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Chapter 18: The Hormone Orchestra
You have been watching hormones work behind the scenes for most of this book. You just didn’t know it yet.
- In Chapter 4, you met auxin, the hormone that makes pruning work.
- In Chapter 7, you watched auxin tell roots which way is down.
- In Chapter 8, it was bossing lateral buds into silence.
- In Chapter 11, it bent stems toward light.
- In Chapter 15, you learned that a tiny gas called ethylene controls whether your banana ripens on the counter.
- And finally, in Chapter 17, you discovered that a cold treatment can trick an artichoke into flowering a whole year early.
Every single one of those stories was a hormone story. We just were not ready to tell the full version yet.
Now we are.
This chapter is not going to reteach everything you already know about auxin and ethylene. You should have a decent understanding of both. Instead, we are going to do something more interesting. We are going to meet the hormones you have not met yet, discover new tricks that the familiar hormones can do, and most importantly, see how all of these chemical messengers work together as a team. Because here is the thing about plant hormones that nobody mentions until you are ready to hear it: no hormone works alone. Ever.
Plants run on a system of chemical conversations happening simultaneously across every root, stem, leaf, and flower. One hormone says grow. Another says stop. A third says wait. A fourth says now. And the plant listens to all of them at once and somehow figures out the right thing to do.
It is, honestly, one of the most impressive things in all of biology.
The Big Five
Botanists have identified dozens of plant hormones and hormone-like compounds, but five major groups do most of the heavy lifting. These are the ones you need to know.
You have already met two of them: auxin and ethylene. The other three are gibberellins (jib-uh-REL-inz), cytokinins (Sie-toh-Kie-ninz), and abscisic (ab-SISS-ik) acid. Each one has its own personality, its own set of jobs, and its own way of interacting with the others. Think of them as five musicians in an orchestra. Each one can play solo, but the real music happens when they play together.
Let us meet the ones you haven’t been properly introduced to yet, and then we will come back and look at how all five interact.
Gibberellins: The Growth Accelerators
If auxin is the plant’s growth coordinator, gibberellins (jib-uh-RELL-ins) are the growth accelerators. They are the hormones that can make a plant grow taller, faster, and more dramatically than almost anything else in the chemical toolkit.
The discovery story is a good one.
A Foolish Seedling and a Fungus
In the early 1900s, Japanese rice farmers had a frustrating problem. Some of their rice seedlings were growing absurdly tall, stretching way above the rest of the crop, pale and spindly and weak. These seedlings would eventually topple over and die without producing any grain. The farmers called the condition bakanae, which translates to “foolish seedling,” because the plants were acting like fools, growing tall for no good reason and wasting all their energy on height they could not support.
You don’t need to watch this entire video, but if you want to see what it looks like, watch the first part to see the seedlings that are growing taller than the rest.
A Japanese scientist named Eiichi Kurosawa figured out that the problem was not coming from inside the rice plants. It was coming from a fungus called Gibberella fujikuroi that had infected them. The fungus was producing a chemical that was forcing the rice plants into overdrive growth. Kurosawa isolated the chemical, and scientists eventually named it gibberellin, after the fungus.
Here is the twist: when researchers started looking more closely, they discovered that plants make gibberellins too. The fungus had stumbled onto a chemical that plants were already using to regulate their own growth. The fungus was just flooding the system with way too much of it, like someone turning the volume knob to maximum on a speaker that was already playing at a reasonable level.
What Gibberellins Actually Do
Gibberellins have a few major jobs in plants, but their most dramatic one is stem elongation. They make internodes stretch, and the result is a taller plant. (Quick refresher from Chapter 8: internodes are the sections of stem between nodes.)
This is where it gets really interesting. You know those dwarf varieties you see in gardens? Short, compact marigolds. Dwarf sunflowers that stay two feet tall instead of ten. Miniature roses. Many of those plants are compact specifically because they produce less gibberellin than their full-sized relatives. Fewer “stretch” signals means shorter internodes means a shorter plant.
Scientists proved this with a beautifully simple experiment. They took dwarf pea plants, the kind that stay short and bushy, and sprayed them with gibberellin. Within days, the dwarf peas started growing tall. Not just a little taller either. They grew to the same height as normal pea plants. The dwarfism wasn’t broken growth machinery. The machinery worked fine. The plants were just missing the chemical signal that told them to use it.
It was like finding out a car with a perfectly good engine had just been sitting in the driveway because nobody turned the key.
Gibberellins and Flowering
Remember bolting from Chapter 17? That’s when a biennial plant freaks out and rockets a flower stalk skyward, usually because a cold snap fooled it into thinking winter was over. Gibberellins are the culprit behind that chaos.
Here’s what happens: the moment a plant flips from “just growing leaves” mode to “time to make flowers” mode, gibberellin levels spike hard. And that spike is what drives the stem to shoot up almost overnight.
You’ve probably seen this with lettuce that’s been left too long in the garden, or a second-year foxglove that suddenly looks like it’s trying to escape the flowerbed. That’s not slow, steady growth. That’s gibberellins flooding the plant and telling every internode to stretch now. The whole point is to get those flowers up high enough that pollinators can actually find them.
Gibberellins and Germination
Gibberellins also have a huge role in seed germination. Back in Chapter 16 we talked about a seed “waking up” when water soaks in. Here’s what’s actually happening under the hood.
When water breaks through the seed coat and reaches the embryo, the embryo releases gibberellins. Those gibberellins travel to a thin layer of cells wrapped around the endosperm called the aleurone layer (AL-yuh-rone). Think of it like a thin jacket surrounding the seed’s food supply. The gibberellins tell those aleurone cells to start pumping out enzymes, and those enzymes break the stored starch down into sugars the embryo can actually run on.
Without gibberellins, the seed is basically sitting right next to a fully stocked fridge with no arms to open the door. The food is there. The embryo just can’t get to it. Gibberellins are the arms.
Seedless Grapes: A Gibberellin Story
Here’s a fun one you’ve probably eaten without knowing the science behind it. Most seedless grapes at the grocery store aren’t naturally seedless. They’re treated with gibberellin sprays!
Grape growers spray their vines with gibberellic acid (a type of gibberellin) at just the right time during fruit development. The gibberellin tells the fruit to keep growing and expanding without waiting for the seeds to finish developing. The berries grow larger, the seeds never fully form, and the result is those plump, crunchy, seedless grapes you grab by the handful.
So next time you’re snacking on seedless grapes, you’re eating a gibberellin success story.
Cytokinins: The Cell Division Crew
If gibberellins are the accelerator pedal for stem stretching, cytokinins are the accelerator pedal for cell division. Their name gives it away: “cyto” means cell (think cytoplasm from Chapter 3), and “kinin” relates to movement or division. Cytokinins are the “make more cells” hormones.
They’re produced mainly in root tips and travel upward through the xylem, the plant’s water highway you learned about in Chapters 5 and 8. This is an important detail. Auxin is made in shoot tips and travels down. Cytokinins are made in root tips and travel up. The two hormones are literally moving in opposite directions through the plant, and as we’ll see in a minute, they’re constantly arguing with each other.
What Cytokinins Do
The most basic job of cytokinins is cranking up cell division. More cytokinins in an area means more cells being produced. But they don’t stop there.
Cytokinins also delay aging in plant tissues. This is why the leaves closest to the roots, where cytokinins are being made, often stay green and healthy longer than the leaves at the top of the plant. The lower leaves are getting a steady supply of “stay young” signals from below. When a leaf stops receiving cytokinins, it starts to yellow and age. Florists actually use this trick. Dipping cut flowers in a cytokinin solution keeps them looking fresh and green in a vase for days longer than untreated flowers.
Now, if you’ve been paying attention, you might already be forming a question. If cytokinins travel up from the roots and delay aging, and lower leaves are closest to the root supply, shouldn’t the lower leaves on every plant be the last ones to turn yellow?
In a lot of plants, that’s exactly what happens. But if you’ve ever grown tomatoes, you’ve watched the opposite play out. The lower leaves on a tomato plant are almost always the first to yellow, wilt, and look terrible, even on a perfectly healthy plant. What gives?
Tomatoes are an exception, and they’re a great example of why one hormone never tells the whole story.
Here’s what’s actually going on. Tomatoes are heavy feeders, especially when it comes to nitrogen. Once the plant starts growing tall and producing fruit, it gets greedy. It starts pulling nitrogen and other nutrients out of the older, lower leaves and shipping them upward to the growing tips and developing fruits. The lower leaves are basically getting raided. The plant strips them for parts, and they yellow and die as a result. Botanists call this progressive senescence (sih-NESS-uhns), and it moves from the bottom of the plant upward. It’s completely normal, and it happens even in well-fertilized plants once fruiting kicks into gear.
On top of that, the lower leaves are getting buried in shade. As the upper canopy fills in, less and less light reaches the bottom of the plant. A shaded leaf isn’t producing much sugar through photosynthesis, so from the plant’s perspective, it’s not pulling its weight anymore. Why keep an unproductive leaf alive when there are fruits to grow and new shoots to feed? The plant makes the call, and the lower leaves lose.
If nutrient theft and shade weren’t enough, the lower leaves also get hit with disease first. Fungal infections like early blight almost always start at the bottom of the plant because those leaves are closest to the soil, they stay wet longer after rain or watering, and they’re already weakened by everything else happening to them. It’s a triple whammy.
So what about the cytokinins? They’re still there, still traveling up from the roots, still doing their job. But in a fast-growing, fruit-heavy plant like a tomato, the demand from all those ripening fruits and new shoots creates a signal so strong that it overwhelms the cytokinin “stay young” message. The cytokinins help, but they can’t win the argument against a plant that’s aggressively funneling every available resource into reproduction.
This is why experienced gardeners don’t panic when the lower leaves on their tomato plants start yellowing. It’s normal. Many gardeners actually remove those lower leaves on purpose, since it improves airflow around the base of the plant and reduces the chance of fungal disease spreading. You’re not hurting the plant. You’re just cleaning up what the plant was already planning to throw away.
Cytokinins also promote lateral (side) bud growth. Remember how auxin from the terminal (top) bud suppresses lateral buds, keeping the plant tall and skinny? Cytokinins do the opposite. They encourage those lateral buds to wake up and start growing. So, the plant is constantly getting two competing signals: auxin from above saying “stay tall, don’t branch,” and cytokinins from below saying “branch out, make more shoots.”
The balance between these two hormones determines whether a plant grows tall and narrow or short and bushy. It’s the tug-of-war that Chapter 17 teased, and it’s happening in every plant you’ve ever looked at.
The Auxin-Cytokinin Tug-of-War
This rivalry is one of the most important ideas in plant biology, and once you know about it, you’ll start seeing it everywhere.
When auxin is winning, the plant grows tall. It stretches upward, keeps its side branches suppressed, and pours its energy into height. That actually makes a lot of sense: if taller plants are blocking your sunlight, the fastest way to survive is to outgrow them. Wasting energy on extra branches would just slow you down.
When cytokinin is winning, the plant goes bushy. It wakes up its lateral buds, grows more branches, and spreads out sideways instead of up. That also makes sense: if you already have plenty of light, spreading out means more leaves, and more leaves means more photosynthesis.
When the two are roughly equal, you get a little of both. Moderate height, moderate branching. That’s actually where most healthy, well-lit plants spend most of their time.
Here’s where it gets wild.
Scientists can control this. When researchers grow plant cells in a petri dish, they can decide what those cells turn into just by changing the ratio of auxin to cytokinin in the gel the cells are sitting in.
Pump up the auxin and drop the cytokinin? The cells grow roots.
Flip it, high cytokinin and low auxin? The cells grow shoots.
Keep them equal? The cells just keep dividing into a shapeless blob called a callus. No roots, no shoots, just… a blob.
Same cells. Same DNA. Same species. The only thing that changed was the ratio of two hormones. And that ratio decided whether you got a root, a shoot, or a blob. That’s not background noise. That’s the hormones acting as architects, designing the plant from scratch.
Abscisic Acid: The Emergency Brake
Every team needs someone who can say “stop.” In the plant hormone world, that someone is abscisic acid, usually just called ABA.
If auxin, gibberellins, and cytokinins are all about growth, growth, growth, ABA is the voice of caution. It tells the plant to slow down, shut things off, conserve resources, and hunker down when conditions get tough. Think of it as the plant’s emergency brake.
ABA and Drought
The most immediate and dramatic thing ABA does is slam stomata shut.
Remember stomata from Chapters 10 and 11? Those tiny pores on the underside of leaves that open to let carbon dioxide in for photosynthesis and close to prevent water loss? That decision to open or close isn’t random. It’s controlled by hormones, and ABA is the one that locks the doors.
When a plant’s roots sense that the soil is drying out, they start pumping out large amounts of ABA. That ABA travels up through the xylem to the leaves, where it tells the guard cells surrounding each stoma to lose water and go limp. When the guard cells go limp, the stoma closes. Across the entire leaf, thousands of stomata snap shut almost simultaneously, and the plant’s water loss drops dramatically.
This is why a plant in dry soil can sometimes hang on for days even in hot weather. The ABA signal is keeping every door in the building locked.
The trade-off, of course, is that closed stomata also block carbon dioxide from entering. So the plant can’t photosynthesize while its stomata are shut. It’s choosing between dying of thirst and going hungry, and it picks going hungry every time. You can survive hunger a lot longer than you can survive dehydration. The plant can catch up on photosynthesis when it rains. It can’t catch up on water loss if it’s already dead.
ABA and Dormancy
ABA is also the hormone behind seed dormancy, something we explored in detail in Chapter 16. Remember those chemical inhibitors inside seeds that prevent germination until conditions are right? ABA is one of the main ones.
When a seed is developing on the parent plant, ABA levels inside the seed are high. This keeps the seed from germinating while it’s still attached to the mother plant, which would be a disaster. Imagine a seed sprouting while still inside the fruit. The seedling would have nowhere to go and no soil to grow in. ABA prevents that by keeping the embryo in a state of enforced sleep.
After the seed is released and exposed to the right conditions (cold stratification, repeated rainfall, time), ABA levels gradually drop. At the same time, gibberellin levels rise. When the gibberellin-to-ABA ratio tips far enough in gibberellin’s favor, the seed finally wakes up and germinates.
So germination isn’t just about one hormone showing up. It’s about the balance between two hormones shifting. ABA says sleep. Gibberellins say wake up. The seed listens to whichever one is louder.
And yet… it does happen occasionally. If you’ve ever sliced open a tomato and found tiny green sprouts growing out of the seeds inside, you’ve seen what happens when ABA loses control. It’s called vivipary (vih-VIP-uh-ree), which literally means “live birth,” and it’s exactly as weird as it looks.
It’s most common in tomatoes, peppers, and sometimes squash. What’s going on is that the conditions inside the fruit somehow tricked the seeds into thinking it was time to germinate. Maybe ABA levels dropped too low. Maybe the fruit sat on the counter too long and the inside got warm and moist enough to override the dormancy signals. Maybe the balance between ABA and gibberellins tipped just a little too far in gibberellin’s favor. Whatever the cause, the seeds jumped the gun.
The sprouts almost never survive. They’re trapped inside a fruit with no soil, no real light, and no way to establish roots. It’s exactly the disaster that ABA is supposed to prevent. But the fact that it happens at all tells you something important: ABA isn’t a perfect lock. It’s a chemical signal, and chemical signals can be overridden if conditions push hard enough. The system works incredibly well almost all the time, but biology doesn’t do “always.”
ABA and Bud Dormancy
ABA does the same thing for buds that it does for seeds. In autumn, as days get shorter, ABA levels build up in the buds of deciduous trees. This is part of what pushes those buds into dormancy for winter. They wrap themselves in protective scales and shut down growth until spring.
And just like with seeds, the return of warm temperatures and longer days causes ABA levels to drop and gibberellin levels to rise. When that balance shifts, the buds break open and new growth begins. That dramatic bud burst you watched in the Chapter 4 time-lapse? That was gibberellins winning the tug-of-war against ABA.
Ethylene: The Rest of the Story
You already know ethylene’s biggest hit: fruit ripening. The banana bag trick, climacteric versus non-climacteric fruits, the whole story from Chapter 15. But ethylene does a lot more than ripen fruit. It’s one of the most versatile hormones in the plant kingdom, and some of its other jobs are just as wild.
Ethylene and Leaf Drop
Remember the abscission zone from Chapter 11? That specialized layer of cells at the base of the petiole that dissolves itself to let the leaf fall cleanly from the branch? Ethylene is one of the main signals that kicks off that process.
As autumn approaches and the tree starts dismantling its leaves to reclaim nitrogen and other nutrients, ethylene production ramps up in the abscission zone. The ethylene tells those cells to produce the enzymes that dissolve the connections between them. It’s the chemical “cut here” signal.
This is also why damaged or stressed leaves sometimes drop earlier than they should. Injury triggers ethylene production, and ethylene triggers abscission. The tree is cutting its losses, literally, by dropping a damaged leaf before investing more resources into trying to save it.
Ethylene and Flower Death
If you’ve ever received a bouquet of flowers and watched them wilt within a few days, ethylene was involved. Ethylene speeds up the aging and death of flower petals. Once a flower has been pollinated, the plant fires off a burst of ethylene that tells the petals to wither and fall off. The flower’s job is done, so the plant is clearing the construction site to make room for fruit development.
This is why florists are so careful about ethylene. They keep flowers cold (cold slows ethylene production), they remove any ripening fruit from the display area (because fruit releases ethylene that can kill nearby flowers), and some even treat flowers with chemicals that block ethylene receptors. A tiny amount of ethylene gas in a flower shop can shorten the vase life of every bouquet in the room.
It’s also why you should never store a bouquet of flowers next to a fruit bowl. That banana sitting on the counter is pumping out ethylene gas 24 hours a day, and your roses are breathing it in and dying faster because of it.
Ethylene and Stress
Plants produce ethylene in response to all kinds of stress: flooding, drought, physical damage, disease, even being touched too roughly. It’s the plant’s stress alarm.
One of the most fascinating stress responses involves flooding. When a plant’s roots are submerged in water, they can’t get enough oxygen (roots need oxygen for cellular respiration, even though leaves produce it). The oxygen-starved roots start producing a chemical called ACC, which travels up to the stem and gets converted into ethylene.
In some plants, this ethylene triggers a remarkable response: the stem grows special air channels called aerenchyma that create internal tubes for oxygen to travel from the leaves down to the drowning roots. The plant is basically building its own internal snorkel system. Rice plants are especially good at this, which is one reason rice can grow standing in water.
The Triple Response
When a seedling pushing up through the soil hits an obstacle like a rock or a hard clump of dirt, it needs to change its growth pattern to get around it. Ethylene coordinates this change through what scientists call the triple response.
First, the stem stops elongating. Second, it gets thicker and sturdier. Third, it starts growing sideways instead of straight up. Once the seedling clears the obstacle, ethylene production drops, and the stem goes back to growing normally: thin, tall, and straight toward the light.
Think of it like walking through a dark room. If you bump into something, you stop, brace yourself, and feel your way around it. Once you’re past it, you keep walking. The seedling is doing the same thing, just with chemistry instead of a nervous system.
Auxin: What We Haven’t Told You Yet
You know auxin’s greatest hits by now: apical dominance, phototropism, gravitropism, etiolation. Quick refresher in case those terms have gotten fuzzy:
- Apical dominance: the terminal bud at the top of a stem bosses the lateral buds below it into staying dormant, keeping the plant growing tall instead of bushy (Chapter 8)
- Phototropism: stems bend toward light because auxin piles up on the shaded side and makes those cells grow faster (Chapter 11)
- Gravitropism: roots grow downward because auxin tells them which way is down, using tiny starch grains as gravity sensors (Chapter 7)
- Etiolation: when a plant doesn’t get enough light, auxin runs unchecked and the stem stretches out tall, pale, and spindly, gambling that it’ll find light before it runs out of energy (Chapter 11)
But there are a few more auxin stories that are too good to skip, especially because they connect directly to things you see and use in the real world.
Rooting Powder: Auxin in a Bottle
If you’ve ever tried to grow a new plant from a cutting, you may have dipped the cut end into a powder or gel before sticking it in soil. That powder is synthetic auxin.
Here’s why it works. When you cut a stem, the plant needs to grow new roots from the cut end. Auxin naturally piles up at the base of a cutting because it flows downward from the tip (remember, auxin always moves from shoot tip toward the base). But sometimes there isn’t enough natural auxin to trigger root formation quickly, especially in species that are stubborn about rooting.
Rooting powder gives the cutting a massive dose of auxin right where it needs it. The concentrated auxin at the cut end tells those cells, “You’re root cells now. Start growing.” It dramatically increases the success rate and speed of rooting. Without it, a cutting might sit in soil for weeks doing nothing. With it, you can see new roots forming in days.
This is one of the simplest and most widely used applications of hormone science in all of gardening. A bottle of rooting powder is basically a bottle of auxin, and it works because we understand what auxin does at the cellular level.
Auxin as Weed Killer
This is one of the wildest hormone stories in agriculture, and it’s beautifully ironic: a growth hormone that kills plants.
In the 1940s, scientists developed synthetic versions of auxin that plants could absorb but couldn’t break down. The most famous of these is a compound called 2,4-D (short for 2,4-dichlorophenoxyacetic acid, but nobody actually calls it that).
Here’s how it works. 2,4-D gets absorbed by broadleaf plants (dicots) much more easily than by grasses (monocots), because of differences in their leaf structure and how they process the chemical. Once inside a broadleaf plant, the synthetic auxin floods the system and can’t be broken down or regulated. The plant goes into chaotic, uncontrolled growth. Stems twist. Leaves curl. Cells divide where they shouldn’t. The plant’s own growth systems, overwhelmed by a signal they can’t shut off, tear the plant apart from the inside.
The plant literally grows itself to death.
Meanwhile, the grass standing right next to it is barely affected. It absorbs less of the chemical, breaks down what it does absorb more quickly, and carries on growing normally.
This is why you can spray a lawn with 2,4-D and kill the dandelions without killing the grass. The dandelion, a dicot, gets overwhelmed by uncontrollable auxin. The grass, a monocot, shrugs it off. It’s selective weed killing based entirely on the biology of how different plants handle the same hormone.
2,4-D is still one of the most widely used herbicides in the world. Every time someone sprays their lawn to kill broadleaf weeds, they’re weaponizing auxin.
Why Auxin Only Flows One Way
Here’s a detail we skipped in earlier chapters because it would have been too much too soon, but it’s worth knowing now.
We’ve said multiple times that auxin flows downward from the shoot tip. But have you ever stopped to wonder how? It’s not like there’s a pipe running down the middle of the stem. And auxin is just a tiny molecule floating around inside cells. So why doesn’t it drift in every direction equally?
Because the plant doesn’t let it.
Every cell in the stem has special protein pumps built into its membrane, and these pumps are only installed on the bottom side of each cell. They grab auxin molecules and push them out the bottom, where the next cell’s pumps grab them and push them out its bottom, and so on, cell after cell, all the way down. It’s like a bucket brigade where everyone is facing the same direction and only passes the bucket one way. Scientists call these pumps PIN proteins, and the whole system is called polar auxin transport.
This is why auxin only flows downward. It’s not drifting. It’s being shoved, one cell at a time, in a very specific direction.
And that one-way flow is what makes almost everything we’ve learned about auxin actually work. It’s what creates the top-to-bottom gradient that drives apical dominance. It’s what allows auxin to pile up on the shaded side of a stem for phototropism. It’s what makes gravitropism work in roots. If those pumps didn’t exist and auxin just sloshed around randomly, none of those responses would function. The plant would have no way to concentrate auxin where it needs it.
The fact that a plant can control the direction of a hormone’s movement at the level of individual cell membranes is pretty incredible. It means the plant isn’t just making chemicals and hoping for the best. It’s actively directing where those chemicals go, with a precision that scientists are still working to fully understand.
The Hormone Orchestra: How They All Work Together
Here’s the most important concept in this entire chapter: plant hormones almost never act alone. They work in combinations, sequences, and balances. The effect of any single hormone depends on what other hormones are present, how much of each one there is, and what type of cell is receiving the signal.
We’ve already seen a few examples. The balance between auxin and cytokinins determines whether cells become roots, shoots, or shapeless blobs. The balance between gibberellins and ABA determines whether a seed germinates or stays dormant. Ethylene and auxin both play roles in leaf abscission (where a leaf falls off a branch in the autumn).
But let’s look at a few more to really drive this home.
Growing a Fruit
After a flower is pollinated, every single hormone we’ve learned about in this chapter gets involved. Here’s how they tag-team to build a fruit.
It starts with auxin. Auxin levels skyrocket in the developing seeds, and that auxin sends a signal to the ovary wall: start expanding. (Remember from Chapter 15 how the ovary becomes the pericarp, the fleshy part of the fruit you actually eat?) Auxin gets the construction project started.
Then gibberellins show up and start stretching cells longer, making the fruit grow bigger. Right alongside them, cytokinins are busy cranking out more cells through cell division. So the fruit is getting bigger in two ways at once: each cell is getting larger, and there are more cells being made. That’s how a tiny green nub behind a flower turns into a full-sized apple in a few months.
While all this growing is happening, ABA stays quiet. Its levels stay low on purpose, because if ABA spiked too early, it could trigger the seeds inside the fruit to go dormant before they’re fully developed. Or worse, remember what we just learned about vivipary? High ABA is what keeps seeds from sprouting inside the fruit. The plant needs to keep ABA at just the right level: enough to prevent premature germination, but not so much that it interferes with growth.
Finally, when the fruit is fully grown and the seeds inside are mature, ethylene takes over. It triggers the ripening cascade you learned about in Chapter 15: starches convert to sugars, acids break down, the fruit softens, colors change, and those amazing fruity smells start drifting into the air.
Five hormones. Each one showing up at the right time, doing its specific job, and then stepping aside for the next one. Take any one of them out of the lineup and the whole process falls apart.
Surviving Winter
Here’s one you can watch happen in your own yard every year.
In autumn, as the days get shorter, ABA starts building up in the tree’s buds. That’s the “time to sleep” signal. The buds shut down and wrap themselves in protective scales for winter. At the same time, ethylene ramps up in the abscission zones and the leaves drop. The tree goes quiet. No growth. No leaves. Just a bare skeleton waiting out the cold.
But something important is happening during those cold winter months, even though you can’t see it. All that ABA sitting in the buds? The long stretch of cold is slowly breaking it down. Week after week, the ABA levels drop a little more. The “stay asleep” signal is getting weaker.
Then spring arrives. Temperatures warm up and gibberellin levels start to rise in the buds. Remember how germination works? ABA says sleep, gibberellins say wake up, and the seed listens to whichever one is louder? The exact same thing happens here. The gibberellins finally overpower what’s left of the ABA, and the buds break open. That’s the bud burst you watched in the Chapter 4 time-lapse.
Now things move fast. The fresh shoot tips start producing auxin, which flows downward and reestablishes apical dominance, telling the tree which branches get priority this year. Down below, the roots are waking up too, and they start sending cytokinins upward, fueling new cell division throughout the whole plant.
Just like that, the whole system is back online. The tree picks up right where it left off, like it never missed a beat.
Responding to a Wound
When a plant takes damage, say a branch snaps in a windstorm, multiple hormones respond almost immediately. Ethylene surges at the wound site, triggering stress responses and sometimes causing nearby leaves to drop. Auxin flow gets disrupted because the transport pathway is broken, which can release nearby lateral buds from apical dominance (this is why damaged trees often sprout new branches near the wound). Cytokinins promote new cell division to heal the wound. And if the damage is severe enough, ABA might trigger a defensive shutdown in surrounding tissues to keep any infection from spreading through the wound.
Four hormones, all responding to the same event, each doing a different job, all at once. The plant can’t call for help, can’t run away, and can’t slap on a bandage. All it has is chemistry. And that chemistry is remarkably good at its job.
Humans and Hormones: Putting the Science to Work
Once scientists figured out what plant hormones do, it didn’t take long to start using that knowledge in agriculture, gardening, and industry. You’ve already seen a few examples (rooting powder, 2,4-D, gibberellin sprays on grapes), but the list goes on.
Growth Retardants
If gibberellins make plants taller, what happens if you block gibberellins? The plant stays short.
Commercial flower growers use gibberellin-blocking chemicals called growth retardants to keep potted plants compact and bushy. That perfectly round, dense chrysanthemum in a six-inch pot at the garden center? It was almost certainly treated with a growth retardant. Without the treatment, it would be tall, leggy, and way less attractive on a store shelf.
The same approach is used on wheat in some parts of the world. Shorter wheat stems are less likely to fall over in wind and rain (a problem called lodging), which means less crop loss. By spraying fields with compounds that reduce gibberellin activity, farmers can keep wheat stalks short and sturdy while the grain heads at the top still develop normally.
Ethylene in the Supply Chain
The banana ripening story from Chapter 15 is actually part of a huge industrial system. Bananas are picked green and hard in tropical countries, shipped across the ocean in refrigerated containers (cold slows ethylene production, keeping them green), and then placed in special ripening rooms at their destination. In those rooms, they get hit with carefully controlled doses of ethylene gas that trigger uniform ripening. This is why every banana in a bunch at your grocery store is the same shade of yellow. They were all ripened together by the same ethylene treatment.
The following video doesn’t mention ethylene by name, but you can see them gassing the bananas at timestamp 1:58.
Tomatoes go through a similar process. Many commercial tomatoes are picked green and hard (easier to ship without bruising) and then gassed with ethylene at distribution centers to turn them red before they hit store shelves. This is why grocery store tomatoes sometimes look perfectly red on the outside but taste bland and mealy. The ethylene triggered the color change, but the tomato never got the chance to develop its full flavor on the vine. A tomato that ripens naturally in the sun, with its own ethylene production happening gradually alongside sugar development and acid changes, tastes completely different.
Preventing Premature Fruit Drop
Orchards sometimes have a problem with fruit falling off the tree before it’s fully ripe. This premature drop happens because ethylene triggers the abscission zone at the base of the fruit stem, the same mechanism that drops leaves in autumn. Growers can spray trees with synthetic auxin, which counteracts the ethylene and delays the formation of the abscission zone, keeping the fruit on the tree longer so it has time to reach full size and maturity.
One hormone being used to cancel out another. Auxin versus ethylene. The farmer is basically taking sides in a chemical argument that the tree is having with itself.
Chapter Wrap-Up
Plants have no brain, no nervous system, no muscles, and no ability to pick up and move. And yet they manage to pull off incredibly complex behaviors: growing toward light, dropping leaves in autumn, ripening fruit at exactly the right moment, going dormant in winter, waking up in spring, healing wounds, fighting stress, and timing their flowering to the season.
They do all of this with hormones. Five major groups of tiny chemical messengers, traveling through the plant in different directions, produced in different places, doing different jobs, and constantly interacting with each other in ratios and sequences that somehow produce exactly the right response at exactly the right time.
Auxin coordinates growth direction and suppresses branching. Gibberellins accelerate stem elongation, trigger germination, and promote flowering. Cytokinins drive cell division and fight auxin for control of the plant’s shape. Abscisic acid slams the brakes during drought and enforces dormancy. Ethylene ripens fruit, drops leaves, kills flowers, and sounds the stress alarm.
None of them works alone. All of them work together. And the result is a living organism that can respond to its environment with a sophistication that scientists are still working to fully map out.
The next time you bite into a seedless grape, prune a rosebush, watch a houseplant lean toward a window, or marvel at autumn leaves drifting to the ground, remember: you’re watching hormones at work. The orchestra is always playing. You just have to know what to listen for.
The following video is actually a lot longer, but I skipped to the part about hormones. If you want a review about cells, you can watch it from the beginning.
You now know how plant hormones work. You know the five major players, what they do individually, and how they team up or fight each other to coordinate everything from germination to fruit drop. But here’s a question we haven’t really answered yet: what happens when the environment fights back?
We’ve touched on pieces of this already. You saw roots chasing water in Chapter 7 and stems bending toward light in Chapter 11. You watched ethylene build snorkel systems for flooded roots and coordinate the triple response when a seedling hits a rock. But those were previews. Individual examples scattered across different chapters.
Chapter 19 is where we pull it all together and go much further.
Because plants deal with a lot more than just gravity and light. They deal with scorching heat waves that threaten to cook their proteins. Freezing nights that could turn their cells into ice crystals. Droughts that last for months. Floods that suffocate their roots. Insect attacks that chew through their leaves. Fungal infections that try to eat them alive. Even other plants trying to poison them through the soil.
And plants can’t do what you would do. They can’t move to the shade. They can’t put on a jacket. They can’t swat a mosquito or run from a wildfire. They’re rooted in place, and whatever the world throws at them, they have to deal with it right where they stand.
So how do they survive?
Some of their tricks are familiar territory that we’ll explore in much more depth. But some are going to surprise you. There are plants that can “feel” being touched and respond in less than a second, folding their leaves shut so fast you can watch it happen in real time. There are plants that, when a caterpillar starts eating them, release chemical signals into the air that attract wasps, and those wasps come and attack the caterpillar. The plant is calling for backup. There are plants that can sense when a neighboring plant is being attacked and start building their own defenses before the attacker even reaches them. They’re eavesdropping on their neighbor’s distress signals.
There are even plants that “remember” past stresses and respond faster the next time around, almost like a primitive immune system.
None of this involves a brain. None of it involves thinking. But the sophistication of these responses will make you wonder just how much a living thing can accomplish with nothing but chemistry, physics, and the right set of molecular tools.
Chapter 19 is the story of how plants fight back against a world that’s constantly trying to kill them. And honestly? They’re better at it than you’d ever expect.