This post was originally published on GMO Answers' Medium page.

By Manjul Dutt Ph.D., and Jude W. Grosser Ph.D.

Manjul Dutt Ph.D., is a Research Assistant Scientist in the Horticultural Sciences department within University of Florida’s Institute of Food and Agricultural Sciences.

Jude W. Grosser Ph.D., is a professor at the University of Florida working on a research program in citrus variety improvement that addresses all major citrus production problems in Florida, and also strives to develop new cultivars that will provide growers with new marketing opportunities. Grosser’s research combines emerging biotechnologies with conventional breeding approaches in efforts to develop improved rootstocks and scions for both processing and fresh market.


Citrus Greening

Genetic engineering is the fastest method of citrus improvement that may be able to save this Florida crop. (Image Credit: GMO Answers)


The Citrus Greening Problem

Florida’s citrus economy has taken a hit from the citrus greening disease (Huanglongbing also known as HLB) epidemic outbreak. The causal agent of citrus greening disease is Candidatus Liberibacter spp. These bacteria live in the plant’s phloem tissue (the plant’s sugar transport system) and the gut of its carrier host, an insect called the Asian Citrus Psyllid (Diaphorina citri). The Asian Citrus Psyllid sucks the sap from the leaf or stem of an infected tree and transmits the bacteria into the healthy tree. This is similar to how malaria is spread by mosquitos in humans. This disease reduces the tree’s vigor and leads to tree decline. Fruits produced on severely infected trees often do not ripen completely and can become misshapen, rendering the fruits unmarketable as the juice becomes bitter. Intensive costly insecticide spray programs are being employed by citrus growers across Florida to control this insect.  However, they are not working efficiently, as the disease has spread from south Miami-Dade County, where it was first discovered in 2005, to most trees in all citrus growing counties.

Hundreds of thousands of affected grapefruit and sweet orange trees have been removed from Florida groves, as they are highly susceptible to HLB infection. As a result, many growers, packing houses and even a few juice processing plants have gone out of business. Some growers have even ventured into alternate crops such as blueberries and peaches. This disease is causing serious damage to the Florida economy, as citrus has been a $10B industry in Florida. 

The Biotech Solution

The citrus trees that we see out in the groves, homeowner yards or in garden centers are produced through the combination of two tree parts. The top of the tree that produces the fruit is called the scion and the bottom of the tree that anchors the tree below ground is called the rootstock. Trees can be grown with different combinations of scions and rootstocks. Researchers have studied the use of various combinations to determine if there are scion and rootstocks or a combination of the two that are more tolerant to HLB infection. Long-term management of HLB lies in the development and deployment of highly tolerant or resistant citrus scions and/or rootstocks. Conventionally bred hybrid scions and rootstocks are currently being evaluated and several show promise, but genetic engineering (familiarly, “GMO”) remains the fastest method of citrus improvement without otherwise changing the original characteristics (cultivar integrity) of the citrus varieties that are popular in Florida. Using genetic engineering technology, one or more genes that confer resistance can be inserted into the genome of any given citrus variety, without changing the variety other than adding the new valuable trait that is otherwise unavailable.

Three Ways GMOs Can Help

  1. Several strategies to control HLB are showing promise.  The most publicized strategy has been the use of the spinach defensin AMP gene by the Southern Gardens Corporation.  AMP stands for ‘anti-microbial peptide’.  AMP gene products are small protein-like peptides that can damage bacterial cell membranes, causing bacterial cell death and thus prevent infection.  Many plants and animals (including humans) produce AMP’s.  For an example, think about your own eyes – a nice warm, open and moist environment – perfect for a bacterial infection; yet it rarely happens – WHY?  Because humans have their own AMP gene products floating around in the eye intraocular fluid that prevent infections.  The AMP gene transferred to sweet oranges showing promise in this case is from spinach, so it is something that we already eat routinely!  This should dampen opposition to the genetically-engineered citrus trees.
  2. Another strategy that we are finding successful is a strategy to augment the citrus trees own defense system.  Just like animals and humans that have immune systems, plants have developed defense mechanisms against invading pathogens.  The primary defense system in plants is called SAR (Systemic Acquired Resistance).  This defense response begins with a mobile signal that is triggered by infection. Once this occurs, the mobile signal initiates a series of steps that activates a number of defense related genes within the plant’s cells. However, HLB somehow has the ability to fool natural citrus trees, and the available defense system does not become fully engaged after infection.  We have found that the insertion of the NPR1 gene isolated from Arabidopsis (a mustard plant relative, again an edible plant) into sweet orange trees increased their SAR defense response, making the trees highly tolerant or resistant to HLB.  Our work has demonstrated that this non-citrus NPR-1 that did not co-evolve with the HLB bacterium facilitates the activation of this mobile signal following infection, thereby enhancing the HLB resistance in these engineered orange trees.  We are currently evaluating the horticultural performance of these trees.
  3. Finally, in a grafted tree, such as citrus, the SAR-induction signal can potentially move from the site of infection in the scion (fruit-producing part of the tree) to the rootstock and trigger the activation of defense genes both in rootstock and scion. If the SAR-induced defense signal is indeed mobile and can be transmitted from one part of the plant to the other, then it is indeed possible that a non-transgenic scion (the top of the tree)  grafted onto a transgenic rootstock expressing the Arabidopsis NPR1 gene could adequately protect the non-transgenic scion against infection. In this case, the fruit or juice from the tree might not be considered a “GMO” product at all.  This would have a positive impact on subsequent consumer acceptance.  Research is ongoing to evaluate this hypothesis. If this hypothesis is validated, then the transgenic rootstock would have the defense genes turned on and ready to assist the non-transgenic top scion following infection, resulting in highly tolerant or resistant citrus trees.

Tolerant rootstocks and scions have the potential to be an effective long term solution to combat citrus greening. The genetic engineering strategies described herein are just some of the many avenues being explored by researchers at the University of Florida and elsewhere. As researchers work together, we are hopeful that a total and sustainable solution will emerge to help save our beloved citrus industry!