Anthocyanin biosynthesis and accumulation in developing flowers of tree peony (Paeonia suffruticosa) ‘Luoyang Hong’

Highlights

• In-vase P. suffruticosa ‘Luoyang Hong’ flowers show color fading instead of natural flower color.

• Anthocyanins content is lower in the petals of in-vase developing flowers.

• Low expression of some anthocyanin biosynthetic genes may be related with color fading.

Abstract

Tree peony (Paeonia suffruticosa) ‘Luoyang Hong’, a typical purplish red-flowered cultivar, is particularly appreciated by Chinese among the hundreds of cultivars. However, in-vase ‘Luoyang Hong’ flowers do not exhibit natural flower color as shown by on-tree flowers, since color fading of in-vase flowers is easily observed with the naked eye. This study was carried out to reveal the underlying molecular mechanism of color fading of in-vase P. suffruticosa ‘Luoyang Hong’. Five regulatory genes and six structural genes involved in anthocyanin biosynthesis were isolated based on a transcriptome library from P. suffruticosa ‘Luoyang Hong’ petals. Compared with fully opened on-tree flowers in field conditions, in-vase flowers (vase condition of 21–23 °C, 50–60% RH and a 16 h photoperiod with an illumination of ∼40 μmol m⁻² s⁻¹) of P. suffruticosa ‘Luoyang Hong’ at full opening stage show significant color fading with 50.20% greater lightness, 37.43% less redness and 36.18% lower chroma in non-blotch parts of the middle petals (the 4th to 6th layer petal). From pre-opening to full opening stage, anthocyanin contents in inner, middle and outer petals of in-vase flowers was 24.49–38.75% lower than those in on-tree developing flowers. With the exception of PsCHI1, another ten anthocyanin biosynthetic genes were preferentially and highly expressed in floral tissues (sepals, petals, stamens or carpels). Temporal expression analysis of anthocyanin biosynthetic genes in on-tree and in-vase developing flowers revealed that only the expression pattern of PsF3′H1 was correlated with anthocyanin content development. On the whole, PsbHLH3, PsWD40-1, PsWD40-2, PsMYB2, PsCHS1, PsF3H1 and PsDFR1 of P. suffruticosa ‘Luoyang Hong’ showed lower transcript abundances in in-vase flowers than in on-tree flowers, suggesting that the low expression of these genes is probably responsible for lower anthocyanin contents, resulting in the color fading in the petals in in-vase flowers.

Introduction

Flower color is an important ornamental trait for ornamental flowering plants. Flower coloration is caused by the accumulation of pigments, including flavonoids, carotenoids and betacyanins within the epidermal cells (Mol et al., 1998). Anthocyanins, a class of flavonoids, are the major coloring factors in many plants (Holton and Cornish, 1995, Winkel-Shirley, 2001a, Winkel-Shirley, 2001b, Davies et al., 2012), and anthocyanin biosynthesis needs two groups of genes: structural genes encoding anthocyanin biosynthesis enzymes which directly participate in the biosynthetic reactions, and regulatory genes controlling the expression of structural genes (Forkmann, 1991, Holton and Cornish, 1995, Winkel-Shirley, 2001a, Matus et al., 2010, Feller et al., 2011, Hichri et al., 2011, Davies et al., 2012).

Tree peony (Paeonia suffruticosa Andr.), belonging to the Paeoniaceae family, is a very popular Chinese traditional flower that has high esthetic value and unique culture symbolization (Li, 1999). It has more than 1500 years’ history of cultivation, and cultivars with various flower colors have been produced by natural selection and conventional breeding. The composition and quantity of petal anthocyanins have been determined for different groups and several wild species of Chinese tree peony (Wang et al., 2001, Wang et al., 2004, Zhang et al., 2007). Previous studies showed that the six major anthocyanins in tree peony are pelargonidin-3-O-glucoside, pelargonidin-3,5-di-O-glucoside, cyanidin-3-O-glucoside, cyanidin-3,5-di-O-glucoside, peonidin-3-O-glucoside and peonidin-3,5-di-O-glucoside (Hosoki et al., 1991, Wang et al., 2001, Wang et al., 2004), but not delphinidin.

Based on the anthocyanin biosynthetic pathway which is well characterized in the model plants maize (Zea mays), snapdragon (Antirrhinum majus) and petunia (Petunia hybrida) (Forkmann, 1991, Holton and Cornish, 1995, Winkel-Shirley, 2001a), putative anthocyanin biosynthetic pathway without the delphinidin-producing branch, is predicted in P. suffruticosa as that in Paeonia lactiflora (Zhao et al., 2012). Six key enzymes are responsible for pelargonidin and cyanidin biosynthesis in the anthocyanin biosynthetic pathway: chalcone synthase (CHS), chalcone isomerase (CHI), flavanone 3-hydroxylase (F3H), flavonoid 3′-hydroxylase (F3′H), dihydroflavonol 4-reductase (DFR), and anthocyanidin synthase (ANS). UDP-glucoside: flavonoid glucosyltransferase (UFGT) and methyl transferase (MT) further catalyze the formation of complex aglycone and anthocyanin composition. The anthocyanin biosynthetic pathway has been well studied in some ornamental plants, such as A. majus (Jackson et al., 1992), Eustoma grandiflorum (Noda et al., 2004) and P. lactiflora (Zhao et al., 2012), but the information on anthocyanin biosynthesis in P. suffruticosa is scarce at present, with only some studies done on the isolation and functional analysis of CHS, CHI and DFR genes in tree peony (Zhou et al., 2010b, Zhou et al., 2011a, Zhou et al., 2011b, Zhou et al., 2014).

‘Luoyang Hong’, a typical purplish red-flowered cultivar of Chinese traditional P. suffruticosa cultivars, is particularly appreciated by Chinese and is considered the first choice for cut flower industry because of the better postharvest quality (Jia et al., 2008). However, previously we found that visible color fading (lighter and paler color, Griesbach, 1992) was observed in the in-vase developing ‘Luoyang Hong’ flowers, i.e. cut flowers developing in the vase after harvest. Since attractive flower color is the most vital postharvest characteristic, color fading impairs the ornamental quality of cut P. suffruticosa ‘Luoyang Hong’. Our present study was constructed to reveal the underlying molecular mechanism of color fading of in-vase P. suffruticosa ‘Luoyang Hong’. For this purpose, petal color and anthocyanin accumulation of on-tree and in-vase developing flowers were measured, and the expression patterns of eleven anthocyanin biosynthetic genes in the middle petals of on-tree and in-vase developing flowers, as well as their expression in different tissues (carpels, stamens, petals, sepals, stems, leaves and roots), were investigated.