Phototropism - Online Tutor, Practice Problems & Exam Prep

Signal Transduction and Response

Animals get a lot of credit for their ability to sense and respond to things going on in their environments. But plants don't get the credit they deserve. So in this lesson, we're going to look at plants' ability to sense and respond to what's going on in their environments. But before we get to the specifics of plants, I want to talk generally about cell signaling. And if you want a bigger refresher than this, I recommend you go and check out our videos on cell signaling that will cover these ideas in greater detail.

Now, signal transduction, which is going to result from molecular signals leading to some change in metabolism, gene expression, or something of the like, can be broken down into three steps. The first is reception, the signal is received. And this happens when a ligand, a signal molecule, binds to a receptor. And you can see we have two ligands being released by our cells here. We have a blue dot and a red square. These are very important biological molecules. I'm kidding, by the way. So these ligands will bind to their appropriate receptors, and notice how the receptors on these cells are the specific shape of these ligands. Right? This cell here has these receptors that will fit those red squares, and this one has receptors that will fit the blue dots, and guess what? That's because receptors and ligands are specific to each other. That is, ligands will only bind to specific receptors, and receptors will only bind specific ligands.

So our second step is transduction. The signal is carried through the cell and you can see that happening here, these little molecules interacting with each other trying to represent here a cascade of molecular interactions that are carrying the signal from these receptors. Lastly, we have some response in the cell. And this response is determined by the receptors present on the cell and the signal transduction pathways available to the cell. So you can see here that our cells are kind of growing outward towards each other, that is their response from those molecular signals they received.

Now, there are many types of signaling molecules, but the one that I want to talk about specifically is the hormones. Hormones are signaling molecules that will affect gene expression, cell division, and growth. So these are super important signaling molecules. As we said about ligands, ligands are specific to receptors, hormones are no exception there. A hormone's structure means that it will only bind to certain receptors that are meant to bind that hormone. So one way that an organism can mediate its cells' response, is with the presence or absence of specific receptors for a hormone. For example, if I release a blue dot hormone and some of my cells don't have any receptors for the blue dot hormone, then they won't receive the signal. So by including or not including the necessary receptor, you can mediate whether or not a cell will respond to that signal.

Now another really cool thing about hormones is their ability to amplify signals. Signal amplification will result when a few signaling molecules have a huge effect. So, in our little diagram we have our hormone here, and this hormone will influence this protein to do something to this molecule, and this molecule will have an effect on two molecules, and each of those blue molecules will have an effect on two pink molecules, and so on and so forth. So this is a huge oversimplification, but hopefully, you get the general idea that a single hormone can lead to an effect on many molecules downstream. That's what we mean by signal amplification, that you can amplify the signal as you carry it.

Often, the signal transduction pathways will involve what are called phosphorylation cascades, which is when you basically have a series of proteins that activate and deactivate each other through the transfer of phosphate groups. And you can see, sort of a model of what that might look like here where, you know, the ligand binding at this receptor leads to the activation of this protein, and that protein activates this protein, and that protein activates another protein, and so on and so forth, and you have a cascade of activation and deactivation, and the whole time they're transferring phosphate groups to essentially turn each other on and off.

You can also have what are called second messengers, and these are intracellular signaling molecules. So, they signal within the cell, and they're going to be involved in various signal transduction pathways. We have a little model for one right here, this is the, you know, an intracellular signal being carried through, you know, these various molecules. So this would be our second messenger, and that's going to activate a series of signaling molecules and elicit some response in the cell.

Photomorphogenesis

It should hopefully come as no surprise that one of the most important things for plants to be able to sense and respond to is light, as that is pretty much the basis for their survival. They need sunlight to carry out photosynthesis, and they need to carry out photosynthesis in order to produce sugars to eat and survive. So, etiolation is going to be the general term for plant responses to the absence of sunlight. These responses include growing towards the sun. If you detect an absence of sunlight, your response, logically, as a plant would be to grow towards the sun. One way they might accomplish this is by having longer internodes. Remember, this is the portion of the stem between leaves. So, if you increase that length, the length of the internodes, you're going to have a lower density of leaves, but that doesn't matter because you're not getting enough sunlight anyway. These are responses to an absence of sunlight. So by having longer internodes, you're going to have more vertical growth, and that's going to allow you to reach the sun. Now, one of the last symptoms, if you want to call it that, is chlorosis, and this is a lack of chlorophyll. You can see an example of that in this very sad-looking plant right here. Now, the reason plants will experience chlorosis as a response to an absence of sunlight is actually kind of an energy conservation mechanism. Why would you want to waste energy producing chloroplasts and chlorophyll if you're not going to be catching sunlight there and thus not performing photosynthesis anyway?

De-etiolation is going to be the opposite of etiolation. It's responses to sunlight, and these are going to be in part regulated by some photoreceptors called phytochromes, and we're going to talk about phytochromes in just a little bit. Now, just because I'm piling on the terminology right here, photomorphogenesis is going to be a big focus in this lesson, and that is essentially plant growth in response to different spectra of light. Hopefully, you recall from the section on photosynthesis that there are different types of light, and plants selectively absorb specific wavelengths of that light, meaning that they're going to have responses to different types of light. Depending on the type of light they're able to receive, they will output different responses and growth patterns based on these different spectra of light, again called photomorphogenesis, and that's photo for light, morpho for form, and genesis for origin. So it's basically the origin of the form due to light. You could think of it that way.

Now, tropisms are a big category of plant responses, and these are just movements of plants in response to something in the environment. Here we're focusing on phototropism. Growth toward or away from light. So essentially, plants responding to light. However, in other lessons, we'll be looking at different types of tropisms. Phototropism is going to be controlled or is going to require, I should say, photoreceptors. These are going to be proteins that respond to stimulation from certain wavelengths of light. So if you're going to grow toward or away from light, you're going to need something to detect that light, and photoreceptors are what plants are going to use. One type of photoreceptor is, or one class of photoreceptor, I should say, are phototropins. These are blue light photoreceptors. Blue light, if you recall, is one of the main lights that plants are going to absorb for photosynthesis. In addition, they also will absorb red light. And hopefully, you also will recall that blue light is what we consider higher energy light, that is to say, it's light of a higher frequency.

Here you can see a nice example of phototropism. We have this plant here that is bending towards the light source it has here, which is a lamp here, you know, obviously could be the sun too. Now, just as it's important for plants to be able to detect, detect light they need for photosynthesis, they also need to detect when they're not getting great light for photosynthesis. So, plants use red light and blue light for photosynthesis, as I've said, and red light, the red light they preferably use is roughly in the range of 600 to 700 nanometers. And blue light, roughly around 430 to 470 nanometers. These are going to be the main bands of light that plants want for photosynthesis. It should seem logical that they are very sensitive to those particular wavelengths. They're going to respond, they're going to have strong responses to those wavelengths.

They also can detect what is called far red light. This light is not absorbed by photosynthetic pigments, meaning it's not going to help photosynthesis. It actually passes through leaves and helps indicate shade. So what that means is, high up leaves that far red light will actually pass through them, and will hit parts of the plant that are underneath those top leaves getting the direct sunlight. So it's a way for plants that are not in direct sunlight to detect that they are in shade. Right? Because the leaves above are going to absorb those photosynthetic wavelengths, but that far red light's going to make it through. So it's going to allow the plant to say, "Oh, hey, I'm not getting the good sun right now. I got to get moving here. I got to do some phototropism."

Now, plants will actually use that far red light for a really nifty thing called the red-far red switch. This is a theoretical idea that red light will promote seed germination, and far red light will inhibit it. It's based on this particular type of photoreceptor mentioned earlier, phytochrome, which is a photoreversible photoreceptor, meaning that it actually is a molecule that has two different forms, and each of those forms reacts to a different wavelength of light. When the phytochrome absorbs red light, it changes its conformation and turns into the far red phytochrome. And when this far red phytochrome absorbs far red light, that will switch it back to the red phytochrome conformation. You can see this is a nifty mechanism to detect light and shade, or dark, however you want to think of it. Now, let me get my head out of the way here, and behind my head, you can see this nifty little graph showing you the wavelengths absorbed by the red phytochrome and the far red phytochrome confirmations. The light stimulations are going to cause phosphorylations and dephosphorylations that will induce these conformational changes. Don't need to worry too much about the biochemistry, just know that when the phytochrome in the red conformation absorbs red light, it switches to the far red conformation, and when that far red conformation absorbs far red light, it flips into the red-absorbing conformation.

All of this is part of a behavior known as shade avoidance, where far red light will actually cause plants to lengthen their stems or induce branching in an attempt to grow into direct light. Lengthening the stems is great if you just need to get up to the light. Right? If the problem is your verticality. But it can also induce branching, meaning, making the plant get bushier, so it has a greater area of absorption, which is sometimes the behavior needed in order to absorb more light. And you can see that these are responses to far red light, or shade, basically. Alright, with that let's flip the page.

Auxin

We can't give photoreceptors all the credit. Their job is to detect the light, but that signal for a plant to actually grow or move towards the light is carried by hormones, and specifically the hormone auxin. This is a super important plant hormone, technically its chemical name is indoleacetic acid. I'm going to call it auxin. You can see auxin right here, this lovely little molecule, and this hormone, again, is going to be responsible for plant growth towards light. Now, it has been found that coleoptiles, which you might recall are going to be those coverings on the cotyledons in monocots. These coverings, as the plant grows, will release auxin and allow seedlings to bend toward light. Now actually, the hypothesis for how this works is known as the Colin D. Wendt hypothesis, named after the scientists who helped develop it. It essentially says that auxin produced at the tip is going to move from the light side to the shade side of the plant, essentially, from the side getting the light to the side opposite the light source. So, as you can see in this figure, we have auxin, which probably can't read this little text, but these little pink dots in here are supposed to be auxin molecules. That auxin is produced at the tip in response to the light, and as the light source, if the light source is off-center from the plant, we'll actually get an asymmetric auxin distribution. You can see that happening here, where the auxin has actually become concentrated on the side opposite the light, so you could call it the shade side. This is the shade side, this would be the light side. So the auxin concentrates on the shade side and causes that side to grow more than the light side. So essentially, you get an asymmetric auxin distribution, and that results in asymmetric growth. And as we can see here, if you envision these little green boxes separated by the black lines on the outside of this plant diagram, as the plant cells, what's going to happen is the cells on the shade side, in response to auxin, are going to grow and be longer, they're going to elongate more than those on the side with light. Now, if you have the cells on one side getting longer than the cells on the other side, that's going to bend the plant away from the side where the cells are getting longer. The result for this is going to be that the actual tip of the plant grows towards the source of light. Now, how does this actually happen? How do these cells expand like this? Well, the leading hypothesis is known as the acid growth hypothesis. Basically, you have proton pumps, that will concentrate protons in the cell wall, and this will eventually lead to more water getting in the cell. Now, before we get ahead of ourselves, let's set up our diagram here. So here we have our membrane, that's the membrane, this is our cell wall, and this is cellulose, which, remember, is the polysaccharide that is going to make up plant cell walls, and the strands of cellulose will bind together with hydrogen bonds. And due to the structure of cellulose and these hydrogen bonds, the strands actually group together really tightly, so tightly that water is unable to get in. Water can't get into the cell wall, it is considered insoluble. Now, what's going to happen is these proton pumps, so this is going to be our proton pump, these proton pumps are going to pump hydrogens out of the cell. So what's going to happen is we're going to wind up with a high concentration of protons in the cell wall. Now there are these proteins in the cell wall called expansins, and their job is to loosen these hydrogen bonds in cellulose and that is going to allow water to get through. Normally, cellulose is watertight, the expansins, in response to this high concentration of protons, are going to loosen those hydrogen bonds and allow water, I'm sorry, allow water to get into the cell wall. Now the other thing that happens, right, if we're pumping protons into the cell wall and increasing our concentration in the cell wall, we're actually also going to be decreasing our concentration inside the cell. Hopefully you see this coming. What we have here, folks, is an electrochemical gradient, ever important in biology, and this electrochemical gradient is going to bring potassium into the cell. So potassium ions are going to enter the cell, and as we've learned, water follows ions. Right? Water moves based on osmotic gradients, osmotic gradients. So those potassiums entering the cell, right, is the inside, the outside, as the potassium ions enter the cell, water is going to follow. Actually, I shouldn't draw it this way because water is going to move through different channels. Right? Called, hopefully you remember, aquaporins. I'll squeeze that in here. Aquaporins.

So just to quickly summarize that, the acid growth hypothesis is essentially that by pumping protons into the cell wall, you will allow, or the plant cells will allow, water through into the cell wall and that water will get pumped inside the cell causing them to swell up, and that is how the plant cells can swell and elongate rapidly in response to auxin. Now that's not the only role auxin plays. Auxin has an important function in many different plant behaviors and functions. Now it's transported in a polar manner, from the shoots to the roots. Right? That's the direction it moves in, and it actually does this regardless of gravity. You could take the plant, flip it upside down, so that gravity is going the opposite way, but it, you know, so the shoots are on the bottom, the roots are on top, the plant's still going to transport oxen from the shoots to the roots. We call that polar transport because it's unidirectional. Now, oxen is going to play a role in a bunch of other functions, as we said, and a lot of these functions are actually also going to be related to light. So, you know, even though oxen plays a role in a wide variety of things, that the theme that ties it all together is light. So auxin plays a role in pattern formation, you know, the forms that develop in a developing plant. Also, phyllotaxy, which is the arrangement of leaves on a stem. It also has a role in something we'll talk about more in a later lesson called 'obsession', which is going to be the shedding of leaves and fruits as well. But hopefully, you can see this theme of light. Right? the arrangement of leaves to absorb that light, the shedding of leaves because they're not getting the light. Now, it also has a role in an idea, mentioned in a previous lesson called apical dominance, which is basically that the central plant stem is over the lateral stems and controls the growth of the plant. So auxin has a wide variety of functions that, you know, help hopefully it helps you remember what they are by thinking of that theme of getting light. Right? Ranging your leaves towards the light, growing towards the light, whatever it is.

Alright. With that, let's flip the page.

Photoderiodism

Plants, like animals, experience circadian rhythms. These are daily cycles that will do things like fluctuate the concentration of a particular hormone based on the time of day. Now these cycles are maintained and generated internally by what you could think of as a biological clock. However, they can be influenced by the external environment, and plants can have their circadian rhythms influenced by these things called cryptochromes. Now, these are photoreceptors that detect blue light and can have an influence on, for example, the turgidity of the plant in response to daylight.

Now, this plant over on the left, as you can see, is perked up, its leaves are open, and it's ready to absorb sunlight. This is going to be its, oops, its daytime condition. At night, the plant will lose turgidity and get droopy. Its leaves will close up, and it will not be in a prime position to absorb sunlight. However, it will give the plant certain other advantages. So this fluctuation over the day-night cycle will allow the plant to maximize its photosynthesis, and also, you know, do things like help protect it from environmental conditions by folding in at night.

Now some plants actually bloom in response to seasonal changes, and they detect this by sensing the lengths of the day-night cycle. Now we call these physiological responses photoperiodism. And some plants are considered long-day plants. These are going to be plants that bloom when the days are longest, which is going to be during the summer. Some plants are called short-day plants because they bloom when the days are shorter, and the days are going to be shorter during the spring and late summer or fall. Now, obviously, the days are going to be shortest in winter, but hopefully, you realize why it's not a great time for plants to be blooming. However, that cold does have an effect on plants' ability to bloom, some plants that is, and we call this vernalization. It's essentially a pretreatment with cold that is necessary for the photoperiod blooming response to take place. Essentially, these plants are still going to be detecting the lengths of those day-night cycles. Right? Examining the relative length of day to night to detect seasonal changes. However, they require a period of cold before that photoperiod response can kick in. And, essentially, this is a way to ensure that they have passed through winter and are going to, for example, bloom in the springtime. And we call this vernalization from the Latin, vernal, for spring.

Now it should be noted that some plants bloom when the plant blooms, but it's thought that it's actually a hormonal signal that causes flowering. Believe it or not, this hormone is yet to be discovered and we simply call the hypothetical hormone, florigen. So essentially, florigen is the hormone yet to be discovered that induces flowering. However, there's good evidence to suggest that such a hormone exists. It can just be trickier than you might realize to actually identify these things when, you know, you don't know exactly what to look for, kind of like searching for a needle in a haystack.