Supplemental red and blue light irradiation positively affects
anthocyanin biosynthesis and accumulation
Red and blue wavelengths have optimal chlorophyll absorption and
photosynthetic efficiency (Massa et al., 2008), and both appear to be
effective in promoting anthocyanin biosynthesis in various horticultural
crops (Bian et al., 2014; Zhang et al., 2018). Our results showed that
both the light treatments up-regulated all the anthocyanin biosynthetic
genes in bilberry fruit, including all the bottleneck flavonoid
biosynthetic genes CHS, F3H and ANS described inVaccinium fruits (Primetta et al., 2015; Zorenc et al., 2017;
Günther et al., 2020). This led to the higher accumulation of
anthocyanins under both red and blue light treatments compared to
control fruits. Furthermore, the expression level of UFGT , the
last gene in the anthocyanin pathway, and F3’5’H , the gene
directing dihydroflavonol precursors to delphinidin biosynthesis, were
found to be highly up-regulated under the red light treatment. This
changed the anthocyanin profile towards delphinidin glycosides in fully
ripe berries under red light. Our results suggest that UFGT andF3’5’H are separately regulated and highly responsive to red
light in ripening bilberry fruit. In climacteric fruits, studies have
shown blue light to influence anthocyanin biosynthesis as shown in pear
(Tao et al., 2018). Whereas, in comparison with non-climacteric fruits,
a recent study in strawberry showed that both red and blue light were
able to increase gene expression levels of flavonoid biosynthetic
pathway with red light found to be slightly more effective in inducing
anthocyanin accumulation (Zhang et al., 2018).
Anthocyanin biosynthesis is directly regulated by the MBW complex andR2R3 MYB expression and to some extend bHLH expression. A
high number of DEGs representing MYB and bHLH genes was
found from our RNA-Seq libraries indicating that their regulation is
strongly influenced by the light spectral quality. MYBA- type TFs
have been identified as the key regulators activating anthocyanin
biosynthesis in fruits including Vaccinium berries (Plunkett et
al., 2018; Die et al., 2020). Also, MYBPA1 has been suggested to
have a regulatory role in anthocyanin biosynthesis based on our previous
studies (Primetta et al. 2015; Günther et al., 2020; Karppinen et al.,
2021 unpublished). In this study, both MYBA1 and MYBPA1were up-regulated by red and blue light. Especially MYBPA1 was
induced by red light, which has been recently suggested as one of the
key genes in regulating delphinidin branch during bilberry ripening
(Karppinen et al., 2021 unpublished). Other bilberry MYB sequences
homologous to related MYBPA2 , MYB5 and MYBC2 genes
were also found among top 500 DEGs which are likely to be involved in
regulatory mechanisms during ripening and flavonoid biosynthesis. Some
of the bHLH TF sequences which were up-regulated in both red and blue
light treatments, such as bHLH79 have been previously reported in
grapes towards light induced anthocyanin biosynthesis (Ma et al., 2019).
Also, bHLH6 (MYC2 ) transcription factor is reported to be
commonly involved in light and ABA signaling pathways (Yadav et al.,
2005).