Cell surface immune receptors: the guardians of the plant’s extracellular spaces

Since the original ‘Zigzag model’, several iterations have been proposed to reconcile both the Pattern Triggered Immunity (PTI) and the Effector Triggered Immunity (ETI) branches of the plant immune system. The recent cloning of new disease resistance genes, functioning in gene-for-gene interactions, which structurally resemble cell surface broad spectrum Pattern Recognition Receptors, have further blurred the distinctions between PTI and ETI in plant immunity. In an attempt to simplify further the existing conceptual models, we, herein, propose a scheme based on the spatial localization of the key proteins (receptors) which function to induce plant immune responses. We believe this ‘Spatial Invasion model’ will prove useful for understanding how immune receptors interact with different pathogen types which peripherally or totally invade plant cells, colonize solely extracellularly or switch locations during a successful infection.