Bitterness in wild cucumber, melon, and watermelon

When cucumbers, muskmelons, and watermelons grow in the wild, their fruit contains bitter cucurbitacin molecules, a family of highly oxygenated tetracyclic triterpenes that deter pests from foraging. Although these defense compounds evolved in the plants millions of years ago, humans have bred them out of the fruits to make them more appealing to our palate. A team of researchers led by Yi Shang and Sanwen Huang of the Chinese Academy of Agricultural Sciences has examined the biosynthetic pathways responsible for cucurbitacins, which are also being investigated as anticancer, anti-inflammatory, and laxative agents. They find that human domestication has successfully interfered in the gene expression of important enzymes in the cucurbitacin pathways in the fruit (Nat. Plants 2016, DOI: 10.1038/nplants.2016.183). In watermelon, for example, a single point mutation in a transcription factor results in a dysfunctional protein and reduced bitter compounds in domesticated varieties of the fruits. Interestingly, cucumber, melon, and watermelon plants continue to make the defensive compounds in other parts of the plant, such as roots, leaves, and stems, using other transcription factors: It’s a useful workaround to satisfy their human masters, while maintaining some armor.