Characterization of anthocyanin and proanthocyanidin biosynthesis in two strawberry genotypes during fruit development in response to different light qualities
Introduction
The cultivated strawberry (Fragaria × ananassa) is one of the most prevalent and economically-valuable berries in the fruit crop market worldwide for its outstanding taste and attractive appearance [1, 2]. Moreover, bioactive compounds, such as flavonols, anthocyanins, flavones, proanthocyanidins (PAs) and other polyphenols in relatively high amounts in strawberry have received increasing attention from researchers, consumers and food manufacturers due to their antioxidant properties [3]. Experimental studies have showed it will help to reduce the risk of several diseases, such as cancer, cardiovascular, neurodegenerative, and other chronic pathologies, if people take in enough bioactive compounds [4]. Anthocyanins and proanthocyanidins are well-known to impact these quality traits of strawberry. The presence of anthocyanins mainly produced at the late stages of fruit ripening contributes to berry coloration. Chemical analyses showed that ripe strawberries contain high amounts of anthocyanins, but less than a large pool of PAs [5, 6], although PAs was decreasing significantly during fruit ripening. In addition, concentration of PAs in strawberry fruits was demonstrated to depend on genotype, indicating the important role played by the genetic background in PAs biosynthetic pathway [7, 8].
The formation of anthocyanins and proanthocyanidins starts with the phenylpropanoid pathway from phenylalanine, which is converted to chalcone by a series of enzymatic reactions sequentially catalyzed by phenylalanine ammonia-lyase (PAL), cinnamate 4-hydroxylase (C4H), 4-coumarate: CoA ligase (4CL), and chalcone synthase (CHS). Chalcone is the first intermediate product in the flavonoid biosynthetic pathway [9, 10]. The branch of the anthocyanin and PA biosynthesis exists in downstream of the flavonoid biosynthetic pathway, mainly concerning dihydroflavonol reductase (DFR), anthocyanidin synthase (ANS, also known as LDOX), and UDP-glucose flavonoid 3-Oglucosyl transferase (UFGT) which are responsible for anthocyanin synthesis, as well as leucoanthocyanidin reductase (LAR) and anthocyanidin reductase (ANR) which are critical in control of PAs production [11]; These enzymes are encoded by the late biosynthetic genes (LBGs) of flavonoid pathway, which are regulated by the MYB-bHLH-WD40 (MBW) ternary transcriptional complex consisting of three classes of regulatory proteins, including R2R3-MYBs, bHLH (basic helix–loop–helix) and WD40 repeat protein [10]. In strawberry, the structural genes have been well-identified and described, but the information of MBW members involved in the control of anthocyanin and PA biosynthesis is limited. It has been documented that FaMYB1 acted as a transcriptional repressor in manipulating the biosynthesis of anthocyanin at the branching-point of anthocyanin/proanthocyanidin biosynthesis in the strawberry fruit [12, 13], whereas FaMYB10 was a strong positive regulatory transcription factor in anthocyanin biosynthesis [14, 15]. FaTTG1, FabHLH3, and FaMYB9/FaMYB11 were required for proanthocyanidin biosynthesis in strawberry fruits and identified to be the respective functional homologues of AtTTG1, AtTT8, and AtTT2 [7].
Multiple factors have manifest influence on the accumulation of anthocyanins and PAs, including exogenous stimuli, climatic condition, plant variety, tillage practice and field property [16]. Light (photoperiod, light quality and light intensity), one of the key environmental elements that affect the growth and development of plants, plays a pivotal role in anthocyanin and PA accumulation, although the quantity and composition of these compounds are strongly determined by genotypes [[17], [18], [19], [20]]. The main pattern of Chinese strawberry cultivation is protected culture which accounts for more than 90% of strawberry acreage, and the protected-cultivation facilities are divided into greenhouse, small tunnel and high tunnel of three types according to different climatic conditions [21]. Therefore, supplementary lighting is indispensable for inadequate natural light and negative influence of cladding (plastic film, glass, insect-proof net, temporal coating) on light quality and light intensity during strawberry protected cultivation. Conventional artificial light sources used in protected facilities usually include high-pressure sodium lamps, metal halide lamps, incandescent lamps, and fluorescent lamps, all of which have wide ranges of wavelengths and other certain disadvantages [22]. Unlike conventional light sources, light-emitting diodes (LEDs) can provide narrowly-centered spectrum with the properties of small size, lightweight, longevity, low energy consumption and heat output [23]. In addition, most studies regarding effects of light spectra on the phytochemical accumulation in plants have mainly focused on blue and red light, because they are more important to drive photosynthesis than other light quality [24].
Here, the influences of red and blue light on anthocyanin and PA content and related gene expression were examined to further understand the regulation of anthocyanins and PAs in strawberry by light quality, which provided more information for improving cultivation techniques and fruit quality.
Section snippets
Plant Materials and Experimental Treatments
Two strawberry genotypes, ‘Toyonaka’ (8x) and ‘Tokun’ (10x) used in this experiment have the bright red and pale yellowish-orange fruit color, respectively. ‘Tokun’ was created from a crossing between two decaploid hybrids, ‘Kurume IH No. 1’ and ‘K58N7‐21’ [25]. The potted strawberries ‘Toyonaka’ and ‘Tokun’ at the 7 days after flowering (7 DAF) were divided into four groups, respectively and transferred into assigned growth chambers with controlled environmental conditions (16 h photoperiod
Anthocyanin and Proanthocyanidin Contents in ‘Toyonaka’ and ‘Tokun’
The change trend of anthocyanin and proanthocyanidin amounts showed strongly consistent developmental patterns, generally increasing anthocyanin and decreasing proanthocyanidin as the fruit developing (Fig. 1, Fig. 2). In ‘Toyonaka’, there was no detectable accumulation of anthocyanin before 21 DAF. At 25 DAF, anthocyanin accumulated and the content in fruit treated by RL, BL and RBL were significant higher than WL (BL > RL > RBL > WL). At 28 DAF, the highest content of anthocyanin was detected
Discussion
Several studies have shown that flavonoid biosynthesis and accumulation in plant are significantly affected by agricultural practices, environmental condition, genotype, biotic and abiotic stress [31]. Light as the unique photosynthetic energy source and a vital environmental signal, has occupied irreplaceably- important position in driving photosynthetic biosynthesis and photomorphogenesis in plant [32]. Therefore, reasonable and suitable light conditions (light intensity, light quality and
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Conflicts of Interest
The authors declare that they have no conflict of interest.
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