Abstract
Highbush blueberry (Vaccinium corymbosum) fruits contain substantial quantities of flavonoids, which are implicated in a wide range of health benefits. Although the flavonoid constituents of ripe blueberries are known, the molecular genetics underlying their biosynthesis, localization, and changes that occur during development have not been investigated. Two expressed sequence tag libraries from ripening blueberry fruit were constructed as a resource for gene identification and quantitative real-time reverse transcription-polymerase chain reaction primer design. Gene expression profiling by quantitative real-time reverse transcription-polymerase chain reaction showed that flavonoid biosynthetic transcript abundance followed a tightly regulated biphasic pattern, and transcript profiles were consistent with the abundance of the three major classes of flavonoids. Proanthocyanidins (PAs) and corresponding biosynthetic transcripts encoding anthocyanidin reductase and leucoanthocyanidin reductase were most concentrated in young fruit and localized predominantly to the inner fruit tissue containing the seeds and placentae. Mean PA polymer length was seven to 8.5 subunits, linked predominantly via B-type linkages, and was relatively constant throughout development. Flavonol accumulation and localization patterns were similar to those of the PAs, and the B-ring hydroxylation pattern of both was correlated with flavonoid-3'-hydroxylase transcript abundance. By contrast, anthocyanins accumulated late in maturation, which coincided with a peak in flavonoid-3-O-glycosyltransferase and flavonoid-3'5'-hydroxylase transcripts. Transcripts of VcMYBPA1, which likely encodes an R2R3-MYB transcriptional regulator of PA synthesis, were prominent in both phases of development. Furthermore, the initiation of ripening was accompanied by a substantial rise in abscisic acid, a growth regulator that may be an important component of the ripening process and contribute to the regulation of blueberry flavonoid biosynthesis.
Highlights
Highbush blueberry (Vaccinium corymbosum) fruits contain substantial quantities of flavonoids, which are implicated in a wide range of health benefits
Flavonols are thought to function as protective chemicals against UV-B light in fruit skin (Solovchenko and Schmitz-Eiberger, 2003), but they can be found in the seed coat (Lepiniec et al, 2006)
Fruit set and ripening initiation in ‘Rubel’ is temperature dependent and asynchronous; fruits were harvested in batches during the two main phases of the growing season and sorted into maturation classes by size and fruit color (Fig. 2A)
Summary
Highbush blueberry (Vaccinium corymbosum) fruits contain substantial quantities of flavonoids, which are implicated in a wide range of health benefits. Flavonoid synthesis in plants is typically controlled by the tissue-specific expression of transcription factors belonging to the R2R3 MYB, basic helix-loop-helix (bHLH), and WD-repeat protein families (Lepiniec et al, 2006). These physically interact as a complex, and such MBW complexes have been shown to be responsible for the regulation of anthocyanin, PA, and flavonol biosynthesis in a variety of species and tissues, including flowers and fruits (Allan et al, 2008; Dubos et al, 2010). There is a need to extend studies of flavonoid gene regulation to other species of fruit
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