How does auxin move from the tip of the plant down the stem to the roots? To understand this question, we're going to deal with the, with the role of auxin in inducing roots. As I mentioned, one of the roles, main roles of auxin is that it induces root development. Auxin that is synthesized in the shoots moves down the stem into the roots and causes new roots to be formed. To understand auxin flow from the tip to the shoot, we have to understand that plants are polar organisms. That there's a difference between the top and the bottom. And that the flow is always unidirectional when it comes to Auxin. This polar basis of plant biology was discovered in the mid 19th century by yet another botanist. Another plant physiologist, Hermann von Vochting, where he reported that if he would take Stems of trees. Cut off the top, cut off the bottom, so we just have the stem, no longer roots, no longer leaves. Place it in a human cha, humid chamber, but turn them upside down. Roots would still form from the top, from the part that was near the original roots, but then, of course, the roots would grow down. Roots never came out of the side that was close to the top of the plant, to the top of the shoot. This shows the polar nature of the plants. The shoot that was next to the roots always knows that it's the bottom, even it'd been turned upside down. So auxin is synthesized in the shoot and is transported down towards the roots to introduce, to induce root development. So how does auxin move? How can we figure out how this is being done? Now, I'm going to describe here some quite advanced experiments. If you don't have a really, If you don't have a strong interest in understanding the mechanisms of plant biology. You can skip ahead at this point to the next section of this lecture. Now, if you decided to stay right now, I want to go into a little more into the advanced tools that we use to understand plant biology. And these advanced tools taken to utilize genes that we can put into plants. And these are not, these are genes that we put into plants for research purposes. One of the problems that we have in modern research is actually visualizing when a gene is active. And one of the ways we can visualize when a gene is active is by manipulating the gene. In general, we could describe a gene as having two main parts. Now this is a huge simplification. We could talk about a gene having what's called a promoter. That's the part of the gene that tells the gene when to turn on, and when to turn off. And then there's the part of the gene that's called the coding region. This is the part that encodes for a protein or a particular function. So for example, a gene that is induced by auxin will have an auxin responsive promoter. And then whatever the gene is supposed to do. Whatever its function would be. Through modern genetic tools, what we can do is separate these two parts, the promoter and the coding sequence. Such that we could take what's called the Auxin responsive promoter. The part of the gene that responds to auxin and then attach this through genetic engineering to a new gene. The gene that I'm going to talk about is called beta glu, beta glucuronidase. We call it GUS. Beta glucuronidase is a gene that when active, causes a blue color to be formed in the plant cells. So what we call this is reporter gene. What I have again I'm going to go through this slowly, we have a promoter that comes from a gene that's induced by auxin. It's attached to a gene called GUS which causes the blue color to be formed. Such that wherever there's a high concentration of auxin, I can visualize through a microscope, a blue color in the roots. And you can see that here in this picture. This is a plant, and transgenic plant that has this new reporter gene. We could that the blue color accumulates in the meristem, in the dividing cells at the tip of the root Where all the auxin that's coming from the tip accumulate. Once again, this is a transgenic plant. And we're visualizing where auxin is active through the use of this reporter gene. So this is the tool that we're going to use. So here's the hypothesis. The hypothesis is that auxin is synthesized in the shoot and it moves to the root by a polar transport mechanism that is independent of gravity. I'll repeat that. It's a polar transport mechanism independent of gravity. With the root acting as a sink, where, for auxin, acting as the place where all the auxin will accumulate independent of gravity. So here's how this experiment can be done. We're going to take stems of these transgenic plants and cut them off so that we have no tip of the shoot and no roots. We just have stems. We know which part was originally close to the stems and which part was originally close to the shoots. Okay. So we can take these stems and put them in a solution of auxin. If we take them and put them in a solution of auxin, normally at the bottom. So that the shoot side is up, and the root side is down. We see in this picture that the blue color is accumulating at the bottom. In other words, the blue color accumulates where there's a high concentration of auxin, which is at the bottom. If we take now, these same plants, take the same type of stems, and turn them over such that the root side is up and the sheet side is down. And now place these in the auxin solution where the shoot side is in the auxine solution, and not the root side. And now we color the plants. To see where the blue color is accumulating. We see that the blue color, the guss, is accumulating at the top. And remember here that the top is actually the area where the roots should be. In other words, somehow or another the auxin is being transported from the bottom to the top here. Which is transporting from the shoot to the root. Again, we go to this experiment. When we have the shoot side at the bottom, we find the auxin activity only at the bottom. Auxin isn't transported up the stem from the shoots, from the roots, excuse me, to the shoots. But if I do the opposite, I turn the, the stems over. Such as the side that was close to the shoots is now at the bottom. I see the auxin is transported up the stem, which is analogous to what would normally be down the stem, towards the area of the roots. Of course if we do a control experiment where no auxin is added, we see very little of the blue color because there's no part of the shoot here that makes auxin. So what we prove in this experiment is that auxin is only transported in one direction from the shoot to the roots and this is independent of gravity. So if we go back to our phototropism experiment, what we can assume now or conclude is that auxin is produced at the tip. It's transported down the stem, we're getting more cell elongation on the sides that are in the dark, which is causing it to bend. But in the end, all the auxin is accumulating at the tip of the root in the meristem, and this is what induces more root formation.
