A highway for war and peace: the secretory pathway in plant-microbe interactions

Secretion of proteins and other molecules is the primary means by which a cell interacts with its surroundings. The overall organization of the secretory system is remarkably conserved among eukaryotes, and many of the components have been investigated in detail in animal models. Plant cells, because of their sessile lifestyle, are uniquely reliant on the secretory pathway to respond to changes in their environments, either abiotic, such as the absence of nutrients, or biotic, such as the presence of predators or pathogens. In particular, most plant pathogens are extracellular, which demands a robust and efficient host secretory system directed at the site of attack. Here, we present a summary of recent advances in our understanding of the molecular details of the secretory pathway during plant-microbe interactions. Secretion is required not only for the delivery of antimicrobial molecules, but also for the biogenesis of cell surface sensors to detect microbes. The deposition of extracellular material is important in the defense against classical bacterial pathogens as well as in the so-called 'non-host' resistance. Finally, boosting the protein secretion capacity is vital for avoiding infection as well as for achieving symbiosis, even though in the latter case, the microbes are engulfed in intracellular compartments. The emerging evidence indicates that secretion provides an essential interface between plant hosts and their associated microbial partners.