Okay. You know what just completely blows my mind? Sunflowers. Sunflowers. Yeah, sunflowers. They do this incredible like daily dance, right? Tracking the sun across the sky. It's like they're these tiny little golden satellites or something. I never thought about it that way, but yeah, they really do. But here's the thing that I find even more amazing. Overnight, they actually pivot back. Like they turn themselves back towards the east, all set and ready for the next sunrise. Oh, wow. I didn't know that. I know. It's like this tiny little miracle that happens every single day right in front of us. And it just goes to show how responsive plants really are. You know, we tend to think of them as being so passive. Total misconception, right? Because they're not just responding to light. I mean, think about it. They're constantly taking in all this information from their environment, gravity, touch, even threats, and then they're reacting to all of that in these totally fascinating ways. Absolutely. It really makes you appreciate the complexity of that still, seemingly quiet green world around us. And that's exactly what we're diving into today. We're cracking open this chapter. It's called plant responses to internal and external signals. And it's basically like getting this backstage pass to the secret lives of plants. You know, how do they actually experience and navigate their world both internally and externally? It really is. This chapter breaks down all those fundamental processes that allow plants to not just survive, but to thrive in all these incredibly diverse environments that they find themselves in. We're going to explore everything from the very first moment a plant cell receives a signal to the intricate chemical communication that goes on inside the plant and of course how they respond to light and all those other physical cues that shape their growth and even how they defend themselves. Right. Okay. So, this chapter lays everything out in a super structured way which I really appreciate. It's like we have a road map for this deep dive. So, first up, we're going to unpack these signal transduction pathways. Basically, it's all about how a plant takes some external cue from the environment and turns it into an internal action. Kind of like a translation surface, right? Yeah. Exactly. It's like decoding the language of the plant world. Love it. Okay. So, then after that, we're going to plunge head first into this world of plant hormones. You know, those teeny tiny chemical messengers that have this surprisingly huge influence on everything the plant does. It really is amazing how these tiny molecules can orchestrate such massive changes in the plant. I know. Okay. And then obviously we've got to shine a spotlight on plant responses to light. I mean, it's kind of the most crucial thing for them, right? For sure. But then we'll also explore how plants react to other stimuli. You know, things like gravity and touch and even environmental stresses, right? The full sensory experience. And last but not least, we'll investigate how plants gear up and defend themselves against all sorts of attacks from hungry herbivores or pesky pathogens. It's like plant warfare. Exactly. Our mission today is to untangle all these intricate processes. By the end of this, we want you to walk away with a much deeper appreciation for how dynamic and responsive plants really are. And hopefully, we'll have a few of those aha moments along the way. You know, where you suddenly see plants in this whole new light. Yeah, pun intended, I'm sure. Okay, so let's dive straight in. First off, signal transduction. Right off of the bat, the chapter tackles this super common but totally wrong idea that plants are just these kind of like static beings, like inert blobs of green just sitting there, right? Like they're not doing anything. Exactly. But nothing could be further from the truth. Right. Definitely not. When you look at a plant cell under a microscope, it's just as intricate and complex as an animal cell. I mean the genome of some plants like paris japonica are absolutely enormous even bigger than the human genome. Wow. Seriously. So what's the main difference then? If they're both so complex, how come we don't see plants, you know, running around or doing all the things animals do? It's not about complexity. The key difference lies in how they express their response to a stimulus. So when an animal reacts to something, it usually involves movement, right? It'll run or jump or fly away, right? But plants on the other hand, they react by changing how they grow and develop. And this classic example of a potato left in a dark cupboard, it perfectly illustrates this. Yeah, I was just thinking about that. You know, those pale, spinly sprouts reaching for any little bit of light they can find. Exactly. That's what we call edilation. And what's fascinating is that those seemingly weak characteristics, the pale stems, the tiny leaves, the limited root system, they're all actually really clever adaptations to growing in darkness. Okay, so let's break that down a bit. Why would a plant want to be all pale and elongated in the dark? What's the evolutionary advantage there? So, imagine you're that potato stuck in a dark cupboard. If you start putting all your energy into growing big green leaves, well, first of all, they're probably going to get damaged as they try to push through whatever's in their way. Plus, there's no sunlight to use for photosynthesis. So, all that chlorophyll production would be a total waste of energy. Ah, that makes sense. And without any light, water loss through those big leaves would be a problem, too. So there's no need for a huge root system just yet. So the plant's basically in energy saving mode. Exactly. It's like laser focused on one thing, reaching the light as fast as possible. And then boom, it breaks through to the light and everything changes. That's datilation, right? Greening as we normally call it. Precisely. Once that chute hits, it undergoes this dramatic transformation. Stem elongation slows way down. The leaves unfurl and expand. the roots start to grow longer and branch out and of course chlorophyll production ramps up turning the plant green. And the key player that kicks off this whole transformation is phytochrome, right? That light sensing pigment. You got it. Okay. So, how does light hitting this phytochrome molecule actually trigger all these changes inside the plant? I mean, it's not like the light's physically pushing and pulling on the plant cells, right? Definitely not. That's where these signal transduction pathways come in. It's like a chain reaction of molecular events. And typically it goes through these three key stages. Reception, transduction, and response. Reception, transduction, response. Got it. Okay. So what happens during reception? So first, a specific receptor protein inside the plant detects the stimulus. In this case, it's light. And when that happens, the receptor protein actually changes shape. It's like a little molecular switch flipping on. Okay, I'm picturing it. So for deilation the main light receptor is phytochrome and it's located in the cytoplasm of the plant cell. So the phytochrome grabs the light signal. What happens next? What's this transduction stage about? That's where the signal gets relayed and amplified. It involves all these other relay proteins passing the message along kind of like a game of molecular telephone. And they often activate each other in a cascade. And then you have these small molecules and ions getting involved too. We call them second messengers. And they help amplify the signal even more. So it's like a chain reaction that keeps growing bigger. Exactly. In deilation, two key second messengers are calcium ions. Those are CO2 plus and cyclic GMP which is cgmp. So when phytochrome is activated by light, it triggers this opening of calcium channels in the cell membrane. So more calcium floods into the cell. Exactly. And at the same time it activates an enzyme called granil cyclists which then produces cgmp. So now you've got this surge in both calcium and cgmp and they both act as signals inside the cell that go on to activate specific protein kinases. Okay, I remember those from bio class. Protein kinases are enzymes that attach phosphate groups to other proteins, right? Like a little molecular post-edited note telling the protein what to do. Yeah, that's a great analogy. And these activated protein kinosis they lead to the final stage the response. And this can happen in two main ways. Transcriptional regulation and post-transational modification. Okay, break those down for me. So transcriptional regulation basically means that the signal pathway controls which genes are turned on or off. It's like the plants deciding which blueprints to use to build new proteins. Makes sense. And post-transational modification, that's where existing proteins get tweaked or modified. It's like taking a protein that's already there and giving it a new job or making it work differently. And phosphorilation, that's where a phosphate group gets added, is a super common way to do this, right? Like adding that molecular post-it note. Exactly. So in deilation all these pathways ultimately lead to the production of all sorts of proteins involved in greening enzymes for photosynthesis proteins that build chlorophyll and even proteins that affect the levels of those plant hormones we were talking about before like oxen and brassinoststeroids. Wait, so the levels of those hormones actually change during this greening process? They do. Interestingly oxin and breaststeroid levels actually go down during deichilation and this helps to slow down stem elongation. So, it's not just about making new proteins. It's about adjusting those hormone levels, too. Wow, that's fascinating. It's like this intricate web of interactions. It really is. And to really tease apart all these steps in the signal transduction pathway. Researchers use all kinds of cool tools. They study mutants like certain tomato plants that have messed up phytochrome. And they use all sorts of molecular biology techniques to track the signals and figure out how it all works. It's amazing how much we've learned about these intricate processes. But okay, plants are clearly masters of communication within their own cells, but they also need to communicate between different parts of the plant, right? I mean, how does a root know what a leaf is doing and vice versa? That's a great point. And that's where those fascinating plant hormones come into play. Okay, so plant hormones, I've heard that the whole concept is a bit more complicated in plants than it is in animals, right? Like it's not quite the same story. Yeah, that's right. It's not a perfect one:one comparison. You see, in animals, hormones are usually produced in specific glands and then they travel through the bloodstream to act on target cells somewhere else in the body. Right. Like a delivery service for hormones. Exactly. But plants don't have a circulatory system like that. So some plant signaling molecules act more locally within the same tissues where they're produced. So it's less like a long-distance delivery and more like a local message. Kind of. Yeah. And that's why you sometimes hear the term plant growth regulator used instead of hormone. It's a broader term, but this chapter sticks with plant hormone mostly. Yeah, they do. But they emphasize that these hormones are still potent signals even though they're produced in tiny amounts. And what's super interesting is that one single hormone can have multiple effects depending on where it's acting in the plant, how much of it is present, and even the plant stage of development. Okay? So, it's not just one hormone, one job. It's a lot more complex than that. way more complex. And to make things even more intricate, a lot of plant processes are regulated by multiple hormones working together. It's like this big hormonal committee making decisions. Okay, I think I'm starting to get a headache just thinking about all that, but let's try to unpack it a bit. Let's run through some of the major plant hormones and their key roles. First up, oxin. It's like the celebrity hormone, right? It is pretty famous. Oxin or IAA, it's short for indoic acid. It's involved in a ton of different processes. Yeah. The chapter mentions it's produced in lots of different places in the plant, like the tips of shoots and roots, young leaves, developing seeds and fruits. Yep, that's right. And one of its most well-known roles is stimulating stem elongation. You know, making those shoots grow taller. It was actually discovered through those early experiments on how plants bend toward light. We call that photo troism. Right. It's like the plant is reaching for the sun. Exactly. But there's a twist. Oxin only seems to have this growthpromoting effect at low concentrations. If there's too much of it, it can actually inhibit growth. So, it's like a Goldilock situation. Not too much, not too little, just the right amount of oxin, right? And aside from stem elongation, oxin does so much more. It helps roots grow. It's essential for fruit development. And it even helps maintain something called apical dominance where the main stem sort of bullies those side branches and keeps them from growing too much. Oh wow. So, oxin is kind of a control freak in a way. Yeah. And it also plays a role in how plants respond to gravity which we call gravitropism. And it can even delay the process of leaves or fruits dropping off which is called obsession. Okay. So oxin is the grow taller, get rootsy and maintain control hormone. Got it. What about cytokinins? What are those all about? Cytokinans are all about cell division. They're the ones stimulating those cells to divide and multiply both in the shoots and the roots. And they're mainly produced in the roots and then transported up to other parts of the plant through the xyllem. The xylem that's the water transport system in plants. Right. Exactly. And interestingly, cytoinins work against oxen when it comes to apical dominance. They promote those side branches to grow out. So it's like they're telling those branches, "Hey, don't let the main stem boss you around." I love that. It's a hormonal power struggle. Totally. And besides that, cytoinins also help move nutrients around the plant. They can stimulate seed germination in some species. And get this, they even have anti-aging effects, delaying the process of leaf scinessence, which is when those leaves start to yellow and die. Okay, so cytokinins are the divide and conquer and stay youthful hormones. Awesome. What about gibberelins? I remember something about foolish ceiling disease in the chapter. Ah yes, gibberelins or gas for sure. They were actually discovered because of this weird disease in rice seedlings where a fungal infection would make the seedlings grow crazy tall and weak. You know, like a foolish seedling. Right. I get it. So plants naturally produce gibberelins in various parts like those growing tips, young leaves, and developing seeds. And they're all about promoting elongation, but in this case, it's more about stem elongation. So they work with oxen to make those stem shoot up. Exactly. They often work together, but gibberelins can also act on their own. And they're important for a lot of other things too, like pollen development, fruit growth, seed development, seed germination. They even play a role in pollen tube growth. And they can influence things like sex determination in some plants and the transition from a juvenile to an adult phase. Okay. Gibbrelins are the grow long and get reproductive hormones. Got it. Now, let's talk about absisic acid, ABA, right? I always think of it as the hormone that makes leaves fall off, but the chapter mentions it does way more than that. You're right. The name can be a bit misleading. Absic acid is actually a growth inhibitor, and it often works against those other growth promoting hormones we've been talking about. It's synthesized in almost all plant cells, and it's involved in a bunch of important stuff like closing those little pores on the leaves called stomata when the plant's stressed out and needs to conserve water. Right. To prevent water loss. Exactly. It's also crucial for seed dormcancy, making sure those seeds don't germinate too early before conditions are right. And it plays a role in leaf scinessence and helping the plant tolerate all sorts of drying out or desiccation stress. So ABA is more of a slow down, conserve resources and batten down the hatches kind of hormone. It's like the plant's emergency break. I like that analogy. And then we have ethylene, which is unique because it's actually a gas, right? Ethylene, the fruit ripening hormone. That's the one everyone's heard of. It's definitely the most famous, but it's involved in way more than just making your bananas go brown. It's produced by most parts of the plant, and it's especially important during those times of stress. You know, like when a plant gets injured or when the fruit is ripening or leaves are falling off. High levels of oxin can also trigger ethylene production. So, it's like the plant's stress signal kind of. Yeah. And besides fruit ripening, ethylene is responsible for what we call the triple response in seedlings. So when a seedling is growing underground and it hits an obstacle, it triggers this triple response. The stem elongation slows down, the stem thickens, and it starts growing horizontally. It's like the plant saying, "Oh, there's a rock in the way. Time to change direction." Exactly. It's a way for the seedling to push past that obstacle and reach the surface. And ethylene also plays a role in leaf absition, that process of shedding leaves. And it's involved in scinessence, which is basically programmed cell death. Plus, it has this cool autoc catalytic effect during fruit ripening, meaning that as fruit starts to ripen, it releases more and more ethylene, making it ripen even faster. It's like a ripening chain reaction. Exactly. That's why one bad apple can spoil the whole bunch. And we've actually figured out how to manipulate ethylene levels to control fruit ripening, which is super important for storage and transportation. That's awesome. Okay, what about brass androids? I remember the chapter mentioning they're kind of similar to steroid hormones in animals. That's right. Brassinossteroids are present in all plant tissues and they act more locally. They're involved in a bunch of things like promoting cell expansion and division, influencing root growth and playing a role in the differentiation of xylem tissue. Xylm, that's the water transporting tissue, right? Yes. And they also promote seed germination and pollen tube elongation. It took a while for scientists to really figure out what brassinossteroids do because some of their effects overlap with oxen. Okay. So, they're like oxen's lesserk known cousin kind of. Yeah, but they're definitely important players. And then there are jasmineates. The chapter mentioned they're connected to the scent of jasmine. Yeah, that's because one of the jasmineates, methyl jasmine, actually contributes to that characteristic jasmine fragrance. Jasmminates are derived from fatty acids, and they're produced in several parts of the plant. They can travel through the phe, which is the sugar transporting tissue, and they're probably best known for their role in plant defense. Oh, right. They help plants fight off attackers. Exactly. But they're also involved in a bunch of other processes like fruit ripening, flower development, pollen production, and even helping those tendrils coil around things. Wow, they really do a bit of everything they do. And they also interact with other hormones, so it's all very interconnected. And finally, we have strigalactones. They're a relatively new addition to the plant hormone family. New on the scene, huh? What's their story? So stringalactones are made from carotenoids and they're mainly produced in the roots especially when the plant is low on phosphate or when there's a lot of oxin flowing down from the chute. Okay. So they're produced under specific conditions right and they're involved in some pretty cool stuff like promoting seed germination, controlling branching and even attracting those beneficial microisal fungi to the roots. Microisal fungi. Those are the fungi that help plants absorb nutrients from the soil. Right. Exactly. But here's a twist. Stringalactones were actually first discovered because of their role in a less than beneficial interaction. They stimulate the germination of certain parasitic weeds like stria, which is also known as witchweed. Oh, no. So, they're helping those pesky weeds grow. Yeah, unfortunately. But understanding how they work could help us develop strategies to control those weeds. That makes sense. Oh, okay. Wow, that was a whirlwind tour of the plant hormone world. I mean, who knew plants had such complex chemical communication going on inside them? It's like a whole soap opera happening in there. I know, right? And it's amazing how much we've learned about these hormones from those early experiments on photo tropism. You know, those classic studies by Charles Darwin and his son Francis and later by Peter Boyce Jensen. Right. I vaguely remember those from my botney class. They were working with those grass seedlings, weren't they? Yeah. Uh they were using koptiles which are basically those protective sheets that cover the emerging chute and they were trying to figure out how plants bend toward light and they discovered that the tip of the seedling was the key. Right. Like that's where the light was being sensed. Exactly. The Darwins showed that if you covered the tip the seedling wouldn't bend towards the light. And then Boyce and Jensen went a step further and showed that there must be some mobile chemical signal being produced in the tip that travels down the chute to cause that bending. So it wasn't just the light itself directly causing the bending. It was this chemical messenger, right? And that chemical messenger of course turned out to be oxin. And we now know that the koloptiles bend toward light because there's a higher concentration of oxin on the shaded side of the chute. This causes the cells on that side to elongate more than the cells on the light side. And that differential growth is what makes the chute bend. And this transport of oxin, it's directional. It only moves from the tip toward the base, not the other way around. So it's like a one-way street for oxin. Exactly. And we call that polar transport. And it's because those oxin transport proteins, they're not evenly distributed in the cell membrane. They're concentrated at the basil end of the cell which creates that directionality. Okay. So oxin is flowing down from the tip. But how does it actually cause those cells to elongate? The chapter goes into this acid growth hypothesis. Yeah, it's a pretty cool explanation. So according to this hypothesis, when oxin reaches a cell, it triggers these proton pumps in the cell membrane. And these pumps, they start pumping hydrogen ions or protons out of the cell and into the cell wall space. So they're basically acidifying the cell wall. Exactly. And this drop in pH, it activates these special enzymes called expans that loosen up the cell wall, making it more flexible. So it's like loosening your belt after a big meal. Huh. Kind of. Yeah. And at the same time, the plant cell is taking up water. So, it's got this internal pressure building up called tur pressure. And with the cell wall loosened up, the cell can now elongate and expand under that pressure. It's amazing how such a tiny molecule can trigger this whole cascade of events at the cellular level, leading to a visible change in how the plant grows. But oxen does so much more than just make stems bend toward light. Right. Right. It plays a huge role in shaping the overall architecture of the plant. It determines where new leaves will emerge, the patterns of veins in those leaves, and even the activity of the vascular cambium, which is that layer of cells that produces the wood and trees. Wow. So, it's involved in both the fine details and the big picture. Exactly. And we've even figured out how to use both natural and synthetic oxins to our advantage in agriculture and horiculture. We use a natural oxin called IBA to help those cutings root when we're propagating plants. Right? Like when you take a cutting from a house plant to make a new one. Exactly. And then synthetic oxins like 20040 are widely used as herbicides because they're really good at killing broadleaf weeds which are mostly diecuts while leaving those grasses relatively unharmed. This is all about targeting specific types of plants. Yep. And oxen is even used commercially to promote fruit development. That's how they make those seedless tomatoes you see in the grocery store. Really? I had no idea. It's pretty cool. And another fascinating aspect of oxen is its role in apical dominance where the main central stem inhibits the growth of those lateral buds. Right. So the top bud is basically telling those lower buds, don't even think about growing. Exactly. And oxin plays a big role in that. But it's not alone. Cytokinins, those cell division promoters we talked about earlier, they actually counteract oxen and promote lateral bud growth. And then struggle which are produced in response to oxin also contribute to that repression of bud growth. It's like this complex hormonal balancing act going on inside the plant and removing that apical bud throws off the balance allowing those lower buds to finally grow. So pruning your plants is basically a way of manipulating their hormones. Exactly. And speaking of cytokinins, they were actually first discovered because of their ability to stimulate cell division in plant tissue culture. You know when you grow those plant cells in a petri dish, right? Well, cytoinins are produced in those actively growing tissues like roots, embryos, and developing fruits and they travel through the xyllem and they work together with oxen to regulate both cell division and cell differentiation. So they're like a team. Totally. And the ratio of cytokinine to oxin actually determines what happens to those undifferentiated plant cells that we call a callus. If there's more cytokinine, they'll develop into shoots. If there's more oxin, they'll develop into roots. Oh wow, that's so cool. Right. And codinins also have those anti-aging effects. They help delay the breakdown of proteins and they stimulate the synthesis of new RNA and protein. It's like the plant's fountain of youth. And then we have gibbrelins which not only cause those super tall ray seedlings but also play a role in fruit development and seed germination. That whole mechanism with the embryo releasing gibberelins to signal the seed to start breaking down starch is so elegant. It really is. And then we have obsisic acid ABA which does the opposite. It promotes seed dormcancy and helps the plant cope with drought stress. So the balance between ABA and gibberons is super important. Right. Like a seessaw of hormones. Exactly. And then there's ethylene which not only ripens fruit but also helps seedlings navigate obstacles underground with that triple response. And the chapter mentioned those arabidopsis mutants that helped us figure out how ethylene signaling works. Yeah, those mutants were super important. They either didn't respond to ethylene at all or they showed that triple response all the time even without any obstacle. And then there are those newer hormones like brass nosteroids, jasmminates and stricolactones which add even more layers of complexity to the story. It's mindblowing how much is going on inside these plants that we never even see. Okay, so we've covered chemical communication. Now let's talk about light because that's obviously crucial for plants. Absolutely. Light is essential for photosynthesis, of course, but it's also involved in all sorts of other developmental processes that we call photomormorphogenesis. Photomorphogenesis, that's a mouthful. What does that mean exactly? Basically, it's all the ways that light shapes the plant's growth and development apart from photosynthesis. Plants can actually detect not just the presence or absence of light, but also its direction, intensity, and even its color or wavelength. M and the concept of an action spectrum helps us figure out which specific wavelengths of light are most effective for certain responses. Okay. So different wavelengths of light trigger different responses exactly and through those action spectrum studies we've discovered two main classes of photo receptors in plants. Blue light photo receptors and phytochromes. Okay. So what are those blue light photo receptors do? So there are two main ones. Cryptochromes and photoropin. They're both pigment protein complexes that absorb mainly blue light. Right. And they're involved in a bunch of important stuff. Photoropin is the one that's responsible for photo tropism. That bending toward light, right? The classic plant move. Exactly. And it also plays a role in stomatal opening. You know those little pores on the leaves that let in carbon dioxide and release oxygen. Right. Gas exchange essential for photosynthesis. Yep. Photoropin also helps move those chloroclasts around inside the cell to maximize light capture for photosynthesis. Cryptochromes on the other hand are involved in slowing down hypocado elongation. Hypercodal elongation. What's that? It's basically when that stem-like structure below the leaves grows longer. So when a seedling emerges from the soil and it's exposed to light, cryptochromes helps slow down that elongation. It's an energy saving strategy. Makes sense. So it's like the blue light is telling the plant, hey, we're above ground now. Time to start focusing on photosynthesis. Exactly. And then we have phytochromes which we touched on earlier with deilation but they're also involved in seed germination and this thing called shade avoidance. Okay. So tell me more about that. How do phytochromes work? So phytochromes they're a type of photo receptor protein that mainly absorbs red and far red light. And they have these two interconvertible forms PR which absorbs red light and PS which absorbs far red light. When prey absorbs red light it gets converted to PFR. And when PR absorbs far red light it switches back to PR. So it's like a light switch that can flip back and forth. Exactly. And the cool thing is the PFR form is the active form. That's the one that triggers most of the physiological responses. Okay. So what about seed germination? How are phytochromes involved there? Well, there were these classic studies on lettuce seed germination and they found that red light would promote germination while far red light would inhibit it. And the crazy thing was that the very last flash of light the seed received, that's what determined whether it germinated or not. Wow. So, it's like the plant is remembering the last light signal it saw. It is. It's all about that PRPFR ratio. And what about shade avoidance? How does that work? So, plants use this PRPFR ratio to sense whether they're in the shade. You see, when sunlight filters through the leaves of other plants, a lot of the red light gets absorbed by chlorophyll. While more far red light passes through. So, the light under a canopy is different from direct sunlight. Exactly. And when a plant senses that there's more red light than red light, it basically means I'm getting shaded. And that triggers this shade avoidance response where the plant tries to grow taller to out compete its neighbors. Like a race for the sun. It is. And if the plant's getting plenty of red light, it knows it's in a good spot and it doesn't need to elongate as much. Fascinating. Okay. So plants are clearly using light for way more than just photosynthesis. But the chapter also goes into this other aspect of light response, biological clocks and circadian rhythms. Do plants really have an internal sense of time like animals do? They absolutely do. Lots of plant processes like stominal opening and closing, enzyme production, and even leaf movements. They all show these daily rhythms that repeat on roughly a 24-hour cycle. And these rhythms continue even if you put the plant in constant light or darkness. So it's not just them responding to the changes in light and dark. They have an internal timer. Exactly. It's called a circadian rhythm.