Why Herbicides Kill Plants (But Not You) – Part 2: Roundup®

Roundup.jpgRoundup® Kills All Plants (Mostly)

The herbicide that most Americans are likely familiar with is Roundup®.

Unlike the auxin-based herbicides I discussed in the previous post, Roundup® is not a selective herbicide. That is, it usually kills all green plants (except if the plant is Roundup Ready® or if the plant is a naturally Roundup®-resistant “superweed” – see below for more).

Roundup® is the Monsanto brand of the artificial chemical glyphosate, which was first synthesized in the 1970’s by Monsanto as a so-called “broad-spectrum” herbicide (i.e., kills all plants).

Glyphosate kills plants by specifically blocking the action of a key enzyme (5-enolpyruvylshikimate-3-phosphate synthase or EPSPS) that plants use to synthesize three amino acids (tyrosine, tryptophan and phenylalanine) that are essential components of all proteins. Without the ability to synthesize these amino acids, the plants will die.

Why doesn’t glyphosate similarly affect animals?

Humans and most other animals don’t have this enzyme, so glyphosate has no specific target as it does in plants*. (Since they do not have this enzyme, animals do not synthesize these three essential amino acids. They get them from their food.)

*Please note: This is NOT to say, however, that glyphosate is totally non-toxic to animals – more below.

Roundup may be losing its effectiveness, however, due to several factors.

Are Your Plants “Roundup Ready®”?

During the 1980’s there was a revolution going on in the plant sciences. Scientists discovered how to insert genes into the genome of some plants by using the bacterium Agrobacterium tumefaciens (At) as a genetic vector. Here’s how:

Scientists were able to use recombinant DNA technology to load genes into the Agrobacterium. And the bacteria were then able to “infect” susceptible plant tissue and deliver the genes straight into the plant cell’s genome. These foreign genes were actually “hard-wired” (stably inserted) into the plant’s genome. Moreover, these genes were able to be passed along to the plant’s offspring.

Simply put, Agrobacterium was like a taxicab, and the DNA was the passenger.

Using such technology, Monsanto scientists discovered a bacterial version of the enzyme EPSPS (see above) that was not affected by glyphosate, isolated the gene coding for it from the bacteria, and then inserted this bacterial gene into soybeans.

Roundup Ready® soybeans were born. Followed by Roundup Ready® cotton and canola. And when scientists learned how to genetically engineer grasses (Agrobacterium doesn’t work so well on grasses) using the so-called gene gun, along came Roundup Ready® corn. And maybe even Roundup Ready® turfgrass for lawns!


Roundup® Ready crops have certainly contributed to the extensive use of glyphosate, making it the most widely used herbicide in the U.S. This has likely increased the evolutionary selective pressure on “weeds”, leading to the generation of naturally glyphosate-resistant plants, a.k.a., “superweeds”. (Figs. 1 & 2 from Ref. 1 below)

Here Come the Superweeds.

Recent articles, such as this one in, of all places, Business Week have discussed implications of the appearance of so-called “superweeds”.


Examples of more recent reports of Roundup®-resistant weeds can be found here, here, and here. The increase in the number of such glyphosate resistant plant species has elicited warnings from environmental groups such as the Union of Concerned Scientists, as well as the international press.

In addition to naturally occurring gylphosate-tolerant weeds, there also is the risk of the spread of the genes conferring herbicide tolerance from GM (genetically modified) plants to native plants. Such events have recently been reported to have occurred in test plots of Roundup Ready® turfgrass in Oregon.

Briefly, the artificial gene conferring gylphosate-tolerance was discovered to be present in some native grass species growing adjacent to the test plots. (The genes presumably traveled via pollen from the GM plants to pollenate the native grasses.)

Thus, the possibility of the creation of superweeds via the escape of herbicide-resistance-conferring gene constructs is a very real possibility indeed.

May Increased Use of Glyphosate Also Be Toxic to Animals?

Though glyphosate has been considered one of the more benign pesticides, its environmental impacts are being reconsidered in light of some recent evidence to the contrary.

News Update (March 14, 2017): Monsanto Weed Killer Roundup Faces New Doubts on Safety in Unsealed Documents

1. Boerboom, C. and M. Owen (2006) “Facts About Glyphosate Resistant Weeds”, Purdue University Extension, publ. GWC-1. (PDF)

Bottom Line: Though glyphosate kills plants by targeting an enzyme not present in animals, its overuse – due in part to GM Roundup Ready® crops – may be harmful to agriculture, to the environment, and, ultimately, to human health.

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