Thursday, September 8, 2016

A gene from an edible fern is highly effective as an insecticide against whiteflies

Bemisia tabaci (also known as the silverleaf whitefly) feeding on a leaf. Via Wikipedia.

When I got home from work last night, I spent a few minutes on Facebook while waiting for dinner to cook and I stumbled on something that floored me on the page We Love GMOs and Vaccines. Researchers in India have developed the first GE crop to produce compounds to kill whiteflies. They began this project in 2007 and developed the first GE cotton plant by 2012; the paper was just accepted a couple of days ago. In their work, the researchers purified a compound, called Tma12, from an edible fern, Tectaria macrodonta, and then fed it to whiteflies. The researchers observed that a very small amount of the purified compound (1.49 μg/ml) was lethal to the whiteflies. From there, the gene for this compound was identified, cloned and later transformed into cotton.


Tectaria macrodonta via Flickr

From there, the researchers tested the GE-Tma12 cotton in field trials and found that plants expressing ~0.01% Tma12 in leaf material were resistant to whitefly feeding. Furthermore, the whiteflies didn't transmit virus to the plants before they died (more on that in a second). To test the safety of this GE plant, the researchers fed leaf material to rats and no histological or biochemical differences were observed between those fed the GE plant and those fed the control. Since Tma12 lacked any known allergen motifs and did not harm the rats, the researchers concluded that this GE trait would pose little risk to the public. 

It's quite a breakthrough for a group to develop a GE trait specific to whiteflies; however, that isn't what stunned me. It's the implication that this trait might have for my own work. Whiteflies are one of many arthropods that vector plant viruses. However, they transmit many plant viruses that are of great economic importance and can cause famines as they infect many staple crops that people grow in developing nations. Cassava gets cassava mosaic caused by several begomoviruses transmitted by whiteflies. Sweet potato gets viruses from three different genera (Begomovirus, Crinivirus and Ipomovirus). Cotton has a number of begomoviruses that infect it as does tomato, which also gets criniviruses. These are just a few examples of the many crops infected by whitefly-transmitted viruses. In fact, there are over a hundred different viral species transmitted by whiteflies. Some viruses, like ipomoviruses, are transmitted shortly after feeding whereas others, like the begomoviruses, are only transmitted after the whitefly has feed for a long time. The Tma12 trait would be most effective at preventing transmission of the viruses that take longer periods of feeding before transmission takes place. For the viruses that take only a few seconds of feeding, this would not control them.

The thing that struck me most is the way that whiteflies feed. They feed by piercing the leaf and sucking sap from the phloem tissue (think of this as the artery of the plant). There are many other insects that feed this way (many in the order Hemiptera), including many plant disease vectors from aphids to mealybugs to plant/leaf-hoppers to psyllids (a psyllid is responsible for transmitting the bacteria that causes citrus greening). It would be interesting to see if Tma12 is effective against other insects, especially these very important plant disease vectors.

But what does this all mean? As is, this new trait will help make cotton production more sustainable. If this trait were deployed in other crops, the impact of whitefly-transmitted plant viruses in food production could be greatly reduced. If it's effective against other vectors, then this could truly revolutionize agriculture and pest management. 

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