Abstract
An unbalanced pigment distribution among the sepal and petal segments results in various colour patterns of orchid flowers. Here, we explored this type of mechanism of colour pattern formation in flowers of the Cattleya hybrid ‘KOVA’. Our study showed that pigment accumulation displayed obvious spatiotemporal specificity in the flowers and was likely regulated by three R2R3-MYB transcription factors. Before flowering, RcPAP1 was specifically expressed in the epichile to activate the anthocyanin biosynthesis pathway, which caused substantial cyanin accumulation and resulted in a purple-red colour. After flowering, the expression of RcPAP2 resulted in a low level of cyanin accumulation in the perianths and a pale pink colour, whereas RcPCP1 was expressed only in the hypochile, where it promoted α-carotene and lutein accumulation and resulted in a yellow colour. Additionally, we propose that the spatiotemporal expression of different combinations of AP3- and AGL6-like genes might participate in KOVA flower colour pattern formation.
Highlights
An unbalanced pigment distribution among the sepal and petal segments results in various colour patterns of orchid flowers
The analysis of morphological anatomy showed that no red or yellow colouration appeared in the perianths and lip before floral development stage 2 (D2; Fig. 1c)
The total carotenoid, anthocyanin, and chlorophyll contents decreased in all flower segments from D7 to D8. These results suggested that cyanins produced via the anthocyanin biosynthesis pathway (ABP) and lutein and α-carotene synthesised via the carotenoid biosynthesis pathway (CBP) contributed to the coloration of KOVA flowers ranging from purple-red to yellow, while chlorophyll provided green colouration
Summary
An unbalanced pigment distribution among the sepal and petal segments results in various colour patterns of orchid flowers. We propose that the spatiotemporal expression of different combinations of AP3- and AGL6-like genes might participate in KOVA flower colour pattern formation. Orchids are exceptional in that the sepals and the lip are usually as colourful as the petals, which results in an unbalanced pigment distribution among different segments of the perianths and lip to show various flower colour patterns. In Orchidaceae, some R2R3-MYBs have been verified to activate ABP structural genes to promote anthocyanin accumulation in Phalaenopsis[5], Oncidium[14] and Dendrobium[15] flowers Carotenoids are another important type of pigment that contributes to colouration ranging from yellow to red in plant tissues[16,17]. We propose that AP3- and AGL6-like genes might participate in colour differentiation in the perianth and lip segments based on the analysis of transcriptome data and a peloric mutant of KOVA
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