Have you ever had one of those real hormonal days where you are feeling bad and you can tell everyone around you knows? What if I told you that plants interact with their environment in a similar way? Yes, plants use signal transduction pathways to interact with hormones in the environment not too differently than how you and I do. Whereas we would respond to stimuli with movement, plants respond to their environment in the form of changing their growth form and habits.
Lets start at the beginning, Charles Darwin knew something was up. He noticed that some grasses would bend towards sunlight but only if the tips were attached still. We know today he was observing Phototropism, when plants grow towards light. Seemed simple enough right? Maybe not, the issue was trying to explain why this would not occur if the tips were removed, surely the grass didn’t have eyes. Unfortunately because of the limited technology at the time, they simply concluded that there is some kind of mechanism in the tip of the grass blade that caused this, but could not pinpoint exactly what it was or where it came from. Eventually in 1926 a scientist named Frits Went extracted the chemical signal that caused phototropism. He named the hormone auxin, and in doing so he inadvertently discovered the first known plant hormone. Then there was a flurry of research to find other plant hormones. Soon after it became clear that, while there are many different plant hormones with different functions, there were five that stood amongst the rest as the most important. Auxin, gibberellins, cytokinins, ethylene and abscisic acid are going to be the main topics of this blog post.
Auxin has multiple functions, and as a matter of fact there are several different hormones all called Auxins. The two main things auxin does is elongate photosensitive cells to shift growth towards or away from sunlight and subverting growth underneath the apex to grow horizontally rather then vertically (In the case of most long living vascular plants). Auxins use ethylene to inhibit the elongation of plant cells, causing stunted growth in that portion, auxin also only moves downward from leaves to roots. Auxins are surprisingly synthesized in the root tips from the trunk and branches and helps determine the size and placement of said organs. Auxins also have a lot to do with leaf development. Auxin combines with other hormones to suppress the development of auxiliary buds. That’s why in bonsai, when we remove the apical bud from our ficus trees for example, the buds hidden under each leaf node suddenly emerges. The auxin stopping that growth from appearing has been removed and now the new buds are trying to compete with each other for apical dominance, each one producing auxin and trying to subvert the others.
Cytokinins are like auxin’s best pals: they appear in actively growing tissues, and together with auxins, help cells grow larger and undergo mitosis. Without auxin, cytokinins will only make cells grow larger. Why this happens is not yet fully understood. When the two hormones congregate in high enough numbers to the same area those cells will begin to form a callus. If there is too much cytokinin then the callus forms into new buds, if there is more auxin it will instead turn into a new root. What a trip.
Gibberellins are kind of funny because they were found separately on a certain kind of fungi first, and thirty years later is when it was also discovered in plants. At this point in 2020 over one hundred different gibberellins have been discovered in different species of plants. They seem to appear in relatively small quantities compared to the other two hormones we discussed, and whats more is they seem to have a large variety in plant functions. To the induction of flowering to the development of such, they also help regulate size,shape, and location of trichomes. The actual germination of seeds is mostly attributed to gibberellins, as well as a major part of stem elongation.
Ethylene was discovered early in the 1800s when people noticed that the leakage from coal powered gas pipes would cause the trees growing nearby to lose their leaves. It is almost unthinkable that this noxious substance would also be a plant hormone but welcome to the strange world of botany. Ethylene plays (if you couldn’t tell already) a part in hibernation. The main function is actually stress response though, rather, physical stress like overgrazing animals or drought, it also helps to respond to floods and infections. Apoptosis, also called cell death, is attributed to ethylene as well. Imagine when all the leaves fall to the ground when the season is ending. Whats happening is ethylene is causing apoptosis in the cells connecting the petiole to the stem, explaining the original question; Why do deciduous trees undergo senescence(leaf shedding) when exposed to ethylene?
Abscisic acid (ABA) is a hormone that slows growth down. The main two functions are seed dormancy and responses to drought. ABA makes it so that seeds will not germinate inside of a fruit, or inside of your gut. They can keep the seed alive but dormant during the winter and then subside in the spring, causing the seed to germinate. In drought conditions, ABA forces stomata to close very quickly, stopping transpiration and therefore conserving water. ABA also moves quickly from roots to leaves as a warning system that the time of drought is nigh.
I’m attempting to draw a picture showing how all five hormones working in unison, but it turns out that’s a little easier said than done, if you couldn’t tell it’s slightly abstract. Hopefully this post has explained enough about the main five plant hormones so that you all aren’t as confused about it as I used to be.