promotion to infected host plants. Furthermore, we were able to demonstrate that CBL7 is an important molecular asset that contributes to potassium distribution in Arabidopsis and to the adjustment of plant defense responses during symbiosis.