Plastid diversity and chromoplast biogenesis in differently coloured carrots: role of the DcOR3 gene.

Distinct plastid types and ultrastructural changes are associated with differences in carotenoid pigment profiles in differently coloured carrots, and a variant of the OR gene, DcOR3 is vital for chromoplast biogenesis. Accumulation of different types and amounts of carotenoids in carrots impart different colours to their taproots. In this study, the carotenoid pigment profiles, morphology, and ultrastructure of plastids in 25 carrot varieties with orange, red, yellow, or white taproots were investigated by ultra-high performance liquid chromatography as well as light and transmission electron microscopy. α-/β-Carotene and lycopene were identified as colour-determining carotenoids in orange and red carrots, respectively. In contrast, lutein was identified as the colour-determining carotenoid in almost all tested yellow and white carrots. The latter contained only trace amounts of lutein as a unique detectable carotenoid. Striking differences in plastid types that coincided with distinct carotenoid profiles were observed among the differently coloured carrots. Microscopic analysis of the different carotenoid pigment-loaded plastids revealed abundant crystalloid chromoplasts in the orange and red carrots, whereas amyloplasts were dominant in most of the yellow and white carrots, except for the yellow carrot 'Yellow Stone', where yellow chromoplasts were observed. Plastoglobuli and crystal remnants, the carotenoid sequestering substructures, were identified in crystalloid chromoplasts. Crystal remnants were often associated with a characteristic undulated internal membrane in orange carrots or several undulated membranes in red carrots. No crystal remnants, but some plastoglobuli, were observed in the plastids of all tested yellow and white carrots. In addition, the presence of chromoplast in carrot taproots was found to be associated with DcOR3, a natural variant of DcOR3, which was previously reported to be co-segregated with carotene content in carrots. Knocking out DcOR3 in the orange carrot 'Kurodagosun' depressed chromoplast biogenesis and led to the generation of yellow carrots. Our results support that DcOR3 is vital but insufficient for chromoplasts biogenesis in carrots, and add to the understanding of the formation of chromoplasts in carrots.