Gene Mapping May Be the Key to HLB-Resistant Citrus

Beating citrus greening disease is on the front burner for all of those involved with citrus, and advancements in genome mapping will open up a whole range of opportunities for both beating the deadly disease in the future and keeping the citrus industry going in the interim. “There’s a lot of research going on in dierent labs around the country and in dierent labs around the world,” that’s aimed at combating citrus greening, says Dr. Fred Gmitter, a UF Research Foundation Professor in Citrus Genetics. Gmitter has been working in genome sequencing for over 10 years and has been involved in dozens and dozens of projects concerning citrus genome research. He was part of the team that sequenced the trifoliate orange genome, and he is working on current projects that will sequence additional citrus genomes to advance the fight against citrus greening, also known as HLB.

Providing Basic Information

Gmitter maintains that genome mapping supplies the basic information—the exact DNA sequences that make up a particular citrus type—that can be used in further research in combating citrus greening. It’s information that can be used by all of those researchers all over the country and the world who are working on tackling citrus greening, and other diseases too.

“Everybody’s got a laundry list of genes that they want to work on,” he shares, and genome sequencing will supply those genes, whether a scientist is interested in conventional breeding or high-tech technology, such as CRISPR.

Trifoliate orange is a citrus relative that displays high HLB tolerance, and its genome map will help researchers to figure out why the tolerance exists and to potentially transfer that tolerance with other citrus varieties. “If we can figure out why trifoliate orange is tolerant, maybe we can do something to sweet orange to make it tolerant or resistant to the disease,” Gmitter says.

Further Research

Gmitter says the trifoliate genome sequence is “the highest quality citrus genome sequence that exists right now in the world,” and that the sequencing of oranges, grapefruit, lemons, and the highly tolerant Sugar Belle variety, are up next. Another genome sequencing project he’s a part of is “using the very latest in genome sequence assembly to get very, very high-quality assemblies.” These full sequences of genes can be useful in precisely understanding how citrus greening aects citrus trees because it supplies the complete sequence of DNA without any gaps or missing pieces.

“A big part of the challenge,” Gmitter says, “is that the interaction of this bacteria with citrus plants is very, very complicated.” Genome mapping will give scientists the pieces of the puzzle to start to understand which genes or group of genes are responsible for HLB susceptibility and tolerance, and to then figure out how to manipulate those genes to create HLB-tolerant citrus, either through conventional breeding or other methods, like CRISPR.

A Word About Orange Juice

Genome sequencing is also revealing a lot about the ancestry of citrus, specifically sweet oranges, especially considering the Department of Citrus rule that orange juice can have no more than 10% additives not derived from oranges is considered. Gmitter maintains that sweet orange “is a complicated admixture” of ancestral citrus species and that it’s actually “70% to 80% mandarin and the rest is pummelo.” Whether this new information will change the Department of Citrus’s rule and relax juice requirements for growers remains to be seen.

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