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Archive for the ‘Plant Signaling’ Category

Venus_flytrap.jpg“Springing the Trap”

In the previous post I suggested that the Venus flytrap works something like a mousetrap. And I described how the “trap” is hydraulically set. (For a more thorough explanation of how the Venus flytrap snaps, please see PDF file here).

But how do the trigger-hairs on the surface of the flytrap’s leaves act to “spring the trap”?

Briefly, the mechanical movement of the trigger-hairs protruding from the leaf activates mechanosensitive ion channels and generates receptor potentials that induce an action potential.

This action potential travels to the midrib of the leaf where it promotes the opening of water channels called aquaporins. This facilitates the rapid water efflux from key cells that hydraulically control the leaf opening.

More simply put, the stimulation of the leaf hairs produce electrical signals that cause the rapid deflation of the water-pressurized cells that keep the leaves open. And, thus, the “trap is sprung”.

(Please see here and here for some recent information regarding the the kinetics and mechanism of the Venus flytrap.)

ligtning_plants.jpgElectrical Signals in Plants?

Do plants have a nervous system?

The short answer is: no. (At least not the complex nervous system of animals.)

But scientists have been able to detect transient electrical signals somewhat analogous to action potentials under certain situations in plants.

Such situations involve the classic examples of thigmosnasty in plants, namely, the “touch-sensitive” plant (Mimosa pudica) and the Venus flytrap.

Charles Darwin described the Venus flytrap plant as “…one of the most wonderful in the world.” on page 231 of his book Insectivorous Plants.

Sparked by a correspondence with Darwin, which included some Venus flytrap plants, the English physiologist John Scott Burdon-Sanderson was the first to discover action potentials in plants following stimulation of a leaf. (Please see reference 1 below.)

electricity_plants.jpgDo Plants Have a Neural Net?

In addition to thigmosnastic plants, all vascular plants may be utilizing electrical signals to regulate a variety of physiological functions.

Many of the biochemical and cellular components of the neuromotoric system of animals has been found in plants. And this has led to the hypothesis that a simple neural network is present in plants, especially within phloem cells, which is responsible for the communication over long distances.

“The reason why
plants have developed pathways for electrical signal transmission
is most probably the necessity to respond rapidly to
external stimuli, for example, environmental stress factors.” (from ref 2 below)

More regarding electrical communication in plants: Novel electrical signals in plants induced by wounding

The Emerging Field of Plant Neurobiology

In 2006, an article was published in the journal Trends in Plant Science that elicited quite a kerfuffle.

This review (PDF) introduced, to the plant scientific community at least, the field of “Plant Neurobiology”. Although this proposal was not without controversy (PDF), the Society for Plant Neurobiology seems to be alive and well (Plant Neurobiology Website). (And if you happen to be in Kitakyushu, Japan, in May this year, you may be able to attend the 6th International Symposium on Plant Neurobiology).

Bottom line: Though plants don’t have a nervous system like animals, plants do have the necessary electrical, biochemical, and cellular components indicative of a neural network, albeit a relatively simple one.

References:
1. Burdon-Sanderson J. (1873) Note on the electrical phenomena
which accompany irritation of the leaf of Dionaea muscipula.
Proceedings of the Royal Society of London 21, 495–496.
2. Fromm, J & S. Lautner (2007) Electrical signals and their physiological significance in plants. Plant, Cell and Environment 30, 249-257. (PDF)

HowPlantsWork © 2008-2011 All Rights Reserved.

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Secret_Life_Plants.gifDo Plants Have a Nervous System?

Back in the heady (hazy?) days of the early 1970’s, a book was making the rounds on college campuses that suggested plants possessed a sort of sentience.

This book was The Secret Life of Plants.

The professor teaching my Introductory Botany class at the time loathed this book. He actually stated in class that any student he caught in possession of this book would receive a grade of “F”. (True story!)

Although this prof’s reaction was perhaps an extreme example, this story serves to illustrate the general attitude within the scientific community against any suggestion that plants possess a nervous system .

The notion that plants could feel pain, for example, or move rapidly in response to stimuli was solely within the realm of science fiction. (see here and here, for example)

2703146160_61f006d6f5.jpgWhat About the Venus Flytrap?

There are, however, some examples of relatively rapid movements in some plants in response to external stimuli, the most famous of which is the Venus fly trap.

Another example is the rapid leaf movements in the touch-sensitive plant Mimosa pudica. (see a YouTube video here)

In this case, as well as in the Venus fly trap, it’s not so much that the plant is moving in response to mechanical stimulation, but that the touch is triggering a sort of spring-loaded mechanism. Think of an old-fashioned mouse trap – gently touch the triggering mechanism, and the trap snaps shut.

In these plants, however, it’s kind of a hydraulic spring-loading. That is, when some cells within a thickening at the base of the leaves called a pulvinus have a high turgor pressure, this causes the leaves to open. And if these cells lose turgor pressure, the leaves close.

But how does mechanical stimulation trigger the rapid loss of cell turgor pressure in these plants?

mouse-trap.jpg“Setting the Trap”

Plant cells, like animal cells, generate an electrical membrane potential across their cellular membranes.

In plants, this is mainly generated by converting the chemical energy of ATP into electrochemical energy by proton pumps. (Your cells use Na/K-ATPases.)

Some of the energy in this membrane potential is used by cells to accumulate solutes such as sugars and mineral ions such as potassium. This accumulation of solutes draws water into the cells via osmosis.

This is how the pulvinus cells in the Venus fly trap and the touch-sensitive plants likely generate their turgor pressure to open the leaves.

Now the leaves are hydraulically “spring-loaded”, and ready….

Next time: How electrical signals “spring the trap”. Also, an introduction to the emerging field of Plant Neurobiology.

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And The Winner Is……….

At the beginning of this series, I wondered if the impressive array of sensors in the new iPhone rendered it “smarter” than the average plant, at least when it came to sensing and responding to its surroundings.

A summary of my comparison is shown in the table below.

iPhone_table.jpg

Briefly, the iPhone has at least one sensor – the magnetometer – that plants don’t have. (Despite all the reports of geomagnetic effects on plants over the years, few, if any, are truly repeatable, and, thus, credible. Please see here for more information.)

Also, one could argue that plants lack a true 3-axis gyroscope.

Therefore, it would appear that the iPhone has more types of sensors than plants.

However, when it come to light-sensors, plants clearly have the advantage. Most flowering plants have at least three different photoreceptors – phytochromes, cryptochromes, and phototropins. (And I’m not even counting pigments such as chlorophyll and carotenes.)

These photoreceptors work by affecting a complex array of biochemical and genetic pathways inside plant cells. Consequently, most plants have the ability to respond in very complex ways to even subtle changes in the quantity and quality of light in their environment.

Plants also have sensitive mechanical and gravity sensors that allow them to alter their development in response to these environmental cues. Again, by affecting complex cellular mechanisms, these gravi- and mechano-sensors are able to elicit sophisticated environmental responses by the plants.

……….Plants!

marigold_twining.jpgThough the iPhone 4 may have a couple of environmental sensors lacking in plants, plants are much more intelligent than iPhones when it comes to how they respond to their surroundings. That is, plants display a much higher level of complexity in their responses to signals from their sensors.

Plants can not only alter their functions in response to light, for instance, but also can actually change their form to adapt to changes in their environment.

Therefore, though iPhones may be able to sense more things in their environment (magnetic “north”, for example), plants respond to their surroundings more intelligently.

(Even though most dogs have a better sense of smell than you do, and cats have better night vision, you’d probably not say that they are more intelligent than you are.)

Bottom line: Though they might not sense the environment in as many ways or as well as some inanimate objects, such as an iPhone, plants – as with most lifeforms – are much more intelligent when it comes to responding to changes in their environments.

HowPlantsWork © 2008-2011 All Rights Reserved.

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gyroscope.jpgWhy They Call It A “Smartphone”

Arguably, the new iPhone 4 is the most advanced smartphone currently available.

But is the iPhone 4 so smart that it’s actually smarter than the average flowering plant? (At least when it comes to sensing and responding to its environment.)

This is the question I posed way back here, starting with the iPhone’s light and proximity sensors. Next we examined the compass and then the accelerometer. Last up: the gyroscope.

For an excellent review of the iPhone 4 gyroscope, I’ll refer you to AppleInsider. An excerpt from which covers the basics:

“The iPhone 4 gyroscope adds an additional new electronic sensor for detecting 3-axis angular acceleration around the X, Y and Z axes, enabling precise calculation of pitch, yaw and roll.

While conventional accelerometers measure linear acceleration as a change in velocity (speed increasing or decreasing over time) apart a change of direction, a gyro measures angular acceleration: a change in both velocity and direction at the same time. In iPhone 4, the gyro enables the device to sense slight degrees of rotation while rejecting linear movements and hand jitters, while its accelerometer senses those linear movements.

Combined with data from the accelerometer and compass, the gyro provides detailed, precise information about the device’s six-axis movement in space: the 3 axes of the gyro, combined with the 3 axes of the accelerometer enable the device to recognize how far, fast, and in which direction it has moved in space.”

Wow!

How could a plant match all of that?

Do Plants Have a Gyroscope?

The simple answer is no. (At least not one like the iPhone 4.)

But a gyroscope basically is a device for measuring and maintaining orientation. Do plants have something analogous to a gyroscope?

The simple answer to this question is yes.

Plants obviously have the ability to sense and respond to the Earth’s center of gravity.

Most roots grow toward the center of gravity and most stems do the opposite.

Perceiving the direction of the Earth’s center of gravity is the “sensor” most plants use to maintain their correct orientation.

So, instead of a “gyroscopes” plants have a “gravisensors”.

How Do Plant “Gravisensors” Work?balance.jpg

The gravity sensors in plants are located in the root cap cells and in some cells within the growing regions of stems.

The generally-accepted explanation is that starch grains within these cells are relatively dense and heavy enough to be affected by the Earth’s gravity. Thus, their orientation within the gravisensing cells allows them to tell which way is “down”, that is, the center of gravity.

This theory has been recently refined to indicate that starch-containing organelles within the gravisensing plant cells, likely plastids called amyloplasts, are the bodies that move inside the cells in response to gravity.

The reorientation of these organelles somehow affects the transport of the plant hormone auxin out of the gravisensing cells, which is the chemical signal that mediates the plant’s response to gravity. That is, auxin either stimulates (stems) or inhibits (roots) cell elongation, causing the stems to grow away from the center of gravity and the roots to do the opposite.

How the gravity-responsive organelles redirect auxin efflux in these cells is poorly understood. But it may have something to do with relative forces on the cell’s cytoskeleton, which, in turn, may affect auxin transport at the cell membrane. Think tugging on one edge of a spider’s web.

So, though plants don’t have gyroscopes, they do have arrays of gravisensors that allow them to accurately perceive and grow in response to Earth’s center of gravity.

Next-time: Well, which is more intelligent when it comes to sensing and responding to its environment – an iPhone 4 or a plant? Summary and conclusions.

References

1. Miyo Terao Morita (2010) “Directional Gravity Sensing in Gravitropism.” Annual Review of Plant Biology Vol. 61, pp. 705-720. (Abstract)

HowPlantsWork © 2008-2011 All Rights Reserved.

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rollercoaster.jpg3D Motion Sensing – iPhone Versus Plant

If an iPhone can sense its surroundings better than a plant can, does that make the iPhone more “intelligent”?

To try to answer this question, in previous posts, I compared the iPhone’s light and proximity sensors and the geomagnetic sensor to the equivalent (if it existed) in plants.

Now, on to the accelerometer…

The iPhone uses an accelerometer to sense “…orientation, acceleration, vibration shock, and falling.”

An excellent description about how the accelerometer works from an online article published by Macworld:

“Today, like everything else electronic, the iPhone employs micro-electromechanical systems (MEMS). These devices have tiny (3 microns thick and 125 to 150 microns long) polysilicon arms with small hammer-like blocks on the end. They act like springs and hold the MEMS structure above a substrate. Acceleration causes the arms to deflect from their center position. And just like in the old electro-mechanical devices, the movement of that tiny mass is detected, by capacitors in this case, and a signal is generated.”

The actual IC board used in the iPhone can be purchased here for about $20. And an example of how some people actually take advantage of the iPhone’s accelerometer can be seen below:

You might think that it’s unlikely that plants would have sensors analogous to the iPhone’s accelerometer. After all, plants are sessile organisms. What conceivable use would a plant have for a motion detector?

Well, if you think this, then you’d be incorrect.

Plants have very sensitive cellular mechanisms to detect the wind, for example, and even to detect touch. Think Venus flytrap, for example.

This is called “mechano-stimulation”, and is nicely summarized in the following excerpt from the abstract of Ref. 1 below.

“In nature, plants are challenged with hurricane winds, monsoon rains, and herbivory attacks, in addition to many other harsh mechanical perturbations that can threaten plant survival. As a result, over many years of evolution, plants have developed very sensitive mechanisms through which they can perceive and respond to even subtle stimuli, like touch.”

Plant responses to this mechano-stimulation range from movement (thigmonasty) to changes in plant development, such as the fact that plants in windy areas tend to have thicker and shorter stems. The latter is an example of thigmomorphogenesis.

Although how mechano-stimulation is perceived by plant cells is currently unknown, several hypotheses regarding these mechanosensory mechanisms are presented here.

Briefly, one hypothesis is based on localized changes in turgor pressure within plant cells as a result of mechano-stimulation. Another hypothesis is that wind or touch may cause the cell membranes to be stretched, which may trigger stretch-activated ion channels in the membranes. Still another involves mechanical perturbation to the plant cells’ cytoskeleton, sort of like pushing one side of a spider’s web.

All of the above lead to complex biochemical interactions within the cells, including enzyme activation and changes in gene regulation, for example, all culminating in the responses that we can observe.

Though plants have cellular mechano-sensors that allow them to detect motion, these sensors really aren’t acting like the accelerometer in the iPhone.

However, the cellular mechanisms that plants use to sense gravity may be more analogous to accelerometers. We’ll have a peek at these next-time, when I get to the last iPhone sensor on the list, namely, the gyroscope.

References

1. E. Wassim Chehab, Elizabeth Eich and Janet Braam (2009) “Thigmomorphogenesis: a complex plant response to mechano-stimulation.” Journal of Experimental Botany Vol. 60, pp. 43-56. (Full Text)

HowPlantsWork © 2008-2011 All Rights Reserved.

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iPhone.jpgIs the iPhone 4 More Aware of Its Surroundings Than a Typical Plant?

That’s the question that I posed in my previous post.

Because of the array of sophisticated sensors included in the new iPhone, could this inanimate object actually be better at sensing its environment than a living plant?

Last time, I started with the iPhone’s ambient light and proximity sensors.

Today, let’s consider the next iPhone sensor on the list:

The Compass

As reported by AppleInsider, the newer iPhones use a sophisticated magnetometer or “digital compass”, specifically, the “…Asahi Kasei’s azimuth sensor No. AK8973, a 16-pin leadless IC package measuring 4mm square and 0.7mm thick…” (An exhaustive description of this IC can be found here (PDF), a very small portion of which: “AK8973 is a geomagnetism detection type electronic compass IC. The small package of AK8973 integrates magnetic sensors for detecting geomagnetism in the X-axis, Y-axis, and Z-axis, and arithmetic circuit for processing the signal from each sensor.”)

Thus, the iPhone 4 can sense the Earth’s geomagnetic field, and using the compass application, can tell you which direction the iPhone is facing.

Can plants sense the Earth’s magnetic field to obtain directional information? And, if so, why?

The ability to sense the Earth’s magnetic field or magnetoreception has been known to occur in birds and other animals for nearly fifty years. “Dozens of experiments have now shown that diverse animal species, ranging from bees to salamanders to sea turtles to birds, have internal compasses.” (from Ref. 1 below)

But what about plants?

compass.jpgCompared to animal magnetoreception, “…little is known about magnetoreception in plants, although early studies on plants were initiated more than 70 years ago….” (from Ref. 2 below) The authors of this review continue: “The central questions in this context, i.e. (1) whether or not plants can perceive the Earth’s magnetic field, (2) what is the physical nature of the magnetoreceptor(s), and (3) whether or not the geomagnetic field has any bearing on their survival, have remained largely unanswered.”

These authors should be commended for plowing through the scientific literature regarding a myriad of alleged geomagnetic effects on plants, which they describe as “bewildering”.

Perhaps the following best describes the history of geomagnetic-sensing research in plants: “The scientific literature describing the effects of weak magnetic fields on living systems contains a plethora of contradictory reports, few successful independent replication studies and a dearth of plausible biophysical interaction mechanisms. Most such investigations have been unsystematic, devoid of testable theoretical predictions and, ultimately, unconvincing.” (from Ref. 3 below)

These investigators were responding to recent reports (abstracts here and here) of cryptochrome-mediated magnetoreception in Arabidopsis. Harris, et al. (ref 3) could not replicate these results and concluded that there is no reliable evidence for such magnetic effects in plants.

So, what do I conclude?

After a brief review of the recent literature on this subject, I found that there is little or no well-founded, reproducible evidence that plants can sense geomagnetic fields. Moreover, there are no clear reasons that it would be to a plant’s advantage to be able to do so. Therefore, I think it is unlikely that plants have geomagnetic sensors.

Score one for the iPhone.

Next-time: The Accelerometer

References

1. Johnsen, S. and K.J. Lohman (2008) “Magnetoreception in animals.” Physics Today, March 2008, pp. 29-35. (PDF)

2. Galland, P. and A. Pazur (2005) “Magnetoreception in plants.” Journal of Plant Research Vol. 118, pp. 371–389. (Abstract)

3. Harris, S.-R., K.B. Henbest, K. Maeda, J.R. Pannell, C.R. Timmel, P.J. Hore, and H. Okamoto (2009) “Effect of magnetic fields on cryptochrome-dependent responses in Arabidopsis thaliana.” Journal of the Royal Society Interface Vol. 6, pp. 1193 –1205. (PDF)

HowPlantsWork © 2008-2011 All Rights Reserved.

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Is the New iPhone More Aware of Its Environment Than a Typical Flowering Plant?

Today I was watching a bit of Steve Jobs’ recent WWDC keynote address introducing the newest iPhone. (Click on image below to view his presentation.)

iPhone4.jpg
About half way through his talk, Steve enumerates the sensors (see the list above) built into the iPhone 4 to provide information about the world around it. Very impressive.

But as an inveterate plant physiologist, I began to ponder whether the new iPhone was actually more “aware” of its environment than a typical flowering plant.

Intelligence is often defined as an entity’s ability to adapt to a new environment or to changes in the current environment.

Intelligence is not a term commonly used when plants are discussed. However, I believe that this is an omission based not on a true assessment of the ability of plants to compute complex aspects of their environment, but solely a reflection of a sessile lifestyle.” (from Ref. 1 below)

If the new iPhone is better at sensing its environment than a typical plant, then does it follow that the iPhone 4 is more “intelligent” than the average plant?

So, of course, the critical question: Is the iPhone 4 better than plants at environmental sensing?

To try to answer this question, I’ll briefly compare each iPhone sensor to the equivalent (if there is one) in plants. Let’s use the list above and work from the bottom up.

Ambient Light and Proximity Sensors

Hidden behind the translucent dark glass above and to the left of the iPhone’s earpiece are two different kinds of sensors, the ambient light sensor and the proximity sensors.

sensors.jpg
These sensors help improve battery life. For example, under low light conditions, the ambient light sensor signals the iPhone’s software to dim the screen. When the phone is placed against your head during a call, the proximity sensor deactivates the touch sensitivity and illumination of the iPhone’s screen.

How do they work?

Both of these tiny photodetectors are able to convert light energy into electrical energy. The ambient light sensor is basically a light meter, that is, it contains a photodiode that produces more electrical energy the brighter the light.

The proximity sensor is more complicated, however. The iPhone uses a reflective photoelectric sensor, thus it contains both a light emitter and a receiver. The light reflected from an object is used for the sensor detection. A near-infared beam is sent from the emitter. When this near-infared beam is reflected off an iPhone user’s head several inches from the sensor, the receiver detects the reflected beam, and the sensor then signals the iPhone’s software to shut down the display.

OK, but what about plants?

Ambient Light Sensors – Plants can also detect and respond to changes in light intensity. For example, inside leaf cells, chloroplasts may reorient in response to changing light conditions, as I’ve mentioned in a previous post. Another example is that leaf stomatal conductance usually increases with increasing light intensity.

Plants use so-called photoreceptors to perceive the light in such cases. These include phototropins, phytochromes and cryptochromes.

Unlike the iPhone photodetectors above, these plant photoreceptors don’t convert light energy into electrical energy. Instead, light absorption actually causes a conformational change in the photoreceptors. These shifts in their 3D structures, in turn, result in a changes in their relative biological activities.

The nature of these activities is too complex to describe here, but suffice it to say that they may elicit dramatic changes in enzyme activity, membrane transport, and gene regulation within the plant cells.

Life_In_The_Shade.jpgProximity Sensors – Plants may use both photoreceptors, chemical sensors, or both, to gauge their relative proximity to other plants and to respond accordingly.

For instance, many plants apparently use phytochrome to measure changes in light quality that result from light reflected from the leaves of adjacent plants.

This phenomenon is called shade avoidance. Plants may respond by increasing their stem elongation, for example.

Plants also produce and detect volatile chemical signals such ethylene and methyl salicylate. Although evidence is not definitive regarding this, it is likely that plants may also use chemical signals to do proximity sensing.

To Be Continued: Compass, Accelerometer, & Gyroscope

Reference

1. Trewavas, A. (2003) “Aspects of Plant Intelligence” Annals of Botany Vol. 92, pp. 1-20. (Full Text)

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