Layered Defence in Plants: How Surface and Intracellular Immune Receptors Cooperate Against Microbial Pathogens

Plants cannot flee from danger, so every cell must be able to detect and answer microbial attack on its own. For two decades the field has described this ability as a two-tier system: receptors on the cell surface that detect conserved microbial molecules, triggering pattern-triggered immunity (PTI), and receptors inside the cell that detect pathogen-delivered effector proteins, triggering effector-triggered immunity (ETI). Recent work has shown that these tiers are not independent. They share signalling components, strengthen one another, and together produce a defence that neither could achieve alone. This review summarises how plants perceive pathogens, how early signals such as calcium influx and reactive oxygen species are generated, how PTI and ETI are now understood to potentiate each other, and where environmental change and crop improvement enter the picture. We close with open questions and priorities for future research.