Research articlePhenylpropanoid metabolites and expression of key genes involved in anthocyanin biosynthesis in the shaded peel of apple fruit in response to sun exposure
Introduction
Anthocyanins are a group of phenolic compounds derived from the phenylpropanoid pathway. Other phenolic compounds derived from other branches of the same pathway include phenolic acids, flavonols, dihydrochalcones, and flavanols. All these phenolic compounds are considered as main factors contributing to the internal and external qualities of apple fruit [1]. Generally, apple peel has much higher concentrations of polyphenols than the flesh [2], [3], [4]. Cyanidin 3-galactoside is the main anthocyanin in red apple peel [5]. In addition to aiding seed dispersal [6], anthocyanins play a role in protecting fruit from photooxidative damage [7], [8], and other environmental stress such as high temperature under high light [9]. From a human nutrition perspective, polyphenolics in apple have been demonstrated to have the potential to improve human health and reduce risks of many diseases, including cancer, cardiovascular disease, diabetes, pulmonary disorders, Alzheimer's disease, and other degenerative diseases [10], [11].
The biosynthetic pathway for anthocyanins has been studied extensively. Phenylalanine is the precursor for the synthesis of many polyphenols, and is enzymatically converted to anthocyanins via many steps. The enzymes involved in this pathway include phenylalanine ammonialyase (PAL, EC: 4.3.1.24), chalcone synthase (CHS, EC: 2.3.1.74), chalcone isomerase (CHI, EC: 5.5.1.6), flavanone 3-hydroxylase (F3H, EC: 1.14.11.19), dihydroflavonol 4-reductase (DFR, EC: 1.1.1.219), leucoanthocyanidin dioxygenase (LDOX, EC: 1.14.11.19) and UDP-glucose: flavonoid 3-O-glucosyltransferase (UFGT, EC: 2.4.1.91) [12]. As a transcriptional factor involved in the regulation of anthocyanin biosynthesis, MdMYB10, and its allelic genes, MdMYB1 and MdMYBA controls apple red color development via regulating the expression of structural genes in anthocyanin biosynthesis [13], [14], [15], [16]. The expression levels of these biosynthetic genes correspond to anthocyanin concentrations and fruit coloration in apple fruit, and the transcript levels of these genes in ‘Orin’, a yellow cultivar, were less than those in ‘Fuji’ and ‘Jonathan’, two red cultivars [17]. Apple skin patterning is also associated with differential expression of MYB10. Both ‘Honeycrisp’ and ‘Royal Gala’ had higher mRNA levels of MYB10 and biosynthetic genes CHS, CHI, F3H, DFR1, LDOX, and UFGT in red stripes compared to green stripes [18].
The dependence of apple coloration on light was recognized a long time ago [19], [20]. It is known that fruit from the top of the tree canopy have higher levels of cyanidin 3-galactoside and quercetin 3-glycosides than those from the canopy interior [21]. For varieties that do not typically develop red color (yellow/green varieties), bagging fruit first for a period of time, and then removing the bag can induce red color development [22], [23]. Only until recently, however, did the underlying molecular mechanism on light regulation of anthocyanin synthesis begin to be understood. MYB transcriptional factors (such as MdMYB1 and MdMYBA) are induced by light [16], [24], [25], and the stability of MdMYB1 is regulated by MdCOPs in response to light [26]. The expression of the down-stream anthocyanin biosynthetic genes is responsive to light [24], [27]. Fruit bagging and debagging has been used to study the responses of key genes involved in anthocyanin synthesis to light, but bagging fruit represents more of an extreme situation where fruit receives very little light. It remains unclear how the expression of the key genes in anthocyanin synthesis in the peel of apple fruit that have been adapted to a low level of light in the canopy respond to sudden exposure to full sun. This sudden change of light exposure happens when the gain of fruit weight on a branch alters the orientation of the branch or summer pruning opens up the tree canopy. In addition, very few studies have looked at how other classes of polyphenolics, in addition to anthocyanins, in apple peel respond to changes in light exposure.
Apple fruit has a sun-exposed side and a shaded side due to self-shading during fruit development. Compared with the sun-exposed side, the shaded side of apple fruit has lower xanthophyll cycle-dependent thermal dissipation and antioxidants of the ascorbate–glutathione pathway [28]. Exposure of the shaded side to full sun leads to up-regulation of both the xanthophyll cycle and the ascorbate–glutathione cycle [29]. This was accompanied by higher anthocyanin levels in the peel, suggesting anthocyanin synthesis is up-regulated. It appears that exposing the shaded peel to full sun by fruit turning provides a highly relevant experimental system for understanding the effects of light exposure on the synthesis of anthocyanins and other phenolic compounds. In addition, quantifying the effects of fruit turning on the levels of individual phenolics and total phenolics in apple peel may allow us to develop a new way of improving apple fruit quality in terms of total polyphenolic content and coloration for maximizing health benefits and consumer appeal of apple. In this study, we used two apple cultivars, red ‘Fortune’ and yellow/green ‘Mutsu’ to determine the expression levels of key genes involved in anthocyanin biosynthesis in the shaded peel of apple fruit in response to full sun exposure, and their relationships with the levels of anthocyanins and other phenolic compounds.
Section snippets
Expression of key genes in anthocyanin biosynthesis
For the control fruit at 0 h, the transcript levels of MdMYB10, MdPAL, MdCHS, MdCHI, MdF3H, MdDFR1, MdLDOX and MdUFGT were much lower in the shaded peel than those in the sun-exposed peel in both ‘Fortune’ and ‘Mutsu’ (Table 1). The transcript levels of MdMYB10 in the shaded peel were about 1/11 and 1/17 of those in the sun-exposed peel in ‘Fortune’ and ‘Mutsu’, respectively (Table 1). Comparing the two cultivars at 0 h, the transcript levels of MdMYB10 in the sun-exposed peel and the shaded
Discussion
Our results show that, in response to sunlight exposure, the expression levels of MdMYB10 and seven structural genes (MdPAL, MdCHS, MdCHI, MdF3H, MdDFR1, MdLDOX and MdUFGT) involved in anthocyanin synthesis were markedly up-regulated in the original shaded peel of both ‘Fortune’ and ‘Mutsu’ apple fruit, followed by red color development and corresponding increases in the level of cyanidin-3-galactoside (Figs. 1 and 2). These findings are consistent with the results obtained on other apple
Plant material and sampling
Two apple (Malus domestica Borkh.) cultivars, ‘Fortune’, a red apple, and ‘Mutsu’, a yellow/green apple were used in this study. They were on M.9 rootstocks and planted at a spacing of 1.8 × 4.2 m, and 2.1 × 4.2 m, respectively, at Cornell Orchards in Ithaca (42°26′N, 76°29′W; elevation 500 m), New York, USA. The trees were 15- and 14-year old, respectively, and trained as a central leader system. They received standard horticultural practices, and pest and disease control. These two cultivars
Acknowledgments
The authors would like to thank Mr. Richard Raba and Dr. Zhenchang Liang for technical assistance with HPLC.
References (39)
- et al.
Cultivar variation in apple peel and whole fruit phenolic composition
Sci. Hort.
(2009) - et al.
Concentrations and characteristics of procyanidins and other phenolics in apples during fruit growth
Phytochemistry
(2007) - et al.
The elevated anthocyanin level in the shaded peel of ‘Anjou’ pear enhances its tolerance to high temperature under high light
Plant Sci.
(2009) A comprehensive review of apples and apple components and their relationship to human health
Adv. Nutr.
(2011)- et al.
Anthocyanin biosynthetic genes are coordinately expressed during red coloration in apple skin
Plant Physiol. Biochem.
(2002) - et al.
Effects of light on flavonoid and chlorogenic acid levels in the skin of ‘Jonagold’ apples
Sci. Hort.
(2001) - et al.
MYB transcription factors that colour our fruit
Trends Plant Sci.
(2008) - et al.
The sun-exposed peel of apple fruit has higher xanthophyll cycle-dependent thermal dissipation and antioxidants of the ascorbate-glutathione pathway than the shaded peel
Plant Sci.
(2003) - et al.
Exposure of the shaded side of apple fruit to full sun leads to up-regulation of both the xanthophyll cycle and the ascorbate-glutathione cycle
Plant Sci.
(2004) - et al.
Molecular cloning and analysis of anthocyanin biosynthesis genes preferentially expressed in apple skin
Plant Sci.
(2003)
Isolation of anthocyanin biosynthetic genes in red Chinese sand pear (Pyrus pyrifolia Nakai) and their expression as affected by organ/tissue, cultivar, bagging and fruit side
Sci. Hort.
Influence of fruit bagging on aroma volatiles and skin coloration of ‘Hakuho’ peach (Prunus persica Batsch)
Postharvest Biol. Technol.
Developmental changes of carbohydrates, organic acids, amino acids, and phenolic compounds in ‘Honeycrisp’ apple flesh
Food Chem.
Biosynthesis of phenolic compounds and its regulation in apple
Plant Growth Regul.
Activity and contents of polyphenolic antioxidants in the whole fruit, flesh and peel of three apple cultivars
Arch. Latinoam. Nutr.
Colour accumulation patterns and the anthocyanin biosynthetic pathway in ‘Red Delicious’ apple variants
J. Hort. Sci. Biotech.
The shaded side of apple fruit becomes more sensitive to photoinhibition with fruit development
Physiol. Plant
Red ‘Anjou’ pear has a higher photoprotective capacity than green ‘Anjou’
Physiol. Plant
Cited by (127)
Study on the white frost formation mechanism during storage of Phyllanthus emblica Linn. fruit based on component analysis and spatial metabolomics
2024, Journal of Pharmaceutical and Biomedical AnalysisResearch progress in understanding the biosynthesis and regulation of plant anthocyanins
2023, Scientia HorticulturaeAntioxidant response and quality of sunburn Beurré D'Anjou pears (Pyrus communis L.)
2023, Plant Physiology and BiochemistryThe relationship between canopy microclimate, fruit and seed yield, and quality in Xanthoceras sorbifolium
2023, Journal of Plant PhysiologyTranscriptomic analyses reveal light-regulated anthocyanin accumulation in ‘ZhongShan-HongYu’ grape berries
2023, Scientia HorticulturaeThe underlying molecular mechanisms of external factors influencing fruit coloration in fruit trees
2023, Scientia Horticulturae
- 1
These authors contributed equally to this work.