Trekking Toward HLB-Resistant Trees

Josh McGill Breeding, HLB Management, Research

By Michael Rogers

Much has been discussed recently about the promise and potential of utilizing CRISPR technology to create HLB-resistant trees for Florida’s citrus growers. The primary objective in citrus gene editing is to develop new varieties that are resistant to HLB disease while remaining non-transgenic.

HLB-resistant trees
Nian Wang examines gene-edited non-transgenic Hamlin orange plants in the greenhouse.

To achieve this goal, researchers make alterations to or eliminate specific genes within the plant responsible for disease symptoms. This results in a distinction from transgenic plants, where genes are added to confer disease resistance. Developing new citrus varieties through non-transgenic gene-editing methods eliminates the additional layer of regulatory and public scrutiny associated with transgenics (GMOs). This is why the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) is placing increased emphasis on gene-editing research programs.

While UF/IFAS researchers continue to refine their methods for expeditiously and efficiently developing gene-edited plants, the current techniques employed are meticulous and time-consuming, involving multiple stages of work. The gene-editing process begins at the microscopic level, where specific genes are modified within individual plant cells. These edited plant cells are then cultured in petri dishes with media to facilitate their growth into new citrus plants.

The transition from a plant cell, invisible to the naked eye, to a young seedling takes approximately 12 months. Once the plants reach this developmental stage, they undergo PCR analysis to confirm the accuracy of the gene edits. Less than 1% of the developed plants will exhibit the desired edits, while the rest are discarded. The “potential winners” are mass-produced in greenhouses for subsequent field trials. Several years of field evaluations are necessary to determine definitively if researchers have successfully developed a new variety resistant to HLB.

Nian Wang of UF/IFAS has refined the CRISPR editing process to create non-transgenic gene-edited citrus plants. He has successfully generated four lines of non-transgenic trees that have exhibited resistance to canker in greenhouse studies. These plants are currently being moved to replicated field trials to evaluate their resistance against both canker and HLB. It will take several years of field testing before their performance against both diseases can be ascertained.

HLB-resistant trees
Citrus embryos grow in a petri dish six to seven months after gene editing.

In an effort to expedite the process, budwood has been provided to the Florida Department of Agriculture and Consumer Services Division of Plant Industry (DPI) to “clean up” these lines, which typically takes a couple of years. This will trim the time needed to produce clean lines for growers. If these lines are successful in the field, then DPI will already have them “cleaned up” and ready to go to growers.

Other UF/IFAS faculty members are also employing biotechnology to develop transgenic disease-resistant citrus varieties. One such faculty member, Zhonglin Mou in Gainesville, has multiple lines of transgenic Hamlin orange and Duncan grapefruit that reportedly exhibit strong tolerance to HLB. While this year marks the commencement of fruit data collection from these lines, the process of deregulation for release of these transgenic varieties to growers has already begun. This has not been done before, so the amount of time it will take is unknown.

While UF/IFAS continues its extensive research efforts, it is important to note that these projects will take time to reach completion. In the interim, there are various therapies available to help sustain commercial citrus operations. In addition to new UF/IFAS sweet orange varieties with better HLB tolerance, practices such as employing individual protective covers, trunk injections, application of PGRs like gibberellic acid, and, of course, maintaining vigilance with nutrition and irrigation can support groves.

For more information, please visit and consider tuning in to the monthly All In For Citrus podcast.

Michael Rogers is a professor and director of the UF/IFAS Citrus Research and Education Center in Lake Alfred.

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