Citrus greening study unlocks key genetic information and potential for faster varietal testing

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Citrus greening study unlocks key genetic information and potential for faster varietal testing

Citrus greening disease has long plagued Florida’s citrus groves, greatly affecting sweet orange varieties used for juicing. Last year, the state saw its lowest production rates and a crop that resembled World War II levels.

Relief could be on the way, however, thanks to a new development by the USDA’s Agricultural Research Service (ARS) and the University of Florida’s Institute of Food and Agricultural Sciences (UF/IFAS). Scientists say they better understand one the key genetic components needed for producing disease-resistant fruit that is also marketable. The issue comes down to flavor.

Citrus greening, known as Huanglongbing (HLB), is caused by the bacterium Candidatus Liberibacter asiaticus, transmitted by insects called Asian citrus psyllids. The disease causes citrus trees to become unproductive and fruit quality to decline. Once the tree is infected, there is no cure for it.

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HLB was first detected in Florida’s Miami-Dade County in 2005 and has since spread. The disease also poses a threat to other U.S. citrus-growing areas, including California, Louisiana and Texas.

Orange juice is one of the most heavily affected categories by HLB. Sweet oranges commonly used for juice stay green when affected by HLB, producing a bitter-tasting liquid that can not be marketed or sold. 

The need for fruit with desirable flavor for juicing has been one of the biggest challenges with developing an HLB-resistant variety that suits the market.

During varietal testing in the 1960s, scientists identified better tolerance to citrus greening in hybrids bred with the cold-hardy trifoliate orange, Poncirus trifoliata. The flavor issue remained, however.

This led scientists to work on isolating the individual chemicals that give orange juice its characteristic flavor. And they did just that, identifying 26 total flavor compounds and seven chemicals called esters, deemed essential to the desired flavor profile of orange juice.

That advance, in turn, enabled the ARS and UF/IFAS team to pinpoint the esters’ master gene, CsAAT1, and make what’s known as a DNA marker for it. That provides a tool that can be used to quickly check for the genetic presence of a desirable trait in germinated seeds without having to wait 10 to 15 years for its physical expression.

“Breeders can use this DNA marker to screen seedlings for desired flavor profiles at an early stage,” explained Anne Plotto and Jinhe Bai, plant physiologists with the ARS Citrus and Other Subtropical Products Research Unit in Fort Pierce, Florida. 

While the first commercial releases of orange-like hybrids with HLB tolerance are still a few years away, researchers are confident that this is a big step in the right direction.

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