Although “Global Warming” is to some people a controversial subject, the one thing that’s not controversial is that the level of atmospheric CO₂ has significantly increased in the past 100 years and will likely continue to increase – at least until humans stop burning fossil fuels. (We’ve previously visited this subject on a number of occasions, here and here, for example.)

OK, so atmospheric CO₂ is at historically very high levels and is going even higher in the decades to come. How will this likely affect plants?

As previously discussed, since green plants use CO₂ as the carbon source in photosynthesis, they will probably do more photosynthesis, i.e., produce more biomass.

But as is often the case with things biological, it’s not quite as simple as that.

Fertilize More, Water Less

For your garden plants to take full advantage of this high CO₂ world, you will probably need to add more nitrogen fertilizer, but you may have to water less often. Here’s why.

For optimal plant growth, plants need sufficient amounts of carbon and nitrogen and water.

In a high CO₂ world, plants will have a sufficient carbon source. But if the availability of nitrogen is limited, then plant growth will be limited. Therefore, to fully take advantage of a high CO₂ world, your garden plants will need to have sufficient amounts of nitrogen (N). In most cases, nitrogen is available to plants in the form of nitrate (NO₃^-) in the soil. So to ensure your plants thrive in a high CO₂ world, add plenty of compost or nitrogen-containing fertilizer.

Plants in a high CO₂ world will also use water more efficiently. This is because the stomates in the leaves need to open less to obtain sufficient amounts of CO₂. This is good, because then the plant transpires less water. The result, in general, is that plants will use less water for a given amount of biomass production in a high CO₂ world.

Don’t Plant Corn

Not all plants will benefit from a high CO₂ world.

So-called C-4 plants already use CO₂ very efficiently. Consequently, their photosynthesis will not be significantly improved with increased amounts of atmospheric CO₂.

Corn or maize is a classic C-4 plant.

Other C-4 plants include sugarcane, sorghum, and so-called ”warm season” grasses.

Other cereals such as wheat, barley and oats are not C-4 plants — they are so-called C-3 plants — and should benefit from a high CO₂ world.

Bottom line: People on this planet show no signs of throttling back their use of fossil fuels. On the contrary, the production of CO₂ from the burning of fossil fuels, especially coal, will likely increase in the coming years.

So, it makes sense to prepare for a high CO₂ (and probably warmer) world by learning more about how plants will likely respond to such changes in their environment.

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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.

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!

Though 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.

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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.

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?

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)

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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)

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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?

Compared 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)

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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.)

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 it’s 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.

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.

Proximity 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.

Next-Time: 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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You’ve likely heard about microbes with the ability to “eat” crude oil (a.k.a., petroleum).

That is, some bacteria have the ability to metabolize most of the the organic compounds present in petroleum.

(For excellent scientific coverage of the oil spill see “The Science of the Oil Spill”.)

Interestingly, most of the bacteria used in the bioremediation of oil contamination are from natural populations, rather than genetically engineered in the laboratory.

But can plants help clean up the oil spill in the Gulf of Mexico?

That is, can plants, like some bacteria, take up and metabolize the organic components of crude oil?

Phytoremediation

Using green plants to help clean up (remediate) soils contaminated with toxic substances, such as heavy metals, radionuclides, or toxic organic compounds, is referred to as phytoremediation.

The use of plants to help clean up the environment has great appeal. The two main reasons why are (1) that the contaminated soils can be treated in situ and (2) that it’s relatively cheap, compared to conventional industrial clean-up methods. (The latter reason is, of course, very enticing to both government and industry.)

Though phytoremediation has a lot of promise, to date, it has had only limited success for several reasons.

Each contaminated site is different. Success in greenhouse studies often can not be replicated in the field, due to all the environmental and biological variables that occur.

Toxicity of site, especially with petroleum-contaminated soils, may kill most plants.

To breakdown toxic organic compounds in the soil, plants must by able to extract them. This is a major limitation.

The solution may be to use the plants’ roots as both a physical and nutritional “scaffolding” for microbes that can metabolize total petroleum hydrocarbons contaminants.

This is called “rhizoremediation”. Briefly, it takes advantage of the fact that plants can form symbiotic relationships with soil bacteria. (More about rhizoremediation later on)

Bottom line: The use of plants for phytoremediation of petroleum-contaminated soils is an emerging technology. Consequently, at the present time plants will likely play a limited role – at least directly – in remediating the Gulf oil spill.

References

1. Collins, Chris D. (2007) “Implementing Phytoremediation of Petroleum Hydrocarbons”, IN: Phytoremediation- Methods and Reviews, Methods in Biotechnology Vol. 23, pp. 99-108. Abstract

2. Gerhardt, K.E., X.-D. Huanga, B.R. Glicka and B.M. Greenberg (2009) “Phytoremediation and rhizoremediation of organic soil contaminants: Potential and challenges .” Plant Science Vol. 176, pp. 20-30. Abstract

3. Van Epps, A. (2006) “Phytoremediation of Petroleum Hydrocarbons”, Environmental Careers Organization, U.S. EPA. (PDF)

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A recent report that a species of aphid can make carotene thanks to a gene it apparently acquired from a fungus got me to thinking about whether genes can flow between different plant species.

When an organism incorporates genetic material from another organism without being the offspring of that organism, this is called horizontal gene transfer (HGT), a.k.a., “lateral gene transfer”. (Vertical gene transfer occurs between parent and offspring.)

HGT is common in bacteria – it’s often how bacteria acquire genes for drug resistances, for instance.

Although much less common, there are cases of HGT between microbes and plants. (An excellent review can be found here.) Interest the subject of HGT in plants has been stimulated by the proliferation of GMO’s, especially transgenic crop plants, see here, for example.

The natural transfer of genes between fungi, bacteria and plants has been established, but to what degree will likely have to await a more complete array of genetically-sequenced plants.

Parasites as a Bridge for Gene Flow Between Diverse Plant Species

Parasitic plants form vascular connections with their host plants via haustoria to allow transfer of nutrients, water, and even mRNAs ( see Ref 1 below). Thus, it has been suspected that HGT of nuclear genes may occur in parasitic plants.

In a recent report, scientists have found evidence for nuclear gene transfer in the parasitic plant Striga. “Striga hermonthica (Del.) Benth. is a devastating parasitic plant that infests members of the grass family (Poaceae), including major crops such as sorghum (Sorghum bicolor) and rice (Oryza sativa).” (from Ref 2 below)

Briefly, these investigators searched for grass-specific genes within the genome of Striga. They did indeed find at least one grass specific gene. Thus, “…our comparative genomics analysis of a eudicot parasite and its monocot hosts presents a clear case for nuclear HGT.” (from Ref 2)

Bottom line: Research over the past decade has provided evidence that gene movement between distantly related plant species occurs, and that plant parasites are likely a vehicle for such movement.

References

1. Westwood, J. H, J. I. Yoder, M. P. Timko, and C. W. dePamphilis (2010) “The evolution of parasitism in plants.” Trends in Plant Science Vol. 15, pp. 227-235. (Abstract)

2. Yoshida, S., S. Maruyama, H. Nozaki, and K. Shirasu (2010) “Horizontal gene transfer by the parasitic plant Striga hermonthica.” Science Vol. 328, p. 1128. (Abstract)

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I could answer this question here, but instead I’ll refer you to an excellent introduction provided by Dr. Colin Osborne. Or you can read the Wikipedia entry. (I’ll wait until you come back….)

OK….now you know that C₄ plants – such as the crops maize, sorghum, and sugarcane – add a preliminary step to regular old C₃ photosynthesis in order to increase the effective CO₂ concentration for the enzyme RuBisCo. This results in much less photorespiration in C₄ plants, which significantly increases their photosynthetic efficiency. Thus, in environments that promote photorespiration (e.g., hot, arid, and/or saline), C₄ plants seemingly have a distinct selective advantage over most C₃ plants.

Where Did C₄ plants Come From?

Although C₃ land plants have existed for nearly 500 million years, C₄ plants didn’t arise until about 25 to 35 million years ago.

Why?

It’s probably because when, for a number of reasons, Earth’s atmospheric CO₂ decreased to the point where photorespiration – which reduces photosynthetic efficiency – became a significant issue for plants, especially in hot, arid environments.

This limitation to photosynthetic productivity was reduced through the convergent evolution of C₄ photosynthesis in nearly 50 independent flowering plant lineages.

This happened independently so many times likely because many C₃ plants may already have had C₄-type photosynthesis occurring in their stems (e.g., see Ref. 1 below).

The first C₄ plants were probably grasses (monocots), followed several millions of years later by C₄ dicots. Today, grasses represent about 2/3 of the roughly 7,500 species of C₄ plants extant, with the rest split about evenly between dicots and sedges.

Despite the fact that C₄ plants make up only about 3% of plant species, they account for nearly 25% of terrestrial photosynthesis (see Refs. 2 & 3 below). “C₄ grasses and sedges dominate nearly all grasslands in the tropics, subtropics and warm temperate zones, and are major representatives of arid landscapes from the temperate zones to the tropics.” (from: Ref. 2)

“The evolution of grasses using C₄ photosynthesis and their sudden rise to ecological dominance 3 to 8 million years ago is among the most dramatic examples of biome assembly in the geological record. A growing body of work suggests that the patterns and drivers of C₄ grassland expansion were considerably more complex than originally assumed.” (from: Ref. 3)

Considering the Increasing Levels of Atmospheric CO₂, Whither C₄ Plants?

Fast forward to today, with the increasing levels of atmospheric carbon dioxide, thanks mainly to the burning of fossil fuels.

Are C₄ plants losing their CO₂ advantage over C₃ plants? And with atmospheric CO₂ levels projected to double or even triple within the next 100 years, could C₄ plants eventually disappear from the landscape? And what about C₄ crop plants such as maize that constitute a major source of food and fuel?

Though C₃ crop plants may benefit from increased atmospheric CO₂, C₄ crop plants will likely not benefit much, if at all. (e.g., see Ref. 4)

And what about native plant communities?

Although there is some evidence that increased CO₂ may promote the displacement of some C₄ grasses by C₃ dicots on some rangelands, a great deal of uncertainty currently exists regarding the effects of CO₂ alone. This is because of the multiple environmental effects (such as increased heat and drought) that accompany increasing levels of atmospheric CO₂. (e.g., see Ref. 5)

Bottom line: Though C₄ plants likely arose as a result of decreased levels of atmospheric CO₂, their fate is very uncertain in the face of the increasing levels of CO₂ that will likely occur in the centuries to come.

References

1. Hibberd, J.M. and Quick, W.P. (2002) “Characteristics of C₄ photosynthesis in stems and petioles of C₃ flowering plants.” Nature vol. 415, pp.451-454. (Abstract)

2. Sage, R.F. (2004) “The evolution of C₄ photosynthesis.” New Phytologist vol. 161, pp. 341–370. (Abstract)

3. Edwards, E.J., Osborne, C.P., Strömberg, C.A.E., Smith, S.A., and C₄ Grasses Consortium. (2010) “The origins of C₄ grasslands: Integrating evolutionary and ecosystem science.” Science vol. 328, pp. 587-591. (Abstract)

4. Leakey, A.D.B. (2009) “Rising atmospheric carbon dioxide concentration and the future of C₄ crops for food and fuel.” Proceedings of the Royal Society B vol. 276, pp. 2333-2343. (Abstract)

5. Bradley, B.A., Blumenthal, D.A., Wilcove, D.S., and Ziska, L.H. (2010) “Predicting plant invasions in an era of global change.” Trends in Ecology & Evolution vol. 25, pp. 310-318. (Abstract)

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Though you can’t easily see them using a light microscope, other organelles such as mitochondria are also streaming, along with the chloroplasts.

Why do this? The streaming is thought to facilitate the transport of materials within, as well as between, cells. This “stirs the pot”, so to speak, so that the cell doesn’t “stew in its own juices”.

How does it work? This movement is on intracellular tracks called microfilaments, composed of actin proteins. The organelles are attached to the actin filaments by myosin, which is a member of a group of proteins called motor proteins. These proteins are able to transform the chemical energy in ATP into mechanical energy. Thus, myosin uses the energy released during the breakdown of ATP to change its conformation and “walk” down the actin filament.

Avoiding the Sun
In leaf cells under bright sunlight, chloroplasts may have the ability to “move into the shade” of other chloroplasts, a phenomenon called photorelocation.
“Chloroplasts gather in areas irradiated with weak light to maximize photosynthesis (the accumulation response). They move away from areas irradiated with strong light to minimize damage of the photosynthetic apparatus (the avoidance response). The processes underlying these chloroplast movements can be divided into three parts: photoperception, signal transduction, and chloroplast movement.” (from Ref 1 below)

Photoperception: Evidence presented in ref. 1 supports the hypothesis that plant blue-light photoreceptors phototropins perceive the light.

Signal transduction: This probably involves calcium signaling.

Chloroplast movement: A recent paper (see Ref. 2 below) implicates other motor proteins in this photorelocation movement that are similar to kinesins. Though kinesins normally interact with other cellular filaments called microtubules, the authors suggest that in plants kinesin-like proteins may be interacting with actin filaments.

References
1. Suetsugua, N. and M. Wada (2007) “Chloroplast photorelocation movement mediated by phototropin family proteins in green plants.” Biological Chemistry Vol. 388, pp.927-935. (Abstract)
2. Suetsugua, N., N. Yamadab, T. Kagawac, H. Yonekurad, T.Q.P. Uyedad, A. Kadotab, and M. Wada (2010) “Two kinesin-like proteins mediate actin-based
chloroplast movement in Arabidopsis thaliana.” Proceedings of the National Academy of Sciences (USA) (Abstract)

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