Transcriptional regulation of the flavonoid pathway in the skin of dark-grown ‘Cripps’ Red’ apples in response to sunlight

Abstract

SummaryFlavonoids, including anthocyanins, flavonols and condensed tannins (CTs), are synthesised by a common biosynthetic pathway. Anthocyanins are responsible for the red colour in apple fruits, while CTs and flavonols are potent anti-oxidants which may be beneficial in the human diet. The effect of sunlight on flavonoid accumulation and gene transcription was investigated in fruit skin of the red apple variety Malus domestica Borkh. cv. ‘Cripps’ Red’. Individual fruit growing on trees and covered with bags 46 d after full bloom (DAFB) failed to accumulate anthocyanins by 147 DAFB, 2 weeks before harvest. In contrast, control fruit, grown under natural light conditions, accumulated anthocyanins up to 140 ng mg–1 fresh weight (FW) of skin. The accumulation of flavonols was moderately reduced in bagged fruit, but the synthesis of CTs was unaffected. Exposure of bagged fruit to sunlight induced anthocyanin synthesis up to approx. 30 ng mg–1 FW of skin after 13 d (harvest), but had no effect on the synthesis of flavonols or CTs. The synthesis of anthocyanins correlated with an increase in the steady-state transcript levels of all flavonoid pathway genes except those specific for CT synthesis. These data suggest that transcription of most apple flavonoid genes is controlled by a common regulatory mechanism that is light-responsive.