When we speak about genetically modified organisms (GMOs), we often think first about herbicide tolerant (Round-Up Ready) or insect resistant (BollGard) crops. But plants can be genetically engineered perform functions other than insect resistance and herbicide tolerance. My first post described one example: using plant genetic engineering to improve the quality of life for celiac disease patients. Bioremediation, the use of GMOs to reduce chemical contaminants in the environment, is another application of genetic engineering.
Bioremediation is the process of using live organisms to degrade toxic compounds. Phytoremediation is bioremediation using plants as the live organisms. In 2010 a research group at Nankai University published the results of their studies on phytoremediation of the herbicide, atrazine.
Atrazine is one of the most widely used herbicides in crops. Limiting weeds in the field reduces competition for water and nutrients. In corn production, for example, weed control using atrazine can lead to yield increases of 1–6%. However, atrazine is a relatively stable compound, degraded slowly in soil by microbes. It may be carcinogenic and has been associated with birth defects and endocrine disruptions. Atrazine accumulation in the soil might damage crops that are planted after atrazine treatment of fields. For these reasons, it is important to have a method to degrade atrazine in soil. One approach to increasing degradation of atrazine and other pesticides is phytoremediation.
Among the soil microbes that naturally degrade atrazine are Pseudomonas and Arthrobacter. These bacteria contain the atrazine chlorohydrolase gene (atzA), that converts atrazine to the non-toxic hydroxyatrazine. The Nankai University researchers genetically engineered (GE) plants to add the atzA gene. They tested the resulting
GE plants for their ability to degrade atrazine in soil.
Non-GE plants grew as well as GE plants if the plants were grown in soil without atrazine. But in soil containing atrazine, non-GE plants were shorter and weighed less than plants genetically engineered to contain the atzA gene. Since atrazine acts by destroying chlorophyll in the leaves, the GE plants were tested for changes in chlorophyll content. When GE and non-GE plants were grown in soil containing atrazine, the chlorophyll in non-GE leaves decreased. The chlorophyll level of GE plants remained unchanged, indicating protection of the chlorophyll and therefore the plant by the introduced gene.
But will the GE plants remediate contaminated cropland? That is, can they remove atrazine from the soil, reducing damage to successive crops and reducing potential effects on human health? It’s still unclear how these plants will perform in the field. However, after growing GE plants in soil containing atrazine for 90 days, no atrazine remained in the soil. While preliminary, these data suggest that GE plants can be used for phytoremediation of an important herbicide in soil.
H Wang, X Chen, X Xing, X Hao, and D Chen. 2010. Transgenic tobacco plants expressing atzA exhibit resistance and strong ability to degrade atrazine. Plant Cell Reports 29: 1391 – 1399. DOI: 10.1007/s00299-010-0924-7
Genetic engineering of plants can reduce the need for chemical insecticides. This, in turn, can improve food safety, and reduce energy inputs and cost. In recent posts I described genetically engineering plants to produce their own pesticides, specifically TMOF and chitinase. I also discussed the effects that the pesticides created by those plants had on insect larvae. Savvy readers would have noticed that the effects on insects have been promising, but not stellar. It’s hard to get very enthusiastic about delayed weight gain as a measure of success!
That’s the reason the Rao research group at Università di Napoli took their project one step further and combined two biopesticides into a single plant. In a paper published in the 2010 Insect Biochemistry and Molecular Biology journal, these researchers tested the combination of plant-made TMOF and plant-made chitinase against larvae of the tobacco budworm.
Classical methods were used to breed genetically engineered (GE) plants producing TMOF with GE plants producing chitinase. The hybrid offspring produced both pesticidal proteins. Tobacco budworm (Heliothis virescens) larvae were fed leaves from the hybrid plants, or from GE plants producing either TMOF or chitinase, or from non-GE plants. The insects that were fed leaves from the TMOF-producing plants or the chitinase-producing plants developed more slowly than those fed with leaves from non-GE plants. But insects that were fed leaves from the dual-biopesticide plants developed even more slowly than the insects fed with leaves from plants producing either TMOF or chitinase alone. Most important for crop protection, approximately 75% of larvae fed on hybrid plants died. This suggests that GE plants producing both TMOF and chitinase protect themselves better against damage from insect larvae than plants producing only one of the proteins.
L Fiandra, I Terracciano, P Fanti, A Garonna, L Ferracane, V Fogliano, M Casartelli, B Giordana, R Rao and F Pennacchio. 2010. A viral chitinase enhances oral activity of TMOF. Insect Biochemistry and Molecular Biology 40: 533 – 540. DOI:10.1016/j.ibmb.2010.05.001
In a recent post, I mentioned the importance of identifying genes and proteins besides Bacillus thuringiensis (Bt) that can be used in the fight against crop pests. One such protein is chitinase.
Chitinase (pronounced ′kītən′ās) is an enzyme that breaks down the protein chitin. Chitin is an important component of insect exoskeletons and fungal cell walls. Functionally, it helps the insect or fungus retain its structure. Destroy the chitin, and the insect or fungus dies.
Scientists from the Rao laboratory at the University of Napoli hypothesized that a plant that made its own chitinase could protect itself against pests. They generated genetically engineered (GE) plants that produced chitinase, then tested the effect of the chitinase-producing GE plants on fungi and tobacco budworm larvae.
The results, published in a 2008 article in Transgenic Research, showed reduced fungal growth and abnormally slow weight gain in the insect larvae after they ingested the plants. This suggests that chitinase-producing GE plants protect themselves against damage from fungi and insect larvae better than non-GE plants.
In a future article, I will report on research which suggests that combining TMOF and chitinase improves crop protection against insect pests.
Corrado, G, Arciello, A, Fanti, P, Fiandra, L, Garonna, A, Diglio, MC, Lorito, M, Giordana, B, Pennacchio, F, Rao, R. 2008. The chitinase A from the baculovirus AcMNPV enhances resistance to both fungi and herbivorous pests in tobacco. Transgenic Research 17: 557 – 571, DOI: 10.1007/s11248-007-9129-4.