Building blocks toward cell surface receptor-mediated immunity in plants

Land plants possess an innate immune system that enables their survival in terrestrial environments and allows them to interact with a wide range of microorganisms, some of which can be pathogenic. Typically, cell surface-localized receptors are used to sense microorganisms by recognizing structurally conserved molecules, whereas intracellular receptors detect effector molecules released by microorganisms that attempt to manipulate plant immune responses. Activation of these receptors in each cell triggers optimal defense responses for organismal survival, utilizing both shared and unique signaling components. Growing evidence indicates that immune receptor genes evolve rapidly, with each plant species harboring a markedly different gene repertoire, likely shaped by the ecological interactions within their respective habitats. Despite this, little is known about when and how the plant immune system was established. In this review, we explore the evolutionary processes that shaped plant immune signaling, with a particular focus on cell surface receptor-mediated pathways, which may have been associated with the transition from water to land.