Earlier in today's class, I asked what would a color blind plant look like? One of the responses I got was from someone named Roger in Phoenix and he wrote, a color blind plant would keep growing straight since it wouldn't see the blue light coming from the side. That is a very interesting hypothesis and the question is what experiment could we do to test this hypothesis? How could we actually find a color blind plant? Before I go into this hypothesis, I want to introduce to you our model system that many plant biologist all over world use to answer such questions and it's a plant called Arabidopsis thaliana. And here, we have an example of a adult Arabidopsis plant. This plant is actually only six weeks old and thousands of labs worldwide grow Arabidopsis, because it's a great plant for growing in the laboratory and it's great for a couple of reasons. One, again, this is a six-week old plant. It's whole life cycle can be completed within eight weeks. We can start from seedlings growing on petri dishes. These are seedlings that are a week old. We have a plant that's four-weeks old. One that's five weeks old, one that's six weeks old. And within another week or two, this plant will produce so many seeds that we can get up to 20,000 seeds per plan. And here again, just in this vial, you can see about 20,000 Arabidopsis seeds. So if you're thinking about doing plant research, I can either work on sequoia's and then maybe finish my PhD when I'm 80? Or I could work on something like a Arabidopsis which you could do tens of generations throughout within a couple of years. So again, in our laboratories, we could grow literally thousands of these just in our room. The second reason that our Arabidopsis has become such an important plant in experimental plant biology is that it had a very small genome, a very small amount of DNA. And in 2000, it was the first plant to have its genome sequenced. And so this sequence of DNA, we'll talk about this in future lectures has really opened up the doors for a whole new generation of plant biologists. And the last reason I want to talk about is that, because it's so small, Arabidopsis is a great genetic system. We can make mutants, just like a lot of researchers have used fruit flies to study animal biology. We can, for example, take these seeds and treat them with chemicals that induce mutations and then try to find plants with different types of characteristics. And for what we're talking about here, we could maybe find plants that would be blind to blue light. Now, how would we do that? You could take the plants like this, the seedlings on a plate. Put thousands of them that have been treated with a mutagen, treated with a chemical that will induce mutations. Put the blue light from the side and see if you can find the one plant that wouldn’t bend. Now, experiments like this have been carried out in several different labs worldwide and this is exactly what we we would find. You see a plant, here is the wild type plant, the normal plant. You put the blue light from the side and it grows towards the blue light. Here, we have a color blind plant. And even though the light is coming from the side, it keeps growing up straight. It's literally blind to the lateral light. Light effects plants more than just with phototropism, the ability to see light coming from the side. I'm going to talk for a few minutes about seedling development and this may be an experiment that you did sometime in elementary school. It's an experiment that you can still do, if you can buy some bean seeds or some pea seeds. If you grow plants, if you germinate them in the light or in the dark, for example, in your closet, it's obvious to see that they have two completely different morphologies. A plant that grows in the light, that germinates in the light. The seedling is short. It has what we'd call a short hypocotyl. That's what you might call a stem. It has open and expanded cotyledons. Those are the first leaves that we see and this type of development will continue on eventually to an adult plant, and finally to flowering. On the other hand, a plant that's germinated in the dark, for example, in your closet, it will have an elongated hypocotyl. Obviously, because it's trying to find where the light is. And its first leaves or first leaf like organs, the cotyledons remain closed, because it actually thinks it's under the soil and it's trying to push its way up. When it grows in the light, we call this photomorphogenesis, because it's getting its structure, its development by the light, photo. And when it grows in the dark, we call this skotomorphogenesis. Skoto is Greek for dark. And once this plant that's grown in the dark get's a little bit of light, then it will initiate photomorphogenesis. Its leaves will open and it will continue on to adult development. Scientists have utilized this system to further dissect light signaling in plants, because it ends up that photomorphogenesis can be induced not only by white light, but also by blue or by red light and also by constant far red light. If you grow a plant in constant far red light not just flashes, but hours and hours and hours of high intensity far red light. It will also go through photomorphogenesis and the way that the scientists have utilized the system is by looking for mutants that are actually blind to one of the colors, and let's talk about this experiment. For example, if you grow a wild type, what we call a normal, Arabidopsis plant under white light, it'll have a short hypocotyl and open expanded cotyl leads. It'll go through photomorphogenesis. And of course, also in red light or in blue light. Now, many labs have isolated mutants that are defective in some of the light signalling. Now, what I'm going to describe is an experiment that was first performed in the laboratory of Martin Corniffe. In Vachenagen in Holland in the early 1980s, but it's since been repeated in many other labs. Martin found several types of mutants. The first type of mutant, which we called mutant A was wrong in white light. It was also long in red light, but was completely normal in blue light. The second mutant was also long in white light, it was short looking at wild type and red light, but it was long in blue light. A third mutant that he worked with was long in white light. It was long in red light and it was also long in blue light. What's going on here with these mutants? What are they defective in? The first mutant here where it's long in red light, but normal in the blue light is defective in phytochrome. It's missing the photoreceptor that absorbs red light and transfers the signal to allow seeding development, which is why we get this elongated phenotype. The second mutant which is normal in red light, but long in blue light is defective in another photo receptor for blue light, which is necessary for seeding development. By the way, this is not the same photoreceptor for phototropism. So the second mutant is blind for seeding development in blue light, but what's going on in the third mutant? We see that it's elongated both in red and blue light. What could be the reason for this? I could think of a couple hypothesis. One hypothesis could be that there's another photoreceptor, which is sensitive to both red and blue light and another hypothesis could be that it's something what we call downstream. If you remember in the beginning of the lecture, we talked about how our brain or how we see the ball coming at us that the light comes from the ball into our eyes to the retina and then is transferred to the brain. So perhaps, this third mutant is lacking something that transfers the light signal from the different photoreceptors and that's actually what it is. This third mutant is not detective in the photo receptor, but it is defective in the signalling of the light.
