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Figure 1. 

The morphological phenotypes of tissue-specific anthocyanin and organic acid accumulation of different fruits. Above: The phenotype differences between non-red-fleshed apple 'Jinshanyilamu' (left), a cultivar that accumulated less malate, and red-fleshed apple 'Yepingguo' (right), a cultivar that accumulated more malate^[17]. Middle: 'LCA', a low citric acid pummelo cultivar (left) that is unable to produce anthocyanins, and 'HCA', a high citric acid pummelo cultivar (right) that is able to produce anthocyanins^[30]. Below: Young leaves, flowers and seeds of 'Corsican Citron' (left) that has very low juice acidity around pH 5.5 and completely is unable to produce anthocyanins, and 'Poncire commun' (right) that has normal juice acidity around pH 2.5 and able to synthesize anthocyanins^[18].

Figure 2. 

Cooperation of TFs involved in co-regulation of anthocyanin and organic acid accumulation in fruit crops, including (a) grape (vitis vinifera), (b) apple (Malus domestica), and (c) citrus (Citrus sp.). Solid and dashed lines indicate proven and hypothetical (yet to be experimentally established) relationships, respectively. Arrows indicate positive regulators; T-bars indicate negative regulators.

Figure 3. 

A simplified model of environmental co-regulation of anthocyanins and organic acids in apple fruit. (a) High nitrate cues trigger MdBT2 to ubiquitinates MdMYB1 and the MYB-bHLH complex, MdCIbHLH1-MdMYB73, which are involved in the regulation of anthocyanin biosynthesis and organic acid accumulation in apple fruit, respectively; Low nitrate cues inhibit MdBT2, thus, MdMYB1 and the MYB-bHLH complex, MdCIbHLH1-MdMYB73, enhances the accumulation of anthocyanin and organic acid in apple fruit. (b) High and low temperature cues inhibit and trigger the MYB-bHLH complex, MdMYB1-MdbHLH3, which is involved in the co-regulation processes of anthocyanin biosynthesis and organic acid accumulation, respectively.