Vesicle exocytosis and endocytosis control the activities and turnover of plasma membrane proteins required for signaling triggering or attenuating at the cell surface. In recent years, the diverse exocytic and endocytic trafficking pathways have been uncovered in plants. The balance between conventional and unconventional protein secretion provides an efficient strategy to respond to stress conditions. Similarly, clathrin-dependent and -independent endocytosis cooperatively regulate the dynamics of membrane proteins in response to environmental cues. In fact, many aspects of plant growth and development, such as tip growth, immune response, and protein polarity establishment, involve the tight deployment of exo-endocytic trafficking. However, our understanding of their intersection is limited. Here, we discuss recent advances in the molecular factors coupling plant exo-endocytic trafficking and the biological significance of balance between exocytosis and endocytosis in plants.
Plant phospholipase Ds (PLDs), essential regulators of phospholipid signaling, function in multiple signal transduction cascades; however, the mechanisms regulating PLDs in response to pathogens remain unclear. Here, we found that Arabidopsis (Arabidopsis thaliana) PLDd accumulated in cells at the entry sites of the barley powdery mildew fungus, Blumeria graminis f. sp hordei. Using fluorescence recovery after photobleaching and single-molecule analysis, we observed higher PLDd density in the plasma membrane after chitin treatment; PLDd also underwent rapid exocytosis. Fluorescence resonance energy transfer with fluorescence lifetime imaging microscopy showed that the interaction between PLDd and the microdomain marker AtREMORIN1.3 (AtREM1.3) increased in response to chitin, indicating that exocytosis facilitates rapid, efficient sorting of PLDd into microdomains upon pathogen stimulus. We further unveiled a trade-off between brefeldin A (BFA)-resistant and-sensitive pathways in secretion of PLDd under diverse conditions. Upon pathogen attack, PLDd secretion involved syntaxin-associated VAMP721/722-mediated exocytosis sensitive to BFA. Analysis of phosphatidic acid (PA), hydrogen peroxide, and jasmonic acid (JA) levels and expression of related genes indicated that the relocalization of PLDd is crucial for its activation to produce PA and initiate reactive oxygen species and JA signaling pathways. Together, our findings revealed that the translocation of PLDd to papillae is modulated by exocytosis, thus triggering PA-mediated signaling in plant innate immunity.
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