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In research institutions in La Plata and Tucumán in Argentina, we and other scientists have been exploring GMO techniques to treat citrus trees sick with deadly and uncurable diseases. Traditionally, producers would use conventional approaches to prevent other trees from becoming infected. Now, these novel GMO methods could help protect the industry of the world’s leader producer of lemons and the second largest producer of citrus in South America.
In Argentina, we have been working to develop citrus trees resistant to several diseases. Last April, we published the first scientific perspective paper in our country on this matter in the journal Crop Science. We reviewed techniques to combat serious pathologies, including the most devastating of all time — citrus huanglongbing or HLB, for short. The bacterial disease, also known as citrus greening, costs billions of dollars in lost produce each year. Plant pathologists still don’t know for sure what organism makes trees sick with HLB, but the bacteria Candidatus Liberibacter sp. is associated with the disease. Currently, HLB has almost destroyed the citrus industry in Florida and now growers in California — the second largest producer in the U.S. — are worried their trees might be next. As if this wasn’t enough, HLB has been reported in citrus areas all over the world including, recently, Argentina.
And it’s not the only threat the fruits face. In our perspective paper, we also discuss potential management for another bacterial disease; citrus canker caused by Xhantomonas citri pv citri. Growers can recognize affected trees by the brown corky lesions and cankers the bacteria produces in fruits, leaves and branches. Eventually, sick individuals die.
Besides the bacterial diseases, we also addressed two caused by viruses. Similarly to what happens to human skin in psoriasis disease, Citrus psorosis virus, makes the infected trees shed its bark. The disorder also causes resin to accumulate in leaf veins, and in vessels in the trunk and branches.
In our paper we also discuss Citrus tristeza virus, which was responsible for the death of more than 30 million trees between the 1930s and 1940s in Argentina and Brazil alone. The disease is called tristeza — sadness in Spanish — because of the crestfallen look of the trees it infects.
Combating these diseases is further complicated because of how commercial citrus trees are grown. To shorten the time needed for citrus to reach sexual maturity and therefore, produce fruits, trees have two parts with different genetic identities. The rootstock is a stem with a well-developed root system to which a scion, or upper canopy, holding the leaves, flowers and fruits, is grafted to. To prevent diseases like the viral ones mentioned above, the Argentine government started a programme to certify that all propagative materials, including scions, were free of pathogens. However, this hasn’t been enough since we still don’t know how the Citrus psorosis virus is spread from tree to tree. Scientists suspect the pathogen might be passed on through an insect or a fungus in the soil, agents known as vectors.
As traditional methods have not been effective enough to control these diseases, we turned to GMO techniques. First, we tried something similar to what vaccines do in humans by introducing genes from the pathogens Citrus psorosis and tristeza virus into the plant genome. This activates the plant’s immune system and prevents the infection at the genetic level. We also added tiny proteins, called antimicrobial peptides (AMPs), from a species of frog and from potato plants into the citrus’ genomes. AMPs have a wide range of inhibitory effects against bacteria, fungi, parasites and viruses in nature and we confirmed they’re also effective to protect citrus. Lastly, we introduced a receptor from rice, or a pathogen-resistance gene called R gene, to citrus trees for them to recognize and withstand an infection of Xanthomonas citri, the causal agent of citrus canker disease.
We tested these approaches and obtained promising results. Young transgenic citrus with AMPs and R genes resisted artificial bacterial inoculations in the laboratory and in the greenhouse between 50 and 75% more than their non-transgenic counterparts. Also, symptoms were less severe and took longer to appear. The “vaccinated” citrus trees were 100% resistant to Citrus psorosis virus and even tolerant with a non-transgenic scion. Moreover, we showed that whole trees can be resistant to the virus and still produce non-GMO fruits in the non-transgenic scion. However, trees carrying genes of Citrus tristeza virus, while being resistant in the lab and in the greenhouse, still got sick in the field.
Despite the favourable results of our experiments, the process to obtain government’s approval for these GMO trees to be planted and commercialized in Argentina will be long. Since the early 1990s, Argentina has been a pioneer in the use of GMO crops and, while the agriculture industry has embraced this technology, citizens and those involved in the agroecology movement, have remained skeptical.
Discussion around GMOs in Argentina
The Argentine regulatory institutions have implemented ways — like environmental and human risk assessments — to ensure the biosafety of these products. However, in some discussions we had amongst ourselves and with the highly regarded plant biotechnologist Dr. Esteban Hopp, we identified some issues. On one hand, local developers belonging to National Universities and Public Institutions aren’t familiar with the general regulatory process and the time necessary for approval. Additionally, using GMOs doesn’t come cheap, making it almost impossible for local scientists and small business to take GMOs to the market. Only multinational companies from developed countries have been successful in introducing their GMOs in Argentina for popular crops such as soybean, corn and cotton.
Given the potential benefit for the citrus industry in Argentina, we wonder: Can the system change to help promote local technologies? Are people willing to consume GMO citrus rather than witness the disappearance of citriculture in our country? Even when local, these questions are relevant for other countries going through the same processes.
Article: De Francesco, A., Sendin, L.N., Gomez, R.L., Reyes, C.A. (2021). Transgenic-Based Solutions for Citrus Disease Management in Argentina. Crop Science https://acsess.onlinelibrary.wiley.com/doi/10.1002/csc2.20530
Agustina is an Argentine researcher in plant pathology, former postdoctoral scholar at University of California Riverside (UCR) and La Plata National University. She studies management and detection of plant diseases, mostly focused on citrus. She is currently collaborating with California Food and Agriculture Enterprise (CAFÉ) at UCR on sustainable agriculture and circular economy. Follow her work on Twitter or her personal website.