The goal is to modify male plants that would mate with wild females and produce nothing but male offspring
Weeds are the Achilles’ heel of farming. They invade crops, reduce productivity, compete for water, nutrients and sunlight, and lead to poor crop quality and reduced yield.
Herbicides are the farmer’s counterbalance but for aggressive weeds like waterhemp and Palmer amaranth, both of which are resistant to 7 and 8 herbicide groups, respectively, they have become increasingly hard to control.
In Ontario, waterhemp is expanding across the province due, in no small part, to the fact that each plant can produce more than four million seeds.
But if chemicals are no longer as effective, in the future genetic control may be.
At the University of Illinois’ College of Agricultural, Consumer and Environmental Sciences, researchers have identified genetic signatures that distinguish male waterhemp and Palmer amaranth plants from female plants. That discovery is critical to developing a genetic control system.
The goal is to introduce genetically modified male plants into a population where they would mate with wild females. The modified plants would contain a segment of DNA coding for maleness so that all their offspring would be male, and, in turn, their offspring would produce male plants. Ultimately, all descendants would be male plants resulting in a collapse of reproduction and the crash of populations.
“The dioecious nature (plants are either male or female) of waterhemp and Palmer amaranth contributes to their genetic diversity (by forcing outcrossing) and, consequently, to their ability to evolve herbicide resistance,” said Patrick Tranel, Ainsworth professor and associate head of the Department of Crop Sciences.
“As someone who has been studying these species for years, and interested in basic molecular-level mechanisms, the genetic underpinnings of their dioecious trait have been of interest to me. The sterile insect technique has already been proven to be a successful insect control strategy. With the advent of CRISPR/Cas 9 technology, the idea of applying something along the lines of the sterile insect technique to these dioecious weed species becomes feasible.”
Tranel said that they have identified DNA sequences linked to maleness in both weed species. “Once we identify the actual gene or genes, and confirm that they alone can cause maleness, then we can develop the strategy whereby the gene(s) would be propagated in the population.”
In the study, the researchers grew 200 plants of each species and each sex, extracted DNA and looked for whether any sequences were unique to a given sex. He said that because both species have male-specific and not female-specific sequences, males are heterogametic. Male plants have an X and a Y chromosome (XY), like humans. Females have two X (XX) chromosomes. Maleness in the plant species is dominant.
“This is favourable for our strategy to spread a gene that would be dominant in converting females to males. We took samples outside of the original focused population to determine if the markers were specific to those plants or more generally applicable to the species.”
With the markers linked to the region of the genome that carries the maleness gene, or genes, the next step is to obtain an assembled genome of a male of each species. With that, they will be able to use their markers to focus on a specific region to find the maleness gene. They would then do further experiments to validate the gene function.
“The best way to validate the gene function is to put the gene into waterhemp/Palmer plants and test whether plants with the gene are male,” said Tranel. “Unfortunately, there currently does not exist a procedure for genetic transformation of these species, so that research is something that we are also doing.”
The concept of genetic control of weeds may, at first glance, seem promising compared to the angst over the constant use of chemical herbicides, especially when weed species become more resistant. But the elephant in the room — or in this case the field — is the potential for unintended consequences.
“The idea of releasing a genetically modified weed raises concerns, as it should. It is important to point out that our objective is not to eradicate either species. We will need to verify that our strategy would not lead to that.”
As well, he said using genetic manipulation to control weeds would not be successful without using all other control strategies.
“Population sizes of these weeds are so high in any given field that it is not feasible to release enough of the modified males in a population to have a significant effect. And, as we have already learned, weeds will evolve to overcome any one strategy, so multiple strategies need to be combined. This is just another tool, another way of diversifying our control tactics, that one day could help keep population sizes more manageable.”