Exocytosis is a key mechanism for delivering materials into the extracellular space for cell function and communication. In plant cells, conventional protein secretion (CPS) is achieved via an ER (endoplasmic reticulum)-Golgi-TGN (trans-Golgi network)-PM (plasma membrane) pathway. Unconventional protein secretion (UPS) bypassing these secretory organelles is also in operation and can potentially lead to the formation of extracellular vesicles (EVs) in plant cells. Although multiple types of EVs have been identified and shown to play important roles in mediating intercellular communications in mammalian cells, there has been a long debate about the possible existence of EVs in plants because of the presence of the cell wall. However, increasing evidence suggests that plants also release EVs having various functions including unconventional protein secretion, RNA transport, and defense against pathogens. In this review, we present an update on the current knowledge about the nature, secretory mechanism, and function of various types of EVs in plants. The key regulators involved in EV secretion are also summarized and discussed. We pay special attention to the function of EVs in plant defense and symbiosis.
Keywords: EXPO; Exosome; Extracellular vesicle; Plant defense; Protein secretion.