Gibberellins are a class of plant hormone involved in the regulation of growth. They induce cell division in the stem, leading to stem elongation. They're also involved in fruit growth and seed germination. Now, with our three plants that you can see right here, we are showing the difference in plant morphology based on exposure to gibberellins, which are confusingly abbreviated GA. I don't make the rules, guys. I don't name these things. So, as you can see, this plant in the middle is a normal-looking plant, and it has normal internode length right there, and this plant received a normal amount of Gibberellins. This plant received a lot of Gibberellins. And that's why it has this crazy big internode length right there and just a generally long stem. In contrast, this little guy over on the end didn't get a lot of gibberellins, and that's why it has a very short stem, and especially has very tiny internode length.
In terms of germination, it's actually water being absorbed into the seed that will stimulate the gibberellins and those, as they are hormones are going to influence the gene expression of the seed, and that will lead to the production of amylase, which will start turning the starch in here into sugars, which will feed this organism and lead to its growth, and of course, breaking free from its seed coffin.
Now, acetic acid is another important hormone that we've mentioned previously. This hormone is involved in stomata opening and seed dormancy. Now, you might recall that abscisic acid is produced in the roots and will make its way up to the shoots where it will influence stomatal closing. Now, we had talked about before how blue light photoreceptors can play a role in stomata opening and closing. Abscisic acid will supersede whatever signal those blue light photoreceptors are giving. So even if the blue light, you know, even if the signal coming from the blue light photoreceptors is contradictory to the abscisic acid, the abscisic acid wins out. That hormonal signal is more powerful. Now, dormancy is that period of arrested growth prior to germination, and it doesn't end until there's the right stimuli and conditions for the seed to respond to. Abscisic acid is thought to actually inhibit germination. And these gibberellins we were just talking about induce germination. So, I hope you're noticing a pattern with these hormones. They often have balancing roles. One will turn something on, and the other will turn something off. This gives an organism a ton of flexibility and control over these important systems.
Lastly, I just want to mention brassinosteroids. These are a class of plant hormone that are involved in, growth as well, but specifically in cell elongation and division. And they're going to play a role in regulating overall plant body size. Now, the last thing I want to talk about is senescence. This is the sort of technical term for aging. This is what you might think of as biological aging, and it's usually marked by gradual deterioration of function. Right? So your cells gradually deteriorate in function as they age, you know, the organism as a whole gradually deteriorates in function as it ages. And there's actually a hormone for plants that is closely associated with this process. That hormone is ethylene. It's actually a gas at room temperature on earth. So, under normal conditions, this is a gaseous hormone, which means it's going to be able to diffuse through the air. That's, well, get back to that in a second actually. So, ethylene plays a role in senescence. One of those roles is in the process of abscission, which is going to be the shedding of part of an organism. In plants, usually we talk about leaf abscission, and leaves as you know, are upsized during the fall. That's why I have this beautiful fall background behind my head. Now the reason for that is because cells in the petiole, which is that stalk on the leaf, those cells are going to react to ethylene, and that ethylene is going to trigger enzymes to degrade the cell walls there in the petiole. That's why the leaf will snap off right perfectly at the base of the petiole. And if you've noticed that, but leaves always break off right at that point. It's because the cells there are degraded due to enzymes, which are signaled by this hormone. Now, fruits ripen when exposed to ethylene. As you can see in this nice image here, we have a ripening and aging banana, and as the fruits are exposed to ethylene, starch is converted to sugar, and the cell walls are broken down. So, that's why if you eat a really super green banana like that, it's going to be tougher and also just not nearly as sweet. It's going to be kind of like potato-y, not very palatable. As starch is converted into sugar, the fruit gets sweeter. That's why these fruits down on, or this banana down in the end, you know, this has been exposed to a lot of ethylene. It's going to be really sweet and sugary if you bite into it. So, that is why, for example, if you have an overly ripe banana near some green bananas, those bananas will go bad really quickly because ethylene is a gaseous hormone. And that overly ripe banana is going to be giving off ethylene and influencing those under-ripened bananas to ripen probably faster than they should. So, a nice little life hack here. If you're ever struggling with unripened avocados, they're too hard, throw them in a bag with a ripe banana. Those guys will get nice real fast. It's all ethylene. It's a beautiful thing. Alright. That's all I have for this video. See you guys next time.