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How do seeds know which way to grow?

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Organic Chemistry | Medicinal Chemistry | Carbon Nanotechnology4.3k points·1 month ago

Plants sense gravity and then bend in response to it. This is called called gravitropism. One of the major gravity-sensing parts of the plant is the root tips.

Plants also sense light, and bend to respond to it. This is called phototropism.

Both processes act in concert to orient a plant appropriately.

The bending is achieved through the action of a growth-regulating plant hormone called auxin. When it's secreted selectively, those regions with the higher concentration of the hormone grow and elongate more quickly, which causes a bend in the overall structure.

I saw Mr. Wizard do this experiment where new seedlings were put on a turntable, and as they grew, they all pointed toward the center. Cool!

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155 points·1 month ago(2 children)
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I loved Mr. Wizard! For me it was late night reruns after Nick at Nite

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Why would they all grow towards the center? Was it horizontal or vertical? Was there a light in the middle?

The centrifugal force from rotating around the turn table simulates gravity so they will grow towards the center because it “feels like” up.

34 points·1 month ago·edited 1 month ago

One of the neatest aspects of gravity and acceleration is that according to science, there is no difference.

If you are inside a box and you feel 1G of gravity in one direction, there is no experiment you can perform that will be able to tell you whether that box is sitting still on Earth, or whether you are in a box in space that is accelerating at 9.8m/s²

Edit: see Oz's reply. He's correct. It's gravity and acceleration. I've corrected my example.

What you mean is gravity and acceleration, for which there is no experiment to differentiate. Centrifugal force from spinning can be easily detected because of the coriolis force

Thanks, you're correct. It's been a while since I refreshed on the material.

You'd also have to assume yourself and the box to be a point mass. Little bit of a nuance but you theoretically could measure the difference in the gravitational potential on earth at different heights if the box was tall enough.

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That's like the driving simulator thing. I thought it would be amazing for VR. Some mechanical seat thing that can tilt back when you go faster in the game. You'll just be sitting there, but the gravity of tilting back makes it feel similar to the pull from driving faster.

26 points·1 month ago·edited 1 month ago

They do something similar for that Mars Mission ride at Universal Studios Disney. When the rocket is “blasting off” they just tilt your seat and spin it really fast to simulate continuous forwards motion, and it honestly feels pretty realistic.

That's at Disney World, not Universal. Unless there's two mars mission rides that use centrifugal force? It's called "Mission: Space" btw

Whoops! I stand corrected. It's been a while since I went, I guessed Universal because "generic Space ride" didn't have an obvious Disney theme.

Yeah it is pretty generic! Which most of the rides at Epcot are. Or rather they used to be, they've been changing them to be more explicitly Disney and changing the retro-futurism theme to a more... futurism theme. It makes me mad but I'm probably alone in that haha.

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Ah of course!! Makes sense. That’s really interesting. Thanks.

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Gravitropism (where roots grow towards gravity and stems grow away) is caused by starch grains in the cells being more dense and 'falling' to the bottom. The centripetal force from the centrifuge would move the grains outwards causing the plant to grow as if outwards was down and inwards was up

The centripetal force from the centrifuge would move the grains outwards

Centripetal means inward.

People who are overly particular about the language used to talk about physics like to say there is no such thing as centrifugal force and that centripetal force is the real force. That's fine when it's used to prompt thinking it through more carefully, and that is an exercise every physics student should go through.

But the take away from that is that centrifugal force is an apparent force experienced in a noninertial reference frame, and a deeper understanding of why it matters whether a reference frame is inertial.

If the takeaway message is that "centrifugal" is the wrong word and we should be saying centripetal, that means the pedicogical approach failed, and was perhaps introduced without going though the full thought process.

Damn my knowledge base is biology not physics but I'm embarrassed I mixed that up. I blame my not-sleeping baby.

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Ikr? You can't swing a dead cat by the tail without someone bringing up centripetal force around here.

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My guess would be that the downwars force exerted on the plant by gravity leading to it growing "up" is essentially replaced by the outward centrifugal force the plant experiences when spinning causing it to grow "in".

Could you provide a link if there is a video?

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108 points·1 month ago

Is this one of the things we’re studying with plants on the ISS? Their ability to grow in microgravity or other non-Earth environments?

If I remember correctly, the first plants to be studied were aboard Skylab, but yes, I’m sure that they are still growing some basic plants. Corn, string beans, and peas I’ve read about specifically, because they both grow quickly and exhibit a great variety of genetic diversity in their offspring.

Astronaut Scott tingle just tweeted a picture of eating lettuce grown up on the iss like a week ago

I currently have an experiment on ISS! We are one of a few ongoing plant experiments up there.. we're basically using a centrifuge to determine the lowest level of gravity that can be sensed by the plants. This will hopefully help us make some predictions on how things will go when we have our first crops on the Moon (0.17g) and Mars (0.38g).

Yes and they showed the lack of gravity does not affect root growth..

Source: https://www.nasa.gov/mission_pages/station/research/news/tages.html

109 points·1 month ago

How do they sense gravity?

Organic Chemistry | Medicinal Chemistry | Carbon Nanotechnology361 points·1 month ago

Certain cells at the tip of the root contain starch-filled compartments called amyloplasts. The starch granules inside the amyloplasts settle to the bottom of the compartment, like snow settling in a snow globe. Somehow (no one really knows how yet), this condition results in the auxin secretion in an appropriate area of the root to cause growth in the direction of gravity.

We actually do know, it's one of the research subjects at my University >_>

PIN proteins, which transport auxin in a polar fashion, are put on the membranes. There's different PINs for different growth effects, they generally all are made polar via endocytosis and recycling to the correct membranes side. I don't remember exactly, since I don't work with plants, but I believe the starch granules have a recognition protein that starts a signalling cascade to locally increase the endocytosis of PIN proteins. Whichever number it is for the gravitropic effect.

So it has nothing to do with water sinking in the soil, so the bottom half of the seed is exposed to more moisture than the top half?

Wouldn't that rely on the water to stop moving once it's reached a half way point?

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Nope, not that I know of. Germination starts with a shoot anyway, so the seed is already broken by the time roots come out.

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Things like this really blows my mind. To think nature developed this by random chance is awe-inspiring.

22 points·1 month ago·edited 1 month ago

Don't forget that innumerable other cell structures developed, most of which were less effective or even deleterious.

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2 points·1 month ago

It's kind of mind warping to think about what random chance is actually and what it means when ascribe it to the development and order of the universe and life.

Take a look at carbon, it's an essential element for life on Earth and if we ever discover life elsewhere, possibly also important, because it likes to mix with other elements a lot more freely than other elements do.

Yup, I worry people get the wrong idea about the "random chance" involved in natural selection.

If you imagine you have a bag full of balls of varying density, of which the least dense can be considered "more fit," then it's not a matter of blindly reaching into the bag and amazingly grabbing the least dense one on the first try.

Instead it's a case of taking that whole bag and dumping it out into water. Then you take back only a certain number of balls closest to the surface (let's say 25%) and duplicate them to fill a new bag. Then you throw that whole bag into water as well and repeat. Do that enough times and you'll eventually have a whole bag full of the least dense type of ball.

People need to understand that while the variations are random their viability is not, and that's what natural selection acts on.

What really gets your mind is that if these things hadn't developed by random chances, we may not be here to study them =p

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Would vibrating or spinning the pot of a plant have an effect on root growth? Are there any known ways of interfering with the auxin production or secretion?

Spinning would. Acceleration is indistinguishable from gravity, and rotation creates acceleration. The direction of the net force on the plant would dictate the direction of growth.

I've also heard of experiments where they simulate zero gravity by slowly "tumbling" the plant so that all orientations average out and the net effect of gravity on its growth is zero. Only works for organisms that are slow to react to gravitational stimulus, though.

I wonder what would happen if you only turn it over say a quarter of the way once a day. So basically every four days it would complete a cycle giving it enough time to settle but then turning it again on the next day. Would the roots grow in a corkscrew fashion?

I'll try to find a link, but I was in a science class and I remember there being a picture of a plant that had grown "up" a little ways, then they turned the plant upside down so it started to bend and grow "down" (oriented to the pot), then as it grew that way they turned it until it was right side up again. Basically ended up with a plant with a loop in the stalk.

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There are auxin inhibitors that have been researched in plants. So when a plant is placed sideways, for example, normally the auxin will be produced and create the plant to curve, however with the inhibitor they will just grow straight and not respond to gravity.

It's really insane how so many biological processes are controlled in such a simple manner!

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As a follow-up to this, my understanding is that growth/division of the cells opposite gravity (the "top" of the root) is accelerated and/or the growth/division of the cells on the "bottom" is inhibited.

When the cells on the top of the root surface grow and divide faster than the cells on the bottom, the overall effect is that the root tip begins to grow downward.

Sort of like putting a weight on a fishing line so it goes down when you feed it out.

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So there are chemicals, called auxins, in root tips and stem shoots. In roots these chemicals, as you would expect, are pulled downward by gravity. So if the root is horizontal the auxins are pulled to the bottom. Root auxins restrict growth meaning the root doesn’t grow as much where they are. This means the top side of the root (which doesn’t contain these chemicals) grows faster causing the root to bend downwards as it grows. Auxins stimulate growth in stem shoots causing the shoot to bend toward light

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This is how In-N-Out gets their crossed palm trees. They let them grow for a while, then rotate them to an angle, upon which the trees will behind growing vertically from the angle they are now at. Then they angle them back upright and let them finish growing straight up. Once the process is complete they transplant them into the soil in front of the restaurant.

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Is it possible for these attributes to be faulty and make the plant grow the wrong way round?

A mutation affecting hormone regulation could alter growth patterns but would likely be deleterious and not remain in the gene pool

I have a cactus growing sideways. I always thought it was special, but now I can say it's just anti-gravitropism.

I wouldn't call it that to its face though. It has enough problems.

Growing sideways? They’ll kinda lean (and stretch) if they need more light but haven’t seen one grow sideways before... do you have a photo?

I would sit it down and let it know. Better to hear from you than from others.

Some cacti do grow sideways, usually look like they’re limp and floppy.

Is that what you kids are calling it nowadays?

You probably didn't rotate it in relation to the light.

My sister had a cactus in college. Turned it a quarter turn every few days to keep it level in the windowsill.

Went away for one week. You could see the bend in growth for over a year.

Copper Sulphate around the seeds will acheive this. It disturbs the plants' ability to orient themselves, potentially making them growing downwards.

So for stuff that has been grown on the ISS, were the seeds/plants unable to find their way to the surface with the absence of both gravity and light? Or is it just guessing and throwing roots out in all different directions?

I only ask because these are the important details one should know for those instances where you find yourself in outer space, during a power outage (only affecting your spacecraft’s lighting), trying to grow some food.

they are under a uv lamp to grow i assume, and the roots grow every which way because no gravity

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So what would happen if you grew a plant in an environment without gravity?

Well it would be low gravity likely. But I think they would grow just fine. Scientists used to think we’d have constant vertigo in space because of the hairs in our ears that help with balance. We found we adapt to low gravity pretty well. Plants would probably do even better if they don’t get too much radiation. They don’t have muscles that require gravity to keep its mass.

The plants roots would grow where there are nutrients. And the plant towards the light. It would just adapt or die. My guess is it would adapt as long as the basics are present.

Do we really have to guess? I'd think they must have grown plants up on the ISS already, and we'd have scientific results by now?

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And then there are bulbs. Reminds me of a conversation with my son when planting bulbs: Son: Which way do we plant bulbs? Pointy side up or down? Me: It doesn't matter. The squirrels don't care.

Radiolab did an episode on the "powers" of plants in the last few weeks. Is it incredible.

For instance, some studies suggest plants have sort of a pavlovian classical conditioning: after getting used to a light being flashed at them with a ventilator turned on, they will follow the wind even if there's no light.

In another study the roots of a tree follows the sound of water.

Another one a plant "learns" that a situation is not dangerous and stops reacting to it.

This is definitely worth your time.

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Isn't it called geotropism?

I thought that they were synonymous, and I normally use geotropism too, but maybe not although I think that that's bollocks, because this page seems to imply that they're interchangable.

I remember the fun I had at school growing peas on clinostats.

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On a related note, there is a German satellite called Eu:CROPIS being launched later this year to simulate plant growth in gravity similar to that on mars and on the moon.

https://www.space.com/35533-space-greenhouses-moon-mars-greenhouse.html

i just have to say that it's more than just sensing gravity. they are growing seeds on the space station in 0 gravity environments and the seeds always sprout and point their roots towards the soil and shoots out of the soil pouches which house them.

On Joe Rogan’s podcast, they talked about how the roots actually grow towards the sound of water. To test this, they had a recording of the sound of water and that was exactly what happened. Apparently the plants have small hair follicles for hearing, similar to the ones on our ear drums! So cool!

I actually knew this one, but couldn't remember what auxin was called. Thanks for reminding me haha

2 points·1 month ago

How are plants affected by being grown in space?

So do seeds go crazy when they spout in a zero g environment like space?

To add onto this I read a while ago that the University of Western Australia did some studies and it seems that many plants can also sense vibration that would generally indicate water sources and their roots grow to meet it.

Could you troll seeds using magnets?

Now I'm wondering how roots know to grow in a certain direction to stabilise the tree when it's lopsided.

I’m going to guess it has something to do with pressure. If a large tree becomes lopsided, the roots would definitely feel pressure on the side the tree is leading. The pressure probably inhibits growth causing the roots to grow on the opposite side and thus creating balance.

This was a wild guess I yanked out of my arse. Sounds good to me! :)

It's worth mentioning that the auxin model is actually flipped for the roots and stems. In the roots, higher concentration inhibits growth

I've done a research exactly about that, specifically Auxin and its effects.

Man, that sure reminded me of forgotten days.

what happens if they're in space?

For the first time I planted some flower seeds with a seed 'kit' and have been watching them sprout and grow in my house. It's really neat to watch the sprouts pop out of the dirt and reach for the light. They are tall enough now to plant them outside.

I thought the way in which they bend differs between the roots and the stems? From what i remember, in the roots auxin causes stunted growth on one side of the root in order to turn it rather than stimulating growth like in the stems.

It probably makes sense to include hydrotropism as well, which means roots can detect water and will grow towards sources of it.

I'm not tryna be a dick or anything, and i could be wrong because I learnt this a while ago, but I just wanna make sure nothing is left out i spose.

Do we know how plants grow low or zero gravity? Does the phototropism completly cover it once they have grown out of the soil? And what happens before they "leave" the soil, do some of the seedlings grow the wrong way?

Thanks!

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Plants have a growth factor (note: NOT hormone) called indole acetic acid that causes cell elongation in shoots by interfering with the hydrogen bonds in the cell wall, so that they slide over eachother thereby lengthening. Sunlight affects distribution of this growth factor such that it exists mostly in the shaded side of the shoot. As a result, the shaded side grows faster than the sunny side, so the plant grows towards the sun.

In roots however, the same substance inhibits growth so that roots grow downwards. Its distribution is affected by gravity. The lower side has a higher concentration so its growth is inhibited more.

Like others have mentioned, this is called a tropism. Phototropism for light, geotropism for gravity.

Hormones are plant growth regulators (PGRs) the differences as to whether they are hormones or not is if they are endogenous. Indole acetic acid (IAA) is a hormone, it is produced primarily is in the apical meristem. Phenylacetic acid (PAA) and indole butyric acid (IBA) are also endogenous auxin hormones. Naphthaleneacetic acid (NAA), 2,4 Dichlorophenozyqcetic acid (2,4 D) and 2,4,5-Trichlorophenoxyacetic acid (2,4,5 T) are synthetic auxin-like compounds that are not technically hormones. They are still considered plant growth regulators and are used in tissue culture as cheap auxin-like alternatives.

Auxin is actually is one of the primary factors in root growth, it is produced in the meristem and transported to the roots. Auxins are involved in root hair growth and in tissue culture are used for the induction of adventitious root growth. Cytokinins (the other major plant hormone or PGR) inhibit root growth.

Also gravitropism is for gravity and is achieved through specialized amyloplast in the roots. These plastids contain starch granules that fall to the bottom of the cell in relation to gravity acting as a signal for the roots to grow in that direction.

Source: PhD student studying plant cells and plant physiology

This is the first response that actually explains it instead of just offering a scientific term. Thanks!

I thought indole-acetic acid was one of the more common forms of auxin, a hormone?

Not to comment on the accuracy of the post you're replying to, but whether a chemical messenger is considered a "hormone" or not depends on how/where it's being used.

Adrenalin (epinephrine) in humans for example, is considered a neurotransmitter when used to carry messages from neuron to neuron across a synapse, but a hormone when released into the blood to carry messages to disparate targets.

In addition to neurotransmitters (seeing as how we're talking about plants after all) There are also chemical messengers that are used in signalling other nearby cells, or even the same cell that excreted them. Traditionally, hormones are only those chemicals that are released broadly, and that carry messages between distant targets (particularly through the circulatory system)

Some feel this classification is overly strict though, so don't be surprised if you see someone using it differently.

Very interesting!! Thank you

There's actually a really cool podcast about this, it's "smarty plants" from Radiolab. They talk, among other things, about some research that suggests plants can hear water. It's one of my favorite episodes, check it out.

Listening to this now. Thanks for the suggestion.

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It's called a "tropism" which is a response to a stimulus. Geotropism; growing straight down towards the centre of the earth and phototropism; growing upwards towards sunlight (& warmth). Plants also grow towards water; hydrotropism. A tropism explains why a pine tree on a mountainside will grow straight up on a 45 degree hillside.

A tropism explains why a pine tree on a mountainside will grow straight up on a 45 degree hillside.

I always wondered how that happened, but now I’m curious: what happens if you have tropisms competing for dominance or control?

For instance, if you planted the seed on a hill, it would seem that the sunlight/warmth would likely be strongest perpendicular to the slope of the hill, as any other (non-perpendicular) path would be longer, effectively being the hypotenuse of a right triangle. (For purposes of my questions, let’s assume the difference between the two distances is not negligible.)

Because trees grow vertical on hills as you mentioned, I assume that the gravitational pull (geotropism) is exerting more force upon the seed than the sunlight/warmth (phototropism).

Is there a way to counterbalance this to grow plants or trees that grow perpendicular to the hillside? For instance, placing the seed closer to the surface in high altitudes (such that the Earth’s gravity exerts less force on the seed)?

Or, since other tropisms also have an effect on seed growth, is there away to manipulate multiple tropisms to get the same effect (plant/tree growing perpendicular to hill)?

I would say that a seed must germinate or it will lie dormant until conditions are right for germination; seasonality, sunlight etc. So I would say a phototropism is necessary for germination but once sprouted and a leaf or two for photosynthesis to take place it must take root concurrently or it will just fall out of the soil (45 deg angle) and cease to exist. Once germination and photosynthesis have taken place the plants must compete for sunlight evidenced in rainforests where they struggle to get enough sunlight to survive by getting to the top of the canopy so they can compete for sunlight to survive in a jungle with other competitors. Some trees do not make it and succumb to become compost for the next generation. Fires provide a rebirth of trees and plants generally who will spring up after a fire and often act as a stimulus for trees to drop their seed (pods) for reproduction.

If a tree grew perpendicular to the hillside, gravity would probably not permit it to survive; it would eventually fall over before maturity.

The best example of a phototropism is the sunflower which will follow the sun as it tracks across the sky everyday.

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This link has a video and an explanation of competition between phototropism and gravitropism. While it does not explain the exact circumstances in which gravitropism becomes dominant, you can infer from the video that it is dominant when the light is principally ambient, and not when it's not.

What about the trees that just grow perpendicular to the ground no matter the slope?

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Some good answers here but I don't see anyone describing the mechanism through which plant roots "know" to grow downwards. Plants contain statocytes at the tips of their roots. Statocytes contain statoliths, starchy organelles that rest on the bottom membrane of the cell due to gravity pulling them down. The location of the statoliths influence the direction of growth.

I had seen not too long ago a news story regarding growing plants underwater in air bubbles / greenhouses. It kept the plants safe in someway? I wonder how this would effect growth of the roots and what the drawbacks and benefits are to this method....

I forget! Anyways, very interesting discussion.

Cellularly, the way to know which way is down, is by using certain granules called statoliths. These granules in the cells of the seed contain concentrated starch and thus are heavier than the cytoplasm, which causes them to fall down in the cell, and then there is a direction of gravity which the cell senses, and then it knows in which direction to grow.

It is tropism There is two types of tropism: The first is geotropsim or gravitropism where the roots grow towards gravity. It is done when auxin is pulled down by gravity and it makes the root elogate towards gravity as auxin effects the cells and stops roots from growing. So as it sits on the bottom of the root the other side still grows and pulls the root towards gravity.

The second is phototropism, which happens in shoots. Auxin causes shoots to grow and it sits at the top of the shoot. Auxin is broken down by sunlight. So if the sunlight is hitting the shoot at 90 degrees it will break down the auxin causing the shady side to grow which curves the shoot towards the sun.

Plant hormones are really cool. Everyone should look into them

The plant relies on phototrophic and geo-trophic senses it has geo-trophic means it senses gravity and that’s what the roots grow towards, the the stem of the pant whilst in the ground grows in the opposite direction but as soon as it breaches the soil it uses its phototrophic senses which means it will always grow towards the Light you can see this in trees that have grown at weird angles to be able to absorb more light energy.

All plants can sense the direction of the gravitational field and orientate themselves accordingly. This is called geotaxis. In mature plants, phototaxis (growing towards the light source) overrides the gravitational impulse for the stalk and leaves, but the roots - and the seed while it is underground - rely on gravity for orientation. The mechanism is thought to be based on either the protoplasm (the living substance inside a cell) exerting a greater pressure on the cell walls at the bottom, or starch grains within the cells settling at the bottom. This question has already been found on the site studydaddy.com also there you can find a lot of useful information.

Interesting..... I learned this phenomenon as gravitropism. Is geotaxis the same thing under a different name, or the underlying mechanism that "drives" gravitropism?

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