# 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
Lipoxygenase (LOX) and alcohol dehydrogenase (ADH) in tomato [78; 79] and alcohol acyl-transferase (AAT) in melon [80] were among the first enzymes to be identified that were associated with production of flavour and aroma volatiles during ripening. Identifying the correct LOX in tomato turned out to be complicated because there were at least five different isoforms involved in different biochemical pathways, including the production of jasmonic acid (JA), which will be discussed later. Antisense technology, using targeting sequences specific for each gene, identified the plastid-located TOMLOX C as playing a critical role in production of lipid-derived C6, and later, C5 volatiles [13]. Alcohol acetyltransferase (AAT1) is important in volatile ester synthesis [80; 81] and the final step in the pathway, conversion of aldehydes to alcohols, requires ADH2 activity [79]. Recently, an increase in FaLOX5 transcripts has been correlated with volatile esters' production in ripening strawberry, and FaMYB11 promoted volatile accumulation partly through the transcriptional activation of FaLOX5 [82]. Many other genes are now known that contribute to the production of flavour volatiles, and it is important to recognise that they are derived from several different biochemical pathways, including fatty acids, amino acids and carotenoids (Figure 3). Key genes that contribute to aroma and flavour in ripe tomato fruits include branched-chain amino acid aminotransferases (_BCAT1_), which is important in the production of branched-chain amino acids [83; 84; 85]. A major group of amino acid-derived flavour and aroma compounds, benzenoids (C6-C1), are synthesised from L-phenylalanine, including benzaldehyde and benzyl alcohol. The first step in this biosynthetic pathway is catalysed by L-phenylalanine ammonia lyase (PAL), encoded by the _PAL3_ gene, which converts phenylalanine to trans-cinamic acid [86]. Aromatic amino acid decarboxylase (AADC1A) mediates the first step in the production of phenylalanine-derived volatiles in tomato fruits [87], and the transcripts of the carotenoid cleavage dioxygenase 1 (_CCD1B_) gene also participate in aroma volatiles from the amino acid pathway. Terpenoids constitute the largest group of plant volatile secondary metabolites [89; 90] and function in attraction and defence during vegetative and reproductive growth. They are synthesised from isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) via two pathways: the cytosol-located mevalonate (MVA) pathway and the methyl-erythriitol phosphate (MEP) pathway in the plastid [91]. Terpene synthases (TPS) catalyse the terminal steps in the pathways and use geranyl diphosphate (GPP), trans-trans-farnesyl diphosphate (trans-trans-FPP, usually referred to as FPP) and geranylgeranyl diphosphate (GGPP) to form monoterpenes (C10), sesquiterpenes (C15) and diterpenes (C20), respectively. There are many different TPSs, and differences in their catalytic mechanisms generate a wide range of different terpenoids [92; 93]. TPS families have been identified in many plants, including Arabidopsis, tomato, grapevine, apple, orange and kiwifruit [94; 95; 96; 97]. Monoterpenes and sesquiterpenes are volatile and contribute to the characteristic aroma of fruits, including citrus [98]. CitAP2.10 [99] and CitERF71 [100] have been reported to regulate the synthesis of the sesquiterpenes valencene and E-geraniol respectively, in citrus. [PERSON] et al. [101] showed that mutation in the NAC binding region of _TPS_ promoters in two kiwifruit species influences their monoter contents.