SIX

What a Plant Remembers

The oaks and the pines, and their brethren of the wood, have seen so many suns rise and set, so many seasons come and go, and so many generations pass into silence, that we may well wonder what “the story of the trees” would be to us if they had tongues to tell it, or we ears fine enough to understand.

—Maud van Buren, Quotations for Special Occasions

Memories often take up a good portion of an average person’s daily mental wanderings. We may remember an especially savory feast, the games we played as children, or a particularly humorous incident at the office from the day before. We can envision a breathtaking sunset that we once saw on the beach, and we also remember significantly traumatic and scary experiences. Our memory is dependent on sensory input: a familiar smell or a favorite song can trigger a wave of detailed memory that transports us back to a particular time and place.

As we’ve seen, plants benefit from rich and varied sensory inputs as well. But plants obviously don’t have memories in the way we do: they don’t cower at the thought of a drought or dream about the sunbeams of summer. They don’t miss being encased inside a seedpod, nor do they feel anxious about premature pollen release. Unlike Grandmother Willow in Disney’s Pocahontas, old trees don’t remember the history of the people who have slept in their shade. But as we’ve seen in earlier chapters, plants clearly have the ability to retain past events and to recall this information at a later period for integration into their developmental framework: Tobacco plants know the color of the last light they saw. Willow trees know if their neighbors have been attacked by caterpillars. These examples, and many more, illustrate a delayed response to a previous occurrence, which is a key component to memory.

Mark Jaffe, the same scientist who coined the term “thigmomorphogenesis,” published one of the first reports of plant memory in 1977, though he didn’t call it as such (instead, he talked about one- to two-hour retention of the absorbed sensory information). Jaffe wanted to know what makes pea tendrils curl when they touch an object suitable to wrap themselves around. Pea tendrils are stem-like structures that grow in a straight line until they happen upon a fence or a pole they can use for support, and then they rapidly coil around the object to grab onto it.

Jaffe demonstrated that if he cut a tendril off of a pea plant but kept the excised tendril in a well-lit, moist environment, he could get it to coil simply by rubbing the bottom side of the tendril with his finger. But when he conducted the same experiment in the dark, the excised tendrils didn’t coil when he touched them, which indicated that the tendrils needed light to perform their magic twirling. But here was the interesting catch: if a tendril touched in the dark was placed in the light an hour or two later, it spontaneously coiled without Jaffe having to rub it again. Somehow, he realized, the tendril that had been touched in the dark had stored this information and recalled it once he placed it in the light. Should this storage and later recollection of information be considered “memory”?

In fact, research on human memory conducted by the renowned psychologist Endel Tulving provides us with an initial foundation from which to explore plants and their unique “recollections.” Tulving proposed that human memory exists on three levels. The lowest level, procedural memory, refers to nonverbal remembering of how to do things and is dependent on the ability to sense external stimulation (like remembering to swim when you jump in a pool). On the second level is semantic memory, the memory of concepts (like most of the subjects we learned in school). And the third level is episodic memory, which refers to remembering autobiographical events, like funny costumes from childhood Halloween parties or the loss we felt at the death of a dear pet. Episodic memory is dependent on the “self-awareness” of the individual. Plants clearly do not make the cut for semantic and episodic memory: these are the memories that define us as human beings. But plants are capable of sensing and reacting to external stimulation, so by Tulving’s definition plants should be capable of procedural memory. And indeed, Jaffe’s pea plants illustrate this. They sensed Jaffe’s touch, remembered it, and coiled in response.

Neurobiologists know quite a bit about the physiology of memories and can pinpoint the distinct (but still interconnected) areas of the brain that are responsible for different types of memory. Scientists know that electric signaling between neurons is essential for memory formation and storage. But we know much less about the molecular and cellular basis of memory. What’s fascinating is that the latest research hints that while memories are infinite, only a very small number of proteins are involved in memory maintenance.

We need to be aware, of course, that what we refer to as “memory” for people is actually a term that encompasses many distinct forms of memory, beyond the ones described by Tulving. We have sensory memory, which receives and filters rapid input from the senses (in a blink of an eye); short-term memory, which can hold up to about seven objects in our consciousness for several seconds; and long-term memory, which refers to our ability to store memories for as long as a lifetime. We also have muscle-motor memory, a type of procedural memory that is an unconscious process of learning movements such as moving fingers to tie a shoelace; and immune memory, which is when our immune systems remember past infections in order to avoid future ones. All but the last are dependent on brain functions. Immune memory is dependent on the workings of our white blood cells and antibodies.

What’s common to all forms of memory is that they include the processes of forming the memory (encoding information), retaining the memory (information storage), and recalling the memory (retrieval of the information). Even computer memory employs exactly these three processes. If we’re going to look for the existence of even the simplest memories in plants, these are the processes we need to see happening.

The Short-Term Memory of the Venus Flytrap

As we saw back in chapter three, the Venus flytrap needs to know when an ideal meal is crawling across its leaves. Closing its trap requires a huge expense of energy, and reopening the trap can take several hours, so Dionaea only wants to spring closed when it’s sure that the dawdling insect visiting its surface is large enough to be worth its time. The large black hairs on their lobes allow the Venus flytraps to literally feel their prey, and they act as triggers that spring the trap closed when the proper prey makes its way across the trap. If the insect touches just one hair, the trap will not spring shut; but a large enough bug will likely touch two hairs within about twenty seconds, and that signal springs the Venus flytrap into action.

We can look at this system as analogous to short-term memory. First, the flytrap encodes the information (forms the memory) that something (it doesn’t know what) has touched one of its hairs. Then it stores this information for a number of seconds (retains the memory) and finally retrieves this information (recalls the memory) once a second hair is touched. If a small ant takes a while to get from one hair to the next, the trap will have forgotten the first touch by the time the ant brushes up against the next hair. In other words, it loses the storage of the information, doesn’t close, and the ant happily meanders on. How does the plant encode and store the information from the unassuming bug’s encounter with the first hair? How does it remember the first touch in order to react upon the second?

Scientists have been puzzled by these questions ever since John Burdon-Sanderson’s early report on the physiology of the Venus flytrap in 1882. A century later, Dieter Hodick and Andreas Sievers at the University of Bonn in Germany proposed that the flytrap stored information regarding how many hairs have been touched in the electric charge of its leaf. Their model is quite elegant in its simplicity. In their studies, they discovered that touching a trigger hair on the Venus flytrap causes an electric action potential that induces calcium channels to open in the trap (this coupling of action potentials and the opening of calcium channels is similar to the processes that occur during communication between human neurons), thus causing a rapid increase in the concentration of calcium ions.

They proposed that the trap requires a relatively high concentration of calcium in order to close and that a single action potential from just one trigger hair being touched does not reach this level. Therefore, a second hair needs to be stimulated to push the calcium concentration over this threshold and spring the trap. The encoding of the information is in the initial rise in calcium levels. The retention of the information requires maintaining a high enough level of calcium so that a second increase (triggered by touching the second hair) pushes the total concentration of calcium over the threshold. As the calcium ion concentrations dissipate over time, if the second touch and potential don’t happen quickly, the final concentration after the second trigger won’t be high enough to close the trap, and the memory is lost.

Subsequent research supports this model. Alexander Volkov and his colleagues at Oakwood University in Alabama first demonstrated that it is indeed electricity that causes the Venus flytrap to close. To test the model they rigged up very fine electrodes and applied an electrical current to the open lobes of the trap. This made the trap close without any direct touch to its trigger hairs (while they didn’t measure calcium levels, the current likely led to increases). When they modified this experiment by altering the amount of electrical current, Volkov could determine the exact electrical charge needed for the trap to close. As long as fourteen microcoulombs—a tiny bit more than the static electricity generated by rubbing two balloons together—flowed between the two electrodes, the trap closed. This could come as one large burst or as a series of smaller charges within twenty seconds. If it took longer than twenty seconds to accumulate the total charge, the trap would remain open.

Here, then, lies the proposed mechanism of the short-term memory in the Venus flytrap. The first touch of a hair activates an electric potential that radiates from cell to cell. This electric charge is stored as an increase in ion concentrations for a short time until it dissipates within about twenty seconds. But if a second action potential reaches the midrib within this time, the cumulative charge and ion concentrations pass the threshold and the trap closes. If too much time elapses between action potentials, then the plant forgets the first one, and the trap stays open.

This electric signal in the Venus flytrap (and the electric signals in other plants for that matter) are similar to the electric signals in neurons in humans and indeed all animals. The signal in both neurons and Dionaea leaves can be inhibited by drugs that block the ion channels which open in the membranes as the electric signal passes through the cell. When Volkov pretreated his plants with a chemical that inhibits potassium channels in human neurons, for example, the traps didn’t close when they were touched or when they received the electric charges.

Long-Term Memory of Trauma

In the mid-twentieth century, some rather obscure work was carried out by the Czech botanist Rudolf Dostál, who studied what he termed “morphogenetic memory” in plants. Morphogenetic memory is a type of memory that later influences the shape or form of the plant. In other words, a plant can experience a stimulus at some point, like a rip in its leaf or a fracture of a branch, and be unaffected by it at first, but when environmental conditions change, the plant may remember the past experience and respond by changing its growth.

This illustration shows three flax (Linum usitatissimum) seedlings. The image on the left shows a two-week-old seedling with two cotyledons and an apical bud (the small bump between the two leaves). The middle picture shows a similar seedling but after the apical bud has been decapitated and the two lateral buds have been growing for about a week. The picture on the right shows a seedling with the left cotyledon removed prior to decapitating the apical bud.

Dostál’s experiments on flax seedlings illustrate what he meant by morphogenetic memory. To fully appreciate Dostál’s experiments in this area, we have to understand a bit more about plant anatomy. Flax seedlings emerge from the ground with two large leaves called cotyledons. In the center of the two cotyledons is what’s called an apical bud, which grows up from the central stem of the plant. As this bud grows up, two lateral buds emerge below it on each side, each facing toward one leaf. The lateral buds are dormant—they don’t grow—under normal conditions. However, if the apical bud is damaged or cut off, then the lateral ones will start to grow and extend, and each one forms a new branch where each lateral bud becomes the apical bud. This repression of the lateral buds by the apical bud is called apical dominance, and releasing this repression is the basis of plant pruning. When you see a gardener pruning the hedges in front of a house, he is actually—if he’s pruning correctly—removing the apical buds from each branch, encouraging more lateral buds, and new branches, to grow.

Flax (Linum usitatissimum)

Under normal conditions, if the apical bud is pruned off, both lateral buds grow evenly. But Dostál noticed that if he removed one of the cotyledons prior to decapitation of the apical bud, the only lateral bud that would grow was the one near the remaining leaf. This result may seem like a classic case of a stimulus followed by a response. But here’s where things get really interesting. When Dostál repeated the experiment and illuminated the plant with red light, the lateral bud closest to the absent cotyledon grew, which revealed that each bud retains the potential to grow.

Dostál’s research was picked up by Michel Thellier at the University of Rouen in upper Normandy. Thellier, a member of the French Academy of Sciences, noticed that after he decapitated the apical bud on his plant of choice, Bidens pilosa (also known as Spanish needle), both lateral buds started to grow more or less evenly. But if he simply wounded one of the cotyledons, then only the lateral bud closest to the healthy leaf would grow. Thellier didn’t have to mangle the cotyledon to get the response; he would just prick the leaf four times with a needle at the same time as the decapitation, and this minor wound was enough to get asymmetrical growth of the lateral buds.

So where does plant memory come in in what appears to be another classic stimulus-response phenomenon? Well, sometimes during these experiments Thellier would extend the amount of time between wounding the leaf and decapitating the main bud—even up to two weeks. And, lo and behold, the lateral bud farthest from the pricked cotyledon would grow out, and not both lateral buds. Thellier knew there had to be some way that the Spanish needle stored this “traumatic” experience and had a mechanism for recalling it once the central bud was removed, even if that happened many days later.

Spanish needle (Bidens pilosa)

The following experiment really sealed the idea that the bud of the Spanish needle remembered which of its neighboring leaves had been damaged. This time, Thellier stabbed one of the cotyledons as he had before, but then he removed both cotyledons several minutes later. He found that the plant retained the memory of the stabbing: once the central bud was removed, the lateral bud opposite the original wounded cotyledon grew more than the one on the side of the wounded cotyledon. The jury is still out on how this information is stored in the central bud, but one promising option is that the signal is somehow connected to auxin—the same hormone that we met in chapter five.

The Big Chill

Trofim Denisovich Lysenko was notorious for his impact on science in the Soviet Union. He rejected classic Mendelian genetics (based on the principle that all characteristics are the result of inherited genes) while championing the idea that the environment leads to the development of adaptive characteristics (such as blindness in moles living in constant darkness) that can be passed on to successive generations. This evolutionary theory, originally put forward by the noted French naturalist Jean-Baptiste Lamarck in the early nineteenth century, fit perfectly with the prevailing ideology at the time of Lysenko’s research, which held that the proletariat could be modified by the environment. The Soviet establishment was so enamored of Lysenko that from 1948 to 1964 it was illegal in the Soviet Union to express any dissent from his theories. Politics aside, Lysenko made a landmark discovery in 1928 that influences plant biology to this day.

Soviet farmers grow what is called winter wheat—wheat that is planted in the fall, sprouts before freezing temperatures in the winter, and becomes dormant until the soil warms in the spring, when it flowers. Winter wheat isn’t able to flower and subsequently produce grain in the spring unless it experiences a period of cold weather in the winter. The late 1920s were disastrous for Soviet agriculture because unusually warm winters destroyed most of the winter wheat seedlings—seedlings that farmers relied on to produce the grain that would feed millions.

Lysenko worked nonstop in an effort to save what little harvest they would have and to find ways of ensuring that warm winters wouldn’t lead to famine in the future. He discovered that if he took winter wheat seeds and put them in a freezer before planting them, he could induce the seeds to sprout and flower without having actually endured a prolonged winter. In this way, he enabled farmers to plant wheat in the springtime, and he ultimately saved wheat yields in his country. Lysenko called the process “vernalization,” which has been embraced now as the general term for any cold treatment, be it natural or artificial.

Wheat (Triticum aestivum)

Other scientists also knew that some plants needed cold weather in order to flower (one of the first reports came out of the Ohio Board of Agriculture in 1857), but Lysenko was the first to show that the process could be artificially manipulated. Many plants rely on the cold temperatures of winter to cultivate their harvest; many fruit trees will only flower and set fruit following a cold winter, and lettuce and arabidopsis seeds only germinate following a cold snap. The ecological advantage of vernalization is clear: it ensures that following the cold of winter, a plant will sprout or flower in the spring or summer, and not during other times of the year when the amount of light and temperature could also support plant growth.

For example, the cherry trees in Washington, D.C., usually have their first bloom of the year around April 1, when there are about twelve hours of daylight. Washington, D.C., also sees approximately twelve hours of sunlight in mid-September, but the same cherry trees never bloom in the fall; if they did, their fruit would never fully develop as it would soon freeze in the approaching winter. Blooming in the early spring, the cherry blossoms are able to give their fruit an entire five months to mature. Although the day’s length is exactly the same in April and September, the trees are able to differentiate between the two. They know it’s April because they remember the preceding winter.

The basis for a wheat seedling or cherry tree remembering winter has only been elucidated over the past decade or so, primarily through research involving the tried-and-true arabidopsis. Arabidopsis grows naturally in a wide variety of natural habitats, from northern Norway to the Canary Islands. The different populations of Arabidopsis thaliana are called ecotypes. Arabidopsis ecotypes that grow in northern climates need vernalization to flower, while those that grow in warmer climates do just fine without it. This need for vernalization is encoded in the genes of the northern ecotypes. If you cross a plant that needs winter in order to flower with a plant that doesn’t, the offspring still need a cold snap in order to flower; genetically, the need for cold is a dominant trait (just as brown eyes is a dominant trait relative to blue eyes in people). The specific gene involved is FLC, which stands for flowering locus C. In its dominant version, FLC inhibits flowering until the plant has undergone a vernalization period.

Once the plant goes through a period of cold weather, the FLC gene is no longer transcribed; the gene is turned off. But that doesn’t mean the plants will immediately start to flower; it only means that the plants could flower if other conditions, such as light and temperature, cooperate. So the plant must have a way of remembering that it once experienced a cold climate to keep FLC turned off, even though temperatures have since warmed up.

Many researchers have tried to understand just how vernalization turns off FLC and how it stays this way once it’s turned off. These investigations have highlighted how epigenetics is intertwined with a plant’s memory of winter. Epigenetics refers to changes in gene activity that don’t require alterations in the DNA code, as mutations do, yet these changes in gene activity are still passed down from parent to offspring.* In many cases, epigenetics works through changes in the structure of the DNA.

(* Epigenetics encompasses a wide range of heritable changes that are independent of DNA sequence. These include chemical changes in histones, chemical changes of the DNA (for example, DNA methylation—see p. 128), different types of small RNAs, and infectious proteins known as prions.)

In cells, DNA is organized in chromosomes, which are much more than simple strings of nucleotides. The double helix of DNA wraps around proteins called histones, forming what is known as chromatin. This chromatin can twist even more, just like an overly twisted rubber band, compacting the DNA and proteins into highly condensed and packed structures. These structures are dynamic: different parts of the chromatin can unravel or pack up again. Active genes (those that are transcribed) are found in areas of the chromatin that are unraveled, while inactive genes lie in regions that are more condensed.*

(* A major differentiator of cell types, like blood cells versus liver cells in people, or pollen versus leaf cells in plants, is the structure of their chromatin, which affects which genes are activated.)

The histone proteins are one of the key factors that determine how tightly knit the chromatin gets, and this is very important for understanding how FLC is activated. Scientists have discovered that cold treatment triggers a change in the structure of the histones (a process called methylation) around the FLC gene, which enables the chromatin to be tightly packed. This turns off FLC, and the plant is able to flower. This epigenetic change (the type of histone around the gene) is passed down from parent to daughter cells over successive generations, and the FLC gene remains inactive in all cells even after the cold weather subsides. Once the FLC gene has been turned off, the plants can wait until the rest of the environmental conditions are ideal for flowering. In perennial plants like oak trees and azaleas, which flower once per year, the FLC gene has to be reactivated once the plant has blossomed to inhibit promiscuous flowering that might occur out of season until the next winter has passed. This involves the cells reprogramming their histone code, which opens the chromatin around the FLC gene, reactivating it. How this occurs, and how it’s regulated, are matters of current research.

This epigenetic mechanism of cellular memory is not specific to plants and is the basis of a great many biological processes and diseases. Epigenetics has caused a paradigm shift in biology because it goes against the classic genetic concept that the only changes that can be passed on from cell to cell are those in the DNA sequence. What’s truly amazing about epigenetics is that it facilitates memory not only from season to season within a single organism but from generation to generation.

In Every Generation . . .

Memories are actively handed down from one generation to the next through rituals, storytelling, and more. But transgenerational memory involving epigenetics is completely different. This type of memory usually involves information about an environmental or physical stress that’s passed down from parents to offspring. Barbara Hohn’s laboratory in Basel, Switzerland, was the first to provide evidence for such transgenerational memory. Hohn and her colleagues knew that conditions that create stress on a plant, like ultraviolet light or pathogen attack, lead to changes in the plant genome that result in new combinations of DNA.

These stress-induced changes make sense ecologically because—like any other organism—a plant needs to find ways to survive under stress. One of the ways a plant does this is through new genetic variations. Hohn’s astounding study showed that not only do the stressed plants make new combinations of DNA but their offspring also make the new combinations, even though they themselves had never been directly exposed to any stress. The stress in the parents caused a stable heritable change that was passed on to all their offspring: the plants behaved as if they’d been stressed. They remembered that their parents had been through this stress and reacted similarly.

This use of the word “remembered” may seem unorthodox, but let’s analyze this in light of the three steps of memory we encountered at the beginning of this chapter: the parents formed the memory of the stress, retained it, and passed it on to their children, and the children recalled the information and reacted accordingly, in this case, with increased genomic changes.

The implications of this study are vast. An environmental stress causes a heritable change that is passed on to successive generations. This fits excellently with the theories of Jean-Baptiste Lamarck, who, as you may recall, claimed that evolution was based on the inheritance of acquired characteristics. Hohn’s plants, following the UV or pathogen stress, acquired the characteristic of increased genetic variation and passed it on to all of their progeny (and a single arabidopsis plant produces thousands of seeds!). This cannot be explained by mutations in the DNA sequence of the stressed plants, because this could at most be passed on to only a very small percentage of the progeny. On the other hand, if the stress induced an epigenetic change, this could happen in all the cells at once, including pollen and egg cells, and be passed on to the entire next generation, as well as many future ones. While scientists speculate as to the nature of the epigenetic change involved in these memories, it remains undiscovered.

Igor Kovalchuk created a follow-up study in which he included other stresses on genetic variation in plants and their progeny including heat and salt. He showed that these different environmental insults increase the frequency of genomic rearrangements not only in the parental generation but also in the second generation. Kovalchuk’s results were fascinating because they revealed even more than this. Not only did the second generation of plants show increased genetic variation, confirming Hohn’s results, but they were also more tolerant to the various stresses. In other words, stressed parents gave rise to offspring that grew better under harsh conditions compared with regular plants. The various stresses almost certainly induce epigenetic changes in chromatin structure in the parents, which they pass on to their progeny. We believe this because Kovalchuk’s group showed that if they treated the offspring with a chemical that wiped out epigenetic information, these same plants lost their ability to thrive under the environmental stress. Hohn’s results were not universally accepted, as is the case with many paradigm-shifting studies in science. The growing consensus, however, is that her results, as well as others, have heralded a new era in genetics. The idea of stress leading to memories that are passed down from one generation to the next is supported by an increasing number of studies, not only in plants, but in animals as well. In all cases, this “memory” is based on some form of epigenetic heredity.

Intelligent Memory?

Plants clearly have the ability to store and recall biological information. Intuitively, we know that this is quite different from the detailed and emotion-filled memories we recall every day. But at a basic level, the behaviors of different plants described in this chapter are remedial types of memory. The tendril’s coiling, the Venus flytrap’s closing, and the arabidopsis’s remembering environmental stress all include the processes of forming the memory of the event, retaining the memory for distinct time periods, and recalling the memory at a later point in order to get a specific developmental response.

Many of the mechanisms involved in plant memory are also involved in human memory, including epigenetics and electrochemical gradients. These gradients are the bread and butter of neural connections in our brains, the seat of memory as most of us understand it. Over the past several years, plant scientists have discovered that not only do plant cells communicate with electrical currents (as we saw in several chapters) but plants also contain proteins known in humans and other animals as neuroreceptors. A perfect example is the glutamate receptor. Glutamate receptors in the brain are very important for neural communication, memory formation, and learning, and a number of neuroactive drugs target glutamate receptors. It was a great surprise, then, for scientists at New York University to discover that plants contain glutamate receptors and that arabidopsis plants are sensitive to neuroactive drugs that alter glutamate receptor activity. At this point we still don’t fully understand what glutamate receptors do in plants, but very recent work carried out by José Feijó and his group in Portugal shows that these receptors in plants function in cell-to-cell signaling in a way that’s very similar to how human neurons communicate with each other. This leaves us to marvel at the evolutionary role of “brain receptors” in plants. Perhaps the similarities between human brain function and plant physiology may be greater than we’ve assumed.

Plant memories, like human immune memory, are not semantic or episodic memories, as Tulving defined them, but rather procedural memories, memories of how to do things; these memories depend on the ability to sense external stimulation. Tulving further proposed that each of the three levels of memory is associated with an increasing level of consciousness. Procedural memory is associated with anoetic consciousness, semantic memory is associated with noetic consciousness, and episodic memory is associated with autonoetic consciousness. Plants clearly do not fit the definition of consciousness associated with semantic or episodic memories. But as stated in a recent opinion article, “The lowest level of consciousness characteristic for procedural memory—anoetic consciousness—refers to the ability of organisms to sense and to react to external and internal stimulation, which all plants and simple animals are capable of.” This leads us to the most intriguing question of all: If plants exhibit different types of memory and have a form of consciousness, should they be considered intelligent?