Vaccinium as a comparative system for understanding of complex flavonoid accumulation profiles and regulation in fruit

Abstract

The genus Vaccinium L. (Ericaceae) contains premium berryfruit crops, including blueberry, cranberry, bilberry, and lingonberry. Consumption of Vaccinium berries is strongly associated with various potential health benefits, many of which are attributed to the relatively high concentrations of flavonoids, including the anthocyanins that provide the attractive red and blue berry colors. Because these phytochemicals are increasingly appealing to consumers, they have become a crop breeding target. There has been substantial recent progress in Vaccinium genomics and genetics together with new functional data on the transcriptional regulation of flavonoids. This is helping to unravel the developmental control of flavonoids and identify genetic regions and genes that can be selected for to further improve Vaccinium crops and advance our understanding of flavonoid regulation and biosynthesis across a broader range of fruit crops. In this update we consider the recent progress in understanding flavonoid regulation in fruit crops, using Vaccinium as an example and highlighting the significant gains in both genomic tools and functional analysis.

Figure 1.

Biosynthesis, inheritance, and regulation of flavonoid traits in Vaccinium.A) The biosynthetic pathway producing the major flavonoid classes reported in Vaccinium species: flavonols, PAs, and anthocyanins. Dihydroflavonols can vary in hydroxylation on the B-ring: dihydrokaemferol (DHK: 4ʹ), dihydroquercetin (DHQ: 3ʹ,4ʹ), and dihydromyricetin (DHM: 3ʹ,4ʹ,5ʹ). The hydroxylation pattern is retained by subsequent compounds in the pathway, corresponding to the 3 major anthocyanidins pelargonidin (Pel), cyanidin (Cy), and delphinidin (Del), and methylated derivatives peonidin (Peo), petunidin (Pet), and malvidin (Mal). LDOX and ANS can refer to the same gene, although some species, including Vaccinium, possess distinct LDOX and ANS genes. B) Anchored QTLs for flavonoids and color detected in blueberry and cranberry. Genomic QTL locations for anthocyanin acylation and glycosylation mapped in blueberry chr2 and chr4, respectively, across 2 mapping populations. QTL regions in blue represent those mapped in NxHP (Montanari et al. 2022), and QTLs in red are those mapped in DSxJ (Mengist et al. 2022b). Markers spanning 2-LOD intervals were used to anchor these QTLs. The sections in a grey shaded box indicate the genomic regions in chr2 (chr2: 8,005,747–11,180,545 bp) and chr4 (chr4: 55,208,167–60,978,559) where the regions spanning 2-LOD intervals for the same trait (e.g. acylated anthocyanin) overlap across the 2 mapping populations/studies. Genomic locations of QTLs for TAcy, PAC, color variation, and color intensity mapped in cranberry across 3 mapping populations and 2 genetic studies (Diaz-Garcia et al. 2018, 2021). QTL regions in blue represent those mapped in CNJ04–02, QTLs in red represent QTLs mapped in CNJ02–01, and QTLs in green represent QTLs mapped in GRYG. Markers spanning the 1.5-LOD interval were used to anchor these QTLs. The grey box sections indicate the genomic regions in chr3 (chr3: 8,700,846–12,176,781 bp, and chr3:16,922,077–20,907,119), where the region spanning the 1.5-LOD intervals for the same or related traits (e.g. TAcy and color intensity) overlap across the populations/studies. C) Flavonoid profiles change during bilberry, blueberry, and cranberry fruit development. The shift from PAs during early, immature stages are linked with SG5/MYBPA2 genes, and anthocyanin accumulation at ripening is linked with SG6/MYBA genes. MYBPA1 has a biphasic expression profile, correlating with both PAs and anthocyanins. D) Anthocyanins and PAs are regulated by MBW TF complexes, which activate flavonoid biosynthetic gene expression. The target genes regulated by MBW complexes are primarily determined by the type of R2R3-MYB. The exact stoichiometry is unknown, but MBW complexes can contain 2 MYBs via dimerized bHLH proteins. E) MBW complexes containing MYBA and MYBPA2 proteins directly regulate MYBPA1 genes, which may then be reinforced by MYBPA1 itself.