Nailing down the Arabidopsis root response to beneficial rhizobacteria

We have billions of micro-organisms in our gut, including bacteria that are fundamental to our health. Plants have billions of micro-organisms around their roots. Like in our gut, these include beneficial bacteria. Beneficial soil bacteria can increase plant growth and plant resistance to disease. Theoretically, these bacteria could therefore be used to increase crop yield. Unfortunately, bacteria that are beneficial to one plant species, are not always also beneficial to other plant species, or even to the same plant species in a different environment. Knowledge on the pathways in a plant resulting in bacteria-induced growth and resilience will enable easier selection of beneficial bacteria. In this thesis, I studied the interaction between the plant Arabidopsis thaliana and the beneficial bacterium Pseudomonas simiae WCS417. When WCS417 is in contact with the Arabidopsis thaliana root, the plant becomes bigger and is more resistant to several different pathogens. In addition, the plant changes the spatial arrangement of its roots. In my first experimental chapter, we uncover signaling pathways that might explain the changes in the spatial arrangement of roots that are observed in WCS417-treated plants. Next, we show that WCS417-treated plants require additional iron uptake to prevent harmful effects of the WCS417-mediated increase in plant growth. Then, we show that there are many similarities between WCS417-induced resistance and iron deficiency-induced resistance, but that these resistances are not activated by the same molecular mechanisms. Finally, we set up a promising method to study the effect of WCS417 and iron deficiency on a different level. The demand for food is increasing worldwide. In addition, we are becoming more aware of the harmful effects of synthetic pesticides and fertilizers. The results presented in this thesis can contribute to using bacteria to increase food production in a sustainable manner