What a plant remembers

While your favorite plant might not remember your birthday, it will certainly know more about the last winter than you do.

As we already know from the previous “What a plant sees” article, plants remember the last color they have been exposed to, what then affects their behaviour. A delayed response to a previous occurrence – that’s the key component of memory. Humans, animals and plants have the exact same three steps in the mechanism of information storing. First, the forming of the memory, which is a process of encoding the information of some event. Second, the retaining of the memory, storing the event information. And third, the recalling of the memory, a retrieval of the event.

However, there are still many criteria for a division of different memory types. We can approach this unit from various viewpoints, from time period length to biological background of such process. And that is exactly what we are going to do in this article, in fact, we will look how these two factors work out together.

Short-term memory

We can show the short-term memory on an example of a venus flytrap plant. The trap starts open and waits for a mechanical stimulation by the prey. After the first stimulation, when the first hair is touched, the trap is getting a receptor potential, which leads to an electrical memory stored as an electrical charge. Over the time, the charge starts going down. If there is a second stimulation, a touch of the second hair, it causes the exact same process. However, if the total electrical memory volume has passed a threshold, it sends an electrical signal throughout the entire trap, causing a number of processes and it results in closing the trap. If the duration between the first and the second touch is too long, the electrical memory volume will be too low to reach the threshold and the trap will remain open.

Long-term memory

Plants growing in regions where the summer is short need to exploit as much time available to complete a seed production, and therefore starting this process as early as possible is crucial. On the other hand, it could be risky to grow and flower too early. This transition through winter allowing plants to flower later is called the vernalization.

The plants use signs or marks to assign to the cold they experience, similarly as human do write to-do notes. Obviously, plants do not have such physical ability to do so, and therefore they use only chemical signs. The marks are placed on the DNA or proteins which anchor the DNA. There are in fact proteins, which function is to place such marks, called writers. These marks cause particular genes to become gradually silent, changing the readout of the DNA sequence without any change of the actual DNA. This is called epigenetic silencing and it results in suppression of some genes and their functions. One of these genes is FLC (flowering local seed) gene, which inhibits flowering. In the end, as winter progress, a writer protein places marks on the FLC gene, silences it, and cancels the inhibition of flowering (a suppressor suppresses a suppressor).

What is amazing about epigenetics is that the effects are passed down from one generation to the next. The various stresses induce epigenetic changes in chromatin structure, which they then pass on to their progeny, resulting in modified behavior and readiness.

Many of the mechanisms involved in plant memory are also involved in human memory, including the epigenetics and electrochemical gradients. The plant cells contain the same proteins in our nerves which are called neuroreceptors. An example of this is the glutamate receptor, a brain protein very important for neural communication, memory formation and for learning. And plants are sensitive to the same neuro-active drugs that alter glutamate receptors in human. This actually leads to a very scary question – if plants exhibit different types of memory, are they conscious?

This is another article based on work published by Professor Daniel Chamovitz, Ph.D. and his popular science book “What a plant knows”, which also served as a base for a course with the same name on Coursera. I highly encourage you to enroll in the course since it can be very useful and interesting source to understanding basic biology of plants.