Crocins with High Levels of Sugar Conjugation Contribute to the Yellow Colours of Early-Spring Flowering Crocus Tepals

Abstract

Crocus sativus is the source of saffron spice, the processed stigma which accumulates glucosylated apocarotenoids known as crocins. Crocins are found in the stigmas of other Crocuses, determining the colourations observed from pale yellow to dark red. By contrast, tepals in Crocus species display a wider diversity of colours which range from purple, blue, yellow to white. In this study, we investigated whether the contribution of crocins to colour extends from stigmas to the tepals of yellow Crocus species. Tepals from seven species were analysed by UPLC-PDA and ESI-Q-TOF-MS/MS revealing for the first time the presence of highly glucosylated crocins in this tissue. β-carotene was found to be the precursor of these crocins and some of them were found to contain rhamnose, never before reported. When crocin profiles from tepals were compared with those from stigmas, clear differences were found, including the presence of new apocarotenoids in stigmas. Furthermore, each species showed a characteristic profile which was not correlated with the phylogenetic relationship among species. While gene expression analysis in tepals of genes involved in carotenoid metabolism showed that phytoene synthase was a key enzyme in apocarotenoid biosynthesis in tepals. Expression of a crocetin glucosyltransferase, previously identified in saffron, was detected in all the samples. The presence of crocins in tepals is compatible with the role of chromophores to attract pollinators. The identification of tepals as new sources of crocins is of special interest given their wide range of applications in medicine, cosmetics and colouring industries.

The carotenoid cleavage dioxygenase CCD2 catalysing the synthesis of crocetin in spring crocuses and saffron is a plastidial enzyme

“…Thus the expression profile of CaCCD2 in stigma and tepals followed the accumulation of crocins in these tissues and the gene exhibited the highest expression levels in both tissues compared with the other CCD enzymes. By contrast, CsCCD2 has been shown to be stigma‐specific (Rubio et al ., ) and this is entirely consistent with crocetin accumulation only in the stigma of saffron flowers (Rubio‐Moraga et al ., ). The expression levels of CaCCD4c and CaCCD4a/b were relatively low in both tissues at all stages analysed.…”

Carotenoid Cleavage Oxygenases from Microbes and Photosynthetic Organisms: Features and Functions

“…Both CCD2 enzymes are plastidic, a major difference with the CCD1 subfamily [118]. No other CCD2 homologues have been identified in other organisms, presumably due to the uncommon ability to synthesize crocetin in plants [120] or bacteria [121]. Besides zeaxanthin, CCD2 is able to recognize and cleave lutein and 3-OH-β-apocarotenals at the C7–C8 position, but it does not cleave β-carotene, lycopene, or β-cryptoxanthin [117].…”

Petals of Crocus sativus L. as a potential source of the antioxidants crocin and kaempferol

“…Crocins form the main pigment of C. sativus stigmas, but have also been identified in petals [22]. Since they seem to be the most abundant compounds in saffron stamens, in this study the main aim was to find whether they are also present in other parts of the flower.…”