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Herbicide resistance poses a challenge to producing enough food for a growing population. Sam Caygill and colleagues at the Gregor Mendel Institute (GMI) of the Austrian Academy of Sciences discovered how a large group of plants, the bryophytes, are naturally tolerant to the herbicide glyphosate – something that had only been known anecdotally from horticulture. Understanding the mechanism through which some plants are naturally tolerant to herbicides gives researchers an important starting point for tackling the problem of herbicide resistance in agriculture.
Growing enough food to feed the world is a herculean task. To increase yield, farmers need to get rid of weeds, which compete with crop plants for sunlight, nutrients and space. While gardeners can afford to painstakingly pick, rake or hoe their plots to keep weeds in check, large-scale crop farmers often resort to using herbicides. However, agriculture is facing a similar dilemma over herbicide resistance as medicine is over antibiotic resistance: The more widely an herbicide is used, the higher the chances that a weed evolves to be resistant - giving it the power to outcompete and outgrow its susceptible siblings.
Glyphosate is one such extensively used and fiercely debated herbicide. Recent overuse has resulted in the emergence of glyphosate-resistant weeds, which have reduced crop yields in parts of Europe, North America and Australia. Anecdotal evidence, however, suggested that bryophytes, a group of non-vascular plants, are tolerant to glyphosate – though this tolerance had not been confirmed experimentally, and the potential mechanism for tolerance was unknown. Understanding such natural tolerance can provide a strategy for tackling herbicide resistance.
AlphaFold helps researchers detect alternative route for protein production
Now, Sam Caygill and Liam Dolan at the GMI experimentally show that bryophytes, a group of understudied plants that includes liverworts, hornworts and mosses, can indeed tolerate glyphosate and grow even when exposed to the herbicide. Glyphosate kills most vascular weeds by inhibiting an enzyme called EPSP synthase, which in turns disrupts amino acid synthesis and protein production. Caygill, a graduate student at the University of Oxford and working at the GMI, reasoned that bryophytes may be tolerant to glyphosate by circumventing EPSP synthase: If an alternative route to EPSP synthase were available for synthesising amino acids in bryophytes, bryophytes could still produce proteins and continue growing even when exposed to glyphosate.
Combining knowledge gained from DNA databases with AI-based structure prediction, Caygill identified MurA as an enzyme that is structurally and functionally similar to EPSP synthase. While MurA genes are present in all bryophytes and all bryophytes tested were glyphosate tolerant, MurA is not present in flowering plants, which are glyphosate sensitive.
The researchers then experimentally tested whether MurA can confer tolerance to glyphosate. Overexpressing MurA in the bryophyte Marchantia polymorpha made the plants more tolerant to glyphosate than control plants with unaltered levels of MurA. Looking at the other side of the coin, M. polymorpha that had been genetically altered to express a non-functional version of MurA were less tolerant to glyphosate than control plants.
If MurA is required for glyphosate tolerance, the researchers then asked, could introducing MurA into naturally glyphosate sensitive plants also confer resistance to the herbicide? The vascular plant Arabidopsis thaliana does not normally express MurA and is sensitive to glyphosate. Introducing MurA into Arabidopsis thaliana indeed rendered the genetically altered plants resistant to glyphosate.
Rewriting the textbooks
Having shown that MurA was responsible for glyphosate tolerance of bryophytes, the team partnered with Thomas Köcher at the VBCF Metabolomics facility to understand how exactly MurA confers glyphosate tolerance. Using highly sensitive mass spectroscopy, the researchers tested if MurA produces the same small compound EPSP that EPSP synthase produces. Indeed, the team showed that both MurA and EPSP synthase catalyze the production of the same product, EPSP. Thereby, MurA provides an alternative route which can maintain amino acid production and protein synthesis in bryophytes, even when EPSPS is inhibited by glyphosate. This discovery was unexpected, as canonically only EPSP synthase was thought to produce EPSP.
“It is one thing to discover a naturally existing mechanism that explains why some plants are tolerant of glyphosate. It is another to discover a new step in amino acid biosynthesis”, says Dolan, senior group leader and deputy scientific director at the GMI. The unexpected discovery underlines why traditionally understudied organisms, like bryophytes, can provide the key to understanding fundamental questions in biology. “Sam and Thomas’s discovery highlights the importance of including a diversity of organisms in our research programs. Natural selection has produced diversity. We should not limit our horizons neglecting this diversity.” In addition, this discovery could lead to new strategies to reduce herbicide resistance in weeds.
Sylvia Weinzettl
Dr. Bohr-Gasse 3, 1030 Wien
T: +43 1 79044 – 4403
Mail: sylvia.weinzettl@gmi.oeaw.ac.at
gmi.oeaw.ac.at
Samuel Caygill, Thomas Köcher, Liam Dolan. MurA-catalyzed synthesis of 5-enolpyruvylshikimate-3-phosphate confers glyphosate tolerance in bryophytes. PNAS. DOI: 10.1073/pnas.2412997121
https://www.oeaw.ac.at/gmi/detail/news/natural-tolerance-to-weed-killer-uncovere...
Marchantia polymorpha, a bryophyte, can resist glyphosate herbicide treatment.
Johannes Hloch /GMI
Johannes Hloch /GMI
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Marchantia polymorpha, a bryophyte, can resist glyphosate herbicide treatment.
Johannes Hloch /GMI
Johannes Hloch /GMI
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