Extracellular Vesicles: Emerging Players in Plant Defense Against Pathogens

Abstract

Communication between plants and interacting microorganisms requires functional molecule trafficking, which is essential for host defense and pathogen virulence. Extracellular vesicles (EVs) are single membrane-bound spheres that carry complex cargos, including lipids, proteins, and nucleic acids. They mediate cell-to-cell communication via the transfer of molecules between cells. Plant EVs have been isolated from many plant species and play a prominent role in immune system modulation and plant defense response. Recent studies have shown that plant EVs are emerging players in cross-kingdom regulation and contribute to plant immunity by mediating the trafficking of regulatory small RNA into pathogens, leading to the silencing of pathogen virulence-related genes. This review summarizes the current understanding of plant EV isolation technologies, the role of plant EVs in plant immunity, and the mechanism of plant EV biogenesis, as well as approaches for how these findings can be developed into innovative strategies for crop protection.

Keywords: cell-to-cell communication; cross-kingdom RNA interference; endomembrane trafficking; extracellular vesicles; plant immunity.

Figure 1

Roles of EV-mediated RNAi in plant–microbial interactions and plant protection. The conventional secretion pathway delivers PAMPs to the extracellular space or transport surface PRRs to PM. In unconventional secretion pathway, MVBs release TET8/9-positive EVs, which contain defense proteins and host-derived sRNAs, into the extracellular space. TET8/9-positive EVs contain a variety of RBPs, including AGO1, RHs, and ANNs, which load sRNAs into EVs. PEN1-positive EVs are secreted into the extracellular space with an unknown mechanism (marked with? in the Figure). EXPO is a novel bilayer membrane organelle that fuses with PM to produce another subtype of EVs. Whether PEN1-positive and EXPO-positive EVs contribute to cross-kingdom RNAi between plant and pathogens (marked with? in the figure) is unknown. On the basis of the knowledge on cross-kingdom RNAi, the spray application of dsRNAs and sRNAs that target pathogen genes can potentially control plant diseases. In SIGS approaches, dsRNAs are applied exogenously or carried by nanocarriers, such as BioClay and artificial vesicles. Exogenous RNAs can either be directly internalized into fungal cells or indirectly via passage through plant cells before transport into fungal cells. dsRNA, double-stranded RNA; EE, early endosome; ER, endoplasmic reticulum; EVs, extracellular vesicles; EXPO, exocyst-positive organelle; MVB/LE, multivesicular body/late endosome; ILV, intraluminal vesicle; PAMP, pathogen-associated molecular pattern; PEN1, Penetration 1; PM, plasma membrane; PRR, pattern recognition receptor; RBPs, RNA binding protein; SIGS, spray-induced gene silencing; siRNA, small interfering RNA; sRNA, small RNA; TGN, trans-Golgi network.