At some point in development the meristem that's been making stems buds and leaves changes its purpose and starts making flowers. This is usually a response to the environment whether to a change in light or temperature or season in some plants it's just a response to change in age. And we don't really have time in this class to go into how this happens from a molecular point of view. But once this switch has been made, the same meristem that once produced leaves, now starts making a complex new organ, the flower. Now there's a huge diversity in flower structure. Flowers could be miniscule several millimeters in size, or they could be massive, over a meter in size. Flowers give us this huge beauty that we see in nature. Flowers allow reproduction in all environments. Even at the edge of a glacier for example. The beauty of flowers, and the structure that we see in flowers has fascinated both scientists and philosophers for ages. And actually the philosopher Goethe, was mixed science and philosophy in his own study of flowers. Goethe was very, very fascinated by the diversity of flower structure in roses. And through his studies, he came to a very clear hypothesis. Goethe published at the end of the 18th century a book called, An Attempt at Explaining the Metamorphosis of Plants. The organs of the vegetating and flowering plant though seemingly dissimilar, all originate from a single organ, namely, the leaf. In other words, what Goethe claimed was that the flower is just a modified leaf. Now, before we attempt to understand Goethe's hypothesis. Let's look more closely at flower structure. But looking at a very simple flower, the flower of the Arabidopsis. All flowers are made of four main structures. I'm going to go from the outside in. The first structure that we see on the outside, these green leaf like structures, we call them sepals. This is like the green cup that hold many flowers. Inside of the sepals we have the petals. These are normally the colored, or lack of colored structures, that give flowers their diversity of color. Arabidopsis has four white petals. Inside of the petals we have the stamens. These are the male sexual organs of the plant. They carry the pollen which is necessary for fertilization. And, inside of the stamens we have the carpels which is the female organ of the plant. Which hold the eggs which hold the ovules. So now looking at this structure, the Arabidopsis flower Is Arabidopsis a monocot or is Arabidopsis a dicot? So, I hope you can remember and can clearly see that Arabidopsis is a dicot flower and you could see that because Arabidopsis has four petals. And as, I said earlier in today's class dicots have flower parts that have four or five numbers of petals. So going back to Goethe's hypothesis, where the leaf represents the basic form from which the stem, leaves, and the flowers diverge. I am going to go back to Goethe's translation of what Goethe wrote. It gets a little complex from 18th century German. Goethe wrote the following in his book. We ought to have a general term with which to designate this diversity of the metamorphosed organ from which to compare all manifestations of this form. We might equally well say that a stamen is a contracted petal, that a petal is a stamen in a state of expansion, or that a sepal is a contracted stem leaf approaching a certain stage of refinement. Or that a stem leaf is a sepal expanded by the influx of cruder saps. So the question we need to ask is, if Goethe's hypothesis is correct, how does a plant turn a leaf into a flower? And to understand how this happens, we needed to have 200 years of science progress so that we would have molecular genetic tools to analyze flower development. What I'm gonna show you now, is the molecular model for flower development, which is termed the ABC model. This model was first proposed by Elliot Myerwitz, and his labmates at Cal Tech University. And it's based on numerous studies of mutants in floral structure. The ABC model claims three types of genes that are needed for flower development. A class of genes, a B class of genes, and C class of genes. If we go back to our model of the Arabidopsis flower, we see that the four types of organs are organized in circles, or in whirls. The outermost whirl are the sepals. Following that we have inside the sepals the petals. The third whirl being the stamens. And the fourth, the internal whirl, being the carpel. In the ABC model, A class of genes are necessary for the formation of the outermost whirl, the sepals. A mixture of the A class of genes and the B class of genes are needed to make the second whirl, which would be the petals. The B class of genes and the C class of genes together are needed to make the third whirl, the stamens. And the C class of genes alone are needed to make carpels. So in this model where the A class genes are expressed only in the outermost whirl, the C class genes only in most inner class whirl, and the B class in the second and third whirls. There's also a hypothesis of mutual inhibition. Where the A class of genes suppresses the C class of genes and vice versa. In other words, the A class of genes inhibit the C class of genes from being expressed in the sepals and the petals, and the C class of genes inhibit the expression of the A genes in the stamens and the carpels. So if this model is correct, we can make some predictions on what would happen if there was a mutant in any of the genes. For example, what would happen in a mutant that was lacking class A of genes. In a mutant, that's missing the A class of genes class C now no longer has a gene to repress it, so the C is now expressed in all four whirls. It's expressed in what should have been the sepals. It's expressed in what should have been the petals, and it has its own normal expression. Morphologically, what this means is when the C class of gene is being expressed in the outer most whirl or getting carpels being formed where there should have been sepals. When it's expressed in the second class whirl, it's being expressed together with the B class genes, so we're gonna be getting stamens where there should have been petals. And in the inner two whirl, we're gonna get our normal organs being formed. What would happen if we have a mutant in the class B of genes? In other words in the first whirl we'll have A genes, in the second whirl we'll have only A genes, in the third whirl we'll have only C genes, and also in the fourth whirl we'll have C genes. And it's such a mutant we actually see that this mutant lacks petals and it also lacks stamens. We have a structure of sepals followed by sepals followed by carpels and followed by carpels. And what happens if we're missing the C class of genes? If we're missing the C class of genes what was inhibiting the A class of genes is missing. So, A is going to be expressed. A is going to be found in all four of the whirls. So, we'll have sepals A alone. With A and B we'll have petals. But, now in the third whirl we'll also have petals being formed. So now we have multiple petals in this floral structure. And then in the final whirl we'll have sepals. So if you look at this Arabidopsis mutant you can see that it's starting to look like some of our cultivated flowers that have multiple petals but are missing stamens. And so now, if we want to take Goethe's hypothesis all the way to the end, what would happen if we're missing both the A genes, the B genes, and the C genes? If none of these floral genes are present, what would be the structure formed? If you're missing, if an Arabidopsis plant is missing both the A, the B, and the C genes, you get a floral structure which is comprised entirely of leaves. So we can see that at a certain level, Goethe was right. All the floral structures are just metamorphisized leaves. And in the absence of the genetic information to cause differentiation, the flower reverts to its original leaf structure. This genetic control of floral development has also been used by humans in plant breeding. But not only for beautiful flowers, such as cultivated flowers which have numerous petals instead of stamens, but also in agriculture. For example, if we look at the structure of a cauliflower, what we see this structure, this bumpy structure, is actually a multitude of meristems which are lacking the ability to finish their differentiation into flowers. This is meristematic tissue which is redividing and making another meristem, and another meristem, and another meristem. While a mutation one gene is keeping it from differentiating into normal flowers.
