Distinctive acidity in citrus fruit is linked to loss of proanthocyanidin biosynthesis

The distinctive acidity of citrus fruit is determined by a regulatory complex of MYB and bHLH transcription factors together with a WDR protein (MBW complex) which operates in the unique juice vesicles of the fruit. We describe a mutation affecting the MYB protein, named Nicole, in sweet orange and identify its target genes that determine hyperacidification, specifically. We propose that the acidity, typical of citrus fruits, was the result of a loss of the ability of Nicole to activate the gene encoding anthocyanidin reductase, an enzyme essential for the synthesis of proanthocyanidins, which are absent in citrus fruit.