# Molecular and Hormonal Mechanisms Regulating Fleshy Fruit Ripening
## 2 Main Physiological and Biochemical Changes Are Regulated by Ripening Genes
### Synthesis and Accumulation of Organic Acids, Sugars and Volatiles Contribute to Taste and Aroma  
The main taste characteristics of fruit are derived from the relative concentrations of organic acids and sugars [69]. The most common sugars are sucrose, glucose and fructose, which are ultimately derived from photosynthetic imported during development. This soluble carbohydrate is often converted to starch and stored in the plastids of unripe fruit. During ripening, the starch is initially metabolised to maltose and glucose, by a network of starch-degrading and sugar-metabolising enzymes. During development, bananas accumulate 20-25% fresh weight of starch, which is converted to sugars during ripening. In some other fruits, photosynthesis by fruit chloroplasts can also contribute to the accumulation of stored starch prior to ripening. Eighteen starch degradation-associated enzymes from banana have been identified bound to the surface of starch granules, and ten showed increased activity during fruit ripening. The accumulation of transcripts and enzyme activities of some of these genes are ethylene inducible. A transcriptional activator MabHLH6 (a basic helix-loop-helix TF) preferentially binds to the promoters of 11 genes encoding starch degradation enzymes and transactivates their promoters [70]. Other types of TFs are also involved in regulating expression of starch degradation genes, since a zinc finger TF, DNA BINDING WITH ONE FINGER (AdDof3), physically interacts with the promoter and transactivates the _AdBAM3L_ (\(\beta\)-amylase) gene in ripening kiwifruit [71]. The metabolism of the large amount of starch in banana not only generates sugars during ripening but also contributes directly to a reduction in firmness, in addition to cell wall changes outlined in Section 2.2. Sugar accumulation often involves the activities of sucrose synthase and sucrose phosphate synthase [72], and appropriate transporters [73] and increases in acid and neutral invertases during ripening of tomato convert sucrose to glucose and fructose, which enhances sweetness [74]. In watermelon, four genes involved in the metabolism of sugars have been identified as being expressed in different watermelon genotypes, including three sugar transporters, SWEET, EDR6 and STP, two sucrose phosphate syntheses and sucrose synthase genes [75]. Unlike sugars, which are imported to the fruit as photosynthate, organic acids are generally synthesised in situ, mostly from sugars, metabolised starch and products of cell wall metabolism [63]. Malate and citrate are the most common fruit organic acids that accumulate in both climacteric and non-climacteric fruits during fruit development, although other acids are found in some fruits. In watermelon, three genes involved in the TCA cycle and pH regulation have been identified that are believed to play a role in organic acid accumulation [75]. During ripening, organic acids can slowly decrease in concentration in some fruits, as the malate, and sometimes citrate, are utilised as respiratory substrates, whereas sugar contents generally continue to increase. It is possible that these sugars and organic acid may have another role, since sugars are known to be signalling molecules as well as being a form of soluble carbohydrate [76]. Furthermore, when malate metabolism in tomato was modified by antisense inhibition of the activities of mitochondrial malate dehydrogenase (MDH), it was found that lines with low malate showed an increased accumulation of starch. Fruit softening and the content of chlorophylls, carotenoids and other pigments were also affected, and the low-MDH tomatoes showed increased susceptibility to _Botrytis cinerea_ infection. It is possible that some of these effects are due to the altered redox state due to lowered malate, leading to several biochemical changes [77]. The characteristic flavour and aroma notes of different fruits are conferred by a range of different volatile components that are synthesised from several different biochemical pathways.