You should spend about 20 minutes on Questions 17–32, which are based on Reading Passage 2.
Memory in Plants
A
Monica Gagliano is an evolutionary ecologist at the University of Western Australia in Perth. In her first experiments with plant behavior, learning and memory, Gagliano decided to test her new subjects the same way she would animals. She started with habituation, the simplest form of learning. If the plants encountered the same unharmful experience over and over again, would their response to it change? At the center of the experiment was the plant Mimosa pudica, which has an unusual protective response to unfamiliar motion or touch: its leaves fold closed and so it takes on a different shape. Gagliano introduced these plants to a new experience: she dropped them. At first the mimosa plants responded by shutting their leaves tight, but as Gagliano repeated the action – seven sets of 60 drops each, all in one day – the plants’ response changed. Soon, when they were dropped, they stopped responding – their leaves no longer folded closed. However, when she prompted them with a light shake, they still shut their leaves tight. It was as if they had learned that being dropped was nothing to be worried about.
B
Three days after this training, Gagliano tested the same plants again and they still did not respond to being dropped. The experiment was repeated six days later and one month later with the same results – their leaves stayed open. Gagliano had not predicted that the plants would retain the training three days later. Whereas in studies of bees, for example, a memory that remains for 24 hours is considered long-term. Therefore, according to the rules that behavioral scientists routinely apply, Gagliano noted that the mimosa plants had demonstrated that they could learn from experience.
C
One of the most well-understood forms of plant memory is vernalization, in which plants retain an impression of a long period of cold, which helps them determine the right time to produce flowers. These plants grow tall through the fall, brace themselves during winter, and bloom in the longer days of spring – but only if they have a memory of having gone through that winter. This idea goes back to the 1920s when Trofim Lysenko, a Soviet Union scientist, discovered early in his career that by chilling seeds he could turn winter varieties of grains into spring varieties. These seeds that were normally planted in the fall and harvested in the spring could now be both planted and harvested in the same season. Although this was not described as plant memory at the time, Lysenko was, in essence, implanting a false memory of winter in plants that need a cold signal to grow.
D
It was not until the 1980s that scientists started talking explicitly about plant memory. A French team led by Mordecai Jaffe, for example, happened upon a type of memory in which a plant recalled a history of damage to a leaf on one side of its stem, and therefore dedicated its energy to growing in the other direction. Since then, scientists have found that certain plants can remember a range of experiences (like exposure to cold and heat or excess light) and modify their responses. For example, when plants experience drought and dehydration, they might retain more water when faced with the same stress again. We now know that plants are capable of much more than they’re given credit for. They can ‘sense’ vibrations, which might help them to recognize insect attacks. They share information by broadcasting chemicals through the air or from their roots. In the study of the memories they form, the next step has been to understand how they do it.
E
In recent years, plant scientists have started using the insights of molecular genetics and, in particular, epigenetics, the mechanisms that switch particular genes on and off. Richard Amasino, a professor of biochemistry at the University of Wisconsin-Madison, US, has studied the complex memory mechanism that controls vernalization and flowering in Arabidopsis thaliana, or thale cress, a plant that is a popular tool in laboratories for understanding many plant traits. This plant has a set of genes that create the proteins that cause flowers to form. Amasino explains that before vernalization, the cells are full of a second protein, named FLC, that represses those key, flower-promoting genes. But when the plant is exposed to cold, its cells slow the production of FLC until it stops, and the balance of protein power then changes. The cells start producing more and more flower-promoting proteins, until the thale cress plant is ready to burst into bloom.
F
Since the early experiments with vernalization, biologists have worked to show that certain plants in certain situations can store information about their experiences and use that information to guide them about when to bloom, and how to grow, develop, or behave. Functionally, at least, these plants appear to be creating memories. Interestingly, a group of plant scientists based in Australia and led by Peter Crisp, recently argued in the journal Science Advances that for plants, forgetting (or not forming memories at all) may be a more powerful tool for survival than memory, and that ‘memory, in particular epigenetic memory, is likely a relatively rare event.’ Crisp, the lead author of the paper, now at the University of Minnesota, US, points out that plants have incredible abilities to rebound from stressful conditions. Crisp argues that ‘having a memory, keeping track molecularly of signals that you’ve received in the past from your environment, does have a cost’. A plant that remembers too much might sacrifice healthy growth to be constantly on guard against extreme weather conditions, salt, and insects. Perhaps it is beneficial for plants to let those types of negative experiences go, instead of always preparing for the worst.