Abstract
As in many other fleshy fruits, the predominant organic acids in ripe peach (Prunus persica (L.) Batsch) fruit are malic and citric acids. The accumulation of these metabolites in fruit flesh is regulated during fruit development. Six peach fruit-related genes implicated in organic acid metabolism (mitochondrial citrate synthase; cytosolic NAD-dependent malate dehydrogenase, and cytosolic NADP-dependent isocitrate dehydrogenase) and storage (vacuolar proton translocating pumps: one vacuolar H+-ATPase, and two vacuolar H+-pyrophosphatases) were cloned. Five of these peach genes were homologous to genes isolated from fruit in other fleshy fruit species. Phylogenetic and expression analyses suggested the existence of a particular vacuolar pyrophosphatase highly expressed in fruit. The sixth gene was the first cytosolic NAD-dependent malate dehydrogenase gene isolated from fruit. Gene expression was studied during the fruit development of two peach cultivars, a normal-acid (Fantasia) and a low-acid (Jalousia) cultivar. The overall expression patterns of the organic acid-related genes appeared strikingly similar for the two cultivars. The genes involved in organic acid metabolism showed a stronger expression in ripening fruit than during the earlier phases of development, but their expression patterns were not necessarily correlated with the changes in organic acid contents. The tonoplast proton pumps showed a biphasic expression pattern more consistent with the patterns of organic acid accumulation, and the tonoplast pyrophosphatases were more highly expressed in the fruit of the low-acid cultivar during the second rapid growth phase of the fruit.
Highlights
Organic acids are important attributes of fruit flavour and, as such, contribute with soluble sugars and aromas to the overall organoleptic quality of fresh market peaches (Sweeney et al 1970)
Malic acid accumulates mostly during the first rapid growth phase that occurs at the end of the cell division phase (Masia et al 1992), whereas citric acid accumulates during the second rapid growth phase, before the onset of ripening (Moing et al 1998)
Fruit development and organic acid and sugar accumulations were thoroughly characterized in both cultivars to allow comparison with gene expression data
Summary
Organic acids are important attributes of fruit flavour and, as such, contribute with soluble sugars and aromas to the overall organoleptic quality of fresh market peaches (Sweeney et al 1970). The predominant organic acids in ripe peach fruit are malic and citric acids, whilst quinic acid accumulates in lower amounts (Moing et al 1998). As in most other fleshy fruits, organic acid accumulation takes place during the early development of peach fruit (Monselise 1986, Tucker 1993). Studies on grape berries, which mainly accumulates malic acid up to a concentration of 150 mM (Diakou et al 2000, Or et al 2000), and on Citrus fruits, which accumulate mainly citric acid up to a total concentration of 300 mM (Sadka et al 2000a, 2000b), have considerably extended our knowledge about organic acid metabolism in fleshy fruits. Phosphoenolpyruvate carboxylase [PEPC; EC 4.1.1.31 (Chollet et al 1996)] and NAD-dependent malate dehydrogenase [NAD-MDH; EC 1.1.1.37 (Miller et al 1998)] are impli-
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