Spatial organization of plant defense at the infection front

Abstract

Plants lack specialized immune cells and instead rely on coordinated cellular responses to restrict pathogen invasion while preserving tissue integrity. How plant immunity spatially organizes these responses remains unclear. Using live-cell reporters in Arabidopsis infected with Pseudomonas syringae, we show that effector-triggered immunity, superimposed on pattern-triggered immunity, establishes a sustained yet spatially confined immune architecture at the infection front. Defense activation persists for days but remains restricted to a narrow ring of cells surrounding viable bacterial microcolonies. Over time, immune activity spreads to adjacent layers, forming a coordinated, multilayered defense zone. This zonation extends beyond transcriptional activation to polarized callose deposition at pathogen-facing cell walls, reinforcing a localized containment boundary that limits pathogen spread. Consistent with previous single-reporter studies, simultaneous visualization of salicylic acid (SA) and jasmonic acid (JA) biosynthesis and response markers reveals a radial hormone gradient, with SA-enriched cells proximal to bacterial colonies and JA-enriched cells in surrounding regions. Individual cells predominantly activate one pathway, indicating that SA-JA antagonism is resolved through spatial compartmentalization across neighboring cells. Together, these findings establish immune zonation as a strategy for robust pathogen containment while minimizing collateral tissue damage. Significance statementPlants lack specialized immune cells and must coordinate defense responses across tissues to contain infection. Although plant immune responses are known to be locally activated, how they are organized across cells to form a stable containment boundary has remained unclear. Using live imaging of fluorescent bacterial microcolonies, we show that plant immunity forms a spatially organized defense zone surrounding viable infection sites. Within this zone, distinct defense activities are arranged in a radial gradient, with salicylic acid-associated responses enriched in cells near the pathogen and jasmonic acid-associated responses in surrounding cells. Rather than being resolved solely within individual cells, antagonistic hormone pathways are separated across neighboring cells. This spatial organization reveals immune zonation as a strategy for confining pathogen spread while preserving tissue integrity.