Co-incidence of Damage and Microbial Patterns Controls Localized Immune Responses in Roots

Recognition of microbe-associated molecular patterns (MAMPs) is crucial for the plant’s immune response. How this sophisticated perception system can be usefully deployed in roots, continuously exposed to microbes, remains a mystery. By analyzing MAMP receptor expression and response at cellular resolution in Arabidopsis, we observed that differentiated outer cell layers show low expression of pattern-recognition receptors (PRRs) and lack MAMP responsiveness. Yet, these cells can be gated to become responsive by neighbor cell damage. Laser ablation of small cell clusters strongly upregulates PRR expression in their vicinity, and elevated receptor expression is sufficient to induce responsiveness in non-responsive cells. Finally, localized damage also leads to immune responses to otherwise non-immunogenic, beneficial bacteria. Damage-gating is overridden by receptor overexpression, which antagonizes colonization. Our findings that cellular damage can “switch on” local immune responses helps to conceptualize how MAMP perception can be used despite the presence of microbial patterns in the soil. [Display omitted] •Arabidopsis roots request cell damage to mount a strong, localized immune response•Damaged cells upregulate pattern-recognition receptor expression in their neighbors•Endodermal barriers compartmentalize immune responses in differentiated cell-types•Damage-gating can minimize immune responses against non-pathogenic root colonizers Roots require both microbial molecular patterns and plant tissue damage in order to mount localized antibacterial immune responses, revealing an effective way to respond appropriately to pathogens while sparing commensals.