Is it animal, vegetable or mineral?

If you've ever played 20 Questions, Charades or any other such guessing game, you've likely posed that question or had it asked of you. Have you ever noticed anything remarkable about it?

It's phrased in descending order. The first category includes the most complex life forms, the second addresses less-than-complex lifeforms and the third isn't living matter at all.

If you're thinking: "Wait a minute, plants aren't complex?", you hit the nail on the head.

Plant life may not think, vocalise or walk around and do stuff but they are quite complex organisms. From their multi-celled structure to the way they feed themselves, reproduce - and yes, even communicate, there's much less that separates plants from animals than you might think.

In the course of your studies, you surely learned that plants have DNA and they express genetic traits; they do it much the same way animals do: with hormones.

What are these hormones and how do they work? That's what Superprof helps you understand today.

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How Do Plant Hormones Work?

You don't have to know much about plant life to know that they never stop growing, they're generally anchored to the earth and that their lives are cyclic. Hormones, the molecules regulating every aspect of plant life, control these processes.

Phytohormones is the correct term to describe plant hormones.

When a carnivorous plant eats, hormones direct their prey's breakdown and digestion processes.
Jasmonic acid is the hormone used to digest the flytrap's prey. Photo credit: Free the Image on Visualhunt

Let's imagine an organism - a human, for instance. Humans are chock-full of hormones; they swirl around inside our bodies, attaching themselves where they need to so that functions necessary to ongoing life will take place.

For instance, the human growth hormone, generated in the pituitary gland, addresses practically every one of the body's tissues, including bone tissue, by stimulating protein synthesis and accelerating the breakdown of fat.

Clearly, that doesn't happen from some hormone command centre that tells each molecule where to go and what to do. Upon their release into our bodies, these hormones course through to where they're needed and get straight to work.

That begs the question: how do they know where to go? You probably know because you've been studying for your Biology GCSE for a long time, right?

It might help to imagine the cell structure of an organism - maybe a human body, as a giant, semi-fluid, interlocking puzzle, wherein each piece has dedicated receptors.

As hormones flow through our bodies, they may only 'dock' on cells with receptors specifically meant for them. For instance, a growth hormone could not lock itself onto a receptor meant for a thyroid hormone and vice versa.

Once a hormone penetrates a cell via its specified receptor, it may directly or indirectly change that cell by causing activation of certain genes (direct change) or by acting as traffic controllers - exciting various signalling paths so other cellular processes take precedence.

As hormones perform a variety of functions and must fit the unique receptors meant exclusively for them, they must be distinct from one another.

Let's take a look now at specific phytohormones - how they work, what part of the plant's life they 'control' and whether any of them ever get crossways with each other.

Auxins

These phytohormones are responsible for aspects of growth. They make it possible for the plant to react to gravity (geotropism) and sunlight (phototropism).

In many plants, auxins establish the apical meristem - comparable in function to what, in animals, are called stem cells. They also set the direction of growth for plants. Most plants grow upwards  but consider also ground cover plants like ivy and other creepers.

Because of their link with growth, auxins are generally concentrated in or closer to the plant's shoots rather than in the roots - although, of course, they can still be found there, just in smaller numbers.

Depending on the type of plant, auxins drive select aspects of their processes. For instance, in fruit trees, they may be particularly active as the tree flowers and then again as the fruit matures and ripens.

For your plant hormone GCSE or other biology exam: how much of an impact do auxins have on photosynthesis and plant growth?

Cytokinins

These phytohormones work with auxins to direct select aspects of a cell's metabolism and also cells' differentiation - what type of cell it changes into. By interacting with the plant's DNA, it hides or expresses certain proteins that allow the plant to create for itself different tissues for its growth and for other purposes.

Where auxins are mostly found in plants' shoots, cytokinins are mostly found in their roots. This balance allows the plant to grow in both directions.

Interesting footnote: scientists experimented with pure cytokinin and auxin applications. With the cytokinin experiment, the plant grew a vast root system but few buds; the inverse was the result in the auxin experiment.

That proves these two hormones balance each other and both are equally necessary for plant development and growth.

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Gibberellins

Gibberellins regulate plant growth, but on a much greater scale than auxins do. Indeed, studies have shown that dwarf plants are generally afflicted with either a lack of gibberellins or, if that hormone is present in sufficient quantity, the plant is unable to use them.

Fun fact: bonsai trees are not dwarf growths; they are deliberately cultivated in small pots to restrict their root growth.

Gibberellins play an important role in waking seeds up when they're dormant. They activate amylases and other enzymes to break starches down into glucose, thus giving the embryonic plant energy to grow.

Gibberellins have many commercial uses, from making fruit bigger (if applied in the right stage of growth) to making sure that every seed germinates. Furthermore, as this phytohormone helps drive plants' s sexual characteristics, breeders can add (or subtract) gibberellins, depending on the outcome they hope for in their selective breeding programmes.

Bonsais are deliberately cultivated to be dwarfs
Bonsais are not genetic mutants; they do not lack any gibberellins. Photo credit: Melinda Young Stuart on Visualhunt.com

Abscisic Acid

Originally named dormin, this hormone is mostly responsible for sustaining the possibility of plant life as it lay dormant. Today, science attributes two more vital functions to this phytohormone: regulating the seed development process and engineering the change from embryo to seedling.

Abscisic acid also plays a vital role in how plants respond to temperature and loss of water.

The higher the temperature, the more water the plant loses through its stoma - tiny breather holes in the leaves of plants. If the plant is in danger of losing too much water, this hormone is produced in greater quantities, which then drive the closure of the stoma, allowing the plant to conserve water.

This function is especially vital for vascular plants.

Also, learn more about plants' leaf structure and function...

Ethylene

Most other plant hormones consist of several different 'ingredients' but ethylene is a 'single' chemical. It was first discovered in the 1960s but it wasn't until around 30 years later that, in its gaseous form, it could stimulate fruits to ripen.

Naturally, that discovery led to wide use in commercial farming. As ethylene gas can be sprayed across entire fields of crops, farmers can better plan their harvests without risking premature collection of their fruits, vegetables or nuts. Indeed, the harvest will not only ripen faster but detach itself from the plant, making for easier pickings.

Ethylene might be compared to our brains' neurotransmitters. It spearheads communication within the plant and with other plants nearby.

When a plant becomes damaged - say, some herbivore decided it looked like a tasty snack, ethylene gas will flow to where the plant was breached and, blown by the wind, will travel to other plants, communicating the damage done to them, which causes the damaged plant's neighbours to engineer defences for themselves.

All plants are regulated by hormones
No matter which variety of plant, their lifecycle is determined by hormones. Photo credit: James Jardine on Visualhunt

Conclusion: What Are Plant Hormones?

These five phytohormones were discovered long ago - well, maybe only relatively long ago. Science had to invent and build the equipment necessary for their discovery and study.

Still, botanists from centuries past intuited that plants developed, expressed certain traits and communicated in specific ways; think Gregor Mendel cross-breeding peas. It took this long to discover the actual chemicals that drive plant processes, to give them names and explore their properties and functions.

Some might say 'exploit their functions', considering the commercial applications that use ethylene and gibberellins. 

The age of plant discovery is not yet finished. In recent years, botanists have discovered four additional phytohormones:

  • brassinolides: similar in function to testosterone and oestrogen in animals
  • salicylic acid: like ethylene, it allows individual plants to communicate
  • jasmonates also function like ethylene
  • systemin mirrors ethylene's role as a defence builder after a plant suffers damage

Keep in mind that all of these are categories of plant hormones. As the plant world is so diverse, of course, different types of plants are going to have hormones that don't drive life in other plants. Moreover, while two plants might have the same hormones, each hormone might have a different function from plant to plant.

For instance, perennial plants - plants that don't have a 'programmed death' have less use for abscisic acid's dormancy function.

Regardless of the type of plant, remember that a hormone cannot bind to a cell unless its receptor protein is present and, the more receptors for a specific hormone a cell has, the more sensitive it is to that hormone. That, in turn, results in a greater cell response when the hormone is present.

Now, discover how hormones help drive plants' transport system.

Need a Biology teacher?

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Sophia

A vagabond traveler whose first love is the written word, I advocate for continuous learning, cycling, and the joy only a beloved pet can bring. There is plenty else I am passionate about, but those three should do it, for now.