# Molecular and Hormonal Mechanisms Regulating Fleshy Fruit Ripening
## 2 Main Physiological and Biochemical Changes Are Regulated by Ripening Genes
### Colour Changes during Ripening  
Visual clues are extremely important for signalling at a distance to frugivores the availability of ripe fruit. A wide range of animals, including seed-dispersers such as bats, rodents, primates and many different types of birds, with different vision systems use visible (380 to 700 nm) or UV light (mainly UVA, 315 to 400 nm) to detect them. Different organisms can have between 1 and 4 functioning colour receptors. Many mammals, including bats and rodents, are dichromats (i.e., have only two types of colour receptor), yet are capable of distinguishing ripe fruits from leaves. Most new world monkeys are also dichromats, whereas most other primates have three types of colour receptors (i.e., they are trichromats), which confer improved colour discrimination. There are four main mechanisms involved in colour change in fruits:  
1. Loss of chlorophyll from chloroplasts, which is accompanied by the disassembly and recycling of some thylakoid membranes and photosynthetic proteins. 2. Accumulation of coloured carotenoids such as \(\beta\)-carotene and lycopene in lipid globules or other characteristic membrane-bound structures in the developing chromoplasts, which arise either by conversion of chloroplasts or develop from other types of plastids [24; 25; 26; 27]. 3. The accumulation of flavonoids or anthocyanins pigments in the cell vacuoles. 4. The production of UV-reflecting components such as surface waxy layers, fatty deposits within cells or from parallel sheets of membranes that generate iridescence due to thin film interferences of light [28]. The synthesis of anthocyanins and carotenoids during ripening has been the most intensively studied at the molecular level and involves the selective expression of genes encoding specific enzymes in their respective metabolic pathways. Much of the knowledge on the regulation of carotenoid biosynthesis has been obtained from tomato and can be inferred to apply to other flesly fruits (Figure 1), although it should be emphasised that structurally different carotenoids can occur in different types of fruits. Fruits rich in carotenoids provide a rich source of dietary pro-vitamin A [3; 29]. The precursor of carotenoid biosynthesis, geranylegradanyl pyrophosphate (GGPP), is synthesised via the methylerythmitrol-4-phosphate (MEP) pathway. Two GGPP molecules produce phytoene, catalysed by phytoene synthase (PSY). Phytoene is converted into lycopene by undergoing a sequential series of desaturation and isomerisation reactions by phytoene desat-urase (PDS), \(\zeta\)-carotene isomerase (ZISO), \(\zeta\)-carotene desatutase (ZDS) and caroten isomerase (CRITSO). Lycopene can be cyclised by lycopene cyclase (LCY) to form \(\alpha\)-carotene, generating lutein, catalysed by caroten hydroxylase (CHIX). Alternatively, lycopene can be cyclised by LCY or chromoplast-specific lycopene \(\beta\)-cyclase (CYCB) to form \(\beta\)-carotene, which is converted into \(\beta\)-cryptokanthin and then into zeaxanthin by CHX [30]. Tomato PSY1 was the first carotenoid biosynthetic enzyme to be identified [31] and the pathways and enzymic steps for carotenoid production are now well-established [30]. In tomato, the PSY1 isoform is involved in synthesis of carotenoids in ripening fruit, whereas PSY2 performs this function in green tissues. Several other key genes are also upregulated during ripening [32; 33] and there is also substantial variation between fruits. A recent review of mutations underlying colour differences between different fruits and varieties highlighted mutations affecting coding sequences and promoters of a number of carotenoid biosynthetic enzymes, including CRTISO, ZISO, CHX, CYCB, etc. [30; 34]. of the phenylpropanol pathway (not shown).