Update on Secretory Pathways in Plant Immune Responses Secretory Pathways in Plant Immune Responses 1

Innate immune receptors in plants detect the presence of microbial pathogens and trigger defense responses to terminate or restrict pathogen growth. The molecular mechanisms of receptor-mediated nonself recognition and subsequent intracellular signaling pathways have received much attention in the past. Less is known about the cellular mechanisms that contribute to the execution of immune responses. Recent studies revealed the existence of a secretory machinery that becomes engaged in the execution of extracellular immune responses. At least two vesicleassociated and SNARE protein-mediated exocytosis pathways appear to drive focal and/or nondirectional secretion of antimicrobial cocktails comprising proteins, small molecules, and cell wall building blocks into the apoplastic space. Both pathways have additional functions in plant development and might have been coopted for immune responses. Bacteria and fungi appear to have evolved counterdefense molecules that intercept the secretion machinery by blocking vesicle formation from intracellular membranes. Independently from this, plant plasma membrane ATP-binding cassette-type (ABC) transporters act in parallel defense pathways and serve as efflux pumps for the targeted delivery of antimicrobials and/or agents promoting chemical cross-linking of plant cell wall polymers. All multicellular eukaryotic organisms are exposed to the danger of microbial pathogens. Unlike animals, plants lack specialized and mobile immune cells to engulf and dismantle microbial intruders. Plants resist microbial attack using elaborate nonself surveillance systems consisting of a repertoire of cell surface and intracellular immune sensors. These receptors detect the presence of parasites and trigger powerful immune responses (for review, see Jones and Dangl, 2006). Microbial parasites have evolved diverse strategies to enter hosts for nutrient retrieval and multiplication. It is conceivable that plants have invented execution mechanisms for immune responses that are fine tuned to microbial entry routes. For example, most