In the late 1990s, molecular biologist Yoram Eyal, and colleagues at The Volcani Center in Israel, identified the key gene and enzymes responsible for producing the bitterness compounds in grapefruit. Now, using the genome editing technology CRISPR/Cas9, the team has inactivated the gene in a type of grapefruit (Citrus paradisi) to eliminate that bitter taste.
“We started from basic research, established the pathway that generates bitter compounds in citrus, and, using genome editing, brought this to agricultural practice to develop new varieties in which the taste was modified,” Yoram Eyal told Refractor in an interview. “I think basically it’s opening up a bigger market for grapefruits or for any kind of future citrus varieties.”
The bitterness receptors on our tongue are significantly more diverse than the other taste receptors. The scientists believe that it allows us to detect a wide array of toxic compounds in our food. Since we still have a limited group of receptors, we “will have off-targets,” explains Eyal. Many nutritious compounds are harmless and healthy, for example, but this survival mechanism perceives them as bitter.
For adults who eat bitter foods like grapefruit, they have likely developed an acquired taste for these bitter compounds. The food becomes pleasant through repeated exposures. However, children are generally more sensitive to the bitterness, which can make them turn away from the healthy foods, missing out on several nutritious perks.
Geoffrey Thomson, a plant genome engineer at Yale University who wasn’t involved in the study, says that removing bitterness in citrus fruits such as grapefruit is a trait that many consumers of citrus juice would find appealing.
However, eliminating a specific trait from a tree is “more technical and difficult” than genome editing in crops, says Eyal. Previous studies have identified a gene called 1,2RhaT, as a key to bitterness, and in the new research, published in The Plant Journal, the team introduced small mutations to this specific gene. This tiny error forced the plant to stop biosynthesizing the protein, effectively abolishing the bitter chemicals.
Analyses of the leaves from the modified plant show no detectable bitter compounds such as naringin, neohesperidin, or poncirin. It should be noted that the study found the metabolic compounds only in the leaves, however the team expects identical results from the fruit as well.
Thompson says that the work is still preliminary because citrus trees have a long juvenile period, so the tree may take several years to produce fruit to test. “It will be important to wait and see how the metabolic composition of the fruit (both flesh and juice) is affected,” the scientist told Refractor.
As for the overall impact, the researchers are not sure whether the genetic modification alters the nutritional benefits of the fruit or the compromises the resistance of the tree.
Since the wild relatives of the citrus fruit are more resistant to cold, Eyal is interested in crossing “wild plants with domesticated citrus to develop more resistance to cold spells.”
