Protecting the Orange
The Disease
A tiny insect has been spreading disease to millions of Florida orange trees since 2005. The creature, known as the Asian citrus phyllid, is small (about the size of an aphid), feeds forty-five degrees from the surface of leaves, and looks like a small thorn on the orange tree (1). The state had been dreading its arrival for many months as it spread throughout countries such as Mexico, China, and Brazil (the world’s largest producer of oranges) (2).
This year, the Asian citrus psyllid Diaphorina citric continued its spread of the “citrus greening” disease that has been devastating orange crops throughout the world. It has already spread the disease throughout all 32 of Florida’s citrus-growing counties; even states such as Texas, California, and Arizona have been affected (2, 3).
The insect itself acts only as a vector for a bacterium called Candidatus Liberibacter asiaticus. Flying rapidly from tree to tree, the psyllid feeds upon the sap of the leaves and inadvertently carries the bacterium within the sap. The insect itself is very hardy, able to carry bacteria for up to a mile without ceasing flight and able to reproduce rapidly. Though they live only for one month, psyllid females may lay up to 800 eggs (1,2).
The trees, once infected, may take years to show symptoms. The leaves turn yellow and fall off. The fruit remains green and falls to the ground prematurely. Eventually, the tree itself succumbs to the disease. Unfortunately, once the tree has been infected, it cannot be cured by any means available to growers. Growers simply must burn or otherwise destroy the tree to prevent the further spread of bacterium-bearing psyllids (1,3).
The effects have been devastating to citrus crops wherever the disease spreads. The U.S. Agriculture Department has classified it next to anthrax and Ebola virus as a potential agent of bioterrorism (1). U.S. Senator Bill Nelson of Florida commented that the $9 billion Florida citrus industry and its thousands of workers are now “totally threatened” and, without a cure, will be eliminated (3).
The Fight
Citrus growers in Florida have already poured $60 million of their own companies’ money into researching the disease. The Florida legislature has approved $8 million in research, and Washington State University has initiated a $9 million project to develop genetically altered psyllids resistant to infection from the citrus-greening bacterium (2,3).
In an attempt to bolster morale and show its commitment to the Florida orange, Coca-Cola (owner of Minute-Maid) has announced a plant to invest $2 billion to plant 25,000 acres of new orange groves in Florida (2).
Currently, efforts to combat the disease center around containment. Growers have chopped down hundreds of thousands of infected trees and sprayed an expanding assortment of pesticides on the psyllid. In fact, the industry has tripled pesticide applications in order to kill the psyllids – the increased dosage has become increasingly worrisome and expensive.
One pesticide lost effectiveness as the psyllid had evolved to become resistant, even as the Florida citrus growers begin their petition to spray young trees multiple times a year in hopes of saving their harvest. The industry has shifted its view towards finding a more permanent solution.
Unfortunately, efforts to find a naturally immune tree to serve as the new progenitor for the Florida orange have failed. The plant pathologist heading a National Research Council task force on the disease remarked that “there is no evidence of immunity” within the scope of “all of cultivated citrus” (1).
Without a natural option, the focus has shifted to laboratory-based approaches. This shift is remarkable given the popularity of oranges in America and the simultaneous skittishness of the American people to support genetically modified organisms (GMO’s).
The Challenge
The manipulation of DNA in laboratories often sparks distrust and fear, according to recent research (1,2). However, the total destruction of the Florida orange crop for the forseeable future is too much to risk for all those invested within the industry. And within the scientific community, there already exists a consensus that genetic engineering will be required to defeat citrus greening effectively (2, 3).
The process of creating a transgenic tree would take much of the next decade. It could cost up to $20 million and require rounds and rounds of testing before approval for use in real fields (2).
Traditionally, plant breeding has been used to select for desirable traits already seen in existing plants. Thus, the absence of a naturally resistant orange tree makes this process extremely difficult. The genetic engineering of resistance will require extraction of a foreign genome from a source organism, isolation of the desired gene, cloning to create thousands of copies of the gene, and ultimately the insertion of the gene into orange trees (4).
To ensure that the gene will function well in the new organism—in this case the orange tree—the gene is modified with sequences, such as a promoter, that will be read by the new organism. Various techniques may be used to insert the transgene into tissue cultures of the plant cells; each technique essentially injects the gene into the nucleus of the cell without killing the cell in the process (4).
Then, the growth of the orange tree begins as a single-line cell cultivar that requires two years of growth in a greenhouse before being planted. It would require many years before bearing fruit. Afterwards, the new orange plant may be bred with the original orange plant to pass down the resistance to a greater number of plants than what may be painstakingly grown from cell culture (2, 4).
Difficulties have already arisen in the process. The plants that seem resistant and healthy in the greenhouse often do not fare well in the grove under a variety of environmental conditions. In these confined field trials, the trees are isolated to prevent their transgenes from spreading rampantly (4).
One stopgap technique created by a scientist at the University of Florida can be used as an alternative means of protecting citrus trees until the day that true transgenic trees become functional. The technique involves making an incision in the bark of a healthy, adult orange tree and inserting a virus that contains the gene for citrus-greening resistance. This virus acts to confer some degree of immunity to the tree (2).
If the pestilence were spreading less rapidly, the growers perhaps could expect to wait for natural immunity to arise within the orange trees. Unfortunately, the rate at which the Florida orange is being threatened is too great, and it is likely that the trees will entirely disappear before developing resistance (2).
Whatever the technique, there still exists much public resistance to transgenic crops. Public opinion polls indicate that as many as a half of Americans would refuse to eat any transgenic crop, a third may refuse to eat fruit modified with another plant gene, and very few would accept plants that contained animal or viral DNA (2, 3).
Public backlash has erupted as of late due to the “Monsanto effect,” a situation where the public disliked farmers using genetically modified plants, such as corn, that were engineered with an internal herbicide, Bacillus thuringiensis (Bt). While these plants reduced the need for harmful insecticides, there still exists trepidation in the “mixing” of genes resulting in the blending of other characteristics from the gene donor (2).
The possibility that the genetically modified orange juice would not be labeled scares those reluctant to consume food created by “unnatural” means. Currently, the Food and Drug Administration supports voluntary labeling of genetically modified foods; however, the citrus industry in Florida is painfully aware of the backlash that may arise from forcing genetically modified orange juice on the market (2, 5).
Thus far, a gene from the spinach plant is showing the most promising results. The gene, though specifically derived from spinach, exists in many forms in plants and animals and functions to produce a protein that will attach the bacteria. In tests in both groves and greenhouses, the trees with conferred resistance remain resistant while the unaltered trees do not (2).
If these resistant trees manage to gain regulatory approval (the gene has already been thoroughly tested on mice and bees, to no ill effect), then they may be ready for planting by 2015 and ready to produce oranges for juice by 2018. The battle with the pestilence of citrus greening will have lasted 13 years in Florida, due to the infiltration of one bacteria-spreading insect (2).
Contact Annie (Yi) Sun at
yi.sun.16@dartmouth.edu
References
1. Citrus Research Board, “The Insect: Asian Citrus Psyllid” (2013). Available at http://californiacitrusthreat.org/asian-citrus-psyllid.php (26 September 2013).
2. A. Harmon, “A Race to Save the Orange by Altering Its DNA”, The New York Times (27 July 2013). Available at http://www.nytimes.com/2013/07/28/science/a-race-to-save-the-orange-by-altering-its-dna.html?pagewanted=all&_r=0 (4 August 2013).
3. M. Ricciardi, “Florida Orange Crop ‘Totally Threatened’ by Bacterium-Carrying Bug” (5 May 2013). Available at http://planetsave.com/2013/05/12/florida-orange-crop-totally-threatened-by-bacterium-carrying-bug/ (4 August 2013).
4. University of Nebraska, “Overview of the Process of Plant Genetic Engineering” (2001). Available at http://agbiosafety.unl.edu/education/summary.htm (27 September 2013).
5. Food and Drug Administration, “Genetically Engineered Plants” (07 April 2013). Available at http://www.fda.gov/Food/FoodScienceResearch/Biotechnology/ucm346030.htm (27 September 2013).