Research progress in the regulation of β-citraurin accumulation in citrus fruits

Abstract：Color is an important attribute for external fruit quality and carotenoids are responsible for the color of some fruits including citrus. β-citraurin is a red carotenoid, and its presence in citrus fruits plays an important role in generating the red fruit color. In this paper, we review the progress in research on the accumulation and regulation of β-citraurin production in citrus fruits. β-citraurin is a C30 carotenoid, which is different from other common plant C40 carotenoids. It was first reported in 1936 in the peel of an orange and thereafter in various other citrus fruits. β-citraurin contains nine conjugated C=C bonds and a C=O group in its molecule, which is responsible for producing the red color of this pigment, with a maximum absorption around 456 to 462 nm. Chromatographically, β-citraurin Rf (for TLC) or retention time (for HPLC) are close to some xanthophylls such as 9-cis-violaxanthin, a predominant carotenoid in citrus peel, and therefore the presence of β-citraurin should be carefully evaluated during analysis to avoid the overlap of HPLC peaks from other compounds. β-citraurin has been reported in various citrus, especially sweet oranges, clementines and mandarins, and can sometimes account for as much as 40% of total carotenoids. However, it is primarily restricted to some citrus and its peel tissues and accumulation is rarely reported in other plants apart from genus Citrus. β-citraurin is generated from the oxidative cleavage of two common C40 carotenoids, β-cryptoxanthin and zeaxanthin, catalyzed by carotenoid cleavage dioxygenase4 (CCD4) , as reported by two recent studies in clementine and Satsuma mandarin. CCD4 is a member of the CCD/NCED (9-cis-epoxycarotenoid dioxygenase) family, which encodes enzymes catalyzing oxidative cleavage of C40 carotenoids. In Arabidopsis, nine CCD/NCEDs have been identified according to their cleavage positions and substrates. CCD1 can result in the formation of aromatic apocarotenoids; CCD7 and CCD8 are involved in the generation of the apocarotenoid hormone strigolactones; NCEDs (NCED2, NCED3, NCED5, NCED6, NCED9) partici-pate in ABA biosynthesis. The CCD4 enzymes catalyze the degradation of carotenoids, which is the same as CCD1, but are targeted to plastids rather than cytosol as with CCD1. Previously CCD4 has been reported to be responsible for the white petal color of a chrysanthemum, the white appearance of a potato tuber and the fruit flesh of various peaches where the symmetrical cleavage of the carotenoids at positions 9, 10 and 9', 10'results in production of only colorless apocarotenoids. In citrus, however, the cleavage catalyzed by CCD4 occurs asymmetrically at either positions 7, 8 or 7', 8'and therefore produces a C30 apocarotenoid, i.e., β-citraurin. Only one CCD4 is present in Arabidopsis but there are five in citrus with different expression profiles. Among them, CCD4 b1 has the strongest expression in the peel and was confirmed to be the member responsible for the biosynthesis of β-citraurin. The accumulation ofβ-citraurin is influenced by various intrinsic or external factors. First, accumulation starts only when the fruit is mature and approaching the color turning stage; secondly, the accumulation is favored by moderate temperatures from 12 to 24 ℃, while is inhibited under either a higher temperature (around 30 ℃) or lower temperature (around 5 ℃) ; finally, ethylene promotes the biosynthesis of β-citraurin while GA3 inhibits its production. In summary, progress in understanding the profiles and underlying mechanisms of accumulation and regulation of β-citraurin in citrus fruits has been recently achieved. Further efforts should be focused on the mechanisms for the unique presence of this pigment in only some citrus species and cultivars and only in peel tissues, and on how the expression of CCD4 b1 is regulated.