Molecular Mechanisms Underlying Fading of the Reddish Color in the Peel of Yamashitabeni-wase (Citrus unshiu Marcow.)

The orange-reddish peel color is an important quality attribute of Yamashitabeni-wase fruit ( Citrus unshiu Marcow.). However, this color begins to fade in January, reducing consumer acceptance and preference for the fruit. To elucidate the molecular mechanisms on fading of the reddish color, changes in the carotenoid metabolism were investigated in the present study. The results showed that the content of β-citraurin, a red apocarotenoid, significantly decreased in January, which was well consistent with the fading of reddish color in the flavedo. Moreover, a decrease in temperature to below 5 °C in January markedly reduced the expression of the β-citraurin biosynthetic gene CitCCD4 . In addition, we investigated the catabolism of β-citraurin and found that β-citraurin was degraded by the CitCCD1 enzyme, producing 3-hydroxy-β-ionone and C17 dialdehyde in vitro. In this study, a complete metabolic pathway of β-citraurin in citrus fruit was proposed for the first time, which contributed to a better understanding of β-citraurin accumulation and peel coloration in citrus fruit.