Biosensors reveal distinct cytosolic pH and redox dynamics across seedlings in response to danger signals and pathogen infection.

Plants perceive danger signals, pathogen- or damage-associated molecular patterns (PAMPs/DAMPs), to activate immune responses such as transient apoplastic alkalinization and reactive oxygen species (ROS) production. However, how these pH and redox changes occur across organs and tissues during pattern-triggered immunity (PTI) remains poorly understood. Using genetically encoded biosensors (GEBs), we monitored cytosolic pH and redox dynamics across whole Arabidopsis seedlings with spatiotemporal resolution. Global treatments with diverse danger signals first induced cytosolic acidification and oxidation in roots, followed by petioles and, later, the hypocotyl, revealing organ-specific responsiveness and a bidirectional response gradient. By contrast, Pseudomonas syringae pv. tomato (Pto) DC3000 induced sustained cytosolic alkalinization and suppressed redox responses in inoculated shoots, even when co-treated with PAMPs. Mutants lacking either a functional flagellum (ΔfliC) or a Type-3 secretion system (ΔhrcC) induced opposite or no long-term responses in the cytosol, respectively. Together, these findings highlight that the immune response of the plant is not uniform but instead follows organ- and tissue-specific patterns which are trigger-dependent, and reflect distinct capacities of seedling cells to activate PTI.