UV-B enhances hyperaccumulation of α- and β-carotene in marigold petals

Marigold (Tagetes patula L.) flowers are rich sources of carotenoids and are widely used as essential components in functional foods. Although numerous studies have examined the quantitative variations in marigold carotenoids among cultivars, carotenoid biosynthetic responses to environmental changes have not been clearly characterized. This study investigated the metabolic characteristics of marigold carotenoids in response to UV-B irradiation and identified specific patterns of carotenoid hyperaccumulation induced by UV-B exposure. Marigold petals contained about 65% carotenoids among total flower components, with lutein being the most abundant. Daily UV-B irradiation time significantly affected the content increase, indicating 20 to 40 min·day^-1 irradiation under 0.5 to 4 W m^-2 UV-B intensities. An efficient UV-B intensity was 1 W m^-2, in the aspect of carotenoid increase and physical damage of the petals. Lutein increased by about 30% after 5 d of 1 to 40 min·day^-1 irradiation under 1 W m^-2 UV-B. Notably, α- and β-carotene increased dramatically after day 5 of UV-B exposure (exceeding 20 min·day^-1) and reached the highest levels, exhibiting approximately 33-fold and 30-fold increases compared with initial petals. Gene expression analysis supported α- and β-carotene hyperaccumulation, showing that the expression levels of most upstream biosynthetic genes such as PSY, PDS, Z-ISO, CRTISO, LCY, and HYD remained high throughout 10 d UV-B treatments and those of CCDs and NCEDs were inhibited. These results suggest that UV-B can regulate carotenoid-containing flowers, producing large quantities of specific carotenoids such as α-carotene and β-carotene in addition to increasing lutein.