**4. Production of apocarotenoids in engineered microbes**

As shown in **Figure 2**, carotenoids can be further converted into apocarotenoids by CCDs or other oxygenases. Here, three apocarotenoids of high commercial values are highlighted here. Retinol, or vitamin A, is one of the most important apocarotenoids to humans. Retinol exhibits an essential function in vision, bone development and also promotes skin health as an anti-oxidant [83]. The other two are aromatic molecules, α-ionone, which naturally exists in raspberry, and β-ionone, which is found in many flowers, for example, rose, osmanthus and violet [84]. The chemical synthesis of these three molecules is not very difficult and contributes significantly to the current market share. However, consumers prefer natural derivatives and are willing to pay higher prices for natural ingredients [19]. As mentioned in the introduction, the extremely low concentrations in natural plant materials make their extraction an extremely expensive process. Consequently, the fermentation of engineered microbes is a promising alternative route.
