Plant hormones are chemicals that promote or inhibit growth in plants. There are 5 main types of plant hormones. Auxin, commonly abbreviated as IAA, gibberellins and cytokininins all promote plant growth. Abcisic acid and ethylene inhibit plant growth. These plant hormones are also produced synthetically and have many commercial uses.
When you want to tell your body to do something, you can use your nervous system to cause very specific reactions, or you can use your hormone system to cause more general longterm reactions. But with plants they don't have a nervous system. The only system that they have to control everything, are their hormones. Now, annoyingly enough for us, when we're having to study them, unlike humans where we have something like the thyroid-stimulating hormone, which pretty much is straight forward. It stimulates the thyroid. With plants, they have these classes of hormones with rather overlapping areas of responsibility. Now before I get into what the different classes of plant hormones do, I want to make sure you understand in general, what a hormone is. A hormone is a chemical that's released by one cell or part of a body and it travels to some target cell, where a receptor protein will bind to that hormone and trigger off some change within the cell of the receiving cell. So the plant hormones fall into two basic categories. These three up here the auxins, gibberellins and cytokinins in general they tend to stimulate plant growth. While abscisic acid and ethylene can inhibit plant growth. So let's look at the first category, the auxins. One commonly mentioned auxin is IAA. And in general, the auxins are involved in cell growth and cell elongation. Especially with various tropisms, plant responses to things like light and gravity. Auxins often causes, say for example on phototropism, the growth of a plant towards light. The side that's in dark tends to capture more and more auxin and elongate those cells moving it towards the light. You'll also see auxin involved in something called apical dominance. The apex of a plant is the tip, say the tip of the stem or the tip of the root. And that's where you get the most growth. Well what prevents the other parts of the plant from growing? Well, because of the build up of auxin in those lower areas. And in fact, I had a wonderful demonstration of apical dominance, happened in my backyard. One of my neighbors had an old tree, where it was starting to rot, so he chopped off the top of the tree to help start breaking it down. Well I discovered that there's a whole bunch of roots from that tree going underneath my backyard. I discovered this because all of a sudden those roots without the apex, say, "Don't do cell growth, don't do cell differentiation." All of a sudden started... shooting up a bunch of new tree branches and tree trunks, basically throughout my backyard and I had to spent a couple of weekends destroying it. In general, the auxins are involved in promoting the growth of fruit and the development of the roots. And they tend to prevent leaves and fruits from being lost. And farmers can use this, if you spray auxins on maturing fruit, it will stay on the tree longer and it won't drop off. As with these other two, auxins are often involved in what's called seed germination. That's when a seed goes from being just a standard seed to being an active seed and growing into a new plant. Gibberellins are involved in stem elongation and increasing the size of plant parts. Growers of grapes can spray gibberellins on to increase the size of the grape yield. It's also involved in breaking something called dormancy. You're probably aware of a lot of trees that lose their leaves during the winter and they enter into a greatly slowed down stage called dormancy. Gibberellins are involved in having the plant become active again. Cytokinins are involved, as you might have guessed from their name, in doing cell division. You may have recognized cytokinesis is sharing common roots. They're involved in promoting cell division and they help prevent what's called senescence or cell aging. Abscisic acid is, unlike these three, promoter of cell growth, abscisic acid tends to inhibit growth and often works in opposition to auxins and gibberellins. It can cause the dormancy that's involved in helping plants slow down for things like winter. And they're involved in helping close the stomata, the openings that plants use everyday to allow carbon dioxide in and oxygen out of the plant. Ethylene gas is involved in inhibiting elongation, inhibiting that growth. And it's actually involved in helping abscission, the dropping of leaves by plants. Typically that happens 'cause you have a slowdown in the amount of auxins and gibberellins being produced and then the ethylene gas gives it the final, kind of push to cause the leaf to drop off. It's also involved in fruit ripening and you can see this happening in some of your grocery stores. If you go and you're buying apples, you may notice that they all seem to be ripe on one side and the other side tends to be a little bit less ripe. And that's because a lot of growers will just pick immature apples off the tree because immature apples are a lot harder. Mature apples are alot softer and so what they'll do is they'll transport them as hard apples, or tomatoes, I've seen this happen as well. And then when they get to the stores, the store owner just goes... with ethylene gas. And if they're not very careful about it, you'll just see the ripening occur on the side that was exposed to the gas blast. That's plant hormones for you.