Billions of years of evolution has led to environmentally specialized organisms across the kingdoms of life: animal, plant, fungi, bacteria, archaea, and protist. To thrive under the inexorable competition for survival, organisms must adapt lest they become extinct. Their adaptations take many forms, including structural or chemical changes that impart unique defensive or offensive tools against predators or prey. But scientists have begun to understand that the fitness of plants and animals is intimately linked to their constant, direct interactions with bacteria, fungi, and archaea.
Microbial organisms can colonize hosts and perform useful or essential functions, and in other cases cause disease and death. Other organisms have learned to productively coexist by providing beneficial services in exchange for resources, protection, or transportation. Whether the interspecies relationship is predatory, commensal, symbiotic, or parasitic, organisms from different kingdoms are in constant communication with one another through physical, chemical, or biological signals that are exchanged. We have already leveraged these evolutionary tools for the benefit of humanity, with examples ranging from potent antimicrobials such as penicillin to advanced therapies such as CRISPR-based gene editing.
However, nature is replete with many less explored examples. For instance, plants are uniquely vulnerable to predators. A plant is unable to outmaneuver attackers, whether the attacker is an herbivorous mammal or a parasitic fungus. In place of flight, plants have developed a variety of elegant tools to repel attacking organisms. Traditionally, plant biologists think of chemicals as the most important plant defense. However, plants have developed cleverer ways of targeting attackers.
One tool they employ is the recently discovered plant extracellular vesicle. Vesicles, originally thought to be artifacts of electron microscopy, are in reality secreted by plants through the nearly impermeable cell wall when the plants are under attack from pathogenic organisms such as fungi. Like mammalian exosomes, they are capable of passing messages from cell to cell. However, in this case the plant preassembles protective biological cargo, such as proteins and nucleic acids, into proteolipid particles that are released across the cell wall toward the invading organism. These particles have unique properties that enable them to protect the delicate biological cargo, target the pathogen, and deliver the anti-infective proteins to the invading organism. In return, fungi have developed their own extracellular vesicles and use them to reduce the ability of the plant to defend itself by packing in small RNAs that silence key plant pathways.
