What makes an orange? A new study finds one gene and seven chemicals

In the United States, for orange juice to be labeled as such, it must be 90 percent sweet oranges, or Citrus sinensis. Thus, U.S. citrus producers have been growing 90 percent of them for a long time Citrus sinensis. But this variety is highly susceptible to the bacteria that cause citrus greening disease, which has devastated Florida's semi-monoculture crop. There is still no way to control the disease; The most effective way to deal with it is to find citrus varieties that are resistant to it and breed them with sweet oranges to give them disease resistance.

Sweet oranges are a hybrid of tangerines and pomelo and are not particularly genetically diverse. However, any disease-resistant citrus we know of does not taste like sweet oranges, so breeding with them will produce fruit and juice with no flavors. These flavors have been difficult to define and measure, because the essential components of proper orange flavor have been difficult to identify and measure.

Now, researchers at the USDA Agricultural Research Service have conducted a comprehensive chemical evaluation of 179 different varieties of citrus fruits — oranges, tangerines, and various hybrids — and compared their chemical compositions with assessments of orange and tangerine flavors in juice samples conducted by a “panel-trained panel.”

Twenty-six compounds were identified by a statistical model as being important in predicting orange versus mandarin flavor. Some of these elements were positively associated with orange flavour, meaning that more of the compound means more orange flavour. Others were negatively associated with them, meaning that having less of those compounds makes the juice taste more orange.

The study identified seven chemicals that distinguish orange flavor from mandarin flavor and one previously undescribed gene that controls the synthesis of six of them. Its activity is stimulated as the fruit ripens.

The work should make it easier to breed disease-resistant but delicious orange hybrids, and a DNA marker for orange flavor could be used to screen seedlings to see which will produce the most flavorful fruit before they ripen.

Science, 2024. DOI: 10.1126/sciadv.adk2051