Lignin‐based barrier restricts pathogens to the infection site and confers resistance in plants

Pathogenic bacteria invade plant tissues and proliferate in the extracellular space. Plants have evolved the immune system to recognize and limit the growth of pathogens. Despite substantial progress in the study of plant immunity, the mechanism by which plants limit pathogen growth remains unclear. Here, we show that lignin accumulates in Arabidopsis leaves in response to incompatible interactions with bacterial pathogens in a manner dependent on Casparian strip membrane domain protein (CASP)‐like proteins (CASPLs). CASPs are known to be the organizers of the lignin‐based Casparian strip, which functions as a diffusion barrier in roots. The spread of invading avirulent pathogens is prevented by spatial restriction, which is disturbed by defects in lignin deposition. Moreover, the motility of pathogenic bacteria is negatively affected by lignin accumulation. These results suggest that the lignin‐deposited structure functions as a physical barrier similar to the Casparian strip, trapping pathogens and thereby terminating their growth. Synopsis Plants employ a multilayered immune system, but the exact mechanisms of how plants restrict pathogen growth remain unclear. In this study, the phenolic polymer and cell wall component lignin is shown to form a mechanical barrier against avirulent pathogens, thereby conferring disease resistance in plants. Lignification is induced during incompatible plant‐pathogen interactions in Arabidopsis . Lignin spatially restricts and encompasses bacteria in the extracellular space Lignin deposition enhances disease resistance. Casparian strip organizer proteins CASPL1D1 and CASPL4D1 are required for pathogen‐induced lignification. Graphical Abstract Lignin deposition is required for innate immune defense during incompatible plant‐pathogen interactions in a manner dependent on Casparian strip organizer proteins.