Plants can respond to mechanical stimulation in several ways. What we've been talking about so far, plant movements, is what's termed thigmonastic responses. Thigmo is the Greek word for touch. Nastic is movement. So, these are movements in relation to touch and these movements are always in one direction, in the same direction. Now, we're going to go into another plant response to touch, which is called thigmomorphogenesis. Which is a permanent change in the structure of a plant in response to mechanical stimulation. This was originally described in the 1960s just, quite by chance. A scientist by the name of Frank Salisbury, who was working at Colorado State University, was studying cocklebur. This is the was the same weed that gives those awful burrs that get stuck in our socks and shoes when we're hiking. He was interesting, interested in the growth processes of this weed and he and his scientists would go out into the fields and daily measure a particular leaf on a plant. They go out, take a ruler, touch the, you know, hold the ruler to the leaf, and measure the size of the leaf, over time. What they noticed, quite surprisingly, was that the leaves that they were measuring were always shorter than the leaves on other parts of the exact same plant. Not only that, the measured leaves, as the experiment continued, often turned yellow and died. But, other leaves on the same plant, ones that had never been measured, were green and flourished. Salisury, Salisbury explained in his own words, we were confronted with the remarkable discovery that one can kill a cocklebur leaf simply by touching it for a few seconds each day. Now, Salisbury himself was not so interested in plants responses to touch, so this type of research was left for other people to continue. And, it wasn't only for another decade when a scientist named Mark Jaffe at Ohio State University recognized this thigmomorphogenesis. He actually coined the term as a general phenomena in plants, that plants can respond in their growth to being mechanically stimulated. Now, why should a plant care if you touch it? Now, it does't really care if I, Daniel Chamovitz is touching the plant, but why should a plant care in general, if it's being touched? When are plants touched, or when are they physically stimulated? Think the most obvious answer is, by the wind. Plants, branches, leaves are constantly stimulated by the wind. And, if there are any hikers among you, you know that a plant, a tree on a top of a mountain and the same species in a valley look vastly different. A tree on the top of a mountain will be stunted, short, a relatively thick trunk, few branches with even fewer leaves, whereas the same species of tree protected in a valley, will be tall, majestic, with many branches and many leaves. What we're seeing here is a response, an adoptive response to the environment. When a tree is exposed to the wind, it responds by retarding its growth and making a thick trunk in order to protect itself. When it's not stimulated by being swayed, it can put its energy into growing tall. We get back to the same point which we've talked about several times, plants are sessile organisms. They're rooted. They're unmoving. They can't escape their environment. And, because they can't escape, they have to adapt biologically. They have to respond by changing their development. And, we can see this change, not only in trees out in nature, but also in our favorite model organism, Arabidopsis, in the lab. What you see here are two pots of arabidopsis. They are exactly the same age. The only difference is that this one was left untreated, whereas this one was touched three times a day. The scientists came, wave the hand over, at three different times during the day, throughout its life cycle. You could see that the one that's untouched, similar to the tree in the valley, is tall. I don't know if you can call Arabidopsis majestic, but at least it's tall and flowering, finishing its life cycle. Whereas the one that was touched, as if it was in the wind on the top of a mountain, is short, and stunted, and it's actually delayed in its development. It's going to take it longer to flower and to set seeds. So, shaking stunts development also, in Arabidopsis. While thigmomorphogenesis, the change in overall growth, takes many days for us to witness, the initial cellular response is actually quite rapid. In fact, Professor Janet Braam and her colleagues at Rice University, have demonstrated that simply touching an Arabidopsis leaf results in a rapid change in the molecular genetic makeup of the plant. What Braam discovered is what we call touch genes, or what she called touch genes. Now, to further understand the importance of this discovery, I'm going to take you on a quick tour of how genes work, in general. The DNA found in the nucleus of each cell that makes up the Arabidopsis plant contains about 25,000 genes. And, this is the same DNA, the same 25,000 genes in each and every cell of the Arabidopsis plant. At the simplest level, each gene encodes for one protein. So, while the DNA is the same in each cell, different cells, though, contain different proteins. For example, the cell in a leaf contains proteins that are different than the cells of a root. A leaf cell contains proteins that absorb light for photosynthesis, while the root cell contains proteins that help it absorb minerals from the soil. So, how is it that each plant cell can have the same genes, but different proteins? The reason is that various cell types contain different proteins, is that different genes are active. Or, more exactly, different genes are transcribed in each type of cell. So that, while each cell has the same number of genes, not the same genes, are not transcribed in each cell. While some genes inter-transcribe, while some genes are transcribed in all cells, for example, like the genes that are necessary to make the cell membrane, most genes are transcribed only in specific subsets of cell types. So again, to repeat this, while each cell has the potential to turn on any of the 25,000 genes, in practice, only several thousand of these are active, making proteins in a particular cell type. Now, we can complicate this even more, because while not only are there cell-type specific proteins, there are also environmentally specific proteins. Many genes are controlled by the external environment. Some genes are only transcribed only in blue light. Some genes are transcribed only in red light. Some genes are transcribed when it's cold. Some genes are transcribed under a heat spell and some genes are transcribed after touch. These are the genes that are transcribed when the, the leaves are touched. And, these are the genes that Braam identified and called them the touch genes. because, these are the first genes that are turned on, the first genes that are transcribed following mechanical stimulation.
