Chapter 28: External Factors and Plant Growth

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Welcome to the Deep Dive.

Today we're embarking on a really a fascinating journey into the secret life of plants.

You might picture plants as, you know, just sitting there, passive observers, but as we unpack this chapter from Raven Biology of Plants, the one on responses, you'll find out they are anything but passive.

They're incredibly dynamic, constantly sensing, responding, adjusting, all to survive.

Our mission really is to distill the key stuff from this material.

We want to translate the complex biology of the terms into language that's just clear and accessible.

We'll explore how they manage everything from bending towards light to, well, knowing exactly when to flower, all without taking a step.

And to kick us off, get this, even genetically identical plants can end up looking totally different based on their environment.

Take fieldbine weed, convolvulus arvensis, it's that rampant invasive weed.

Its stems are slender, they twine, they climb aggressively.

Using other plants, basically a scaffolding, actively searching for light.

It's a perfect example of a plant really interacting with its world, even though it's stuck in one spot.

Indeed.

And what's so fascinating when you dig into it is how plants evolved these incredibly sophisticated internal mechanisms.

They perceive really subtle cues, light quality, temperature shifts, you name it.

And then they translate that into precise growth changes, developmental shifts.

Well, it's a remarkable testament to adaptation.

It gives us a shortcut, really, to understanding that complex dance between a plant and its habitat.

Okay, let's dive in then.

The first big concept,

tropisms.

So basically, a tropism is a growth response.

A plant part bends or curves.

And the key thing is the direction of that bending is dictated by some external stimulus.

Towards the stimulus, that's positive, away from it, negative.

That sets it up perfectly.

But it immediately brings up a big question for me.

You know, how do they do it?

How do they get this precise directional growth without a nervous system like ours?

And the answer, it lies in this really intricate cellular communication network, hormonal regulation.

That's how they coordinate it all.

Right.

And the most obvious one, the one we probably all see, is phototropism.

Just picture a seedling, you know, curving its shoot tip right towards the light.

This bending, it's largely orchestrated by a key plant hormone, auxin.

What happens is, auxin makes the cells on the shaded side of that shoot tip grow longer, faster than the cells on the lit side.

So it literally pushes the plant towards the light.

Early experiments building on Fritz Wentz work started to figure this out.

Researchers like showed that when light hits one side of a coelioptile tip, that's the protective sheath, auxin actually moves sideways and migrates laterally from the light side over to the shaded side.

They even used a tiny piece of glass as a barrier to prove that the sideways movement was crucial.

And later experiments using auxin tagged with radioactive carbon -14 confirmed it.

It was migration, auxin moving, not being destroyed on the light side.

Exactly.

And to build on that,

this really precise redistribution of auxin,

it then travels down to the part of the stem that's actually elongating.

And that's what accelerates growth on the shady side, slows it on the sunny side, giving you that classic curve towards the light source.

Now the actual photoreceptors doing the sensing, there are special pigment -containing proteins, specifically two flavor proteins.

They're called phototropins one and two.

And they're tuned to absorb blue wavelengths of light,

roughly 400 to 500 nanometers.

Actually studies on Arabidopsis mutants, plants that couldn't sense blue light instrumental in finding and cloning the genes for these phototropins.

Okay.

So that's light, but plants also respond really strongly to a gravity, right?

That invisible pole, that's called gravitropism.

You lay a seedling on its side, what happens?

The roots grow down, positive gravitropism.

The shoot grows up negative gravitropism.

We've all seen it.

And what's really clever here is how auxin, that same hormone, is involved again, but with a twist, a fascinating one.

In the shoot, if you have more auxin collecting on the lower side, it stimulates those cells to expand, pushes the stem up, makes sense.

But in the roots, which are much more sensitive to auxin levels, that same higher concentration on the lower side actually inhibits cell expansion.

So the cells on the upper side of the root expand more, causing the root to bend downwards.

It's a really striking example, isn't it?

Different tissues, totally opposite responses to the exact same signal.

That auxin duality is, yeah, it's a brilliant biological hack, but it leads to the next puzzle.

How do they even sense gravity?

The main theory, the one with the most evidence, is the starch -stetolith hypothesis.

It proposes that gravity perception is linked to the settling, the sedimentation of amyloplasts.

These are basically dense, starch -filled plastids.

They act as gravity sensors, or statoliths, and they're found in specialized cells called statocytes.

So imagine these heavy little amyloplasts inside the cells at the very tip of the root cap, the colonella cells.

In a root growing straight down, they settle at the physical bottom of those cells, but turn the roots sideways.

These amyloplasts slide down, settling against what used to be the vertical sidewalls.

And that movement, that pressure change perhaps, is thought to kick off a signaling cascade.

The evidence.

Well, studies on Arabidopsis mutants are key here.

If you remove those colonella cells, or use mutants that can't make starch, the gravitropic response is messed up, impaired.

We now understand that the downward curve starts really close to the root tip, in the distal elongation zone.

Elongation gets boosted on the upper side, suppressed on the lower.

And specific proteins, like Pin3, are critical for shuttling auxin around asymmetrically in response to that gravity cue.

Okay, light, gravity.

What about water?

Plants obviously need water.

Do they grow towards it?

Yes, that's hydrotropism.

Roots actively growing towards moisture.

But this one was apparently quite tricky to study for a long time.

Why?

Because that gravity response, gravitropism, is usually much stronger and masks it.

Right, it was tough to isolate.

A big breakthrough came from a mutant pea plant.

It was called a neotropum.

As the name suggests, it just didn't respond to gravity.

And this mutant clearly showed a hydrotropic response.

It grew towards water.

That confirmed hydrotropism was real, and also showed the root cap was involved.

Other studies done in microgravity think space station experiments also helped.

They nullified gravity's effect on normal roots.

And again, you could clearly see positive hydrotropism.

What's really interesting here is the auxin connection again.

Oxin is essential for both hydrotropism and gravitropism.

But the polar transport of auxin, how it's actively moved directionally cell to cell, which is required for gravitropism, that doesn't seem to be required for hydrotropism.

It suggests auxin might be playing a different kind of role in sensing water, maybe related to concentration gradients rather than directional transport.

Fascinating.

So light, gravity, water.

How about just touching things?

Physical contact.

This brings us to sigmatropism, a growth response to touching a solid object.

This is how roots manage to grow around rocks in the soil.

Or think about climbing plants, like peas or vines, how their tendrils wrap around a trellis or another plant for support.

You can actually watch a tendril coil around something in less than an hour.

It's pretty fast.

What's happening is the cells that are touching the support shorten just a bit, while the cells on the opposite side elongate more, causes that coiling.

And get this, some tendrils can even sort of store the memory of being touched for a few hours.

Okay, so we've covered these directional growth responses, the tropisms.

But plants have other tricks up their sleeves, right?

What about an internal sense of time?

We see it all the time.

Flowers opening in the morning, closing at night.

Leaves unfolding by day, folding up at dusk.

These are called nictonastic movements, sleep movements.

And apparently a Greek soldier noticed this way back in the fourth century BCE.

What's truly amazing, though, is that these daily rhythms often continue even if you put the plant in constant conditions.

They're like constant dim

temperature.

These regular roughly 24 hour cycles, those are circadian rhythms.

Exactly.

And they're controlled by an endogenous internal timing mechanism.

Call it the circadian clock.

It's not just reacting to the environment.

It's an internal oscillator, keeps its own time.

Now, its own natural period might not be exactly 24 hours in Arabidopsis.

It's often somewhere between 22 and 29 hours.

But this internal clock gets synchronized or entrained by external cues, primarily the daily cycles of light and dark, and also temperature changes.

This entrainment keeps the plant's internal schedule aligned with the actual 24 hour day.

And it lets the plant adjust to seasonal changes in day length.

Very important.

Two other key features of this clock.

One is temperature compensation, it keeps pretty accurate time across a decent range of temperatures, and the other is gating.

This means the plant's sensitivity, or how strongly it responds to a stimulus -like light, can change depending on the time of day according to its internal clock.

In Arabidopsis, which is like the lab rat of the plant world, scientists have found the oscillator involves at least three interlocking feedback loops of gene activity.

Key genes like TOC1, LHY, and CCA1 regulate each other's transcription.

For instance, dawn light turns on LHY and CCA1.

These then suppress TOC1.

As LHY and CCA1 levels naturally drop during the day, TOC1 expression goes up, which then helps stimulate LHY and CCA1 again for the next morning cycle.

It's intricate.

And this whole clock system controls the timing of other genes when they are turned on or off.

Like the genes for chlorophyll at binding proteins, essential for photosynthesis, so it links timekeeping directly to vital processes.

Wow, so that internal clock isn't just for opening and closing flowers.

Its main job, maybe, is letting the plant measure how long the days are getting, which is absolutely critical for timing seasonal things, especially flowering.

This ability to measure day length is called photoperiodism.

It was discovered back in the 1920s by Garner and Allard.

They figured out plants basically fall into three groups.

Short -day plants, SDP, think cocklebur, chrysanthemums, they flower only when the light period is shorter than some critical length, so they tend to flower in early spring or fall.

Then you have long -day plants, LDP, like spinach or henbane.

They flower only if the light periods are longer than their critical length, mostly summer bloomers.

And finally, day -neutral plants, cucumbers,

sunflowers are examples.

They just flower when they're ready regardless of the length.

Here's where it got really interesting thanks to work by Hamner and Bonner.

They found out it's not actually the length of the light period that matters most, it's the length of the uninterrupted dark period.

That's the critical factor.

They showed that even a brief flash of light, like just one minute in the middle of the long dark period, could stop a short -day plant from flowering or trigger a long -day plant to flower.

Completely flipped the script.

That discovery was huge.

It immediately focused research.

Okay, what is the molecule in the plant sensing light and, crucially, measuring this dark period?

Research, particularly at the U .S.

Department of Agriculture, led to phytochrome.

That's the main photoreceptor for photoperiodism.

The clues came from studies on lettuce seeds.

These seeds only germinate if they see light.

They found red light made them germinate, but far -red light stopped germination.

Phytochrome, it turns out, exists in two forms that can switch back and forth.

There's PR, which absorbs red light, around 660 mm, and converts to PFR.

And then there's PFR, which absorbs far -red light, around 730 mm, and converts back to PR.

PFR is the biologically active form.

Think of it like a switch.

Red light flips it on to PFR, promoting things like germination or flowering and long -day plants.

Far -red light flips it off back to PR, inhibiting those things.

The very last flash of light the plant sees determines the state of the switch.

And crucially, in darkness, the active PFR form either slowly reverts back to the inactive PR form or it gets broken down.

This slow decline of PFR levels during the night is exactly how the plant measures the length of the dark period.

And Phytochrome does way more than just photoperiodism.

It's critical in shade avoidance syndrome.

Plants use it to detect if they're being shaded by neighbors.

Under a canopy, red light gets absorbed, but far -red light passes through, so the ratio changes.

Phytochrome senses this shift, and the plant responds by elongating its stems and leaf stalks, trying to grow up and out of the shade to find better light.

It's like foraging for light.

Phytochrome is also vital for preventing etiolation.

That's the pale, spindly, weak growth you see when seedlings sprout in complete darkness.

Just a brief exposure to red light converts PR to PFR, and that triggers the switch to normal, green, healthy growth.

The signaling pathway is complex, but it involves Phytochrome interacting with other proteins called Phytochrome Interacting Factors, PIFs.

Active Phytochrome often leads to breakdown of these PIS, which in turn changes which genes are expressed, leading to all these different growth responses.

Okay, so the leaves are doing the day -length calculation using Phytochrome, but flowering actually happens at the shoot tip, the merism, so there must be some kind of signal, right?

A messenger traveling from the leaf to the tip.

Is that the floral stimulus or florigen that people talked about?

I remember hearing about early experiments by Chylockian, maybe, showing something moved from the leaves.

You got it.

The idea of a mobile signal, this florigen, was around for decades thanks to experiments like Chylockian's.

Grafting experiments were key, too.

They showed you could induce a non -flowering plant to flower just by grafting it to a plant that was induced by the correct day length, but only if they had a living connection, suggesting it moved in the flomum.

But identifying it chemically was elusive for ages.

The breakthrough, again largely from work in Arabidopsis, reveals florigen is actually a small protein.

It's called FT, which stands for flowering locus T.

This FT protein is made in the companion cells of the flomum and the leaves.

Then it travels through the flomum sieve tubes all the way up to the shoot apical meristem.

And there, at the meristem, it interacts with another protein, a transcription factor called FD.

The FTFD complex then activates the genes that switch the meristem from making leaves to making flowers.

A pretty elegant system.

So, day length is huge, but temperature also plays a role for flowering.

You mentioned vernalization.

That's the process where needing a long period of cold makes a plant able to flower later, like winter rye.

You plant it in the fall, it lives through the cold winter, then flowers in summer.

But if you just take the germinating seeds and chill them for a while, you can trick them into flowering even if you plant them in late spring.

Exactly.

Vernalization, that cold requirement, is common in winter annuals and biennials.

Things like carrots or cabbage need it, too.

And it serves a really important purpose.

It stops them from flowering too early.

If there's a warm spell in the autumn, that would be a waste of resources.

What's different from photoperiodism is where the sensing happens.

Day length is sensed in the leaves.

Vernalization, the cold sensing, happens right in the cells of the shoot apical meristem itself.

In Arabidopsis, there's a key gene called flowering locus C, or FLC.

It acts as a break, inhibiting flowering.

Vernalization, the prolonged cold, works by shutting down FLC expression.

It silences the gene.

Once FLC is silenced, the break is off, and the plant is now competent to flower when it gets the right signal, like long days in spring.

Researchers have even found a specific long non -coding RNA molecule involved called cold air that helps establish and maintain the silenced state of FLC after the cold treatment.

Makes sense.

Plants don't just grow all year.

They need ways to survive tough seasons.

Which brings us to dormancy.

That's like a state of suspended animation, right?

Arrested growth.

An adormant bud, or a seed, needs specific environmental cues to wake up.

It doesn't just happen.

This is crucial, isn't it?

It stops them sprouting during a random warm week in January, only to get killed by the next freeze.

The idea is they have internal endogenous inhibitors that need to be broken down or counteracted first.

Precisely.

So what are those cues?

Well, for seeds, breaking dormancy often requires that period of cold we just talked about,

stratification.

Or sometimes they need a period of drying out.

Or the seed coat might be really hard and needs to be physically worn down, that scarification.

Think birds eating berries and passing the seeds.

Some desert plants even need a good amount of rain to literally wash inhibitory chemicals out of the seed coat before they'll germinate.

And seeds can remain viable, dormant but alive, for incredible lengths of time.

You mentioned the 2000 -year -old palm seed.

That's amazing.

Understanding dormancy is vital for things like the fall -barred global seed vault.

They store backup copies of crop seeds from all over the world.

Knowing how to maintain and break dormancy is essential for safeguarding that genetic diversity.

It's a very practical knowledge.

And for buds on trees and shrubs in temperate zones, dormancy is absolutely essential for winter survival.

The main trigger for buds going dormant in the fall is decreasing day length.

That signal initiates acclimation, the process of becoming cold hardy.

Many of these dormant buds also need a chilling period, exposure to cold, to be able to break dormancy in the spring.

That's why you can't grow things like apples or cherries in tropical climates.

They never get the cold needed to break bud dormancy.

Although not all bud dormancy needs cold.

Think about potato -wise, they're buds.

They mainly need a period of dry storage to break dormancy.

The whole process, entering and exiting dormancy, usually involves a really complex balance inside the plant between growth -inhibiting hormones and growth -stimulating ones.

Okay, one last category of movements, gnastic movements.

You said these are different from tropisms because the direction of the movement isn't related to where the stimulus comes from.

We already mentioned nectanasty, the sleep movements of leaves folding at night.

Right, and the mechanism behind those sleep movements, like in wood, sorrel, or beans, is pretty cool.

It involves changes in turgor pressure, water pressure inside specialized cells.

These cells are located in swellings in the base of the leaf or leaflet, called pulvini.

They act like little hydraulic joints.

Rapid fluxes of ions, like potassium and chloride, move into or out of these cells, water follows by osmosis, and the cells either swell up or shrink, causing the leaf to move.

Beyond sleep movements, there are also thigmonastic movements, which are responses to touch or mechanical shock.

The sensitive plant, Mimosa pudica, is the famous example.

Tuts is leaflets, and they rapidly fold up, the whole leaf might even droop down.

Again, it's due to a sudden loss of turgor pressure in the pulvini motor cells.

It's thought this sudden movement might startle insects or herbivores trying to eat it.

And then there's the Venus flight trap, Dionea muccipula.

Truly spectacular.

It catches insects by snapping its modified leaves shut.

But it's clever about it.

Inside the trap are trigger hairs.

An insect has to touch two different hairs, or touch the same hair twice, within a short time window like 0 .75 to 40 seconds.

This prevents the trap closing on just a raindrop or random debris.

When triggered correctly, it generates electrical signals, like a nerve impulse almost, that activates ATP use, proton pumps, or rapid rush of water, and the trap slams shut.

You can even tell the difference between struggling prey and, say, a pebble dropped in.

Amazing.

And just the general effect of being bumped or blown by the wind, that has an effect too.

You mentioned thigmomorphogenesis.

Botanists noticed plants grown outside are often shorter, stockier than ones coddled in a greenhouse.

Studies show that regular rubbing or bending, like from wind or even raindrops, actually slows down stem elongation, but stimulates the stem to get thicker, wider.

It's the plant's way of building a tougher, more resilient structure to cope with mechanical stress.

Makes sense.

And finally, lots of plants track the sun, right?

Solar tracking,

or heliotropism.

Sunflowers turning their heads are the classic image.

These movements often involve those pulvini, using changes in turgor similar to sleep movements, allowing leaves or flowers to orient themselves for optimal light absorption throughout the day.

So yeah, from bending to light, sensing gravity's constant pull, knowing the time of day, the season, even snapping shit on flies, plants are anything but passive.

This deep dive really uncovers a whole world of sophisticated biological engineering.

It's quite humbling.

It really is.

And connecting it all back, the sheer variety and the precision of these responses, it just highlights their incredible adaptability, their evolutionary success, how they've managed to thrive in almost every environment on earth by fine tuning these responses.

It definitely makes you stop and think,

what other subtle cues are they responding to?

What else are they sensing that we haven't even figured out yet?

That's a great thought to end on.

There's always more to learn.

Well, thank you for joining us on this deep dive into how plants sense and respond to their world.

We really hope you've gained a new appreciation for just how dynamic

responsive plant biology truly is.

From the Last Minute Lecture Team, thank you for listening.