Anthocyanin Profile of Euphorbia Pulcherrima (Poinsettia) and the Effects of High Temperature on Anthocyanin Content and Dihydroflavonol 4-Reductase (DFR) Gene Expression
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2024
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Abstract
Poinsettias are a symbol of the Christmas season worldwide and the most popular holiday plant in the United States. Breeders of this commercially valuable commodity are continuously creating new and improved cultivars for the market, and the color of the bracts is one of the most important traits targeted in their breeding programs. Poinsettia growers face difficulties in production when plants do not color in time for Christmas sales, an issue that can be caused by high temperatures delaying flower initiation and bract coloration, termed heat delay. Color in poinsettia bracts is conferred by anthocyanins, a group of water-soluble flavonoids contributing to the color of a wide range of plant tissues. The goal of this research was to expand the knowledge base of anthocyanins in poinsettia bracts by examining the physical aspects of their spatial location and distribution, conducting a quantitative and developmental profile of modern cultivars including the newer E. pulcherrima × E. cornastra interspecific hybrids (EIH), investigating the role of the dihydroflavonol 4-reductase (DFR) gene in anthocyanin production, and determining how high night temperatures affect anthocyanin accumulation and DFR gene expression. Microscopic examination of the cross-sections of bracts of 11 cultivars infiltrated with polyethylene glycol revealed anthocyanin accumulation in both adaxial and abaxial epidermal layers, with some pigmentation within the mesophyll in red and pink E. pulcherrima cultivars and pink EIH. White cultivars did not show any anthocyanins in the cross-sections. Epidermal cell shape in all cultivars were domed with varying amounts of cell elongations. EIH had the most elongated epidermal cells and white E. pulcherrima cultivars had the flattest epidermal cells. Spatial location of pigmented cells in bracts of modern poinsettia and EIH cultivars has not been previously reported.
High-performance liquid chromatography (HPLC) analysis was conducted on anthocyanidins extracted and purified from poinsettia bracts to identify and quantify the pigments contributing to their color. Cyanidin and pelargonidin were identified in red, pink, and white bracts of six of the seven E. pulcherrima cultivars examined (‘Premier Red’, ‘Premier White’, ‘Premier Pink’, ‘Freedom Red’, ‘Polly’s Pink’, ‘Orange Spice’), with the total amounts of each component and the proportions of the two compounds influencing not only the color of the bracts, but the darkness/lightness of that color and where on the color spectrum it falls. Red bracts can shift towards purple or orange and pink bracts can shift towards red or orange. The genetic and breeding background of the cultivars were explored and related to the amounts and proportions measured in the different cultivars. No anthocyanidins were detected in ‘Polar Bear White’, ‘Princettia Max White’, or E. cornastra, and very small amounts of cyanidin were found in the pink EIH cultivars 'Princettia Dark Pink’ and ‘Luv U Pink’. The anthocyanin profiles of a range of modern poinsettia cultivars is reported while the main contribution to the bright pink color of EIH cultivars remains unclear.
Chlorophyll content measured using the Soil Plant Analysis Development (SPAD) meter, anthocyanin content measured using HPLC analysis, and DFR relative gene expression measured using Real-Time Quantitative PCR (qPCR) in ‘Prestige Red’ poinsettias throughout development from the beginning of inductive short-days to post-anthesis after 10 weeks were used to build a developmental profile and quantify the relationship between these parameters. The timing of DFR expression increases paralleled anthocyanidin production increases with peak DFR activity at 8 weeks and total anthocyanidin amounts reaching highest concentrations at 10 weeks. These results support previous evidence that the DFR gene is critical to anthocyanin synthesis in poinsettia bracts under short-day conditions.
Temperature effects on these same parameters were studied by applying high night temperatures of 28 °C at the start of inductive short-days to induce heat delay of flower initiation and bract coloration. Anthocyanin accumulation and DFR expression increase were slightly delayed in the high night temperature treatment plants, and the overall levels of expression were lower throughout development. However, quantity of anthocyanidins measured in the high night temperature treatment were greater than those in the moderate temperature treatment, in contrast to the DFR expression levels in the two treatments. These results imply that other factors may be involved in anthocyanin accumulation in poinsettia bracts that are more critical than DFR. Overall, the results of this research begin to illuminate the underlying molecular mechanisms of heat delay temperature conditions on anthocyanin synthesis gene expression in poinsettias.
The anthocyanin research undertaken in this study will inform breeders on how anthocyanins operate in poinsettias and assist in producing new and improved cultivars both for growers facing anthocyanin related production issues and for consumers to enjoy during the holiday season or even year-round.
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Horticulture, Plant sciences, Agriculture, anthocyanin, dihydroflavonol 4-reductase, Euphorbia pulcherrima, heat delay, poinsettia, temperature
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268 pages
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