In Pyrus, there are three major species, P. communis L. (pear or European pear), P. bretschneideri Rehd. or P. uusuriensis Maxim. (Chinese pear), and P. pyrifolia Nakai (Japanese pear: Nashi), which are commercially cultivated in temperate zones. The attributes that constitute good quality in one species may differ from that in another, as is the case with European and Asian pears. The attributes of European pears, for example, include its soft buttery texture, whereas those of Asian pears include its juicy and crisp flesh. The most distinctive characteristic of Asian pears is the fact that they mature on the tree in contrast to European pears, which usually require exposure to chilling temperatures for initiation of ripening.
Pear fruits can be classified as climacteric or nonclimacteric according to their ripening characteristics. European pears are climacteric (Jackson, 2003), whereas Asian pears are thought to include climacteric and nonclimacteric cultivars because climacteric type fruits showed a rise in respiration and ethylene production, and nonclimacteric-type fruit did not show a rise in respiration and ethylene production during fruit ripening (Downs et al., 1991; Itai et al., 1999, 2003a; Kitamura et al., 1981). In Asian pears, climacteric-type fruits have a low storage potential, whereas nonclimacteric fruits maintain fruit quality for over 1 month in storage (Itai et al., 1999, 2003a; Kitamura et al., 1981). Therefore, fruit storage potential is closely related to the maximum level of ethylene production in Asian pears.
We previously cloned three 1-aminocyclopropane-1-carboxylate (ACC) synthase genes (PPACS1, 2, and 3) and studied their expression during fruit ripening (Itai et al., 1999, 2003b). PPACS1 was specifically expressed in cultivars showing high ethylene production (>10 nL·g−1·h−1, usually over 50 nL·g−1·h−1), and PPACS2 in cultivars showing moderate ethylene production (0.5 nL·g−1·h−1 to 10 nL·g−1·h−1). Moreover, we previously identified restriction fragment-length polymorphism (RFLP) markers linked to the ethylene production in ripening fruit using RFLP analysis with two ACC synthase genes (PPACS1 and PPACS2) (Itai et al., 1999). These RFLPs were designated A (2.8 kb of PPACS1), which is linked to high levels of ethylene, and B (0.8 kb of PPACS2), which is linked to moderate levels. Based on this analysis, we classified 35 Asian pear cultivars into four RFLP types (AB, Ab, aB, and ab), of which types AB and Ab show high levels, aB a moderate level, and ab a low level of ethylene production during fruit ripening (Itai et al., 1999). We further transformed these two RFLP markers into more convenient easy-to-use polymerase chain reaction (PCR)-based CAPS markers (A: 1.57 kb and 0.63 kb of the PPACS1 fragment; B, 0.83 kb and 0.35 kb of PPACS2 fragment) (Itai et al., 2003b).
These markers are useful for predicting the ethylene levels of Asian pear cultivars and they enable identification of low ethylene producers with an enhanced postharvest storage ability. However, available information on ethylene production is limited to certain cultivars. Therefore, in the present study, we conducted CAPS analysis of 152 cultivars using two ACC synthase genes to evaluate the ripening characteristics and to improve breeding and posthandling information on Asian pears.
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