Plant memory

Physiology of Plant Memory

The physiology of plant memory is documented in many studies and is understood to have four main physiological mechanisms that work together in synchrony to provide the plant with basic memory functions, and are thought to be precursors to advanced memory functions found in animals. These four mechanisms are the storing and recalling, habituation, gene priming or epigenetics, and the biological clock.

Storage and Recall

The storage and recall method of memory occurs when a plant, in response to a stimuli, reduces or increases the concentration of a chemical in certain tissues, and maintains this concentration for a certain period of time. The plant then uses this concentration of chemical as a signal for a recall response. Stimuli known to create a store and recall responses like this are touch, damage, temperature, and even electromagnetic radiation. It is suspected that Ca2+ signalling plays a key role in this form of plant memory. A proposed mechanism of this is that the presence or absence of Ca2+ acts as a long term on/off switch for cellular processes in response to stimuli for storing genes. Ca2+ along with electrical signalling, is also integral as a signalling pathway for plants to transmit signals of the original stimulus between cells or tissues throughout the plant. An example of short term electrical memory store and recall function can be seen in the trap mechanism of the Venus flytrap. When one hair on the trap is touched, an electrical is generated and retained for 20 seconds. The trap requires that at least one more hair is brushed within this 20 second period in order to reach the charge threshold required to close the trap. Electrical signaling from cell to cell in plants is controlled by proteins in the cell membrane. Protein Memristors are biological resistor proteins that can depend on the electrical history of the cell, and are a class of protein that are shared between plants and animals in electrical memory function.

Habituation

The process of habituation in plants is very similar to the store and recall function, but lacks the recall action. In this case, information is stored and used to acclimate the plant to the original stimulus. A great example of this is research done on mimosa plants by  and their leave’s acclimated response to being dropped by Gagliano et. al.. In this study the plants initially reacted to being dropped by closing their leaves, but after the stimulus had been experienced a number of times the plants no longer responded to being dropped by closing their leaves.

Epigenetic Memory

The third aspect of plant memory is Epigenetics, where the plant, in response to a stimulus, undergoes Histone and Chromatin modification leading to changes in gene expression. These changes lead to a subsequent change in what proteins are made by the plant and establish a way for the plant to respond or be affected by stimuli from past experiences. These experiences can be passed down heritiarily from parent plant to offspring, giving an even longer term memory of a stimulus such as a stressor or other environmental stimuli. It is important to note that these changes are different from genetic changes because they can be reversed in response to new stimuli or environmental conditions.

Biological Clocks

Plants use biological clocks to perform certain actions at times they will be most effective. The two most well documented biological clocks in plants are the day and seasonal cycles which are usually established by Photoreceptors . Once a plant has established a pattern of light, they can effectively memorize nighttime, daytime, or longer periods like seasons. A clear example of this can be seen in the ability of plants to over winter, cease leaf growth and then activate leaf growth in the spring when environmental conditions favor growth. These cycles, or Circadian rhythms are controlled by genes associated with different spatial times that are activated when an environmental cue for that time is present. These genes control what proteins are made at certain times, as well as electrical and chemical signals that are produced to control motor proteins and other proteins. The overall result of these processes are subsequent changes in how the plant functions.

Summary

The combination of these four mechanisms of plant memory are proposed to work together to form different functions of memory in a plant. The overall proposed mechanism of this memory is a signal or environmental cues lead to a signal (Chemical concentration, Calcium waves, electrical, Small RNAs, or Phytohormones), and this eventually leads to the activation or deactivation of memory associated genes (store and recall, epigenetics, habituation, or circadian rythms). The protein products of these genes then go on to produce actions based on the memory of the initial stimuli. The production and actions of these proteins to a past stimuli is the core of observable plant memory in action. Much is still unknown as to how these four aspects work together, and research as to how they interact should be pursued.