Can genetic engineering save Florida citrus?

Citrus greening threatens to destroy Florida's $10.7 billion citrus industry. (Credit: Jim Mullhaupt/Flickr)

New genetically modified citrus trees appear to appear able to fight back against the highly destructive citrus greening bacterium—and have the potential to resist canker and black spot, as well. However, the commercial availability of those trees is still several years away.

When researchers used a gene isolated from the Arabidopsis plant, a member of the mustard family, to create new trees, the trees exhibited enhanced resistance to greening and reduced disease severity. Several trees even remained disease-free after 36 months of planting in a field with a high number of diseased trees.

“Citrus crop improvement using conventional breeding methods is difficult and time-consuming due to the long juvenile phase in citrus, which can vary from four to twelve years,” says Jude Grosser, a professor of plant cell genetics at the University of Florida. “Improvement of citrus through genetic engineering remains the fastest method for improvement.”

Citrus greening threatens to destroy Florida’s $10.7 billion citrus industry. The diseased bacterium first enters the tree via the tiny Asian citrus psyllid, which sucks on leaf sap and leaves behind the greening bacteria. The bacteria then move through the tree via the phloem—the veins of the tree. The disease starves the tree of nutrients and damages its roots. The tree then produces fruits that are green and misshapen and unsuitable for sale as fresh fruit or, for the most part, juice.

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Citrus greening was first detected in Florida in 2005. Florida has lost approximately 100,000 citrus acres and $3.6 billion in revenues since 2007.

For the study published in the journal PLOS ONE, researchers used sweet orange cultivars Hamlin and Valencia and created plants that defend themselves against pathogens utilizing a process called systemic acquired resistance, or SAR. SAR provides protection against a broad spectrum of microorganisms and is associated with the production of anti-pathogen proteins. Utilizing SAR has already resulted in the production of transgenic canker-resistant trees. Transgenic trees are those into which DNA from an unrelated organism has been artificially introduced.

Disease resistance to greening, also known as huanglongbing or HLB, in this study was evaluated in two ways.

First, in a greenhouse study, several hundred trees (clones from several independent transgenic plant lines) were exposed continuously for two years to free-flying, greening-positive psyllids. Trees were routinely pruned and fertilized to stimulate new leaf production. These trees were evaluated every six months for two years for the presence of greening. The insects were also randomly evaluated during this study for the presence of the greening bacterium.

Approximately 45 percent of the trees expressing the Arabidopsis gene tested negative for greening. In three of the transgenic lines, the greening bacterium was not detected at all. Control trees tested positive for the presence of greening within six months and remained positive for the entire duration of the study.

In the second concurrent study, selected transgenic trees and controls were cloned, grown, and planted in fields with a 90-percent HLB infection rate. These trees were similarly evaluated every six months for three years for the presence of the greening bacterium.

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In this study, one transgenic line remained greening-free for the duration of the study, except for the 24-month sampling period when it tested positive. A second line tested positive at the 30-month sampling period while a third line tested positive at 30 months, but was greening-free at 36 months. Neither of these lines declined in health, and both showed continued growth with periodic flushes.

“In addition to inducing resistance to greening, this transgenic line could potentially protect our trees from other important citrus fungal and bacterial diseases such as citrus canker and black spot,” says Manjul Dutt, a research assistant scientist.

The next steps include transferring this gene into additional commercial varieties and rootstocks that are commonly grown in Florida. In addition, researchers must “stack” this gene with another transgene that provides resistance to the greening bacterium by a completely different mechanism. That will prevent the pathogen from overcoming the resistance in the field. It will still be several years before such trees will be available for commercial use.

The Citrus Research and Development Foundation funded the work. The foundation is funded by the citrus box tax, a tax growers pay on each box of citrus they sell.

Source: University of Florida