Prions – those infamous proteins linked to mad cow disease -may be responsible for memory in plants.
The proteins may help plants change their activity based on past events, helping them decide when to flower, for instance.
That plants have memory is well known. For instance, certain plants flower after a prolonged exposure to cold. But if the conditions are not right following the cold, the plant will delay flowering until temperature and light are just right. This suggests that plants “remember” the exposure to cold.
You can even take tissue from such plants and grow a new plant, and it, too, will remember the encounter with the cold, and flower accordingly. The biological state is somehow perpetuated in both the original and new plants.
“Plants have lots of states that they self-perpetuate,” says Susan Lindquist of the Massachusetts Institute of Technology. “They have memory in some ways.”
A prion protein can fold in two ways: it has a normal form and a prion form. Once it folds into a prion, it can then cause similar proteins to change their folding, turning them into prions too.
Lindquist’s team already knew that yeasts use prions as a form of memory, and suspected that plants might too. Unlike in Creuzfeldt-Jakob disease, the human equivalent of BSE or “mad cow disease”, where prions multiply in the human brain with terrible consequences, prions in yeast are beneficial. They can help the organism use different nutrients and grow in new places.
Crucially, this ability persists over generations. “It could be state that only lasts for 50 generations, or it could last for thousands and thousands of generations,” says Lindquist.
The team applied techniques developed for finding prions in yeast to Arabidopsis thaliana, a flowering mustard plant. Their method involves using specialised algorithms to search the full complement of proteins expressed by the plant.
The researchers found four proteins involved in flowering that had portions that resembled prion-specific sequences in yeast.
Next, the team replaced the prions in yeast cells with the prion-like protein sequences from Arabidopsis, and confirmed that the three of the four plant protein fragments did indeed behave like prions.
This is the first time a prion-like protein sequence has been found in plants. “We don’t know what it’s actually doing in the plant, so we are trying to be cautious,” says Lindquist. “That’s why we call it prion-like.”
The finding is “very significant”, says Frantisek Baluska at the University of Bonn, Germany, an expert on plant intelligence. “In fact, I was expecting the discovery of prions in plants.”
“Prions, we think, are responsible for some really broad, really interesting biology,” says Lindquist. “We have only seen the tip of the iceberg so far.”
Journal reference: PNAS, DOI: 10.1073/pnas.1604478113