Study Guide

Plant Biology - Plant Hormones

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Plant Hormones

Teenagers aren’t the only ones with raging hormones. Plants are full of hormones too, but lucky for them they don’t get pimples. In plants, hormones are responsible for all sorts of things, like helping the plants sense light, forming lateral roots, and triggering flower development and germination, just to name a few. If a plant had a facebook account, it might write updates like "OMG my axillary branches are shooting up so fast" or "just tricked a bee into pseudo-copulating with my flower, lol."

However, plants don’t have facebook, so they rely on hormones to be their messengers. Hormones are signaling molecules that are produced in small amounts and sent to other parts the plant body, like tiny messengers running around.

Why should anyone care about plant hormones? Plant hormones are really important in creating the green world around us, and providing the fruits we eat and other plant products we enjoy on a daily basis. Many things about plant hormones are still unknown, so it is a great field for a budding plant biologist (no pun intended…well actually it was, sorry).

Here we will discuss five types of plant hormones:

  • Auxin
  • Cytokinins
  • Gibberellins
  • Abscisic Acid
  • Ethylene


Scientists were interested in how plants respond to light; if plants don’t have eyes, how do they sense where light is and which way they should grow? It is a common observation that plants grow toward light, but for a long time no one knew why.

One of the first people to experiment with this concept was Charles Darwin, who along with his son, Francis, was interested in figuring out how plants respond to stimuli (in this case, light). They noticed that coleoptiles, which are sheaths that protect grass stems as they germinate, bend toward light. They tried covering the coleoptile with foil and found that when covered, the coleoptile didn’t bend. When uncovered, it bent again! From this the Darwins concluded that the tip of the grass coleoptile senses light.

Even though it doesn’t seem very exciting now, in the 1800s this was just as scandalous as Lady Gaga’s meat dress. The idea that plants could do something as brilliant as respond to their environment was shocking in an age when Man was exerting control on all things wild.

Later work by another scientist, Frits Went, determined that the signal responsible for bending toward light was a mobile chemical, and Went went ahead and gave it the name auxin. These days, auxin is sometimes referred to by its chemical name, indoleacetic acid (IAA).

Auxin does a couple different things in a plant, but its main role is to work with another type of hormone (cytokinins) to stimulate elongation of stems. If auxin is helping cells elongate, it is likely found in a place where a lot of new cells are forming. Where would that be? The shoot apical meristem, of course! The shoot apical meristem is a major source of auxin, but not the only one. Developing seeds also produce auxin, which leads to fruit development. When fruits such as tomatoes are grown inside greenhouses where there are no insect pollinators, synthetic auxins are used to help fruits develop normally.

Another commercial use of auxin is in the vegetative propagation of plants from cuttings. Instead of planting seeds, people can grow some plants by just cutting a leaf or stem; spraying the detached leaf or stem with auxin induces root production, and a whole new plant is formed.


Auxin helps cells elongate, but it doesn’t work alone. Auxin’s partner in crime is a class of hormones called cytokinins. Cytokinins promote cell division (cytokinesis) and are produced in roots, embryos and fruits, or wherever there is actively growing tissue. However, cytokinins need auxin to induce cell division. The ratio of cytokinins to auxin determines where cells will develop. If cytokinin levels increase, shoots form; if auxins increase, roots form. By themselves, cytokinins don’t cause any new tissues to form.
Cytokinins do a couple other things too: they help delay aging in plants by increasing the amount of new protein that is made and decreasing the amount of old protein that is demolished. Because of this, cytokinins are sprayed in flower shops to keep leaves green and cut flowers fresh.


Gibberellins are most important in stems, fruits and seeds. In stems, they work with auxin to cause stem elongation. Gibberellins and auxin also work in concert when fruit is developing. In fact, green seedless grapes are usually sprayed with gibberellins to make them bigger. Maybe that’s what Snooki sprays on her hair, too.

Seeds have the problem of not knowing when conditions are right for germination; after all, they don’t come with calendars and thermometers. Lucky for the seeds though, they do have lots of gibberellins, which are released after seeds take up water (perhaps after a heavy spring rain). After gibberellins are released, the outer layer of the endosperm releases digestive enzymes that break down nutrients in the endosperm. These nutrients feed the embryo as it germinates and grows into a seedling.

Abscisic Acid

It looks like a scary name, but abscisic comes from the word abscise, meaning to cut off or to fall away. On a plant, both leaves and fruits fall off, and abscisic acid (ABA), got its name because scientists originally thought that ABA caused leaves and fruits to fall off. It turned out later that other hormones (see ethylene, below) are mainly responsible for abscission, but the name stuck.

ABA does do some important things, even though it doesn’t do what it’s named for. ABA slows growth, and is the main player in seed dormancy. Since plants can’t exactly nurse their young and sing them lullabies like humans can, seeds have to be a bit more independent than a lot of animal babies. In fact, seeds are so good at taking care of themselves, they don’t even start growing until conditions are right (temperatures are warm, or there is a lot of rain, or they get free tickets to Disneyland). The abscisic acid in a seed keeps it dormant (sleeping, basically). Certain things, such as water, light, or even prolonged cold temperatures, cause the ABA to break down and cue germination of the seed.

ABA has another important role in plants: drought tolerance. When water gets scarce and leaves start wilting, ABA production is cranked up in the roots. ABA moves up the plant to the leaves. As it accumulates in the leaves, ABA causes stomata to close, preventing more water loss. When water is plentiful again, the ABA breaks down and stomata reopen.


Where would we be without ethylene? We would have many unripe fruits, for starters. And without ripe fruits we would have no strawberry milkshakes, pineapple-mango smoothies, or Fruit Ninja. Ethylene helps fruits ripen by making them softer, through the breakdown components of the cell walls, and sweeter, through the conversion of starches to sugars. Unlike the other plant hormones, ethylene is actually a gas and is distributed through the air, not through the plant body.

One of the coolest things about ethylene is that it is released in a positive feedback loop: a little bit of ethylene causes more to be released, which causes even more to be released, and so on. A benefit of this fact is that you can take an unripe fruit (a pear, plum, or peach, for example) and put it in a paper bag with riper fruit (bananas work well for this) and ethylene will accumulate, making the unripe fruit soft and sweet.

Brain Snack

Worldwide, billions of dollars of produce spoils every year before it is eaten. Thanks a lot, ethylene.

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