Biosynthesis, regulation, and domestication of bitterness in cucumber

Cucurbitacins are triterpenoids that confer a bitter taste in cucurbits such as cucumber, melon, watermelon, squash, and pumpkin. These compounds discourage most pests on the plant and have also been shown to have antitumor properties. With genomics and biochemistry, we identified nine cucumber genes in the pathway for biosynthesis of cucurbitacin C and elucidated four catalytic steps. We discovered transcription factors Bl (Bitter leaf) and Bt (Bitter fruit) that regulate this pathway in leaves and fruits, respectively. Traces in genomic signatures indicated that selection imposed on Bt during domestication led to derivation of nonbitter cucurbits from their bitter ancestors. A molecular explanation is given for how sweet domestic cucumbers were derived from bitter wild cucumbers. Biosynthetic pathway holds roots of domestication The wild cucumber is a spiky, bitter relative of what we now grow in our gardens. The bitterness comes from cucurbitacin, which helps the plant to ward off herbivores. Cucurbitacin is also useful to people for its anti-tumor properties. Shang et al. have now worked out the biosynthetic pathway of cucurbitacin. Along the way, they discovered genetic traces of the domestication process and unraveled the mystery of why some cucumbers, if grown in chilly conditions, become bitter. Science, this issue p. 1084