An "Acceptable" GMO?

Apr 05 2013 Published by under Issues in Science, Science

Soybean Field

For many opponents of genetically modified foods, the idea of fiddling with an organism’s genome doesn’t quite sit well in their stomachs. The type of genetic tweaking that renders soybean plants resistant to the herbicide Roundup strikes some not only as unnatural but something that borders on playing God. Similarly, another common objection to genetic engineering is that the transfer of genetic material/DNA genes violates a so-called “species barrier.” Such is the case for Bt corn, which harbors the bacterial gene for Bt toxin, a compound that is poisonous to insect pests. This argument, however, disregards the fact that Nature ignores this barrier all the time. In the wild, DNA is often transferred between species through processes collectively known as horizontal gene transfer. So, not even Nature plays by antiGMO rules.

But what if an already existing gene variant with a desired trait from one organism is genetically engineered into another organism of the same species? Would this make GMOs a little bit more palatable to their detractors?

Soy is one of the most important crops grown in the US and it is nearly ubiquitous in the market. It’s in our food, drinks, biodiesel fuel, even cosmetics. If you rummage through my mom’s kitchen you’ll find soy sauce in the pantry, tofu in the fridge, and edamame in the freezer. Back in the day, she used to keep soybeans on hand for when she’d press her own soy milk.

Soybean cyst nematode and egg SEM

"Low-temperature scanning electron micrograph of soybean cyst nematode and its egg. Magnified 1,000 times."

Latte drinkers, vegetarians, and us Asians aren’t the only ones who love soy, however. Lurking underground are parasitic worms known as soybean cyst nematodes, which find the roots of the soybean plant irresistible. These agricultural pests invade the roots of the soybean plant where they do a bit of their own agriculturing. These nematodes are capable of inducing the root cells on which they feed to divide thereby creating a steady supply of food for themselves. Whereas males leave the comforts of their “root homes” in order to find mates, females remain there where they continue to feed and swell in size until eventually their bodies burst through the root. Once mated and having laid her eggs, the female dies and her cuticle hardens to form characteristic the cysts on the roots of the soybean plant. The damage to soy crops is damages to the tune of $500 million to $1 billion annually in the US alone.

Segment of soybean root infected with soybean cyst nematode. Signs of infection are brown-white females or cysts with egg masses that are attached to root surfaces.

"Segment of soybean root infected with soybean cyst nematode. Signs of infection are brown-white females or cysts with egg masses that are attached to root surfaces."

Soybean plants aren’t entirely defenseless, however, as there are soybean plant strains, such as the Forrest cultivar, that are resistant to nematode attack. In this cultivar, the feeding cells that the nematodes “cultivate” in the roots of the soybean plant die off and the worms starve before they can reproduce. (Conversely, there are also soybean cyst nematodes that are resistant to resistant soybean plants. It wouldn’t be Nature without the wrinkles, now would it?)

While exactly how the feeding cells in the Forrest cultivar degenerate in response to soybean cyst nematode is unknown, a team of scientists led by Shiming Liu (Southern Illinois University) and Pramod Kandoth (University of Missouri) has recently identified mutations in the serine hydroxymethyltransferase (SHMT) gene that are responsible for nematode resistance. Serine hydroxymethyltransferase is an enzyme involved in the shuttling of one-carbon units between molecules--folate in particular--until the carbon is ultimately freed up for the cell to use in important processes such as DNA and protein synthesis. For instance, one of the consequences of serine hydroxymethyltransferase activity is the conversion of the serine to glycine, both of which are amino acids found in proteins.

One of the reactions that serine hydroxymethyl transferase catalyzes is the conversion of the amino acid serine to glycine.

One of the reactions that serine hydroxymethyl transferase catalyzes is the conversion of the amino acid serine to glycine.

Since the mutations in the Forrest SHMT gene are located near the active site, or the “business end” of the SHMT protein where the shuttling of carbons occurs, it’s possible that the mutations affect the activity of the SHMT protein. Since To test this model, Liu and Kandoth expressed the mutated SHMT Forrest gene to see if it could restore the growth of an E. coli strain that is unable to survive because it can’t manufacture it’s own glycine. They found that the mutated version of SHMT was less effective than the unmutated form of SHMT in restoring growth of the E. coli strain indicating that the activity of the Forrest SHMT protein was probably reduced due to mutations.

More importantly, soybean plants that were susceptible to SCN infection became resistant when Liu and Kandoth transferred the Forrest SHMT gene in the susceptible plants. This demonstrated that the mutated Forrest SHMT was responsible for soybean cyst nematode resistance. The scientists speculate that the decreased activity of the mutated SHMT reduces either the “nutritiousness” of the feeding cells or their ability to divide, and as a result the nematodes that infect the Forrest cultivar starve to death.

So, this brings me back to my original question of what, if anything, would constitute an “acceptable” GMO to opponents of genetic engineering? Would detracters object to a scenario where an already existing mutation* that confers resistance to an agricultural pest is engineered into other soybean plants. Directly transferring the existent Forrest SHMT variant would be more efficient over traditional methods of breeding, since only the Forrest SHMT gene would be introduced into another soybean plant without carrying over any unwanted traits or genes. There are, after all, many different cultivars of soybean plants used for different applications that may benefit from nematode resistance.

*I’ll avoid saying “naturally-occurring” since the Forrest cultivar was developed by a USDA breeding program.

 

Reference

Liu, S., Kandoth, P., Warren, S., Yeckel, G., Heinz, R., Alden, J., Yang, C., Jamai, A., El-Mellouki, T., Juvale, P., Hill, J., Baum, T., Cianzio, S., Whitham, S., Korkin, D., Mitchum, M., & Meksem, K. (2012). A soybean cyst nematode resistance gene points to a new mechanism of plant resistance to pathogens Nature, 492 (7428), 256-260 DOI: 10.1038/nature11651

Crossposted at Amasian Science.

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