Biotechnology To the Aid of Weed Resistance

You are likely familiar with the amazing results in medicine where mRNA (messenger RNA) Vaccines instruct our body to produce the spike protein needed to fend off COVID-19. If you are exposed to the virus, your body will recognize it and know how to fight it off. After the mRNA delivers the instructions, your cells break it down and get rid of it.

Molecular biology is also being used in other areas, including plant science, for a number of purposes, including ongoing research using another type of RNA, RNAi, that can revert a herbicide-resistant weed back to a herbicide-sensitive weed.

Let’s take a look at the current state of weed resistance and why this new genetic tool is being explored. To date, most herbicide-resistant weeds have been selected by the continued use or overuse of certain herbicides, allowing a resistant biotype to flourish. What is important to know is that there are two main mechanisms of herbicide resistance to weeds (target site and non-target site), which may require different strategies for management.

📸: @nicolas_ via Canva

Target site resistance

The most common is target site resistance, which means that a specific site in a weed has been altered so that the herbicide that previously affected a particular weed can no longer do so.

Management of target site resistance has been achieved by using herbicides with different sites of action that will control the resistant biotype. In addition, layering residual herbicides and implementing cultural practices such as the use of cover crops, crop rotation, and deep tillage have all helped to keep herbicide-resistant weeds at bay.

Non-target site resistance 

There is a big concern about non-target site resistance, as this can result in a weed population change so that they are now resistant to numerous sites of action at once. Non-target site resistance is the result of a plant's physiological change that does not allow a lethal dose of the herbicide to reach the site of action and control a weed.

This type of resistance is being found more often, especially with waterhemp and Palmer amaranth, where their ability to outcross produces a tremendous diversity of offspring. With more diversity comes the chance of a unique weed-resistant biotype. The mechanism of target site resistance can include a change in herbicide absorption, translocation, sequestration, or metabolism.

Metabolism-based resistance means that the weed has evolved and is now able to break down or metabolize the herbicide before it can control the weed. In this case, it may not matter what herbicide target site is involved, as most weeds would not be affected by a sub-lethal dose of an herbicide, so it is possible that a newly evolved metabolism-based resistant weed could be resistant to four or five different herbicide modes of action at once.

The cytochrome P450 enzyme is known to be responsible for the rapid breakdown of many herbicides, both in a good way that can safen an herbicide in corn but also in a negative way that overexpression in a weed can lead to resistance. When you hear of five- and six-way herbicide site of action resistance, it is often due to metabolism-based resistance.  

📸: @nicolas_ via Canva

Using RNAi to battle weed resistance

Management of non-target site resistance using RNAi, which stands for RNA interference, is a novel approach to attacking weed resistance and is under evaluation. This has not yet reached commercialization, but the concept has been proven. By spraying the weeds with a mixture of an herbicide and small RNAs capable of silencing genes, called "spray-induced gene silencing" RNA (known as SIGS), researchers have found they can change the weed's genetic makeup so it can once again be controlled by the same herbicide that previously could not due to resistance.

Here are some examples utilizing biotechnology that are being researched:

1. Kansas State University Department of Agronomy and Plan Pathology researchers are looking at using RNAi to reverse glyphosate resistance by silencing production of the EPSPS gene in weeds. This form of gene therapy can be used to silence or turn off a single gene. For example, in the case of glyphosate resistance, where normally a weed is controlled due to the binding to the EPSP gene inhibiting EPSP production, a glyphosate-resistant weed can over-express or amplify EPSP production to the point where glyphosate is not effective. If a weed is treated with an RNAi that will silence the EPSP gene, glyphosate will once again control the weed.
2. Non-target site resistance in an Illinois waterhemp population has been attributed to metabolism of metolachlor by cytochrome C-450, resulting in metolachlor-resistant waterhemp. Researchers at the University of Illinois Department of Crop Sciences are working to identify the gene responsible for using RNAi to silence the production of cytochrome C-450.
3. 2,4-D and MCPA resistance Palmer amaranth in Kansas is also likely a metabolism-based resistance due to cytochrome P-450. Kansas State University researchers are working to identify the gene involved and could potentially use the specific RNAi that will silence the gene.

This is just another reminder to use all the tools you have today to manage herbicide resistance. If you are not already implementing these practices on your farm, please consider them, as they may avoid you having to resort to some of the more complicated and likely expensive methods in the future.  

1. Scout your fields and be aware of which weed species are escaping so you can better select herbicides that will have activity on your problem weeds.
2. Use a strong residual herbicide, such as Zidua Pro® herbicide or Verdict® herbicide at or before planting to keep weed populations down and provide a manageable situation for successful post-applications later in the season.  
3. Apply a residual herbicide, such as Outlook® herbicide or Zidua® herbicide tank, mixed with your post-application to provide layered protection of weed control.
4. Use cultural practices such as the use of cover crops, crop rotation, and deep tillage if you can help keep herbicide-resistant weeds at bay.

Always read and follow label directions. Grow Smart, Zidua, Verdict and Outlook are a registered trademarks of BASF. Copyright 2024 BASF Corporation. All rights reserved.

📸: @JJ Gouin via Canva/Featuring waterhemp weed emerging in a soybeans field.

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BASF provides the information in this article as a service to its customers; however, the views expressed by guest writers are their own and do not necessarily reflect the views of BASF.