Weed control is essential to modern crop production practices. Without control, weeds can
How can weeds be controlled in crops?
However, in the last 50 years, since the introduction of 2,4-D, a synthetic auxin, to control broad leaved weeds (dicots) in grasses (wheat, etc.) in the late 1940s, chemical weed control through herbicides has become predominant. In fact herbicides are now the most widely used group of pesticides. Why has this occured?
However, there are several areas of concern about the use of herbicides in crop production including: the unknown and potentially damaging effects of herbicides, and pesticides in general, on the environment; contamination of groundwater; development of herbicide resistant weeds; an increasing interest in lowering the inputs in crop production.
Clearly there are alternatives to using herbicides for weed control. The reasons why these alternatives have not been developed further, or been more widely implemented, are complex and may not necessarily reflect the effectiveness of these methods. The financial investment in research on chemical weed control methods (herbicides) has been much greater than that in alternative methods. One reason for this may be that you can sell a herbicide every year and make a profit from these sales. However, it is much more difficult to make a profit from selling information on how to use cultural practices to control weeds. If you can sell that information at all, you can likely only sell it once!
The essential properties of any herbicide is that it can effectively inhibit the growth of, or kill, at least some plants. Herbicides function by interfering with some essential process in the biochemistry of plant cells:
There are thousands of chemicals that can inhibit plant growth, but there are relatively few commercial herbicides. What makes a "good" herbicide?
All of these are critical to the successful development of a commercial herbicide. And as regulatory standards have changed, meeting the last safety criterion has become more important.
Herbicides can be classified in a number of ways:
In order for a herbicide to selectively kill weeds but not affect the crop, the crop plants must have some mechanism to prevent the herbicide from causing the damage that it inflicts on weeds. How can any plant survive or tolerate a herbicide? To answer this question, let us consider what the herbicide must do in order to be effective.
Herbicides can be selective if any one of these processes does not occur in a plant. For example, if a herbicide is not taken up by a plant, then it will not be effective. If the herbicide is not transported throughout the plant, it may kill cells where it is taken up, but it may not kill the whole plant. If the herbicide is metabolised into an inactive form, it will not affect the plant (atrazine could be used in corn because corn is able to metabolise this herbicide). Finally, if the target site of action (enzyme or other biochemical activity) is not inhibited by the herbicide then it will have no effect on plant growth. So each of these steps (and perhaps others) can allow selective use of herbicides.
There are now many programs aimed at developing crop plants that are resistant to specific herbicides. These are focused on several crops and, usually, at introducing resistance to broad spectrum herbicides.
Why is there such interest in developing these herbicide resistant crops? If you look at who is doing this research and development, you will be able to get some ideas. Most of the research is conducted by chemical companies that develop and market herbicides. For example, Monsanto, the maker of Rounup, is developing crop plants with resistance to Roundup. If this and similar projects are successful, it will lead to:
It may also lead to reduced use of other, more toxic herbicides. We will discuss the advantages and disadvantages of development of herbicide-resistant transgenic crops in a future lecture.
We will first examine the development of crops with resistance to Roundup as the primary example, but we will also look at the development of plants with resistance to other herbicides.
Roundup is the commercial name, glyphosate is the technical name of this herbicide. It is patented by Monsanto and is one of their biggest products. The patent is, I believe, due to expire in the year 2000, at which time other companies will be able to sell this herbicide, and the price is likely to drop.
Glyphosate is a very effective broad spectrum herbicide. It is readily taken up through the leaf, and is transported throughout the plant. There is some concentration and accumulation of the herbicide in shoot meristems, and these tissues are among the most sensitive to glyphosate. Glyphosate inhibits an enzyme that is involved in making one group of amino acids. Unlike humans and many other animals, plants synthesize from scratch all of the amino acids that are required for protein synthesis and other metabolic functions.
Overhead of shikimate pathway
Glyphosate inhibits 5-enolpyruvyl-shikimate-3-phosphate synthase, better known as EPSP synthase. This enzyme is one step in the pathway that leads to synthesis of the aromatic amino acids, those that contain a benzene ring, tryptophan, tyrosine, phenylalanine. These amino acids are clearly required for protein synthesis but are also used as precursors for hormones, lignin, protective chemicals and other compounds.
EPSP synthase uses PEP and shikimate-3-phosphate to make EPSP. Glyphosate interferes with binding of one substrate, PEP, to the active site of the enzyme. If PEP cannot bind to the enzyme, synthesis of EPSP and the aromatic amino acids derived from EPSP is blocked. And if a plant cannot make amino acids that are required for protein synthesis, it will die.
The way glyphosate works to inhibit plant growth indicates why it is such an effective and relatively safe herbicide.
How could we make glyphosate resistant plants? As glyphosate is a broad-spectrum herbicide, all (or nearly all) plants are killed by this herbicide. If you don't know how to make plants resistant, one approach is to identify or select organisms with increased resistance to glyphosate and ask "How come these organisms are now able to survive this herbicide?".
Are there plants with resistance to glyphosate? For reasons that are not entirely clear, there are few well documented examples of glyphosate resistance in plant. However, it is possible to select plant cells that grow in culture that are resistant to glyphosate. What is the reason for their resistance? They produce more of the target enzyme, EPSP synthase. The enzyme is still inhibited by glyphosate, but producing more of the enzyme allows the plant cells to still make amino acids in the presence of glyphosate.
See overhead
If more enzyme makes plant cells resistant, whole plants that produce more EPSP synthase might also be resistant. How can we make plants that produce more EPSP synthase?
In the first results of these experiments, the transgenic plants did have increased glyphosate resistance (see overhead). This research was first published in 1986. However it took almost ten years before crop plants were marketed that had been genetically engineered for RounUp resistance. We will discuss the reasons for this long development time, as well as other ways to produce glyphosate resistance in plants, in the next lecture.