Preharvest long-term exposure to UV-B radiation promotes fruit ripening and modifies stage-specific anthocyanin metabolism in highbush blueberry

Taishan Li,Ryutaro Tao,Hisayo Yamane

doi:10.1038/s41438-021-00503-4

Abstract

Ultraviolet-B (UV-B) light (280–315 nm) is an important environmental signal that regulates plant development and photomorphogenesis, while also affecting the flavonoid pathway, including anthocyanin biosynthesis. Regarding the effects of UV-B radiation on fruits, the effects of a short-term or postharvest irradiation on fruit quality have been well-documented, but the effects of a long-term preharvest UV-B irradiation on fruit growth and coloration remain unclear. Thus, in this study, we investigated the effects of a long-term treatment involving an environmentally relevant UV-B dose on highbush blueberry (Vaccinium corymbosum) fruit. The preharvest UV-B treatment quickly promoted fruit growth and sugar accumulation, which is not commonly observed in other fruit tree species. The UV-B exposure also accelerated fruit ripening and coloration. The dual-luciferase assay proved that in blueberries, expression of VcUFGT encoding anthocyanin biosynthesis key enzyme, is positively and negatively regulated by VcMYBA1 and VcMYBC2, respectively. Throughout the fruit development stage, the UV-B treatment up-regulated VcMYBPA1 expression, which increased VcUFGT expression via VcMYBA1. In the green fruit stage, the UV-B treatment increased HY5 encoding UV receptor, which up-regulates VcMYBPA1 and down-regulates VcMYBC2, thereby promotes the accumulation of anthocyanins. On the other hand, excessive anthocyanin synthesis was inhibited by increased VcMYBC2 levels in mature fruits when exposed to UV-B light through HY5-independent pathway. In conclusion, anthocyanin-related MYB activators and repressor may coordinately balance the accumulation of anthocyanins in blueberry fruits, with UV-B treatments possibly influencing their effects in a stage-specific manner. The potential utility of preharvest UV-B treatments for improving blueberry fruit quality is discussed herein.

Highlights

Blueberry (Vaccinium spp.) is known for its relatively high anthocyanin contents that can decrease the risk of developing coronary artery disease or gut inflammation, while positively affecting the nervous tissue in the brain[1,2]
We focused on the molecular regulatory network underlying UV-Btriggered anthocyanin metabolism, which revealed the stage-specific UV-B-responsive fruit coloration mechanism and a novel regulatory network fine-tuning the anthocyanin accumulation in blueberry fruits
The accumulation of sugars significantly increased in green fruits only in response to the low UV-B dose, whereas it increased in the mature fruits following the treatments with low and high UV-B doses (Fig. 1f)

Summary

Introduction

Blueberry (Vaccinium spp.) is known for its relatively high anthocyanin contents that can decrease the risk of developing coronary artery disease or gut inflammation, while positively affecting the nervous tissue in the brain[1,2]. Anthocyanin biosynthesis is mediated by the anthocyanin pathway involving structural genes and regulatory transcription factors (TFs) contributing to the production of diverse anthocyanin components. This pathway has been thoroughly characterized[12], with the genes encoding the following anthocyanin biosynthetic enzymes having been identified: chalcone synthase (CHS), flavonoid 3′-hydroxylase (F3′H), flavonoid 3′,5′hydroxylase (F3′5′H), dihydroflavonol reductase (DFR), anthocyanidin synthase (ANS), uridine diphosphate (UDP)-glucose:flavonoid-O-glycosyl-transferase (UFGT), flavonol synthase (FLS), leucoanthocyanidin reductase (LAR), and anthocyanidin reductase (ANR). Recent research on blueberry has mainly focused on the gene encoding UGFT, with its substantial increase in transcription in mature fruits, suggesting the UFGT is important for the rapid accumulation of anthocyanins in maturing fruit[13,14,15].

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