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About this Lesson
- Type: Video Tutorial
- Length: 10:13
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- Posted: 07/02/2009
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Taught by Professor George Wolfe, this lesson was selected from a broader, comprehensive course, Biology. This course and others are available from Thinkwell, Inc. The full course can be found at http://www.thinkwell.com/student/product/biology. The full course covers evolution, ecology, inorganic and organic chemistry, cell biology, respiration, molecular genetics, photosynthesis, biotechnology, cell reproduction, Mendelian genetics and mutation, population genetics and mutation, animal systems and homeostasis, evolution of life on earth, and plant systems and homeostasis.
George Wolfe brings 30+ years of teaching and curriculum writing experience to Thinkwell Biology. His teaching career started in Zaire, Africa where he taught Biology, Chemistry, Political Economics, and Physical Education in the Peace Corps. Since then, he's taught in the Western NY region, spending the last 20 years in the Rochester City School District where he is the Director of the Loudoun Academy of Science. Besides his teaching career, Mr. Wolfe has also been an Emmy-winning television host, fielding live questions for the PBS/WXXI production of Homework Hotline as well as writing and performing in "Football Physics" segments for the Buffalo Bills and the Discover Channel. His contributions to education have been extensive, serving on multiple advisory boards including the Cornell Institute of Physics Teachers, the Cornell Institute of Biology Teachers and the Harvard-Smithsonian Center for Astrophysics SportSmarts curriculum project. He has authored several publications including "The Nasonia Project", a lab series built around the genetics and behaviors of a parasitic wasp. He has received numerous awards throughout his teaching career including the NSTA Presidential Excellence Award, The National Association of Biology Teachers Outstanding Biology Teacher Award for New York State, The Shell Award for Outstanding Science Educator, and was recently inducted in the National Teaching Hall of Fame.
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Once Richard Went discovered the first auxin, we started to realize that there are a lot of different chemicals that seem to work in plant growth and plant regulation and this whole idea of plants having chemicals that send messages, like animals, was kind of a breathtaking discovery, that indeed, plants regulate and cells talk to each other in plants also. And since the days of Richard Went, we have discovered several plant hormones, and we have discovered some of their functions, and I just want to tell you one thing. The world needs some more plant physiologists because there is just so much left to be discovered about plant hormones and the way plants regulate.
We have this chart that you can download, and I really strongly recommend that you do because it's going to give you a couple of things. The first thing it's going to do is it's going to lead you through some of the hormones, for example, auxins, and it's important to realize that auxin is kind of a generic term. There are several different kinds of auxins. One generally referred to, that Richard Went discovered is called indolacetic acid, IAA. But one of the things that you'll be able to get from this chart is the name of the hormone and the structure of the hormone. For example, you see that cytokinins have this generalized structure with the two rings and when we go to another plant growth hormone, like gibberellins, we add a third ring and yet, none of these are unbelievably difficult complex structures. Some of them are acid, as in abscisic acid and some of them are very simple, like ethylene.
See, ethylene - there's a great story associated with ethylene - but look at how simple ethylene as compared to something like the brassinosteroids, with its four ring structure, that looks very much like your prototypical lipid, thus the name steroid. And most importantly, this chart is going to tell you a lot about function. I just want to take a brief overview of some of the functions of some of these hormones. What kinds of things - if I sat you down right now and I said, make a list, what would be some of the things you would right down in terms of what might be controlled by plants or by hormones in plants. What kinds of behaviors do plants exhibit? What kind of regulation must a plant do in order to survive?
I think the list will surprise you. Certainly, probably the first things you might write down would be something like those things that control the tropisms, the phototropism, the bending toward light, the gravitotropism, the bending of a root that goes downward, and it always does. Roots, even if you turn a seed upside down, or you put a stem lying on its side, it will eventually bend upward toward the light and away from gravity and a root will always go down. Those are fairly obvious and you know what controls those things. You know that these steroids are going to control cell division, because in order to make new tissue, you have to do cell division. And you know that they are going to control differentiation, and if you link back to some of our discussions on plant development, you know that plants go through this whole series of changes just like animals do, where they have actual germ layers, like protoderm and procambium and ground maristem, that will differentiate into things like vascular tissue and dermal tissue, and tissues like that, and you certainly know that plants cells elongate, thinking back to some of the discussions of growth from a meristem and the fact that every meristematic zone also is followed by a zone of elongation. So, plant cells must be able to elongate. That's got to be driven by something. And we know it's driven by hormones, particularly things like the auxins.
But there are some other very cool things. I love to garden. And this spring, I was planting my garden out and I planted this new kind of lily-like plant called a salpaglossus and right now it is blossoming like crazy. And the reason it's blossoming like crazy is because I knew about apical dominance. Apical dominance simply means this, that two of these hormones are constantly battling for superiority. The auxins are dripping down from the tip, but there is another hormone called the cytokinins. And the cytokinins want to grow lateral buds. When you plant a plant and there is the tip of the plant and then there's a branch here or there's a leaf here and a leaf here, right here and right here, there are lateral buds that will take off but won't because - and cytokinins are stimulating that - but auxins are repressing it. But what's very cool is if I chop - we call it pinching off the tips, if you pinch off the tips of a plant and remove the very tip, you will then get side-shoots from that plant. And so what I did with my salpaglossus is, I pinched off the tips after I transplanted them and they branched off and they became much thicker plants. And that's the way to get nice thick plants. So, apical dominance is something that has to be controlled by plant hormones. Senescence and aging. Senescence is literally the aging of a plant. Think about this. Why do leaves turn brown? If you pick a flower, or you pick a plant and eventually the leaves start to, even if you put them in water, they start to kind of turn brown and flowers do the same thing. Well, how important is it for a florist to know a little bit about senescence and know what kind of hormones control that? So, for example, we might spray cytokinins over plants at a florists so that their leaves stay green. Because the cytokinins will be absorbed and those are senescence preventers. They're like an anti-aging hormone. I wish I had one of those. Maybe I ought to try some cytokinins, might work on....
So, senescence is another one. Here's another cool one. Fruit growth. And maturity and development. I don't know if you've ever seen wine grapes, but wine grapes, when they grown, they grow really close to each other and they grow kind of small. But did you ever notice that when you get these Thompson seedless grapes in the grocery store, that they're big and they're kind of spread out. You want to know why? We spray them with gibberellins. We use gibberellins, one of those plant hormones, to cause bigger, more spread out fruit. And it just makes for bigger, juicer, maybe not sweeter, but more convenient fruit.
And certainly germination. What about seeds? One of the things if you're ever growing seeds - as a gardener, I know this, too - some seeds need light and some seeds need dark. Did you know that? Did you know that some flowers and, I think it's lettuce, need to be exposed to the light before it will grown. And that's why when you plant lettuce in your garden, you don't plant it real deep, because light triggers some kind of hormonal reaction and I've grown flowers in my basement and I also read the seed packets and it always says, keep it in the dark until it starts. Or, expose to light for 48 hours and then cover with soil. Why? Hormones. Sometimes, we're going to get, believe it or not, grown inhibition. Why would a hormone want to inhibit growth? Well, I come from a part of the country where it gets pretty cold around August, it seems. And what starts to happen then is the plants take the primordial tissue that should be growing new stems, and they make bud scales over them. We call them winter buds. And that is triggered by a plant hormone, to inhibit growth to form buds in the winter.
Here's a good one. I call it the Michael Jackson hormone, ethylene. Why would I call it the Michael Jackson hormone? You guys remember the Jackson 5, don't you? Sure you do. And the Jackson 5 used to sing this song "One bad apple won't spoil the whole bunch, girl". Well, Michael Jackson should have taken the Thinkwell course, because one bad apple will spoil the whole bunch because of ethylene. You see, developing fruit gives off ethylene and it causes ripening. So, if you have a bunch of apples that you don't want to ripen too much, you take the rotten apple out. And in the autumn, when I pick my tomatoes, and it's just about to get a frost and I don't want them to die, and I have all these green tomatoes, you know a great way to get green tomatoes to ripen? Take an apple and put it in a bag with the tomatoes. The apple gives off ethylene. The ethylene causes the ripening of the tomatoes. It's kind of a domino effect. And when we ship fruit, what we often do is we'll take this fruit as green fruit, like a green tomato, we'll ship it green and then when you get it to the stores, you put it in a room full of ethylene and it ripens.
See? You guys thought plants were boring. And last, but not least, is abscission. Abscission is the loss of leaves from a tree in the autumn. And there is one hormone called abscisic acid. And it was named that because they though it caused abscission. Well, they were wrong. It doesn't have anything to do with abscission as you'll find out if you download our chart. But there are other hormones that cause abscission. So, you know, that's the story, you guys. Plant hormones control an unbelievable variety of behaviors in plants. The world needs some good plant psychologists.
Plant Systems and Homeostasis
Plant Hormones Page [2 of 2]
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