WHAT A PLANT KNOWS (COMPLETE)

Chamovitz, Daniel. What a Plant Knows: A Field Guide to the Senses. New York: Farrar, Straus and Giroux, 2012.

This book is 228 pages in length. As noted in the prologue, the author was motivated to take on the study of parallels between plants and humans in terms of how they respond to light, among other things, after discovering that both organisms share genes that regulate responses to light. The unique aspect of plants is that, unlike animals, they are immobile and unable to choose their environments and therefore must develop adaptations in terms of complex sensory and regulatory systems to survive and thrive where they are.  As he noted in the prologue, “a head of lettuce has to know if there are ravenous aphids abut to eat it up so that it can protect itself by making poisonous chemicals to kill the pests”. In each chapter, the author uses latest research information to describe how plants demonstrate “senses” that allow them to take in light, chemical, and temperature information and adjust and respond to their environment.

In the first chapter, the author describes observations and experiments showing how plants responds to light and that they contain phytochromes that allow them to distinguish different regulatory responses depending on the color of light, even to the level of far red light versus just red light.  In Chapter 2, the focus is on the ability of plants to “smell” or detect chemical vapors and use it as a regulatory signal in producing chemicals that deter insect attacks for instance. Controlled experiments show that parts of plants from a beetle-infested leaf as well as neighboring plants use a chemical emitted by affected leaves as a signal to make nectar that attracts arthropods that eat beetles. In Chapter 3, the author shows how plants can detect touch or mechanical stimuli and describes how they respond to it.  The extent of biological response to mechanical stimuli between plants and animals is quite different at the organism level but similar at the cellular level. In both cases, the stimuli triggers an electric signal that travels from cell to cell and results in the coordinated functions of calcium and potassium ion channels and gene transcription for certain proteins. However, because animals have brain that provide another level of response, animal responses can be subjective while plant responses derive purely from development of distinct physiological responses to deal with the perturbations. In chapter 4, the author explores the question of whether plants can hear or detect and have any use for sounds.  No research has shown any data to indicate so and the author argues that the ability to detect sound and use it for information imparts no evolutionary advantage to plants: they are immobile and so cannot flee even with the sound of potential danger nor do they have a need for rapid communication as their time scale of living is slower than that of animals. In Chapter 5, the author tackles the topic of how plants know in which direction to move: roots grow down, shoots grow upward, stems bend toward light, etc. In humans, this ability for the brain to be aware where every part of the body is relative to others is called proprioception.  Studies that go back to Darwin have shown that plants have gravity receptors in the form of statoliths, heavy dense ball-like structures in the cell that fall toward the direction of gravity. Roots bend or grow in the direction that the statoliths fall.  Furthermore, experiments have detected slow plant movement in a general spiral oscillatory manner that does not require gravity but whose speed and degree can be modulated and amplified by gravity.  As the author summed up in the end, “Just like Newtonian physics, the position of any part of the plant can be described as the sum of the force vectors acting upon that tell a plant both where it is and which direction to grow”. Chapter 6 explores the growing evidence of some level of memory formation and keeping in plants.  The coiling of tendrils, the shutting of Venus flytrap, and the ability of parents to pass on stress response mechanisms to their offspring through epigenetics are some of the examples provided by the authors that are carried out using the three basic steps in the process of memory: encoding, retention, and retrieval.  These memory examples in plants are procedural types of memories that are triggered by external stresses and other environmental conditions.  Just like in humans, the encoding, retention, and retrieval processes involve the formation of electrical gradients that control the memory process as detailed in the chapter.  From the biochemical aspect, researchers have also found glutamate receptors in plants that normally function in humans as neuroreceptors participating in neural communication, memory formation, and learning. It is not clear yet how these receptors function in plants but preliminary studies show that they may be involved in cell-to-cell communication.  In light of all this, in the epilogue, the author ponders the question of “intelligence” in plant but leaves the reader with his final emphatic thoughts on the stretching the parallels between humans and plants to an extreme.  In 2005, a new controversial field of biology, plant neurobiology, was defined to study information networks in plants based on similarities between plant anatomy and physiology and neural networks in animals. The primary criticisms in the creation of this field stem from the claim of the field’s flawed theoretical basis and stretching the parallels between plants and animals to an extreme. Proponents argue that the term itself is designed to provoke/encourage new ways of thinking with regards to ways that plants and animals process information. The author points out, however, that regardless of extensive genetic similarities between plants and animals, “they are two very unique evolutionary adaptations for multicellular life, each of which depends on the unique kingdom-specific set of cells, tissues, and organs”. The author believes that exploring “intelligence” in plants, a term with no solid objective definition, is not the right question to ask but rather, “Are plants aware?”. Based on evidence he presented in the book, the author believes that plants are aware of and, in fact, respond to, and remember stimuli from their environment and humans simply are a part of the “external pressures that increase or decrease a plant’s chances for survival and reproductive success”.  This very basic level of awareness precludes applying anthropomorphic subjective emotions to plants (happy, sad, suffering, etc.). Emotions are responses to external stimuli the formation of which requires the action of a brain to interpret the stimuli and produce the response and as the author emphasizes, plants have no brain. To this end, the author writes that “A shared genetic past does not negate eons of separate evolution. While plants and humans maintain parallel abilities to sense and be aware of the physical world, the independent paths of evolution have led to a uniquely human capacity, beyond intelligence that plants don’t have: the ability to care”.

Even though the book is about plant biology, I added it to my reading list because it confirmed my expectation that plant “senses” ultimately have a biochemical basis.  It also contains interesting vignettes that I can share with my class which most of the time has a number of biology majors who have taken or will be taking botany.

From the book’s section on “A Note about the Author”

Daniel Chamovitz, Ph.D., is the director of the Manna Center for Plant Biosciences at Tel Aviv University. He grew up in Aliquippa, Pennsylvania, and studied at Columbia University before receiving his Ph.D. in genetics from the Hebrew University of Jerusalem. He has been a visiting scientist at Yale University and at the Fred Hutchinson Cancer Research Center in Seattle, and has lectured at universities worldwide. His research on plants and fruitflies has appeared in leading scientific journals.

CHAPTER ONE: WHAT A PLANT SEES

In this chapter, the author describes how plants detect light, dark, and colors and regulate functions in response to light changes.  The author begins this chapter by explaining what it means to have sight and to detect not just light and dark but color as well.  He spends a few paragraphs describing visible light and how human retina contains the photoreceptor proteins in the form of rods and cones that allow us to detect and interpret different wavelengths of light as different colors. The human retina contains about 125 million rods and 6 million cones; this number of light detectors is equivalent to a digital camera resolution of 130 megapixels! Rods, containing the protein rhodopsin, are the more sensitive of the two and they are what allow us to see under low light conditions at night.  Cones, containing photopsin proteins, on the other hand, allow us to differentiate bright light into colors. 

One of the earliest researchers of plant response to light was Charles Darwin who stated in his final book “The Power of Movement” that most plants move toward light in the process now called phototropism.  In 1864, Julius von Sachs discovered that, in particular, they bend toward blue light but not others. Experiments carried out and published in 1880 by Darwin and his son indicated that the plant’s light sensor is on the tip of the plant’s shoot, demonstrating what the author called “rudimentary sight” in plants.

In the next section, the author describes the phenomenon of the Maryland Mammoth, a strain of tobacco plant first observed in 1906 that “just kept growing” to a height of 15 feet, all leaves and no flowers. While the abundant leaves were a boon to the farmers, no flowers means that they cannot harvest seed for re-planting for the next season. Subsequent experiments in 1918 showed that when exposed to an “artificial” short days of light, these “mammoths” stopped growing and started flowering in a phenomenon now called as photoperiodism. This phenomenon pertains to plants’ ability to measure light they take in and respond accordingly as ‘long-day” (irises and barley) or “short-day” (chrysanthemums and soybeans) plants when it comes to flowering.

The initial observation of photoperiodism provided a basis for scientists to control the flowering process in plants simply by artificially changing the amount of light they receive.  The flowering process in short day plants on short days can be suppressed simply by shining light on them even for a few minutes in the middle of the night.  The reverse can be done for the long-day plants during winter. These experiments suggested that plants measure the amount of continuous darkness and not the amount of light they are exposed to. Further probing revealed that plants only record red light shone on them at night to measure the length of continuous darkness and ignores blue and green flashes of light.  Thus, blue for bending and red for measuring the length of darkness. It got even more interesting when scientists in the early 1950’s found out that a longer wavelength of red light cancels any effect of red light on the flowering process, in alternating ways as if longer wavelength red light is an off switch for flowering and shorter wavelength red light is an on switch for flowering for long-day plants. By the 1960’s, further experiments revealed that phytochrome receptors are responsible for the plant’s response to red light and longer-wavelength red light (far red light).  This process acts as a way for the plant to measure the amount of darkness based on how long ago it last saw red light. Further experiments showed that shining the light on a single leaf is sufficient for the entire signal to get propagated  to the entire plant and to induce the effect but shining the light on the tip or stem had no effect.

Humans have 5 photoreceptors: rhodopsin for light and shadows, red, blue, and green photopsins, and cryptochrome for regulating internal clocks.  Plants have directional blue light receptors (called a phototropin) and red and far-red light phytochrome receptors but later genetic experiments on Arabidopsis thaliana showed that it had at least 11 different photoreceptors, each one regulating a function (e.g., when to flower, when to bend to light, etc.).

Because plants are immobile, they need to grow toward their food (sunlight). In order to survive and thrive, they have developed fine tuning methods to regulate their functions as a function of light detected, the amount and quality of which change depending on the time of day, season of the year, and their local environments.

Plant photoreceptors can detect a wider range of electromagnetic light, both shorter and longer beyond the tight mostly visible spectrum that humans can detect.  Plants, however, do not have a nervous system that allows them to “see” but rather they use the light detection process to gain signals for regulating function. As the author notes, “sight is the ability not only to detect electromagnetic waves but also the ability to respond to these waves”.  Using this definition, one can then say that plants have “sight” as well, as they respond to light and translate these signals into “physiologically recognizable instruction”.  While human red photopsin and plant phytochrome both absorb red light, they have different functions and chemistries.  Structurally, they are similar in that they are both proteins to which a chemical dye is attached.  Both humans and plants also have the blue-light receptors cryptochromes that control both human and plant internal (circadian) clocks using light signals but with different function responses. An interesting example of this is the evolutionary development of circadian clock regulation of cell division to only occur at night, possibly to the damaging effects of UV rays in sunlight.

CHAPTER 2: WHAT A PLANT SMELLS

In humans, while we need only 4 photoreceptors to detect and interpret color and light, there are hundreds of olfactory receptors to detect smell. Our nervous system allows us to interpret these smells.  This is not true for plants.

Since ancient times, it has been observed that fruits ripen faster when they are in the presence of opened ripe fruits.  In the early 20^th century, Florida growers found that lighting a kerosene lamp near their citrus induced ripening. Smoke from a lighten incense was also used by the Chinese. In 1924, Frank Denny, a USDA scientist, determined that the ripening agent is the compound ethylene which is released by ripe fruits and by burning kerosene and incense. Denny found that even at a very low ethylene concentrations of 1 to 100 million lemons can be induced to ripen.

Ripe fruits release ethylene and thus communicate to other fruits its current state of ripeness. The physiological functions of the hormone ethylene are as follows:

·         Regulates the response of plants to environmental stress such as drought and wounding

·         Regulates the aging process that produces the coloration of autumn foliage (leaf senescence)

·         Produced in the ripening process to ensure that the fruit ripens uniformly and that neighboring fruits ripen as well to attract seed dispersers.

The plant Cuscuta pentagona, also known as the dodder plant, thrives despite the fact that it does not have any leaves and lacks chlorophyll so it is unable to photosynthesize.  It is a parasitic plant that takes food from the host plant by inserting an appendage into the vascular system of the host. After a seed germinates, the shoot tip bends and rotates while growing until it finds the leaf of a host plant.  Then it bends down to find the stem, attaches itself to it, and imbeds microprojections that penetrate into the phloem so that the dodder plant can suck some of the sugar water.  The host plant eventually wilts as the dodder thrives.

An experiment by Dr. Consuelo de Moraes at Penn State showed that the dodder plant can detect chemical vapor (“smell”) that tells it that there is a tomato plant next to it for instance.  Even in the absence of an actual tomato and just with cotton swab containing chemicals from the tomato plant can induce the dodder plant to grow toward it.  When given a choice between wheat and tomato, the dodder plant chose tomato. Both wheat and tomato contain beta-myrcene, a volatile chemical that induces dodder plant to grow toward it. The two chemical hypothesis for this preference is that the tomato plant emits 3 volatile chemicals that are attractive to the dodder plant.  Also, the wheat emits (Z)-3-hexenyl acetate which actually causes the dodder to grow away from the wheat more effectively than the beta-myrcene attracts it.

Another widely popularized observation about chemical signaling in trees was made in 1983. The first observation was a natural one: caterpillars avoided trees that have neighboring trees already infested.  The unaffected trees were found to have phenolic and tannic acids in their leaves.  Later a controlled experiment was done where two populations of trees were placed in the same plexiglass case (seedlings), one population had their leaves torn.  It was observed later that the remaining leaves of the damaged trees and the leaves of undamaged trees both showed large increases in phenolic and tannic compounds.  The leaves of a control population in a separate glass container did not show this.

In another set of observations by Martin Heil’s team in Mexico concerned the lima bean. When a lima bean is eaten by beetles, it emits a mixture of volatile chemical in the air and the flowers release a nectar that attracts beetle-eating antrhopods. In a controlled experiment, the scientists examined the mixture of vapors around 4 leaves using a gas chromatograph-mass spectrometer.  They found the atmosphere around 3 of the leaves, the beetle-eaten leaf, an unattacked leaf from the same plant, and a leaf from a neighboring plant, contained the same mixture of volatile chemicals while the isolated control leaf had none of these gases around it. The healthy plants were less likely eaten by the beetles. In another set of experiments, they were able to show that when these gases from beetle-eaten plants were not allowed to reach the healthy plants, the unaffected plants did not produce nectar.  They were able to reverse this by blowing gases from the affected plants to the healthy plants and subsequently observing nectar production. The hypothesis is that the gases given off are for the survival of the plant by causing the healthy leaves to make themselves resistant to the attack.  Neighboring plants simply benefit because of their proximity (up to a few feet).

Identification of these emitted gases showed that a similar mixture of gas was released by bacterial and beetle attacks except for two gases. Under bacterial attack, the leaves emitted methyl salicylate (a volatile form of salicylic acid) and under beetle attack, the leaves emitted methyl jasmonate.  When a plant is attached by bacteria, the methyl salicylate produced at the exact spot of infection acts as a defense hormone that is sent as a signal to the rest of the plant through the vein. The plant then constructs a protective wall of dead cells around the affected area on the leaf to prevent further spread (looks white).

This production of methyl salicylate had also been observed in tobacco leaves that have been infected by a virus.  Methyl salicylate “inhaled” by neighboring plants through their leaf openings have been shown to be converted to the water soluble salicylic acid.

Plants’ flowers and fruits produce all kinds of aromas that function to attract pollinators and seed spreaders.

As of 2011, only the ethylene receptor has been found in plants. The author argues that this can be a form of olfaction because other plants detecting this chemical converts it into a signal that regulates a physiological response.

CHAPTER THREE: WHAT A PLANT FEELS

Plants have a sense of touch by way of detecting a tactile sensation. They can differentiate between hot and cold, vines rapidly start growing when they “feel” a fence to attach to, and a Venus flytrap closes its jaws when an insect lands on it. Some plants have tactile sensations 10 times more sensitive than humans: the burr cucumber vine can feel a string weighing 0.25 g while humans can only feel a string on the finger if it weighs about 2 g.

Humans have many different types of receptors that then report to the neurons what the tactile sensation is; the nervous system does the interpretation.  Mechanoreceptors are the sensory neurons for touch. Neural communication is based on electrical signals.  The ions involved are potassium, sodium, and calcium.  The ion gradient between the outside and the inside of the cell creates a charge: there are more sodium ions outside and potassium ions inside.  When a stimulus is created, e.g. by a finger touching a surface, mechanoreceptors are activated and ion channels open up at the contact point.  Sodium ion enter the cell changing the electrical charge and causing more sodium ion channels to open up leading a flux of sodium ions into the cell.  This creates a depolarization that travels like a wave from neuron to neuron until it hits the brain.  Neuron communication involves calcium ions.  The action potential [“In response to the appropriate stimulus, the cell membrane of a nerve cell goes through a sequence of depolarization from its rest state followed by repolarization to that rest state. In the sequence, it actually reverses its normal polarity for a brief period before reestablishing the rest potential.”,http://hyperphysics.phy-astr.gsu.edu/hbase/biology/actpot.htmlcreated by the sodium ion flux [?] causes a rapid increase calcium ion concentration which results in a release of neurotransmitters. These neurotransmitters initiate new waves of action potentials in the next neuron to which it is bound.  “These spikes in electrical activity exemplify the ways in which nerves communicate, whether from a receptor to the brain or from the brain to a muscle to cause a movement.”

In humans, tactile sensation and interpretation involves cells in different parts of the body that sense the pressure and convert this information to electrical signals that travel to the brain where the sensation is interpreted and where a response is initiated.

Human skin contains different receptors for different kinds of touch (mechanoreceptors) and also different kinds of pain. Pain receptors are called nociceptors “require a much stronger stimulus before they send action potentials to the brain”. Pain relievers work to block the signal from nociceptors but not mechanoreceptors.

The Venus flytrap is probably the most well-known insectivore plant.  Much of what we know now about this plant is owed to Charles Darwin. The closing of its leaves to trap an insect is triggered black hairs on the pink surface of each lobe.  At least two of these hairs need to be disturbed within 20 seconds of each other to activate the lobe closing. The hairs are sensitive but also very selective. Darwin learned a great deal about the Venus flytrap except the mechanical basis of the signal and the rapid closing of the lobes. A contemporary of Darwin, John Burdon-Sanderson discovered that pushing on two hairs produced an action potential much like that observed in nerves when animal muscles contract, producing depolarization in both lobes. A hundred years later, experiments were able to show that the shutting process of the lobes was caused by the electrical stimulation caused by the depolarization.

In a later study of leaf movement in the plant Mimosa pudica, it as shown that the electrical signal acts on motor cells called pulvinus in leaves. Pulvinus cells can act as mini-hydraulic pumps that control the pressure against the cell wall as water is pumped in and out of these cells.  The changing pressure against the cell wall causes the motion of the leaves: when filled with water, they push the leaflets open and when they lose water, the decreased pressure causes retraction of the leaflets. The leaves stay open and erect when the pulivinus cells are filled with water.  Electrical signals stimulated by touching the leaves cause the potassium ion channels to open allowing water and potassium ions to leave the cells and causing a decrease in volume and pressure against the cell wall.  When the electrical signal dies, potassium ions are pumped back inside the cell causing water to enter and engorge the cell again through osmosis.

Calcium ions play a role as well as they regulate the opening of the potassium ion channels.

An example where touch affects the growth of a plant is in Arabidopsis thaliana and in cocklebur. In Arabidopsis, it was observed that plants that are touched a few times a day causes them to grow squatter and to flower later. In cocklebur, touching causes growth retardation and eventually plant death.  The term thigmomorphogenesis is used to refer to the general effect of mechanical stimulation on plant growth.  Experiments later showed that simply touching an Arabidopsis leaf causes the activation of about 2 percent of its genes. The touch-activated genes (TCH) include one that is responsible for making proteins involved in calcium signaling in plant cells.  Calcium in plant cells helps maintain turgidity and is also part of the cell wall (in humans, they propagate electric signals from neuron to neuron and is necessary for muscle contraction). In plants, calcium ion concentration spikes when the plant is mechanically stimulated, e.g. shaking a branch or a root hitting a rock. Calmodulin is the protein that is produced when a TCH gene is activated by mechanical stimulus. In humans, calmodulin binds with calcium and regulates the activity of proteins involved in memory, inflammation, muscle function, and nerve growth. In plants, presumably, calmodulin is produced to bind the calcium ions released during the action potentials triggered by the mechanical stimulation.

An interesting note about variation of subjective responses to touch in humans: “While plants feel touch, they don’t feel pain. Their responses are also not subjective. Our perception of touch and pain is subjective, varying from person to person.  A light touch can be pleasurable to one person or an annoying tickle to another. The basis for this subjectivity ranges from genetic difference affecting the threshold pressure needed to open an ion channel to psychological differences that connect tactile sensations with associations such as fear, panic, and sadness, which can exacerbate our physiological reactions.”

Plants’ responses to mechanical stimulation are not subjective because they lack the brains to provide different interpretations. Mechanical stimulations in plant result in development of a response to the perturbation. For example, in tomato plants, “wounding” a single leaf causes the transcription of proteinase inhibitor genes in the unaffected leaves. The two hypothesis as to how this is communicated include a chemical signal transported through the vein to the rest of the plant and the other is through an electric signal. An experiment showed however the measurement of an electric signal in parts of the plant that did not receive any injury.

The extent of biological response to mechanical stimuli between plants and animals is quite different at the organism level but similar at the cellular level. “Mechanical stimulation of a plant cell, like mechanical stimulation of a nerve, initiates a cellular change in ionic conditions that results in an electric signal” which can be propagated from cell to cell and results in the coordinated functions of calcium and potassium ion channels and gene transcription for certain proteins.

CHAPTER 4: WHAT A PLANT HEARS

In humans, hearing involves the mechanoreception of vibrations in the air by tactile-sensitive hair cells in the inner ears and the interpretation of the sound information by our brains.  There are two types of information received by these mechanoreceptors: volume and pitch.  Volume depends on the amplitude of the wave: the louder the sound, the higher the wave amplitude, the more the stereocilia bend.  Pitch is the interpretation based on the frequency of the vibrations: the higher the pitch, the higher the frequency of the pressure wave, the faster the stereocilia bend back and forth. The bending motion of stereocilia trigger action potentials that are relayed to the auditory nerve and travel to the brain where the sound is processed into information.

The first 13 pages of this chapter were devoted to describing and debunking the content of a popular book published decades ago that purported that music had an effect on plant growth.

In 2000, the sequence of the approximately 120 million nucleotides that make up the DNA of Arabidopsis was finally completely determined, taking 4 years and 70 million dollars. In 1990, the NSF chose Arabidopsis as the first plant whose genome is sequenced because it contains the least amount of noncoding genes but contain the same number of about 25,000 genes as most plants and animals in just approximately 120 million nucleotides.  Wheat contains about the same number of genes but in 16 billion nucleotides.  Almost all of the 25,000 genes found in Arabidopsis are found in economically and agriculturally important plants.  Some of these genes are known to be involved in human diseases and disabilities: the BRCA gene involved in hereditary breast cancer, the CFTR gene involved in cystic fibrosis, and a number of genes involved in hearing impairments.  The human genome contains a gene involved in plant development including a group of genes involved in plant responses to light.  A gene does not “cause” a disease but rather a disease arises when a gene does not function normally.

Some of the Arabidopsis genes discovered are involved in human hearing impairment. These genes encode the protein myosin, nanomotor proteins that help move different proteins and organelles within the cell. One of these myosins form the hair cells in the inner ear.  Here is the connection: “when a mutation occurs in one of the four Arabidopsis “deaf” myosin genes, the root hairs don’t elongate properly, and consequently the plants are less efficient at absorbing water from the soil”. Human need myosin to make sure that inner ear hairs function properly while plants need myosin to produce root hairs that function normally to absorb water and nutrient from the soil.

An example of research involving sound and plants include studying the effect of sound waves on vineyard yields. The buzzing of bees has been known to stimulate the release of pollen but these vibrations involve actual physical contact with the flower.  It is conceivable, however, that the sound of vibrations could be affecting the plant is some undetected way.  Trees are known to emit ultrasonic vibrations during a drought but these passive sounds result from physical forces related to changes in water content.

No hard scientific have yet shown that plants have the ability to “hear” or detect sounds.  From an evolutionary point of view, hearing has the advantage of allowing animals to detect potentially dangerous situations and communicating by sound even from far away. Both of these do not impart any known advantage to plants. They are immobile and has no ability to escape even with the sound of potential danger. Because they function and live at a slower time scale, they have no need for rapid communication.

CHAPTER 5: HOW A PLANT KNOWS WHERE IT IS

Plants know that roots grow down and shoots grow up.  It has been observed that even when plants have been turned upside down, it will reorient itself slowly.  Plants also are aware of in what directions its branches are growing and where tendrils can attach.

In humans, the process of proprioception allows us to have a sense of where the different parts of our body are relative to each other based on the internal status of the body. Under intoxication, the ability for proprioception is impaired and this is shown by reduced hand-eye concentration. “Proprioception involves the coordinated input of signals from the inner ear, which communicates balance, along with signals from specific nerves throughout the body that communicate position. The author provides a detailed anatomical explanation of the mechanics of how we are able to sense the position of our head.  The proprioceptive nerves keep everything coordinated while proprioceptive receptors relay information to the brain about the positions of limbs. “Proprioception encompasses not only our sense of balance but our coordinated motion as well”.

During the 18^th and 19^thcenturies, studies and observations showed that seedlings reorient themselves so that shoots grow upward and roots grow downward, that gravity is the cause of this directionality, and the receptor that detects the direction of gravity is at the root tip.

Later studies showed that there are two places that detect gravity for the plant to correctly orient its growth: in the roots, it’s the root tip and in the stem, it is the endodermis. In the roots, the endodermis is a selective membrane that regulates what and how much of substances (ions, water, minerals) enter the xylem tubes for transport to the green parts of the plant.

How do these parts of the plant detect gravity? In the root tip, cells in the central area of the root cap contain statoliths which are small dense ball-like structures that are heavier than other parts of the cell. Experiments using a high-gradient magnetic field to simulate gravity have shown that root growth follows the same direction that statoliths move. Experiments done in space shuttle where there is no gravity showed the statoliths don’t fall and remain naturally distributed throughout the cell – no gravitropic bending is observed in plants.  Thus, this shows that plants need to detect gravity through statoliths to know where down is.

In the 1930’s, scientists identified auxin as the movement hormone, the messenger molecule that light causes to accumulate on the dark side and causes cells to elongate resulting in bending. In root tips, gravity causes the auxin to accumulate on the up side of the root causing it to grow down.

Darwin’s meticulous tracking of plant on a glass plate marked with tip location and in more modern times using a time-lapse video have shown that plants move at varying speeds and lengths in a spiral oscillatory track. Darwin called this motion circumnutation. Experiments on seedlings germinated in space showed that gravity is not needed for the spiral oscillatory motion of plants but gravity modulates and amplifies the endogenous movements of plants.

“Just like Newtonian physics, the position of any part of the plant can be described as the sum of the force vectors acting upon that tell a plant both where it is and which direction to grow”.

CHAPTER 6: WHAT A PLANT REMEMBERS

While plants do not have memories in the same sense that humans do, they can exhibit a component of memory in the way that they exhibit a delayed response to a previous event. The examples given by the author include how plants can “remember” the color of the last light they saw and willow trees know that other trees have been attacked by caterpillars.

Human memory is exhibited at three different levels: procedural, semantic, and episodic. Semantic and episodic can probably be considered not present in plants but something akin to procedural memory probably exists. In the case of the Venus flytrap, as discussed before, for the trap to shut close, two hairs must be touched within 20 seconds of each other. This requires that information regarding the first hair being touched must be encoded and stored for up to 20 seconds and later on retrieved to trigger the trap-shutting mechanism. Information encoding, retention, and retrieval are all processes common to all forms of memory (sensory, short-term, long-term, and muscle-motor). An experiment later showed the mechanism for encoding the memory of the first hair trigger: an electrical current triggers the closing of the trap and that at least 14 microcoulomb of charge must pass within 20 seconds. A model proposed for the memory encoding and retention posits that touching the first hair triggers an action potential that causes calcium channels to open causing a rapid increase in ion concentration that then steadily decreases until another hair is triggered within 20 seconds to supply the addition calcium ion needed to trigger trap-closing.  The memory of the first hair trigger is retained by keeping a high enough calcium ion concentration; the memory is lost if a second hair trigger does not add more calcium ions within 20 seconds to keep a calcium concentration above the threshold needed for trap-closing. A subsequent experiment inhibiting potassium channels resulted in the trap not closing even with the application of electrical charges. The author summarizes it better and more succinctly: “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.”

Studies done on cotyledon damage and the corresponding response indicated that plants have a way to store information about “trauma” and retrieve it later based on delayed response. In another example, cherry trees only bloom in April after having gone through a winter season despite April and mid-September having twelve hours of day required for blooming. This delayed response to a stimulus weeks after indicates that cherry trees have some way of encoding, retaining, and later retrieving having gone through vernalization.  The gene responsible for inhibiting the flowering process until after the plant has undergone vernalization is the FLC gene.  Scientists have found that cold weather causes the histone around the FLC gene to change structure such that the chromatin packs tightly and prevents the gene from being transcribed. When not transcribed, the plant is able to flower again but only after the cold weather has turned off the FLC gene. However, flowering does not take place right away but rather waits for the right conditions to exist. This change in the gene activity of the FLC gene that does not require a change in DNA code (in this case only a change in DNA structure) is an example of epigenetic mechanism of cellular memory that is passed down from parent to daughter cells so that all FLC genes are inactive in all cells even after the cold weather passes.  This is what the author has to say about the importance of epigenetics in biology: “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.”

A more profound example of “memory” described by the author is the phenomenon of transgenerational hand-off from parents to offspring of epigenetic changes in plants caused by environmental or physical stress (“epigenetic heredity”).These genetic changes produce new combinations of DNA that are passed on to and expressed in offspring even though the new generation plants did not experience the stress themselves. “They remembered that their parents had been through this stress and reacted similarly”, notes the author. Why memory? The parent encoded the memory of the stress and retained this through genomic changes, passed it on to the offspring that can then retrieve the information and express it as a response to a similar stress. In the seminal experiment by Hohn, plants subjected to UV or pathogen stress acquired increased genetic variation that was then passed on to the Arabidopsis progeny of thousands of seeds, a process likely induced by an epigenetic change that occurs in all cells all at once including pollen and egg cells and inherited by progeny as opposed to a mutation that is passed on to only a very small percentage of progeny. Further experiments by Kovalchuk showed evidence that stressed parents produced progeny that are more tolerant of harsh conditions, suggesting that various stresses experienced by the parents induced epigenetic changes passed on to offspring.

“Plants clearly have the ability to store and recall biological information…The tendril’s coiling, the Venus flytrap’s closing, and the arabidopsis’ remembering environmental stress all include processes of forming the memory of the event, retaining memory for distinct time periods, and recalling the memory at a later point in order to get a specific developmental response.”

Some similarities in biochemical make-up related to memory in humans with plants have been studied in the last several years.  One of this is the discovery that plants also contain glutamate receptors that function in neural communication, memory formation, and learning in humans.  It is not clear yet what the function is of glutamate receptors in plants but preliminary results indicate that they are involved in cell-to-cell communication.

EPILOGUE: THE AWARE PLANT

In 2005, a new controversial field of biology, plant neurobiology, was defined to study information networks in plants based on similarities between plant anatomy and physiology and neural networks in animals. The primary criticisms in the creation of this field stem from the claim of the field’s flawed theoretical basis and stretching the parallels between plants and animals to an extreme. Proponents argue that the term itself is designed to provoke/encourage new ways of thinking with regards to ways that plants and animals process information. The author points out, however, that regardless of extensive genetic similarities between plants and animals, “they are two very unique evolutionary adaptations for multicellular life, each of which depends on the unique kingdom-specific set of cells, tissues, and organs”. The author believes that exploring “intelligence” in plants, a term with no solid objective definition, is not the right question to ask but rather, “Are plants aware?”. Based on evidence he presented in the book, the author believes that plants are aware of and, in fact, respond to, and remember stimuli from their environment and humans simply are a part of the “external pressures that increase or decrease a plant’s chances for survival and reproductive success”.  This very basic level of awareness precludes applying anthropomorphic subjective emotions to plants (happy, sad, suffering, etc.). Emotions are responses to external stimuli the formation of which require the action of a brain to interpret the stimuli and produce the response and as the author emphasizes, plants have no brain. To this end, the author writes that “A shared genetic past does not negate eons of separate evolution. While plants and humans maintain parallel abilities to sense and be aware of the physical world, the independent paths of evolution have led to a uniquely human capacity, beyond intelligence that plants don’t have: the ability to care”.