Abstract
BackgroundIn wine grape production, management practices have been adopted to optimize grape and wine quality attributes by producing, or screening for, berries of smaller size. Fruit size and composition are influenced by numerous factors that include both internal (e.g. berry hormone metabolism) and external (e.g. environment and cultural practices) factors. Combined physiological, biochemical, and transcriptome analyses were performed to improve our current understanding of metabolic and transcriptional pathways related to berry ripening and composition in berries of different sizes.ResultsThe comparison of berry physiology between small and large berries throughout development (from 31 to 121 days after anthesis, DAA) revealed significant differences in firmness, the rate of softening, and sugar accumulation at specific developmental stages. Small berries had significantly higher skin to berry weight ratio, lower number of seeds per berry, and higher anthocyanin concentration compared to large berries. RNA-sequencing analyses of berry skins at 47, 74, 103, and 121 DAA revealed a total of 3482 differentially expressed genes between small and large berries. Abscisic acid, auxin, and ethylene hormone pathway genes were differentially modulated between berry sizes. Fatty acid degradation and stilbenoid pathway genes were upregulated at 47 DAA while cell wall degrading and modification genes were downregulated at 74 DAA in small compared to large berries. In the late ripening stage, concerted upregulation of the general phenylpropanoid and stilbenoid pathway genes and downregulation of flavonoid pathway genes were observed in skins of small compared to large berries. Cis-regulatory element analysis of differentially expressed hormone, fruit texture, flavor, and aroma genes revealed an enrichment of specific regulatory motifs related to bZIP, bHLH, AP2/ERF, NAC, MYB, and MADS-box transcription factors.ConclusionsThe study demonstrates that physiological and compositional differences between berries of different sizes parallel transcriptome changes that involve fruit texture, flavor, and aroma pathways. These results suggest that, in addition to direct effects brought about by differences in size, key aspects involved in the regulation of ripening likely contribute to different quality profiles between small and large berries.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-016-2660-z) contains supplementary material, which is available to authorized users.
Highlights
In wine grape production, management practices have been adopted to optimize grape and wine quality attributes by producing, or screening for, berries of smaller size
Because of the observed differences between small and large berries at the physiological level and of the potential role of some of the differentially expressed (DE) genes on determining fruit and wine quality, we focused our discussion on pathways related to hormone metabolism and signaling, cell wall modifications, and flavonoid, stilbenoid, and fatty acid biosynthesis
NCED4 gene in small compared to large berries from 103 days after anthesis (DAA) onwards might reflect the lower abscisic acid (ABA) biosynthetic capacity of small compared to large berries at 103 and 121 DAA
Summary
Management practices have been adopted to optimize grape and wine quality attributes by producing, or screening for, berries of smaller size. The grape berry is extremely rich in secondary metabolites ranging from anthocyanins, carotenoids, norisoprenoids, tannins, terpenes, and other volatile organic compounds. These metabolites are very important to wine production as they affect wine quality by determining its color, aroma, and flavor [1]. The reasoning being that this higher skin to berry weight ratio results in higher concentrations of key secondary metabolites accumulated in the skin [2, 3] These practices are becoming increasingly popular and machines are even sold to automatically sort berries based on size
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