Tropisms and Hormones - Online Tutor, Practice Problems & Exam Prep

Plants exhibit various tropisms, such as phototropism and gravitropism, allowing them to respond to environmental stimuli. Gravitropism involves statoliths, dense amyloplasts that help roots and shoots sense gravity. Auxin distribution influences growth direction, with roots bending toward higher auxin concentrations. Additionally, plants respond to touch through thigmotropism, utilizing action potentials for rapid movements. Hormones like gibberellins promote stem elongation and seed germination, while abscisic acid regulates dormancy and stomatal closure. Ethylene, a gaseous hormone, plays a role in senescence and fruit ripening, demonstrating the intricate balance of plant growth regulation.

Gravitropism

Hi. In this lesson, we'll be talking about tropisms and plants' responses to certain important hormones for growth. Now tropisms, you might recall, are an organism's movement in response to an environmental stimulus. We talked about phototropism, or plants' response to light, but plants can also respond to gravity, and we call this movement in response to gravity gravitropism. This phenomenon is found both in roots and in shoots. One of the hypotheses for how this works is called the statolith hypothesis. A statolith is a specialized type of amyloplast. An amyloplast is an organelle used to store starch granules. Of course, starch is the eventual product of the sugars that plants produce by photosynthesis. Statoliths are a special type of amyloplast that is super dense in order to cause it to sink in a cell, and these are going to be used to detect gravity. The idea behind the statolith hypothesis is that because statoliths are denser than water, they'll sink to the bottom of cells and activate a sensory signal there, which will allow the plant cell to sense the direction of gravity. Cells in the root cap contain statoliths, and respond accordingly. In this image, we have a plant cell, and you can see these blobs at the bottom here, these are statoliths. They obviously sink and set off some sensory signal.

I've also shown a past and present photo. This photo on the left is from the past; the one on the right is from the present. These photos are part of an art installation at a wonderful art museum in Massachusetts called the Massachusetts Museum of Contemporary Art, where these trees were initially planted upside down. However, over time, you can see from the photo behind my head, those trees have warped and bent to right themselves, essentially. This is going to be in part due to their ability to sense gravity. This is also in part due to phototropism. A combination of the two, but I think it really nicely shows how powerful these tropisms can be, especially if you give the plants enough time to respond to them.

Auxin might also have an influence on gravitropism. It's thought that the distribution within the root will influence the direction that the root grows, in a similar manner to how auxin distribution in the sprout or the shoot tip will influence which direction that shoot tip grows. Auxin in a vertical root is going to have an even distribution, and that's what this is supposed to indicate. Even auxin distribution. And basically, the idea behind that is that the arrows on either side are the same. Right? That's supposed to be the indicator of the amount of auxin moving through the root.

In a case where we have uneven auxin distribution, like this root here, you can see that it is not vertical, which is going to cause its auxin distribution to be uneven. You can see that there's more auxin on the underside. More auxin on the underside and less auxin up top, and that is going to cause this root to bend down towards the side with more auxin. So, similar idea to the shoots except it's actually going to work in the opposite way. Remember, the shoot tips will bend away from the side with more auxin. That shaded side that has more auxin, the shoot tips are going to bend away from that. Here, the root is actually going to bend towards the side with more auxin. Similar concept, uneven auxin distribution, but it actually works in the opposite way. With that, let's flip the page.

Thigmotropism and Cytokinins

Plants are able to sense and respond to physical stimuli, like touch or wind. Growth or movement in response is called thigmotropism. Climbing plants use this in order to climb. They grow out tendrils, like you can see here, and when they touch a surface or something to grab onto, there's a response that causes them to grow and wrap around, and grip that object or surface, whatever they're touching. Venus flytraps will also use this in order to shut on an unsuspecting insect or some form of prey. And this actually requires something pretty special because, as you can imagine, if you've ever tried to swat a fly, they can move pretty quickly. So, the plant has to respond to that sensation of touch and move very fast to shut closed on the fly. So what a plant is going to need to do is use an action potential, and this is an electric signal, like what goes through the nerves in our bodies, but plants don't have nerves. They actually will move their action potential through plasmodesmata. So it's not going to be quite as fast as moving through a nerve. However, it will still be quite fast signal, and if you're wondering how they generate that electrical signal, it's actually done by moving ions across the membrane. The specifics of this get pretty complicated. I wouldn't worry about it too much here. Just know that by moving ions around the membrane, they can create electric signals that they're able to transmit. And we will learn much more about action potentials when we talk about the nervous system.

Now it's also worth noting that plants that get a lot of physical stimulus, for example, generally speaking, they often will not grow as tall. This is because of their response to wind. If they're getting a lot of physical stimuli, it could be from lots of wind gusts. And in order to not get blown over or have the plant body get broken in the wind, plants will actually restrict their vertical growth so that they are less susceptible to being damaged by the wind. So this comes in many different flavors and varieties. All you really need to know is that plants are able to sense and respond to physical stimuli.

Now plant growth is hormone regulated, and it is going to be regulated by a hormone, or a class of hormones, called cytokinins, which actually regulate growth by regulating the cell cycle. If you don't remember the cell cycle very well, I highly recommend you go back and check out the videos on cell division, especially the video on regulation. If you remember that stuff, hopefully, you recall that there is a gate at G2. This is sort of like a fail-safe mechanism to make sure that, right before mitosis happens (that's what that 'M' is), that all systems are go; this is the point of no return, and cytokinins are actually going to cause cells to pass that G2 checkpoint and continue dividing. They are the gatekeepers essentially. This hormone is produced in the roots and is transported through xylem to target tissues. It is going from roots up. And it's thought that in terms of apical dominance, which you might recall is in part regulated by auxin, cytokinins actually play a role in there too. And the ratio of cytokinins to auxin is, in part, what determines that apical dominance. It's thought that, cytokinins play a role in growing outward, in growing bushiness, whereas auxin plays a role in vertical growth.

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

Plant Hormones and Senescence

Gibberellins are a class of plant hormone involved in the regulation of growth. They induce cell division in the stem, leading to stem elongation. They're also involved in fruit growth and seed germination. Now, with our three plants that you can see right here, we are showing the difference in plant morphology based on exposure to gibberellins, which are confusingly abbreviated GA. I don't make the rules, guys. I don't name these things. So, as you can see, this plant in the middle is a normal-looking plant, and it has normal internode length right there, and this plant received a normal amount of Gibberellins. This plant received a lot of Gibberellins. And that's why it has this crazy big internode length right there and just a generally long stem. In contrast, this little guy over on the end didn't get a lot of gibberellins, and that's why it has a very short stem, and especially has very tiny internode length.

In terms of germination, it's actually water being absorbed into the seed that will stimulate the gibberellins and those, as they are hormones are going to influence the gene expression of the seed, and that will lead to the production of amylase, which will start turning the starch in here into sugars, which will feed this organism and lead to its growth, and of course, breaking free from its seed coffin.

Now, acetic acid is another important hormone that we've mentioned previously. This hormone is involved in stomata opening and seed dormancy. Now, you might recall that abscisic acid is produced in the roots and will make its way up to the shoots where it will influence stomatal closing. Now, we had talked about before how blue light photoreceptors can play a role in stomata opening and closing. Abscisic acid will supersede whatever signal those blue light photoreceptors are giving. So even if the blue light, you know, even if the signal coming from the blue light photoreceptors is contradictory to the abscisic acid, the abscisic acid wins out. That hormonal signal is more powerful. Now, dormancy is that period of arrested growth prior to germination, and it doesn't end until there's the right stimuli and conditions for the seed to respond to. Abscisic acid is thought to actually inhibit germination. And these gibberellins we were just talking about induce germination. So, I hope you're noticing a pattern with these hormones. They often have balancing roles. One will turn something on, and the other will turn something off. This gives an organism a ton of flexibility and control over these important systems.

Lastly, I just want to mention brassinosteroids. These are a class of plant hormone that are involved in, growth as well, but specifically in cell elongation and division. And they're going to play a role in regulating overall plant body size. Now, the last thing I want to talk about is senescence. This is the sort of technical term for aging. This is what you might think of as biological aging, and it's usually marked by gradual deterioration of function. Right? So your cells gradually deteriorate in function as they age, you know, the organism as a whole gradually deteriorates in function as it ages. And there's actually a hormone for plants that is closely associated with this process. That hormone is ethylene. It's actually a gas at room temperature on earth. So, under normal conditions, this is a gaseous hormone, which means it's going to be able to diffuse through the air. That's, well, get back to that in a second actually. So, ethylene plays a role in senescence. One of those roles is in the process of abscission, which is going to be the shedding of part of an organism. In plants, usually we talk about leaf abscission, and leaves as you know, are upsized during the fall. That's why I have this beautiful fall background behind my head. Now the reason for that is because cells in the petiole, which is that stalk on the leaf, those cells are going to react to ethylene, and that ethylene is going to trigger enzymes to degrade the cell walls there in the petiole. That's why the leaf will snap off right perfectly at the base of the petiole. And if you've noticed that, but leaves always break off right at that point. It's because the cells there are degraded due to enzymes, which are signaled by this hormone. Now, fruits ripen when exposed to ethylene. As you can see in this nice image here, we have a ripening and aging banana, and as the fruits are exposed to ethylene, starch is converted to sugar, and the cell walls are broken down. So, that's why if you eat a really super green banana like that, it's going to be tougher and also just not nearly as sweet. It's going to be kind of like potato-y, not very palatable. As starch is converted into sugar, the fruit gets sweeter. That's why these fruits down on, or this banana down in the end, you know, this has been exposed to a lot of ethylene. It's going to be really sweet and sugary if you bite into it. So, that is why, for example, if you have an overly ripe banana near some green bananas, those bananas will go bad really quickly because ethylene is a gaseous hormone. And that overly ripe banana is going to be giving off ethylene and influencing those under-ripened bananas to ripen probably faster than they should. So, a nice little life hack here. If you're ever struggling with unripened avocados, they're too hard, throw them in a bag with a ripe banana. Those guys will get nice real fast. It's all ethylene. It's a beautiful thing. Alright. That's all I have for this video. See you guys next time.

Here’s what students ask on this topic:

What is phototropism and how does it affect plant growth?

Phototropism is the growth response of a plant to light direction. Plants exhibit positive phototropism when their shoots grow towards light, optimizing photosynthesis. This response is primarily regulated by the hormone auxin, which redistributes to the shaded side of the plant, causing cells there to elongate more than those on the light-exposed side. This differential growth causes the plant to bend towards the light source. Phototropism ensures that plants maximize their light absorption, which is crucial for their energy production and overall growth.

How do statoliths contribute to gravitropism in plants?

Statoliths are dense amyloplasts that play a crucial role in gravitropism, the growth response of plants to gravity. Found in specialized cells in the root cap, statoliths settle at the bottom of these cells due to their density. This settling triggers a sensory signal that helps the plant determine the direction of gravity. The plant then adjusts its growth accordingly, with roots growing downward (positive gravitropism) and shoots growing upward (negative gravitropism). This mechanism ensures that roots anchor the plant and access water and nutrients, while shoots reach for light.

What role do gibberellins play in plant growth and development?

Gibberellins are a class of plant hormones that significantly influence growth and development. They promote stem elongation by stimulating cell division and elongation in the stem. Gibberellins also play a crucial role in seed germination by activating genes that produce enzymes like amylase, which convert stored starches into sugars, providing energy for the growing seedling. Additionally, gibberellins are involved in fruit growth and can break seed dormancy, ensuring that seeds germinate under favorable conditions. Their overall effect is to enhance various growth processes, contributing to the plant's development.

How does ethylene influence fruit ripening and leaf abscission?

Ethylene is a gaseous plant hormone that plays a key role in fruit ripening and leaf abscission. During fruit ripening, ethylene promotes the conversion of starches to sugars, softening of cell walls, and changes in color, making the fruit sweeter and more palatable. In leaf abscission, ethylene triggers the production of enzymes that degrade cell walls in the petiole, causing the leaf to detach from the plant. This process is particularly noticeable in the fall when leaves shed. Ethylene's ability to diffuse through the air allows it to coordinate these processes efficiently across the plant.

What is the role of abscisic acid in plant dormancy and stress responses?

Abscisic acid (ABA) is a plant hormone that plays a critical role in inducing dormancy and managing stress responses. In seeds, ABA promotes dormancy by inhibiting germination until conditions are favorable. It also regulates stomatal closure in response to drought stress, reducing water loss by limiting transpiration. ABA is produced in the roots and transported to the shoots, where it overrides signals from blue light photoreceptors to close stomata. This hormone helps plants survive adverse conditions by conserving resources and delaying growth until the environment is suitable for development.