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biosynthesis based on our previous studies (Primetta et al. 2015; Günther et al., 2020; Karppinen et al., 2021 unpublished).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).