Lipid-bilayer vesicles: guardians of plant immunity proteins and RNAs
Dr. Roger Innes, a leading scientist in plant molecular and cellular immunity studies, revealed the power of unexplored plant components called exosomes (also called “Lomasomes” or extracellular vesicle [EVs]) in plant immunity. On October 27, 2016, Dr. Innes presented his research to members of the Dept of Plant Science and Landscape Architecture . He focused on exosome appearance, its constituents, and how these vesicles, 50 to 200 nm sized lipid-bilayer spheres, are involved in mediating the transfer of proteins and miRNAs during pathogen attack.
Reading about exosomes in animals and humans prompted Dr. Innes to look for their role in plants. He was particularly interested in biogenesis and the function of EVs in plants, mainly in the context of the plant immune response. Electron microscopical studies showed that vesicles may arise from diverse sources, fuse with each other and get trapped between the plant cell wall and plasma membrane (An et al., 2007; Raposo & Stroovagel, 2013).
Dr. Innes developed a method for specifically purifying exosomes from plant apoplastic fluids and characterizing them. Purifying these vesicles was challenging and tedious, but thanks to his PhD student (Brian Rutter), they were able to follow a purification protocol having a series of centrifugation and filtration processes. After extraction, they studied its constituents and the role of these constituents in plant immunity. By proteomics and Promoter:reporter gene fusion techniques, they identified PEN1 and PEN3 proteins inside the vesicles, which might be involved in the execution of apoplastic immune responses in order to limit pathogen entry.
Moreover, by artificially inoculating Arabidopsis plants with P. syringae, they found that infection enhances EVs productions. A proteomics study showed that EVs are enriched in stress-response proteins and that PEN1 contributed to the production of more EVs. In the latter, they were also able to unravel a fundamental novel aspect of plant biology, i.e. plant vesicles also carry miRNA and play a central role in cell signaling (Rutter et al. 2016, unpublished). This could be one of the first and breakthrough investigation in plant immunity studies.
The present research contributes to our current knowledge of the existence and function of exosomes in plants, particularly that exosomes act as mediators to transfer cytosolic proteins, lipid, and most importantly, mRNA and miRNA in order to silence pathogen genes during colonization.
An, Q., van Bel, A. J. E., Hückelhoven R., (2007). Do Plant Cells Secrete Exosomes Derived from Multivesicular Bodies? Plant Signaling & Behavior 2(1): 4 - 7.
Raposo, G., and Stoorvogel, W., (2013). Extracellular vesicles: Exosomes, microvesicles, and friends. JCB, 200(4): 273 - 283.