Plant memory

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Plant memory is the ability of a plant to store information from previously experienced stimuli. While memory is a word often used in an human-centric light, this basic definition can be extended to many other organisms that exhibit delayed responses to a stimulus, including plants. Many examples can be seen across nature like from plants timing their leaves to rise in synchrony with the sun rising, to producing new leaves in the spring after overwintering. Plant memory is different from human and animal memory in that it does not involve the storage of memory in a brain structure, but does function similarly by using experiences to benefit the organism's survival later in life.  Human memory is often thought to be a very complex process mechanistically, but very little is actually known about how memories are created and stored. When discussing human memory, there are three important categories to consider. This first type of memory is sensory, which involves rapid input of stimulus around a person. Sensory memory is constant, but fleeting.  This would include sounds, light, smells, and other basic stimuli. The next type of memory in humans is short term memory, which is maintained much longer than sensory memory. Another category of human memory is long term. This includes the ability to recall life moments weeks, months, and even years after an experience. Each of these types of human memory can be broken down to a basic process of retention and response. This pattern of processes can also be observed in plants. The most basic learning and memory functions in animals have been recorded in plants, and it is proposed that the development of these basic memory mechanisms were developed by an early organismal ancestor. Plants not only have developed conserved ways to use functioning memory, but some species have also developed unique ways to use memory function depending on their environment and life history. In this way, one can see that there is another similarity between the evolution of plant memory and the evolution of animal memory among the different species of each. The use of the term plant memory sparks some controversy in the field of plant biology, as some researchers believe this function only applies to organisms with a functioning brain. Some researchers believe that comparing plant functions that resemble memory in humans and other higher division organisms may be too direct of a comparison, while others believe that since the function of the two are essentially the same this comparison can act as a basis of knowledge for further understanding how these functions in plants work. In order to better define the parallels and differences of these functions in both plants and animals, more studies are necessary.

History[]

One of the first experiments to delve into the idea of plant memory was not intended to do so at all. Scientist Mark Jaffe was conducting an experiment to get to the bottom of the mechanism behind the curling of pea plant tendrils. Jaffe knew that pea plants would coil around objects that would give them support and help them grow. In his experiments, Jaffe rubbed one side of a pea plant tendril to mimic a support object and to see if the contact would induce the coiling behavior.^[1] He also decided to test the effects of light on this process and that is when he stumbled upon an example of plant memory. When Jaffe rubbed the tendrils in light he witnessed the expected coiling response. Interestingly, when he did the same procedure in the dark, the pea plant tendrils no longer responded to the physical contact. When the tendrils from the dark experiment were brought back into light hours later, they exhibited a coiling response without any further stimulus.^[1] This demonstrated that somehow the pea tendrils retained the stimulus that Jaffe had provided and responded to it at a later time. After this discovery, many scientists wanted to further pursue how exactly plants could retain information.

The Venus flytrap suggests one possible mechanism. The carnivorous plant has many tiny hairs along its trap’s surface. When these hairs are touched, the response of shutting the trap is triggered. One important caveat to this process is that when just one hair is touched the response is not triggered. This made many scientists, pause and wonder what mechanism was behind reaching the threshold for closing the trap. They wondered how the plant retained how many stimuli had occurred. Dieter Hodick and Andrias Sievers wanted to get to the bottom of this in the late 1980s. They proposed a model by which the memory of the stimulus was retained through calcium concentrations.^[2] Similar to human action potentials, they hypothesized that the first stimulus led to an increase of calcium. These levels are retained, allowing memory. If the second stimulus takes too long to occur, then the combined calcium levels will not surpass the threshold required to trigger the trap shutting. In this way, the memory would be lost. If the second stimulus occurred quickly enough, then the calcium levels would breach the threshold and trigger the trap to close. This demonstrated delayed response to the stimulus.^[2] The maintenance of the initial trigger through calcium levels is an example of a mechanism by which short-term memory can occur in plants. Even though this is not what many would consider human memory, the fundamentals of what have occurred are the same. The storage of the input, whether it be a smell to a human or a touch to a plant, is fundamentally the same in terms of the previously discussed definition of basic memory.

As this field of study moves forward, some new questions regarding plants’ abilities to retain information have arisen. One such question surrounds the length of plant memory. While evidence supported short term retention of signals, some scientists wanted to see if plants could retain information for longer periods of time similar to the way that humans can. Monica Gagliano’s research is often at the forefront of this discussion. Gagliano performed experiments from which she observed plants retaining memory for up to a month. Gagliano used the mimosa pudica plant for her experiment. This plant is very sensitive and will curl up its leaves in response to touch or shaking. Gagliano performed an experiment in which these plants were dropped sixty times. She observed that over time, the response of leaf curling when dropped decreased in her plants.^[3] She controlled for the plants simply being worn out by shaking them after the dropping experiments, which elicited the defense response of curling up their leaves.^[3] This showed that the plants still had the energy necessary to respond, but that they remembered that the dropping stimulus didn’t require expending it. This first part of her experiment demonstrates short term memory in the plants. At the very least, this part of the experiment shows that from the time of one drop to the next, the plants retained that the drop was not a threat She concluded that the plants were able to remember from the previous responses that the dropping was not dangerous. Gagliano went further with this experiment and wanted to see how long the plant would retain the information for. In order to test this, she waited a month and then repeated the dropping experiment again and observed that the plants had retained the memory of not needing a defense response when provided the dropping stimulus.^[3] This research set a new precedent for the length of plant memory. Originally thought to only occur for short durations, Gagliano’s work supports the need for further research into the extent of time over which plants can retain information.

Another question is whether the nature of plant memory is ‘intelligent’. When thinking of human memory, it is often moments of importance that then stick long term. Making a human memory involves some levels of deciding what information is important to keep and what information will not be as helpful in the future. Such evaluations are part of what is considered intelligent memory. Further progress in this field of study will likely focus on whether or not the nature of plant memory is considered intelligent, which presents the issue of how intelligence itself is defined.

Physiology[]

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.^[4]

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.^[4] Stimuli known to create a store and recall responses like this are touch, damage, temperature, drought,^[5][6] and even electromagnetic radiation.^[4] It is suspected that Ca2+ signalling plays a key role in this form of plant memory.^[4] 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.^[4] 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.^[7] 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.^[8]

Trauma Reaction Example:

Long-term trauma memory in plants has been an area of interest for several years because of its potential to understand other types of memory. In the mid-twentieth century, Rudolf Dostal and Michel Tellier conducted a set of experiments which revealed interesting results. Under normal conditions, the decapitation of the apical bud of a plant leads to symmetric growth of the lateral buds. However, Dostal and Thellier found that removing the cotyledon on one side of the plant, or simply wounding it, resulted in asymmetrical growth towards the healthier side of the plant.^[9] This trauma memory hypothesis was solidified when Thellier showed that past damage can be remembered by the plant even after removing both cotyledons, suggesting that trauma memory is stored in the bud. Dostal and Thellier were pioneers in understanding trauma memory in plants but the physiological and molecular processes involved are still unknown. In this article, we propose several potential mechanisms that could explain how information about past trauma is stored in the bud.

One proposed mechanism for plant memory storage in the bud is the relative rise in Ca2+ concentration within the cytosol of plant cells via calcium waves.^[4] The cytoplasm of plant cells has a relatively low concentration of Ca2+, but the cell has stores of Ca2+ throughout. These stores of Ca2+ are usually membrane bound organelles that have inositol-3-phosphate (IP3) dependent Ca2+ channels within their membranes. These channels only open when they are bound to IP3, a molecule that is produced through intracellular processes governed by the enzyme phospholipase C. Some of the Ca2+ channels are not IP3 dependent and open in response to other stimuli like electrical gradients or the stretching of a membrane. Once a CA2+ channel (IP3 gated or not) opens, it can briefly stimulate the opening of adjacent channels. This allows for a wave of calcium to be released from these Ca2+ stores into the cytoplasm of the plant cell. Calcium will flow down its concentration gradient, from an area of high Ca2+ concentration to an area of low Ca2+ concentration, through the now open Ca2+ channels. Because (1) there are many forms of Ca2+ channels on these storage organelles (2) these channels respond to different stimuli and (3) depend on the relative concentration gradients of these stimuli, the kinetics and magnitude of calcium waves are specific to the stimuli that triggered them. The respective elevation in the concentration of Ca2+ in the cytoplasm “stores” the memory of the stimuli. This proposed mechanism is a form of plant memory storage.

Another potential mechanism involves electrical signals and calcium. When Ca2+ floods the cytoplasm it allows the plant to store a memory, the duration and amplitude of this Ca2+ wave is determined by the type of stimulus that was perceived and how the plant will store the memory.^[4] Electric signals are induced by variation potentials which are stimulated by an injury such as heat or a cut. There are also action potentials but these are stimulated by non-damaging stimuli such as temperature. The variation potential travels through the xylem and is regulated by both hydraulic pressure and system potentials, such as the Ca2+ flux. Two methods by which a signal may travel over a short distance is by propagating over the cell membrane through the plasmodesmata, or secondly the current of one cell membrane may depolarize a neighboring cell membrane without having to be in direct contact. But for memory purposes the electric signal needs to travel over a longer distance, sieve tubes are used since they have pores and a continuous plasma membrane which makes sieve tubes low resistance. With a trauma the xylem can have a change in hydrostatic pressure which leads to turgor changes in the neighboring parenchyma cells and then via mechano-sensors membrane potential changes. Sheath cells protect the signal when traveling from the mesophyll to the phloem which contains the sieve tubes. Although there are many suggested methods by which long distance electric signals may travel, the exact ion channels that are used are still unknown, but recently GLR genes have been proposed to mediate wound stimulus through cation channels.^[10] Another possible method on storage memory and recall is the hormone auxin and how it reacts to a trauma.

While there are many hormones that dictate major plant processes and eventual changes in physiology, auxin remains the most prevalent hormone. Auxin serves to increase cell length, stimulated by light or gravity in processes known as phototropism and gravitropism, respectively. In terms of plant memory, Auxin may act as the mechanism as to which plants respond to stimuli previously encountered. Auxin moves to different sides of the cell, depending on the particular cell type and process initiated.^[11] In phototropism, auxin is transported to the side of the plant shaded. This is accomplished through the PIN transport proteins. PIN proteins act as a conduit for auxin, allowing auxin to flow between cells. As auxin accumulates on the shaded side of the plant, the hormone promotes cell elongation in the cells. Auxin does this by stimulating the expansibility of cell walls. This allows cells to expand and elongate, making the plant bend in one direction. This also occurs in the roots, under a different stimulus. Auxin also plays a role in regulating gene expression. The genes that are regulated are correlated with cell expansion biochemistry and physiology. In What a Plant Knows, David Chamovitz describes an experiment in which they test a plants long term memory regarding past trauma.^[9] While the experiment (stated above) concluded that plants can store trauma memory, the exact mechanism is unknown. Chamovitz, along with the original perpetrators of the experiment Rudolf Dostal and Michel Tellier, postulate that Auxin may play a pivotal role in trauma memory because of the hormone's role in regulation of growth through multiple mechanisms including long term gene expression.

Trauma memory is a great example illustrating the physiology of memory storage and recall in plants. While the mechanisms responsible for it have yet to be determined, our current understanding of plant physiology allows us to propose three pathways to explore: Ca2+ flux, electrical signaling, and auxin stimulation. Long-term trauma memory is only one of the many distinct types of memory in plants and understanding its physiological and molecular mechanisms could reveal discoveries pertinent to other memory processes such as immune memory or musclemotor memory.

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..^[12] 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.^[8] 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.^[8] 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, , 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 rhythms).^[4] 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.

See also[]

• Plant cognition

References[]