Photo by David Bartels, USDA
It’s one thing for a hybrid citrus tree to tolerate HLB, but quite another thing for it to produce orange-like fruit that makes delicious orange juice (OJ). That holy grail of traits could be nearer, thanks to a team of U.S. Department of Agriculture Agricultural Research Service (USDA-ARS) and University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) scientists.
In Florida, sweet oranges are the main varieties used to make OJ, and sweet orange is highly susceptible to HLB. Scientists are investigating countermeasures to provide Florida with a ray of hope. USDA-ARS efforts indirectly took root in the 1960s, when agency scientists created citrus hybrids using a relative named Poncirus trifoliata (the cold-hardy trifoliate orange) to shore up trees’ cold tolerance.
The start of the HLB epidemic in 2005 revealed that the hybrids also appeared to tolerate the new disease. That prompted intensive research by USDA-ARS and UF/IFAS to understand why and how this is related to fruit quality.
Initial field tests and flavor evaluations showed that some of the Poncirus-derived hybrids — with the notable exception of US SunDragon — tended to produce juice with an undesirable off-flavor but aroma profiles similar to sweet orange. So, the scientists decided that, in addition to using data from analyses of juice-aroma compounds, they needed to get a better handle on the individual chemicals that give OJ its characteristic flavor. They identified 26 flavor compounds and seven chemicals called esters deemed essential to OJ’s desired flavor profile.
That advance enabled the team to pinpoint the esters’ master gene, CsAAT1, and make what’s known as a DNA marker for it. The marker can be used to quickly check for the genetic presence of a desirable trait in germinated seeds versus observing its physical expression in 10- or 15-year-old mature plants.
Breeders can use this DNA marker to screen seedlings for desired flavor profiles at an early stage, according to Anne Plotto and Jinhe Bai, plant physiologists with the ARS Citrus and Other Subtropical Products Research Unit in Florida. They said that by incorporating this gene into the genetic makeup of HLB-tolerant hybrids derived from Poncirus trifoliata and mandarin, or from many other possible crosses with the same objective, breeders can ensure the new hybrids possess HLB tolerance and also maintain the characteristic sweet orange flavor.
A full description of the team’s approach was published Feb. 28 in the journal Science Advances.
Plotto and Bai, the principal investigators, co-authored the paper together with 11 other collaborators. USDA-ARS collaborators are from the U.S. Horticultural Research Laboratory in Florida and the U.S. Pacific Basin Agricultural Research Center in Hawaii. UF/IFAS collaborators are from the Citrus Research and Education Center and Gulf Coast Research and Education Center.
The researchers cautioned that even with the use of high-tech tools like machine-learning, the first commercial releases of orange-like hybrids with HLB tolerance will be contingent on several more years of testing and refinement.
Source: ARS
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