A closer look at glyphosate and resistant crops

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cereal rye spray
Farm and Dairy file photo.

 Glyphosate is a common name for the chemical, N-phosphonomethylglycine. That is why we simply call it glyphosate. This chemical was discovered in 1970 as a broad-spectrum herbicide and at the time, and also today, could be used as a burn down herbicide.

I come from a plant physiology background so I like to know what the plant does with the nutrients/hormones/pesticides that a plant is exposed to.

One of the main nutrients supplied to crops is nitrogen; some people have referred to nitrogen as “plant candy.” When a plant takes up nitrogen, it greens up and starts budding out new growth from just about all growth points.

The main use for nitrogen in a plant is to make proteins and the pigment chlorophyll. Most of the proteins synthesized in the plant are used for the photosynthetic machinery and thus are found in the leaves; one reason why alfalfa is so rich in protein.

Ramping up growth

So when a plant has plenty of nitrogen, it will ramp up production of proteins and enhance photosynthesis, leading to greater vegetative growth. Plants, unlike us, have to synthesize all 20 amino acids from scratch. We are able to eat food and obtain the amino acids that we utilize to build proteins used for our own vital functions.

Therefore plants have pathways that do not exist in animals. One such pathway is the shikimate pathway, which is used to build three essential amino acids (tryptophan, phenylalanine, and tyrosine) along with lignin and a plant developmental hormone called auxin.

We don’t make any of those things but plants cannot live without them, and they are each vital to plant growth/development. Glyphosate is a molecule that essentially stops the activity of an enzyme referred to as EPSP Synthase and shuts down the production of all those previous products mentioned.

This leads to a protein deficiency as well as auxin and lignin deficiencies. So as the plant receives photo-damage, it is unable to repair itself and basic plant cell upkeep is halted due to the shutdown of protein production.

The chemical is also translocated pretty quickly throughout the plant and the plant dies within a few days of exposure. It sounds pretty scary, but if we look at the lethal dose in mg/Kg to kill 50 percent of a rat population (LD50) of glyphosate it is >5000 mg/Kg. Gasoline is 50 mg/Kg.

More toxic

We handle gasoline all the time, usually without gloves and it is 100 times more toxic. When examining LD50 levels; the bigger the number the safer the product.

The mode of action a pesticide works is in the biochemistry of the organism exposed (plant, insect, mollusk, etc…). When we have a headache, we take an aspirin. Frankly, aspirin does not do much of anything for a plant, and its LD50 level is 1200mg/Kg.

Shared pathways

There are many pathways shared across animals and plants, however. Reading the label and following the safety recommendations will protect you from the product and allow for proper usage, leading to better results. Well let’s summarize: we have a plant pathway that is responsible for several vital functions such as protein, lignin, and auxin (plant hormone) synthesis.

There is a compound called glyphosate that stops the pathway and kills the plant quickly. It is relatively nontoxic to us due to its effect on a pathway that we don’t have. Glyphosate was thus used as a broad-spectrum pesticide to get rid of unwanted plant growth.

So what happened next after glyphosate was discovered in 1970? In the early 1980s scientists knew of a bacterium in the genus Salmonella known to have the same shikimate pathway as plants, but was more tolerant to the compound glyphosate, due to essentially one amino acid change in the protein.

The gene was cloned and transformed into tobacco plants by Rose and Stalker et al.

This gene was now able to reactivate the pathway in the presence of glyphosate and survive the herbicide exposure. The current market varieties of glyphosate-resistant plants have come from transformations from other species with a similar tolerant enzyme as the Salmonella sp. used in Rose and Stalker’s experiment.

Glyphosate tolerance

However this was one of the major breakthroughs in creating a plant that was tolerant to glyphosate for commercial use, the paper was published in 1985.

To date there are several crops that have the glyphosate-resistant genes in them: corn, soybean, cotton, canola, alfalfa, and sugarbeets that are in commercial production. Glyphosate is also widely used in the home garden as a chemical means of weed control.

The chemical does not have residual effects like many herbicides. For most crops, the label recommends about a week before replanting the area. Different formulations and crops will require variances on replanting, however. Consult the label for specifics.

Sun exposure

Glyphosate will be broken down by sun exposure and soil microbes. When glyphosate comes into contact with soil particles, it binds to the particle where it will degrade away.

The glyphosate molecule has a high affinity for soil and will not leach through the water very easily. Also, once it is bound to the soil it becomes inert and will not have any herbicide effects on plants grown in the area because the plant will not be able to uptake the chemical in its system.

Eating genetically modified plants that contain the glyphosate resistance gene will not harm you. Genetic modification is a new (30 years) realm of technology that has been incorporated into modern agriculture.

Along with being a new technology, it can be used to modify food. Food is strongly tied to culture and is necessary for life.

It is expected that people will have concerns and questions. Concerns about how food is handled are why agencies like USDA, ODA, and Extension exist. In Extension, our mission is to engage and strengthen lives through research based information. Our offices are receptive to questions all over the country.

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