Membrane Trafficking in Plant Immunity

Abstract

Plants employ sophisticated mechanisms to interact with pathogenic as well as beneficial microbes. Of those, membrane trafficking is key in establishing a rapid and precise response. Upon interaction with pathogenic microbes, surface-localized immune receptors undergo endocytosis for signal transduction and activity regulation while cell wall components, antimicrobial compounds, and defense proteins are delivered to pathogen invasion sites through polarized secretion. To sustain mutualistic associations, host cells also reprogram the membrane trafficking system to accommodate invasive structures of symbiotic microbes. Here, we provide an analysis of recent advances in understanding the roles of secretory and endocytic membrane trafficking pathways in plant immune activation. We also discuss strategies deployed by adapted microbes to manipulate these pathways to subvert or inhibit plant defense.

Keywords: endocytosis; membrane trafficking; plant immunity; secretion.

Figure 1. Membrane trafficking during plant immune responses

Defense molecules are synthesized in the ER and transported via Golgi to the sorting hub TGN/EE. Vesicles derived from the TGN/EE, including MVBs, fuse with the PM/EHM to deliver soluble cargo to the extracellular space and membrane cargo to the PM/EHM. MVBs and EXPOs contribute to paramural accumulation of exosomes, whose membrane burst in the extracellular space discharges contents with anti-microbial activities. Effector proteins secreted by pathogens into host cells inhibit multiple steps of the secretory trafficking pathways to block defense activation. PM-localized immune receptors, such as PRRs, undergo constitutive endocytosis before activation. Once activated, these receptors are internalized into clathrin coated vesicles (CCVs) and undergo clathrin-mediated endocytosis (CME). The CME sorts activated receptors to the central vacuole through TGN/EEs and MVBs. Both constitutive endocytosis and CME contribute to basal immunity and are targeted by pathogen effectors. Arrows indicate steps of the secretory (green) and the exocytic (red) pathways.