Genetic Response to Chemical Control

Don Lee Author

09/23/2019 Added

25 Plays

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This video details how a population of weeds develop genetic resistance to various forms of chemical control.

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[00:00:00.000]Farmers, Consumers, and GMO Super Weed Evolution Part 2. So, we've thought about why farmers are motivated to control weeds, why the use of chemistry to control weeds provides them with advantages,
[00:00:24.900]and therefore is something they'd like to be able to do as they plan on how they're going to manage their crop. So now we're going to think about the fact that weed populations do not give up.
[00:00:37.220]They can evolve and become these super weeds, and therefore create problems for the farmer that they didn't used to have. So, let's take a look at this and how farmers, if they understand this,
[00:00:50.080]and recognize where they're making mistakes, can do a better job, be better stewards of herbicide technology.
[00:00:54.880]So, again, thinking about what crop plants need and what weed plants need, and recognizing that they need the same things at the same time and will compete with each other,
[00:01:07.960]also sets us up for thinking about what they have in common biologically that allow them to control the way they grow and develop and get through their lifecycle.
[00:01:20.440]So, whether it be a domestic version of a plant,
[00:01:24.860]or a wild plant,
[00:01:26.840]plants and any living thing on the planet will tend to change as the planet changes.
[00:01:34.260]Living things are designed to be able to evolve.
[00:01:38.080]Now, it's not individuals that evolve,
[00:01:40.640]it's populations -- groups of inter-mating individual -- that evolve.
[00:01:44.140]And we're going to see where it requires two primary factors for evolution to occur.
[00:01:51.380]You need to have genetic variation amongst the individuals
[00:01:54.740]in that population
[00:01:55.680]and something called selection pressure.
[00:01:58.560]And I think the weed population response to a herbicide application
[00:02:04.600]is a really excellent way to understand the biology of evolution,
[00:02:09.720]particularly the short-term evolution that we can often see happen in just a few years.
[00:02:17.960]So the key to start our biology thinking is to remember that genes are made of
[00:02:23.420]double-stranded DNA
[00:02:24.700]molecules and that DNA sequence codes the information that living cells use to
[00:02:31.060]assemble amino acids together to build a particular protein. And then the protein
[00:02:36.940]will have a specific job that it will perform in the cell based on
[00:02:42.760]how those amino acids were put together. And that job will therefore allow the
[00:02:47.700]cell to have its functions and contribute to the life
[00:02:54.600]cycle completion of the living thing. Genes encode proteins, proteins control
[00:02:59.980]traits. But you can have genetic variation occur when mutations happen in
[00:03:05.460]in gene sequences. When those gene sequences change, the DNA sequence is
[00:03:11.940]altered and we create what are called alleles, new versions of genes, alleles.
[00:03:16.860]And depending on how that genetic change occurs that new DNA sequence can instruct
[00:03:24.500]a cell to assemble the protein together using amino acids in a slightly
[00:03:29.960]different way and that altered protein can have a couple of
[00:03:35.180]consequences. In most cases the altered protein doesn't work very well and
[00:03:40.420]there's no advantage for a cell that carries that mutant
[00:03:49.080]allele. But in other cases and in the case that we're going to look at --
[00:03:54.400]how the plant responds to glyphosate herbicide -- you can have alterations that
[00:04:01.140]actually provide an advantage. So the normal version of the gene that encodes
[00:04:07.920]the protein that glyphosate herbicide binds to would encode what we call the
[00:04:14.360]sensitive version of the protein or a sensitive version of that EPSPS enzyme.
[00:04:20.520]And if the herbicide is binding to
[00:04:24.300]the protein and these aren't really drawn in scale the protein will
[00:04:28.080]actually have about 300 amino acids and one of these glyphosate molecules is
[00:04:33.720]about the same size as an amino acid so they're not drawn to scale but the idea
[00:04:38.980]is that that there's an ability of the herbicide to bind to the protein and
[00:04:45.300]that prevents the protein from doing its its normal function. The version of the
[00:04:51.200]protein is sensitive to the herbicide. But what happens
[00:04:54.200]with the altered protein? If the alteration is done in just the right way
[00:04:58.380]by chance the glyphosate herbicide won't be able to bind to it so we call that
[00:05:05.140]version of the protein resistant to the glyphosate herbicide. So you have genetic
[00:05:10.820]variation that influences the way the enzyme will react to the herbicide.
[00:05:16.180]That's where selection pressure come can come in if the farmer is using this
[00:05:21.600]herbicide. Plants that have the ability
[00:05:24.100]to make this [mutant] version of the enzyme because they have the mutant or the
[00:05:28.440]resistant allele have a selective advantage. They can complete their
[00:05:32.500]lifecycle and pass those genes on compared to plants that do not have the
[00:05:38.740]resistant version. So selection pressure combined with the genetic variation can
[00:05:44.440]cause the population to evolve. So let's take a look at this from from the
[00:05:49.660]population point of view. We're only going to show the weeds here
[00:05:54.000]in our field, not the crop plant, but in in a situation where the farmer hasn't
[00:05:59.400]used the herbicide or is just starting to, most of the plants in our
[00:06:05.760]population, these genes are in pairs and every plant has two versions of these
[00:06:10.600]genes in every cell. Most of them have two copies of this sensitive version of
[00:06:16.740]this gene encoding the glyphosate binding enzyme.
[00:06:23.900]But there's one plant here that happens to carry the resistant allele. So if the
[00:06:31.700]herbicide is applied, what will happen is what we're showing here. Hey all the
[00:06:38.180]plants were sensitive to the dose of herbicide that the farmer applied [tended to die]. If the
[00:06:44.240]farmer applied a labeled rate that was determined to be effective at
[00:06:48.560]controlling the weeds that were typically in their field,
[00:06:53.800]no matter what gene combination, genotype the plants had, the weeds had, they were
[00:07:00.940]sensitive to the herbicide. We do have a couple of survivors here, but these
[00:07:04.540]individuals survived just because they escaped. Maybe they germinated a little
[00:07:09.700]bit later and were not present when the herbicide was applied. Maybe they
[00:07:17.460]germinated a little bit earlier and were bigger than the other plants and
[00:07:22.680]therefore
[00:07:23.700]could withstand the dose of the herbicide that the farmer applied. Or
[00:07:30.200]maybe it was just luck. Maybe they just didn't happen to be in a part of the
[00:07:35.600]field where their leaves encountered a toxic enough, high enough dose of
[00:07:41.840]of the herbicide to disrupt their growth. They escaped. They can complete their
[00:07:47.020]life cycle. They can pass on their genes, but the genes they have that control and
[00:07:53.600]encode the enzyme that the herbicide will react with did not contribute to
[00:08:00.360]their ability to survive. They escaped because they were lucky, just because of
[00:08:04.280]chance. So in this situation what will happen is the farmer will get
[00:08:09.740]good weed control and the population won't change from one season to the next.
[00:08:14.840]This would probably be considered the ideal situation for a farmer, but
[00:08:21.140]sometimes farmers can make mistakes.
[00:08:23.500]Let's take a look at one possible mistake. A farmer can apply what's called
[00:08:28.000]a sublethal dose. So what if they worry about those escapes and they say well I'm
[00:08:34.480]gonna apply the herbicide just a little bit later when more of the weeds will
[00:08:39.400]have germinated and be present in the population. Well that means more of the
[00:08:44.140]weeds are a little bit bigger. Or they may say you know that labeled rate of
[00:08:50.860]the herbicide worked really well. I wonder if I can
[00:08:53.400]cut back on that labeled rate and get a little bit more out of the dollars I'm
[00:08:58.860]spending on herbicides and still get effective control. So one way or the
[00:09:04.260]other they may apply what's called a sublethal dose. So when it's a lower
[00:09:09.200]rate that the plant encounters it is possible that even if it has just one
[00:09:14.400]copy of the resistance allele that is sufficient to give it a selective
[00:09:19.640]advantage under that low rate situation. So those individuals
[00:09:23.300]that just carry the susceptible allele can survive. They can get through their
[00:09:27.740]life cycle. They can pass their genes on to the seeds to their offspring.
[00:09:32.780]So then the next year when the the offspring become the population of weeds
[00:09:38.360]that the farmer is dealing with, it's changed. Some of the individuals are just
[00:09:43.100]like the surviving parent. They carry one copy of the resistant allele but other
[00:09:48.320]individuals just by the way the genes are passed on can carry
[00:09:53.200]two copies of the resistant allele and if they have two copies then if the
[00:09:58.760]farmer again applies the herbicide, and even if they apply it at the labeled
[00:10:04.600]rate, individuals with two copies of the resistance allele will survive. And if
[00:10:11.200]they're surviving and if we continue to apply the same herbicide whether we
[00:10:17.300]apply it at a low rate or a higher rate those individuals will survive and
[00:10:23.100]take a look at what's left in our population's gene pool, we're going from
[00:10:28.200]having high frequency of those sensitive alleles to all resistant alleles the
[00:10:36.300]population has evolved as a result of the farmers use of the herbicide. So
[00:10:43.860]the farmer made the mistake of repeated application, they may have also made the
[00:10:48.060]mistake of applying low rates of the herbicide. Those two mistakes in
[00:10:52.340]combination
[00:10:53.000]will end up in creating a population in which all of the individuals can resist
[00:11:02.680]the application even if the farmer would apply higher rates they may be able to
[00:11:08.000]withstand that application and therefore this herbicide is no longer an effective
[00:11:13.580]way for them to control weeds. And farmers recognize that it was their use
[00:11:20.360]of the herbicides that applied the selection pressure to the populations
[00:11:24.440]and therefore they have some responsibility in the outcome of the
[00:11:31.640]superweed populations evolving. So the biology is very predictable and
[00:11:40.220]it's something that we've seen before with herbicide resistance that was
[00:11:47.420]controlled by genetic variation in the crop plant that was entirely due to the
[00:11:53.600]occurrence of mutations -- not due to genetic engineering whether it be
[00:11:58.460]genetically engineered resistance
[00:12:01.520]or resistance that occurs from mutations in genes the crop plants already have.
[00:12:08.900]The same kind of situation will occur. If farmers make those mistakes we'll
[00:12:16.680]see the populations evolve resistance and they can evolve resistance at a
[00:12:22.280]fairly quick pace. A fairly rapid pace. So farmers are motivated to learn from those mistakes
[00:12:31.400]and ask the question 'what can we do differently? If new technologies become
[00:12:37.700]available for me to use herbicides to control weeds, are there ways I can do a better job
[00:12:43.940]of using those technologies and allow those technologies to work for me longer during my
[00:12:52.520]career as a farmer?' So let's take a look at what some of those better use practices can
[00:13:01.280]be for the farmer. The first one is crop rotation so farmers often have a choice in which crop they're
[00:13:08.900]going to grow in one year in the field and the next year in the field. And it's possible then
[00:13:14.780]that that one crop is not very competitive with the weed because it grows at a similar rate with
[00:13:24.140]the weed and therefore it's constantly competing with the weed for essential resources.
[00:13:31.160]An alternative crop, a different crop, may be able to grow at a different pace and therefore do a
[00:13:37.040]better job of competing with a given type of weed. So rotating crops is known to keep weed
[00:13:44.660]populations off balance and weed populations that do very well in one year with one crop
[00:13:50.300]don't do as well the next year if you're rotating to crops that are quite different
[00:13:55.820]in the way they grow and develop. And so farmers in the Midwest have typically rotated
[00:14:01.040]crops like soybeans and corn and get the benefit of keeping pest populations out of balance. And
[00:14:09.860]that could be pest populations like weeds or insects or even diseases. Crop rotation has a
[00:14:15.900]lot of advantages. However if we think about the use of roundup resistance in our crop plants, the
[00:14:24.680]same genetic engineering technology applied to create genetically engineered soybeans can
[00:14:30.920]be used to create genetically engineered corn. The same gene from from the herbicide resistant
[00:14:39.320]microbe can be modified, but placed in the tissue culture cells of corn. You can clone
[00:14:46.340]out herbicide resistant roundup resistant corn plants. And the breeders then can have those
[00:14:54.260]as parents and cross that herbicide resistance to any hybrid that a farmer would want to
[00:15:00.800]grow. So what that means then is the ability to control weeds with that EPSPS enzyme-
[00:15:10.120]binding herbicide with glyphosate can be used no matter what crop the farmer is growing. And
[00:15:17.540]when that situation occurs then the farmer is applying glyphosate repeatedly year after year
[00:15:25.400]after year. It promotes the farmers to make
[00:15:30.680]that repeated application mistake. So what can the farmer do when they rotate crops? They can
[00:15:38.840]choose to use a second mode of action in the way they're going to control the weeds. So in
[00:15:48.060]addition to using glyphosate to control weeds, they could use another amino acid
[00:15:54.320]inhibitor -- glufosinate -- if their crop plant is also resistant to glufosinate and
[00:16:00.560]if they use that second mode of action, what will happen is the the individuals
[00:16:07.680]that might be resistant to glyphosate are not going to be
[00:16:14.060]resistant to the second mode of action, to glufosinate. And so even though there
[00:16:19.160]might be survivors those individuals that survive survive because they
[00:16:23.360]escaped not because of the resistance genotype that they had for either
[00:16:30.440]of the herbicides that each attack the plant with a different mode of action. So
[00:16:39.260]using that second mode of action prevents the level of selection pressure
[00:16:47.040]that we have when we use just one mode of action. So farmers can opt to do that,
[00:16:53.060]but the key is that they have to use those two modes of action at the very
[00:16:59.200]beginning. They
[00:17:00.320]have to use that second mode of action before the population has
[00:17:06.140]evolved resistance to the first mode of action otherwise they're really only
[00:17:12.120]using one mode of action. So if they they use both modes of action before the
[00:17:20.180]population has has evolved, they will see the the population remaining sensitive
[00:17:30.200]to both herbicides for a longer period of time and therefore they can utilize
[00:17:36.620]those technologies in their weed control program for a longer time period. So
[00:17:41.980]using two modes of action is an essential part of weed management. So
[00:17:48.020]farmers can avoid sublethal doses by applying the label rates of herbicides.
[00:17:54.440]They can have avoid single modes of action by using more than one type of
[00:18:00.080]herbicide that block different critical functions in the same growing
[00:18:06.560]season, so they're they're using both ways to control their their weed
[00:18:12.080]populations. And then they can avoid using the same crops, growing the same
[00:18:18.200]crops, year after year by rotating their cropping system. If they use all of these
[00:18:24.260]better practices in combination they're going to be better stewards of the
[00:18:29.120]technology
[00:18:29.960]the technology of herbicide based weed control will work in their fields for a
[00:18:34.960]longer time period,
[00:18:37.280]ok? This is really an important idea for farmers now to recognize because with
[00:18:45.440]new advances in genetic engineering technology there are new ways of using
[00:18:52.640]genetic engineering to identify genes that encode proteins that will allow the
[00:18:58.400]plant to resist
[00:18:59.840]herbicides. So for example there have been genes discovered from
[00:19:05.080]microorganisms that encode proteins that will degrade certain herbicides and
[00:19:11.240]those can be transferred to our crop plants, providing them with the second
[00:19:15.320]way to to protect the crop plant from the herbicide, and therefore setting up
[00:19:24.320]the farmer to use multiple modes of action when they
[00:19:29.720]grow their [crops]. But it's very critical that they use these new
[00:19:35.800]technologies in the proper way and avoid the mistakes that they've made in
[00:19:43.280]the past by relying on a single mode of action and not using that that single
[00:19:51.560]mode of action in a proper way. So wise farmers are going to do the best
[00:19:56.420]possible thinking they can to maintain the
[00:19:59.600]use of these technologies by combining the new technologies with optimal
[00:20:04.100]practice. The key really for for the consumer, for the for the general public,
[00:20:10.220]is to understand that farmers are going to be motivated to be successful in weed
[00:20:17.180]control. The question is -- do we do we trust the farmers to use this best
[00:20:25.720]practice based on their own decision-making?
[00:20:29.480]Or do we feel like we need to level some control that that dictates to the
[00:20:37.680]farmer how they're going to use these new technologies? I think that's an
[00:20:43.360]important question to consider because in the United States we have regulatory
[00:20:50.300]agencies that that are designed to help protect the interests of the public. And
[00:20:57.080]it's possible that regulatory
[00:20:59.360]agencies could be involved here in guiding the farmer's use of these
[00:21:05.720]technologies by creating regulations. And the ability to to communicate what these
[00:21:15.860]regulations are to the farmer and then enforce these regulations would be an
[00:21:21.140]anticipated part of the process. So that's the question. How do we expect
[00:21:27.260]farmers to operate going
[00:21:29.240]forward? Do we expect there to be some level of control over how they're going
[00:21:34.000]to use these technologies? Or are farmers in position to use their own decision
[00:21:39.620]making on the best use of these technologies and we can come and we can
[00:21:44.120]trust their wise use of that technology?
[00:21:46.640]Alright so that summarizes our farmers consumers in GMO segments on superweed
[00:21:54.480]evolution and hopefully gives us a better idea of the challenge that
[00:21:59.120]farmers have and the choices that they can make in in the future in working
[00:22:06.500]with these technologies.

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Genetic Response to Chemical Control

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