Melon varieties (Cucumis melo L.) differ in a range of physical and chemical attributes. Sweetness and aroma are two of the most important factors in fruit quality and consumer preference. Volatile acetates are major components of the headspace of ripening cv. Arava fruits, a commercially important climacteric melon. In contrast, volatile aldehydes and alcohols are most abundant in cv. Rochet fruits, a nonclimacteric melon. The formation of volatile acetates is catalyzed by alcohol acetyltransferases (AAT), which utilize acetyl-CoA to acetylate several alcohols. Cell-free extract derived from Arava ripe melons exhibited substantial levels of AAT activity with a variety of alcohol substrates, whereas similar extracts derived from Rochet ripe melons had negligible activity. The levels of AAT activity in unripe Arava melons were also low but steadily increased during ripening. In contrast, similar extracts from Rochet fruits displayed low AAT activity during all stages of maturation. In addition, the benzyl- and 2-phenylethyl-dependent AAT activity levels seem well correlated with the total soluble solid content in Arava fruits.

The aromas of fruits, vegetables, and flowers are mixtures of volatile metabolites, often present in parts per billion levels or less. We show here that tomato (Lycopersicon esculentum Mill.) plants transgenic for a heterologous Clarkia breweri S-linalool synthase (LIS) gene, under the control of the tomato late-ripening-specific E8 promoter, synthesize and accumulate S-linalool and 8-hydroxylinalool in ripening fruits. Apart from the difference in volatiles, no other phenotypic alterations were noted, including the levels of other terpenoids such as ␥-and ␣-tocopherols, lycopene, ␤-carotene, and lutein. Our studies indicate that it is possible to enhance the levels of monoterpenes in ripening fruits by metabolic engineering.In addition to the four basic flavors-sweet, sour, salty, and bitter-that humans recognize in foodstuffs, aromas also have an important influence on people's choice of foods. Food aromas are perceived in humans by the nasal olfactory epithelium, a relatively small area of the mucous-covered inner surface of the nasal cavity. The threshold for human perception of a volatile molecule can be as low as 0.007 g L Ϫ1 in water (Buttery, et al., 1971). Thus, unique combinations of volatiles, as well as the specific proportions of each of the volatile components, determine aroma properties of fruits and other foods (Thomson, 1987).Most of the research on fruit and vegetable breeding carried out during the last few decades has focused on obtaining desirable agronomic characteristics such as resistance to environmental stresses, pests, and pathogens (Stevens and Rick, 1986). Breeding for improved flavor of fruits such as tomatoes (Lycopersicon esculentum Mill.) has mainly been directed toward controlling sugar to acid ratios and improving texture and storage characteristics of the products (Jones and Scott, 1983;Stevens and Rick, 1986). Tomatoes lacking a characteristic or distinctive "tomato" aroma have often given rise to public complaints about the quality of the produce. Nevertheless, conventional breeding to improve the aromas of agricultural products is often impeded by the large number of genes involved in aroma formation, the significant environmental and developmental effects on aroma, and the lack of consistent, simple, and cheap methodologies to probe both aroma preferences of the public and the chemistry involved.Although many specific flavor and aroma compounds have been identified in fruits and vegetables, the enzymes and genes controlling their production and their pattern of inheritance have scarcely been studied and are therefore little understood. However, based on studies in many plants, including tomato, it is known that volatile compounds found in fruits are mainly derived from three biosynthetic pathways (Croteau and Karp, 1991). The formation of the hedonically important short-chain aldehydes and alcohols, such as cis-3-hexenol or n-hexanal, takes place through the action of lipases, hydroperoxide lyases, and cleavage enzymes on lipid components, followed by the action of alcohol...

Vegetable cultivation favored the inclusion of pleasant aromas in the produce, whereas unpleasant aromas were selected against. Introgression lines, generated by hybridization of a cultivated tomato (Lycopersicon esculentum) to its wild relative L. pennellii, were used to map quantitative trait loci (QTL) that influence tomato aroma. A marked undesirable flavor was detected by taste panelists in L. pennellii fruits and was related to an introgressed segment from the short arm of chromosome 8. Analysis of the ripe fruits' volatiles of chromosome 8 introgressed lines revealed an up to 60-fold increase in the levels of 2-phenylethanol and phenylacetaldehyde, as compared to the cultivated tomato. This effect was associated with a 10 cM segment originating from the wild species. Although 2-phenylethanol and phenylacetaldehyde have favorable contribution to tomato aroma when present at low levels, phenylacetaldehyde has a nauseating objectionable aroma when present in levels >0.005 ppm. The loss of the ability to produce high levels of phenylacetaldehyde contributed to the development of desirable aroma of the cultivated tomato. The findings provide a genetic explanation for one of the aroma changes that occurred during the domestication of the tomato.

Apple (Malus domestica Borkh.) cultivars differ in their aroma and composition of volatile acetates in their fruit flesh and peel. Cv. Fuji flesh contains substantial levels of 2-methyl butyl acetate (fruity banana-like odor), while the flesh of cv. Granny Smith apples lacks this compound. Granny Smith apples accumulate mainly hexyl acetate (apple-pear odor) in their peel. Feeding experiments indicated that Fuji apples were able to convert hexanol and 2-methyl butanol to their respective acetate derivatives in vivo, while Granny Smith apples could only convert exogenous hexanol to hexyl acetate. Differential substrate specificities of the in vitro acetyl-CoA:alcohol acetyl transferase (AAT) activities were also detected among cultivars. In Granny Smith apples, the AAT activity was detected only in the peel, and its specificity was almost exclusively restricted to hexanol and cis-3-hexenol. In Fuji apples, the AAT activity was detected in both peel and flesh and apparently accepted a broader range of alcohols as substrates than the Granny Smith enzyme activity. Our data strongly suggest that different AAT activities are operational in apple tissues and cultivars and that these differences contribute to the variation observed in the accumulation of volatile acetates.