A growing branch of research on how plants use so-called exosomes to interact with their environment is opening up a new field of plant biology, writes Amanda Keener in a recent article published in The Scientist.
By the end of 2014, plant biologists Rutter and Innes at Indiana University Bloomington had developed a working protocol that allowed them to describe the content of so-called purified plant exosomes for the first time. Since then, Innes’s group and others have advanced the understanding of these vesicles by leaps and bounds.
The vesicles could explain, for example, how plants send out defensive proteins and RNAs to protect themselves against pathogens, and they may turn out to be a treasure trove of undiscovered proteins, says Hailing Jin, a plant scientist at the University of California, Riverside. “There are a lot of open questions that previously [plant biologists] couldn’t answer,” she says, “and now they feel like exosomes could provide a very good explanation.”
Researchers, including Innes and Jin, have just begun to scratch the surface of what plant exosomes do. Some scientists are also beginning to see their potential as useful tools in agriculture and even medicine.
Some of this is explained in an infographic published in The Scientist.
As fungal pathogens sink their feeding apparatuses into host cells, plants can fight back by shooting out vesicles packed with defensive molecules.
- Exosomes start out as vesicles within multivesicular bodies (MVBs), whose source in plants is currently unknown, but may be the trans-golgi network (TGN), which sorts proteins for delivery to various cellular locations. MVBs fuse with the plant plasma membrane to release exosomes into the apoplast, which includes the cell wall and the space between cells.
- Mounting evidence suggests exosomes have several roles in plant defense. For example, when the haustoria, or feeding structures, of pathogenic fungi penetrate a plant’s cell walls, exosomes may deliver molecules to reinforce the cell wall. Through an unknown mechanism, exosomes can also cross the cell wall and enter nearby fungal cells to deliver fungal defense proteins and short interfering RNAs that disrupt the translation of fungal proteins.
- Plant-derived exosomes also help establish a protective barrier around fungal haustoria that have established themselves within the cell wall by delivering antifungal molecules and materials such as complex polysaccharides.
When identifying vesicles thought to be exosomes, plant biologists look for a number of hallmark traits.
- Size: 30 to 150 nanometers in diameter
- Molecular markers: Membrane trafficking proteins and proteins derived from the trans-golgi network (TGN) and the multivesicular body (MVB)
- Examples of known cargo:
- Cell wall–building enzymes that may be involved in plant growth and development
- Pathogen defense proteins that are delivered to sites of infection
- Short interfering RNAs that can silence pathogens’ genes