WHAT A PLANT KNOWS (Chapter Three)

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.

Even though the book is about plant biology, I added it to my reading list because I suspect that plant “senses” ultimately have a biochemical basis.  It also may contain 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.