share interactive transcript request transcript/captions live captions
download
|
MyPlaylist
KEN MUDGE: Hello. My name is Ken Mudge. I'm with the Department of Horticulture here at Cornell University, and we're standing in the greenhouse conservatory of the LH Bailey Hortorium. I'd like to welcome you to this cyber tower study room called the natural and human history of plant cloning.
What I mean to do is introduce you to my view that the way mankind has learned how to propagate plants is fundamentally by observing natural processes, and then modifying and adapting those natural processes to the increase of the plants that we grow agriculturally. What I'd like to focus on is the subset of propagation called asexual propagation or plant cloning. Let me tell you what I mean by that.
Asexual propagation is basically any propagation technique that does not involve the sexual process of genetic recombination, gamete formation, and male and female gamete fusion to form a new plant. Instead, asexual propagation refers to any of the other techniques, like grafting or cutting propagation and so forth. So throughout the course of this cyber tower study room, we're going to talk about a number of different asexual propagation techniques, including division, cutting propagation, layering, grafting, micropropagation, and the process of asexual embryogenesis.
Agriculture originated 10,000 to 12,000 years ago in the Fertile Crescent, which is in the Middle East region, and it began with the domestication of a number of crops that were propagated from seed, including wheat, barley, peas, and a number of others. But it took a while. It took several thousand more years for crops to be domesticated that were propagated asexually.
Now, why would the crops be propagated asexually rather than from seed? Well, as it turns out, those early crops-- wheat and barley and so forth-- they naturally self-pollinate so they are inbred. That is, they're genetically homogeneous. And if you allow wheat or barley to produce its own seeds, germinate those seeds, they'll come out true to type. In other words, the seedling offspring will be more or less identical to the parent plant because they're self-pollinating.
On the other hand, a lot of the potential crops that may have been producing useful fruits and so forth-- these were trees and so forth that were naturally cross-pollinating rather than self-pollinating. And when plants cross pollinate, there's basically recombination of the genes so that the seedling offspring are not like the parent plant at all. They are liable to be quite different. So it really wouldn't have been successful to domesticate fruit crops like figs, pomegranates, and so forth if seed propagation were the only way.
By cloning these, reproducing them asexually, every single one of the offspring is genetically identical to the parent plant. And so that led to the domestication of several very important fruit trees-- olives, pomegranates, figs, and grapes. All of these occurred more or less in the area of the Middle East.
Asexual propagation today is just as important as it was thousands of years ago. Here to the right, you can see a table showing both sexually reproducing crops and asexually reproducing crops. The ones noted with the asterisk are the 12 most important-- economically speaking-- cultivated plants, and you can see about half of them are sexually reproduced. But the other half, including a number of different fruit trees, are propagated asexually. The grapes, the figs, apple trees, and so forth.
We've been talking about how early agriculturists learned to propagate plants asexually by observing natural clonal reproductive processes in nature. It was several thousand years after the earliest domestication of seed-propagated crops that this began to happen with fruit trees, like the figs and so forth. So let's go now to the rare and manuscript section of the Crop Library at Cornell, and take a look at some of the early examples where this principle of learning from nature was actually explicitly stated in the 17th century.
Here we are the Crop Library. We're in manuscript section where I'd like to show you an example of an early explicit statement of this relationship between observing nature and applying understanding a natural phenomenon to plant propagation. One of the earliest explicit statements that I found is a book by Robert Sharrock, published in 1660. It's called The History of the Propagation and Improvement of Vegetables by the Concurrence of Art and Nature.
Now Sharrock was an Anglican cleric at Oxford University and a friend of Robert Boyle, who is well-known for his contribution to the scientific revolution and Boyle's gas law. It was, in fact, Boyle who encouraged Sharrock to write this book about gardening.
Sharrock writes, "the ways of propagation are either natural or artificial. For even in the ways of propagation that are most artificial, there is more of nature than art. Industry and art may bring materials and place them fitly for it, but nature works them. And therefore as one sayeth, it is the great art of man to find out the arts of nature that are secret. Many of her secrets have been found out and are followed by the artists, the improvement of the traders, and the powers of man."
Now I find Sharrock's motivation in writing this book rather interesting. Unlike earlier agricultural societies, the ancient Egyptians and the Aztecs, in the 17th century Europe-- influenced by Christiandom-- they felt that agriculture was more of a curse, a punishment for Adam's fall. Sharrock writes, "this is our own happiness that whereas before man's transgression, the Earth spontaneously and without human industry and culture brought forth all manner of useful plants according to the command of God in Genesis. Now since the curse, it bringeth forth of its own accord only those that are less useful, and the rest-- are not without the art and sweat of man."
Now there's a rather interesting illustration in this book by Sharrock. The only illustration in the book is a copper engraving of a rather fantastic tree that shows many different types of grafting and budding techniques, as well as several other methods of propagation. If you look at it carefully, you can probably count quite a few.
Well, here we are back in the greenhouse, and I'm happy to be here. I'm a little bit more comfortable here than in the library. What I'd like to do now to conclude this section is to talk about the reproductive biological processes that underlie the plant propagation techniques that we use, and try to show you how they relate to the propagation techniques that we're going to be talking about later on.
I've already made the distinction between sexual and asexual propagation. We're not going to talk about sexual reproduction anymore. Let's focus on the asexual or cloning techniques.
And basically, there are two underlying developmental strategies that a plant can use in order to reproduce clonally or asexually. One is regeneration. The other is fusion.
Why regeneration? What I mean is the formation of new plant parts. Now, that can be done either of two ways. One is asexual embryogenesis, and the other is adventitious organogenesis.
So asexual embryogenesis is just the process of regenerating a new embryo. But instead of the normal sexual embryo, we're talking about regenerating an asexual embryo-- a clonal embryo, if you will. That can occur either by the process of apomixis where a flower produces a seed that is not a sexual seed, but rather an asexual seed.
The other way that asexual embryogenesis can occur is through the process called foliar embryogenesis, where you have some plants that produce embryos without seed-associated structures directly on the leaves and other parts of a plant. Here, for example, is a fern called asplenium bulbiferum, and you can perhaps see the small plantlets that are occurring at the tips of these leaves. This is not a branch. In fact, it's an entire plant, small plant that we call a foliar embryo.
It includes not only stem tissue, but root tissue as well. The roots are very small, and these foliar embryos can either fall off the plant naturally, or they can be detached and planted in soil. And they will grow into full fern plants that are absolutely genetically identical to the parent plant. So that's the process of foliar embryogenesis. It occurs in a number of species, although it isn't all that common.
Here are some of those foliar embryos that I've detached from a plant, and they can be potted up in soil and they'll grow into entire fern plants that are genetically identical to the parent that we got them from. OK, so that's asexual embryogenesis through foliar embryos. Now, let's talk about the other process, adventitious organogenesis.
And this is the type of regeneration where we don't regenerate an entire embryo, but instead a single organ is regenerated. So that in the case of layering, for example, you can see at the bottom of this diagram adventitious roots occur on stems or leaves while the stem or leaf is attached to the parent plant. When that occurs when the plant is detached, that's called shoot cutting propagation. And we'll be spending some more time on both layering and shoot cutting propagation.
Or when adventitious organs form on the roots. That is to say, a bud or a shoot forms on a root cutting or an intact root. That's also adventitious.
When a shoot forms on the intact root system of a plant in the ground, we call that suckering, basically. And those sucker plants can be dug up and transplanted. Or when we take a root cutting, remove it from the ground, put it in the proper environment, we can induce the formation of adventitious shoots. And that's simply root cutting propagation.
So in the next sections, we'll be talking about the practical applications of these different asexual propagation techniques. First, we'll be talking about layering and cutting propagation. Then we'll do a section on propagation by grafting. And finally, we'll be talking about micropropagation.
In this Room I share with you my fascination with plant reproductive biology and its application to horticulture and related disciplines.
I begin by dispelling the widely held oversimplification that "plants grow from seeds" - indeed many of them do, but quite a few have evolved the capacity for asexual (clonal) reproduction. Even before the origins of agriculture, about 12,000 years ago, mankind has been observing wild plants performing feats of asexual reproduction.
From this increasingly sophisticated understanding of the natural history of cloning, early agriculturists domesticated a number of fruit, nut and other food crops and eventually a host of ornamentals as well. The Room includes hands-on demonstrations of clonal propagation by layering, cuttings, grafting and micropropagation.
This video is part 1 of 7 in the Natural and Human History of Plant Cloning series.
