Decreased Protein Abundance of Lycopene -Cyclase Contributes to Red Flesh in Domesticated Watermelon.

Red-fleshed watermelons () that accumulate lycopene in their flesh cells have been selected and domesticated from their pale-fleshed ancestors. However, the molecular basis of this trait remains poorly understood. Using map-based cloning and transgenic analysis, we identified a lycopene β-cyclase () gene that controls the flesh color of watermelon. Down-regulation of caused the flesh color to change from pale yellow to red, and overexpression in the red-fleshed line caused the flesh color to change to orange. Analysis of single-nucleotide polymorphisms using 211 watermelon accessions with different flesh colors revealed that two missense mutations between three haplotypes ( , , and ) were selected and largely fixed in domesticated watermelon. Proteins derived from these three haplotypes were localized in plastids to catalyze the conversion of lycopene to β-carotene and showed similar catalytic abilities. We revealed that ClLCYB protein abundance, instead of transcript level, was negatively correlated with lycopene accumulation. Different amounts of ClLCYB protein degradation among the haplotypes were found in ClLCYB transgenic Arabidopsis () lines. After treatment with the proteasome inhibitor MG132, the concentration of ClLCYB increased noticeably compared with other ClLCYB proteins. These results indicate that natural missense mutations within influence ClLCYB protein abundance and have contributed to the development of red flesh color in domesticated watermelon.