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BP.jpgSadly, A Timely Topic

You’ve likely heard about microbes with the ability to “eat” crude oil (a.k.a., petroleum).

That is, some bacteria have the ability to metabolize most of the the organic compounds present in petroleum.

(For excellent scientific coverage of the oil spill see “The Science of the Oil Spill”.)

Interestingly, most of the bacteria used in the bioremediation of oil contamination are from natural populations, rather than genetically engineered in the laboratory.

But can plants help clean up the oil spill in the Gulf of Mexico?

That is, can plants, like some bacteria, take up and metabolize the organic components of crude oil?

Phytoremediation

Using green plants to help clean up (remediate) soils contaminated with toxic substances, such as heavy metals, radionuclides, or toxic organic compounds, is referred to as phytoremediation.

New_Wave.jpgThe use of plants to help clean up the environment has great appeal. The two main reasons why are (1) that the contaminated soils can be treated in situ and (2) that it’s relatively cheap, compared to conventional industrial clean-up methods. (The latter reason is, of course, very enticing to both government and industry.)

Though phytoremediation has a lot of promise, to date, it has had only limited success for several reasons.

Each contaminated site is different. Success in greenhouse studies often can not be replicated in the field, due to all the environmental and biological variables that occur.

Toxicity of site, especially with petroleum-contaminated soils, may kill most plants.

To breakdown toxic organic compounds in the soil, plants must by able to extract them. This is a major limitation.

The solution may be to use the plants’ roots as both a physical and nutritional “scaffolding” for microbes that can metabolize total petroleum hydrocarbons contaminants.

This is called “rhizoremediation”. Briefly, it takes advantage of the fact that plants can form symbiotic relationships with soil bacteria. (More about rhizoremediation later on)

Bottom line: The use of plants for phytoremediation of petroleum-contaminated soils is an emerging technology. Consequently, at the present time plants will likely play a limited role – at least directly – in remediating the Gulf oil spill.

References

1. Collins, Chris D. (2007) “Implementing Phytoremediation of Petroleum Hydrocarbons”, IN: Phytoremediation- Methods and Reviews, Methods in Biotechnology Vol. 23, pp. 99-108. Abstract

2. Gerhardt, K.E., X.-D. Huanga, B.R. Glicka and B.M. Greenberg (2009) “Phytoremediation and rhizoremediation of organic soil contaminants: Potential and challenges .” Plant Science Vol. 176, pp. 20-30. Abstract

3. Van Epps, A. (2006) “Phytoremediation of Petroleum Hydrocarbons”, Environmental Careers Organization, U.S. EPA. (PDF)

HowPlantsWork © 2008-2011 All Rights Reserved.

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Roundup.jpgThe Last Roundup?

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!

RR_1.jpg

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”.

RR_2.jpg

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.

Reference
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.

HowPlantsWork © 2008-2011 All Rights Reserved.

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