So, we've seen that ethylene is involved in aging processes such as fruit ripening, leaf abscission leading to the autumn foliage. It's also involved in plant responses to environmental stresses like drought and wounding. We won't really have time to go into that here. And its last role is that, ethylene inhibits seedling elongation or cell elongation, and the reason I'm bringing you this in, is because it was important, this characteristic was important for understanding how plants smell, how ethylene's actually, how plants actually sense ethylene in the air. Again, a short review. We learned that we smell in our nose, because of the membrane receptors in the, in the back of the nose, which send a signal to the brain. So where is the nose? Where are the receptors of the plant? These were discovered thanks to molecular genetics using the same plant that we learned about last week, Arabidopsis. So as I mentioned, ethylene can also inhibit seedling elongation. If we grow in the laboratory or you could actually do this at home in one of your closets, a seedling in the dark it's very long and spindly. But in the laboratory if I grow it in darkness but then add ethylene into the air around it; rather than elongating, the seedling will be rather short and fat. We seem to get reduced stem elongation. We get a thicker stem and it actually ends up bending around to the side. Now one of the reasons we think it does this ecologically is that as a seedling is germinating, coming up through the ground. If there's something blocking it, say a rock, there would be a local increase in ethylene, which would then let it bend to get around it. This is what we think is happening in nature. But we don't know. But, anyway, getting back to the lab. We can actually utilize this in order to understand how plants respond to ethylene. So again, this is a picture of Arabidopsis, that have been grown in the presence of high ethylene, but in the dark. You could see that they're all short, fat and sort of bent. But here we have mutants that were grown under the exact same conditions, and we can see that these mutant Arabidopsis are elongated. In other words, they've lost the ability to sense the ethylene. We can think of them as being, as anosmic plants. Anosmia is what happens when we lose our sense of smell. These plants have lost their sense of smell for ethylene. They've lost the ability to respond to ethylene. There have been several different types of mutants that have been found. There are mutants, for example, that are insensitive to ethylene. They've lost the ability to smell. There are mutants though that make too much ethylene, so that even under normal conditions they think there's ethylene around them all the time. And then there's mutants that think that there's always ethylene being given to them, even when they're not. These mutants are always short and fat and bending over, it doesn't matter what's going on around them. But let's get back to, what is the ethylene receptor. These mutants, for example, that are I don't want to say blind, but have lost the ability to respond to ethylene, are mutant in what's called the ethylene receptor. Now, the ethylene receptor was the first protein that was really shown to be a receptor of a hormone in plants. And this was then back in 1993. And this receptor, similar to the way human olfaction receptors work, is found in the plant membrane. This receptor which is called ETR1 is bound by the plant cell membrane and has part of the protein that goes inside the cell. Such that when the ethylene binds the ethylene receptor, it causes some type of signal to go into the cell. So, I want to go in more in-depth of how this ethylene is being sensed by the receptor and what it's doing to the cell. Now, I'll go through this a couple of times, because it can be a little complex. So as I said, the ethylene receptor is sitting in the cell membrane, communicating information into the cell. Now in the absence of ethylene, what these receptors are doing is that they're on. They're active. But their activity is something negative. I'll say this again. So these receptors are on. They're communicating information. But when they're on is they're communicating something negative. What they're doing is they're inhibiting the ethylene response. So in the absence of ethylene, the ethylene receptor is turning off the ethylene response pathway. Now, once ethylene is produced, it is binds to the receptor, and when it binds to the receptor it inactivates it. And once the receptor is inactivated it then stops inhibiting the ethylene response. So in other words, the ethylene receptor is a repressor. It has a negative effect. The ethylene receptor, when it's on, it's repressing the ethylene response. Ethylene molecule, the phytohormone itself comes, binds to the ethylene receptor, on the membrane of the plant cell, turns off the repressor, which then allows of the rest of the ethylene response to continue. Physically what's happening is the ethylene is actually going into the middle of this protein, into the middle of the ethylene receptor, and when it binds in the middle of the receptor it's actually changing the shape. Such that in the absence of ethylene, the shape of the receptor allows it to be an active repressor. Once ethylene has bound to the repressor, and we actually know where it is specifically binding in the protein, it changes the shape of the protein. It's turned off, it can no longer communicate anything. And the rest of the ethylene response can continue. To make a long story very short. We now know most of the pathway involved in ethylene signalling. From its reception to transducing this material to other proteins within the cell, down through the cytoplasm until we get into the nucleus. Such that there are specific genes that are being turned on or being turned off, dependent on whether ethylene is present around the cells, or whether it's not. So these experiments showed, quite clearly I think, that a volatile chemical is sensed, it's smelled. Ethylene is smelled by the plant, and this starts a complex signal transduction pathway that regulates plant development. But, as I asked earlier, does this prove that plants communicate with each other through volatile chemicals?