Abstract
Artificial pigmentation of apple fruits has been intensely evaluated to generate less pigmented red apples, which are profitable because of the changes in fruit quality. In this study, we analyzed the diversity of flavonoids and the patterns of flavonoid metabolic gene expression under light irradiation with or without methyl jasmonate (MeJA) treatment in immature (S1) and color-turning (S2) staged ‘Fuji’ apples. Further, we assessed the metabolic regulation at the gene level between anthocyanin and flavonol in light-responsive apple skins. UV-B exposure within 3 days was found to significantly stimulate anthocyanin accumulation in apple skin compared to other light exposure. S1 skin was more sensitive to UV-B and MeJA treatment, in the aspect of indaein accumulation. The enhancement of apple pigmentation following treatment with adequate levels of UV-B and MeJA was maximized at approximately 72 h. Red (range from 4.25 to 17.96 µg·g−1 DW), blue (range from 4.59 to 9.17 µg·g−1 DW) and UV-A (range from 3.98 to 19.12 µg·g−1 DW) lights contributed to the induction of idaein content. Most genes related to the flavonoid pathways increased their expression under UV-B exposure, including the gene expression of the transcription factor, MdMYB10, a well-known upstream factor of flavonoid biosynthesis in apples. The boosted upregulation of MdMYB10, MdCHS, MdF3H MdLDOX, and MdUFGT genes due to MeJA in UV-B was found and may contribute the increase of idaein. UV-A and UV-B caused higher quercetin glycoside content in both S1 and S2 apple skins than longer wavelengths, resulting in significant increases in quercetin-3-O-galactoside and quercetin-3-O-glucoside. These results suggest that the application of adequate UV-B with MeJA in less-pigmented postharvest apples will improve apple color quality within a short period.
Highlights
Introduction iationsEpidermal color distribution in red apples is regarded as an important factor for evaluating apple quality
In the aspect of overall results (Figure 9), it was confirmed that the pigmentation of epidermal skin in postharvest apples could be rapidly performed by appropriate treatment of UV-B with methyl jasmonate (MeJA)
It was found that the boosted up-regulation of MdMYB10, MdCHS, MdF3H MdLDOX, and MdUFGT genes due to MeJA in UV-B, which may contribute the increase of idaein
Summary
Epidermal color distribution in red apples is regarded as an important factor for evaluating apple quality. An appropriate skin color contributes the preference of consumers, and the merit of high anthocyanin use due to various health benefit activities. Red apples mainly accumulate cyanidin-3-O-galactoside (idaein), a representative anthocyanin in apple skin for pigmentation, in their epidermal cells during the ripening process. Pigmentation is well known to be directly affected by environmental factors, such as altitude, temperature, and especially exposure to light [1,2]. It has been reported that expression of anthocyanin biosynthesis regulatory genes can be induced by light in red apple [3].
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