Plant memory

Experiments involving the curling of pea tendrils were some of the first to explore the concept of plant memory. Mark Jaffe recognized that pea plants coil around objects that act as support to help them grow. Jaffe's experiments included testing different stimuli to induce coiling behavior. One such stimulus was the effect of light on the coiling mechanism. When Jaffe rubbed the tendrils in light, he witnessed the expected coiling response. When subjected to perturbation in darkness, the pea plants did not exhibit coiling behavior. Tendrils from the dark experiment were brought back into light hours later, exhibiting a coiling response without any further stimulus. The pea tendrils retained the stimulus that Jaffe had provided and responded to it at a later time. Proceeding these findings, the idea of plant memory sparked interest in the scientific community. The Venus flytrap may suggest one possible mechanism for memory. Venus flytraps have many tiny hairs along the trap's surface that when touched, trigger the trap to close. But the process requires more than one hair to be touched. In the late 1980s, Dieter Hodick and Andrias Sievers proposed a model for memory retention in Venus flytraps involving calcium concentrations. Comparing the phenomenon to human action potentials, they hypothesized that the first touch of a hair leads to an increase of calcium in the cell, allowing for a temporary retention of the stimulus. If a second stimulus does not occur shortly after the initial increase of calcium, then the calcium level will not surpass a certain threshold required to trigger the trap to shut, which they likened to a memory being lost. If a second stimulus occurs quickly enough, then the calcium levels can overcome the threshold and trigger the trap to close. This demonstrated a delayed response to an initial stimulus, which could be likened to short-term memory. But when shaken by hand, the plants still curled their leaves. Gagliano observed that when young pea plants were grown in a Y-maze task where the light source came from the same direction as a fan, that when the pea plants were placed into a Y-maze task with only a fan, the pea plants grew in the direction of the fan. It appeared that the pea plants had learned to associate the fan with light. Markel cited the differing provider of the Pisum sativum seeds and brand of fan as possible variables that affected the outcome of her experiment. Markel did suggest that further verification of Gagliano's work was necessary before Gagliano's conclusions could be fully accepted. == 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. Plant Memory due to Environmental Stimuli The ability to respond to environmental stimuli is a skill that many plants possess. Exposure to mild stress is a good primer to plant memory. This memory that becomes developed from recurrent exposure better prepares a plant for future stressful factors. Plants also have a capacity for the amount of memory that they are able to possess and when they no longer are exposed to a certain factor they may "forget" what they learned to make room for new memories. Such stresses that plants remember and prime themselves for are drought, excessive light, oxidative stress, abscisic acid, and cold and warm climates. Memories are formed in plants through metabolites or through transcription factors in the plant. Changes in gene expression due to methylation and/or paused RNA pol II may also play this role in memory formation. Free calcium responds to internal and external stimuli creating an electrochemical gradient through channels and pumps in the plant. This response is thought to be stored and ingrained within the plant for future recall to similarly stressful situations. 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. and even electromagnetic radiation. 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. There is also a neuroreceptor found in plants called glutamate, glutamate functions as a neurocommunicator of memory and learning in humans. In plants, glutamate functions as a signaling molecule that responds to multiple stressors such as salinity, temperature, drought conditions, pathogens, and wound stress. Experiments conducted showed expression and activation of glutamate receptors when subjected to stress. 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. 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. 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. 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. 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 genetically 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 night time, 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. Phytochrome, a receptor which is activated by red light and inactivated by far-red light, is one of the ways that plants use to control their flowering cycle. 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 rhythms). The protein products of these genes then go on to produce actions based on the memory of the initial stimuli. Memory's molecular processes are not completely known, and several routes and interactions are thought to be involved, including these four aspects. However, more crucially, the systems outlined must be integrated in order to provide a complete knowledge of the memory process. == See also ==