Spatially Restricted Immune Responses Are Required for Maintaining Root Meristematic Activity upon Detection of Bacteria

Summary Plants restrict immune responses to vulnerable root parts. Spatially restricted responses are thought to be necessary to avoid constitutive responses to rhizosphere microbiota. To directly demonstrate the importance of spatially restricted responses, we expressed the plant flagellin receptor (FLS2) in different tissues, combined with fluorescent defense markers for immune readouts at cellular resolution. Our analysis distinguishes responses appearing cell autonomous from apparently non-cell-autonomous responses. It reveals lignification as a general immune response, contrasting suberization. Importantly, our analysis divides the root meristem into a central zone refractory to FLS2 expression and a cortex that is sensitized by FLS2 expression, causing meristem collapse upon stimulation. Meristematic epidermal expression generates super-competent lines that detect native bacterial flagellin and bypass the weak or absent response to commensals, providing a powerful tool for studying root immunity. Our manipulations and readouts demonstrate incompatibility of meristematic activity and defense and the importance of cell-resolved studies of plant immune responses.