Apocarotenoids contribute to fruit color and aroma, which are critical quality and marketability attributes. Previously, we reported that the red peels of citrus fruits, which are characterized by higher expression levels of a carotenoid cleavage dioxygenase 4b (CitCCD4b) gene, accumulate higher levels of β-citraurin and β-citraurinene than yellow peels. Here, we identified and quantified 12 apocarotenoids, either volatile or nonvolatile, in citrus peel using liquid chromatography-mass spectrometry (LC-MS). Our results show that red peels contain also dramatically higher amounts of β-apo-8'-carotenal, crocetin dialdehyde known from saffron, β-citraurol, β-cyclocitral, and 3-OH-β-cyclocitral and up to about 17-fold higher levels of 3-OH-β-cyclocitral glucoside (picrocrocin isomer). The content of these apocarotenoids was also significantly increased in different CitCCD4b-overexpressing transgenic callus lines, compared with corresponding controls. Transient expression of CitCCD4b in Nicotiana benthamiana leaves resulted in a striking increase in the 3-OH-β-cyclocitral level and the accumulation of picrocrocin. Thus, our work reinforces the specific function of CitCCD4b in producing C10 apocarotenoid volatiles and C30 pigments in citrus peel and uncovers its involvement in the biosynthesis of picrocrocin, C20 dialdehyde, and C30 alcohol apocarotenoids, suggesting the potential of this enzyme in metabolic engineering of apocarotenoids and their derivatives.
Bibliographical noteKAUST Repository Item: Exported on 2021-02-11
Acknowledgements: This work was supported by baseline funding to S.A.-B. from King Abdullah University for Science and Technology (KAUST). The authors acknowledge the financial funding from the National Key Research and Development Program of China (2018YFD1000200) and the National Natural Science Foundation of China (Nos. 31930095, 31630065) to support X.D. of Huazhong Agricultural University (HZAU). The authors greatly appreciate Qitong Feng for his excellent technical assistance and Dr. Hendrik N.J. Kuijer, and Dr. Yagiz Alagoz for valuable comments