Biologists have argued over the thinking power and consciousness of plants for centuries. This argument is still very much alive today, as scientists continue to study plants they steadily find new examples of “thinking”.
Plants and Memory
Some of the strongest evidence of plant intelligence is their ability to not only learn from external stimuli but to then also store that knowledge in a functional long term memory. The most prominent example of a plant’s long term memory is seen in Mimosa pudica, otherwise known as a touch-me-not. When a touch-me-not receives a stimulus it deems dangerous (including actions as harmless as a simple touch) it will fold up its leaves to protect the plant. If researchers take a Mimosa pudica and agitate the plant so that it responds defensively and folds up its leaves but actually cause no harm to the plant, then eventually the plant will learn that that particular stimulus is actually harmless, and will no longer respond defensively. The plants are able to remember that a stimulus is harmless weeks later, even if exposed to a different environment.
Similarly to animals, the more extreme the stimulus, the quicker the plants response is learned . Plants do not have a nervous system like animals, however, they do have complex calcium-based signaling systems. These calcium signaling systems are not enough to explain plants long term memory for environmental stresses, however, researchers are discovering that epigenetic and biochemical modifications provide another possible explanation for these long-term memory systems .
What Are Prions?
Prions were originally discovered while trying to find the exogenous nucleic acid responsible for mammalian neurodegenerative diseases called transmissible spongiform encephalopathies (TSE). Instead, they found infectious proteins that work in the absence of nucleic acids  . These proteins can assume two different forms of the same protein folded in two distinct ways (isomers). It is not uncommon for a protein’s structure to be altered, the unique thing about prions is that the altered isomer of the protein can cause other prions that it comes into contact with to conform to the isomer, and build up toxic aggregates in the cell, eventually leading to brain disease and death. In the context of diseases. This is what allows prions to be infectious.
These diseases can be spread through digestion, for example, by eating a cow that has Mad Cow Disease (the most famous prion disease in cows). They can also be hereditary, but the reality is that most cases of these extremely rare diseases seem to occur spontaneously  . Despite being very deadly, prions have been shown to play important roles in memory in humans. As prion-like proteins (proteins that are similar in structure/function to the original prion protein discovered in mammals) were studied in non-mammals, it was realized that they have been evolutionarily conserved because they perform important beneficial functions  . In fungi, they act to control the mixing of nearby fungal colonies, which might serve to limit the damage caused by viral infections  .
Currently, researchers are attempting to uncover the cellular pathways and interacting physiologies of prion-like proteins . Prion-like proteins can serve as a tool of molecular memory because of their ability to be transiently expressed as well as strongly influence the phenotype of the organism  .
Discovery of Prion-like Proteins in Plants
When researchers at MIT, Whitehead Institute, and Howard Hughes Medical Institute realized that prion-like proteins had been discovered in a diverse selection of organisms but never in plants. They decided to investigate the model organism Arabidopsis to see if prion-like proteins could explain plants memory capabilities. The researchers were particularly interested in long term memory with respect to flowering because flowering is a process that must be highly controlled for the current environment.
The main process that regulates flowering that they focused on was vernalization, the process of flowering in the spring after experiencing extreme cold indicating a harsh winter. Vernalization is interesting because even if only the callus of the plant experiences the cold, the resulting plant will still know when to flower. Across all known prion-like proteins, part of the protein’s amino acid sequence (primary structure) has similar elements known as prion-like domains (PrDs). By algorithmically searching through the parts of the Arabidopsis genome that code for proteins (the proteome) they identified 474 proteins that contained PrDs .
After finding candidates for plant prion-like proteins yeast cells were used as “living test tubes” where phenotypical levels that would be altered by prion-like protein activity was measured. Proteins were confirmed to exhibit prion-like behavior, by causing conformation of proteins that control transcription or controlling RNA stability. It was not yet determined whether the prion-like proteins play a biologically significant role, such as controlling flowering, however, it is a distinct possibility.
Discovering prion-like proteins in plants not only deepens our understanding of plant biology but it may provide an exciting path to better understand the consciousness and long-term memory of plants.