Devangshu Datta: Planting the seeds of memory

Plants 'remember' by storing information using a type of protein called the prion, otherwise associated with neurological diseases in mammals, says a study

How do plants "remember" things at the biochemical level? This is important, as plants need to adapt reproductive patterns to previously encountered environmental conditions. Farmers and botanists have long known that plants respond to changed conditions by altering flowering and germination patterns.

A plant with memory of certain conditions will respond differently in terms of flowering, compared to a plant without that memory. Also, such changes in characteristics are inheritable. The processes by which such "memories" are stored are not well-understood however.

Now a new study (pnas.org/content/early/2016/04/20/1604478113) shows that plants "remember" by storing such information at molecular levels. Amazingly, plants may store memories by using a type of protein called the prion, which is notorious for causing neurological diseases in mammals.

The new study has been carried out by a group of scientists based at the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts. The group - which includes a lead author of Indian origin named Sohini Chakrabortee - led by Susan Lindquist was investigating prion-like proteins in plants.

Prions are oddly twisted proteins associated with terrible, neurological conditions such as Mad Cow Disease in the case of mammals. These twisted proteins can influence the shape of nearby proteins and thus lead to clusters of misshapen proteins called prion domains.

Such prions and prion domains are often found in fungi where they have been studied before. Prions have also been studied in yeasts, mice and fruit flies. It was discovered that prions are associated with the memory function even in animals and do occur in healthy nerve cells in mammals. Nobel laureate Eric Kandel has a hypothesis that prions or prion-like proteins help develop memories and that long-term memory is maintained by (healthy) prion clusters in the brain and nerves.

The Whitehead group investigated prion domains in plants using some of the techniques employed to locate prion domains in yeast. Prions can twist proteins into more complex assemblies, that is, higher-order oligomers in yeast, and these higher-order oligomers may have inheritable characteristics. One of the most-studied and best-understood of the yeast prions is called Sup35.

The group used a computational algorithm to identify where such prion domains may exist in plants. The Arabidopsis thaliana plant was investigated, and close to 500 such candidate prion domains were identified in the genome of the Arabidopsis.

Arabidopsis is the name for plants of the Brassicaceae family, which includes edible, commonly available plants such as watercress and mustard. Arabidopsis thaliana, a flowering mustard plant, was the first plant to have its genome completely sequenced. It thrives in a wide range of conditions, suggesting that it has a "good memory".

The group sifted through 20,000 proteins before it narrowed down to four prion-like domains where higher-order oligomers exist in Arabidopsis. These four were associated with the flowering process in the mustard plant.

These four were then isolated and spliced into yeast where they were used as substitutes for Sup35. It was discovered that three of these four actually behaved like prions when spliced into yeast. The group found that the so-called Luminidependens prion domain had similar characteristics to the Sup35 and seemed to perform exactly the same functions. The study also suggests that such prion-like switches are preserved in evolution and could perform a wide range of normal biological functions.

The discovered protein is called Luminidependens (LD), because it enables the responses to sunlight and heat. This is essential to the germination and flowering process and controls times when the plant blooms. As the study points out, molecular memories of winter must be stored and accessed to regulate flowering. For example, plant memory may help the plant distinguish between an unseasonal cold night and the sustained winter cold.

Although this is an important result, it is a preliminary one. There is no certainty that this mechanism operates in other plants and that the proteins responsible are prions. Also, it is unknown what other normal biological processes are controlled by similar mechanisms in this and other plants. But this study is fundamental. It opens the field up to further research from several new angles.