‘Fupingjianshi’ persimmon (Diospyros kaki L.) is one of the important astringent persimmon cultivars in northwest China. The cultivar is very fast to ripen at ambient temperature after harvest. Softening is a very important feature of persimmon fruit ripening, providing juicy and melting textures for edible quality. It has been well known that fruit softening involves depolymerization and solubilization of cell wall components including matrix polysaccharides (hemicellulose and pectins) (Brummell and Harpster, 2001). The final hydrolysis and breakdown of cell wall components result from the action of cell wall hydrolases including polygalacturonase [electrical conductivity (EC) 188.8.131.52], pectin methylesterase (EC 184.108.40.206), β-galactosidase (EC 220.127.116.11), and xyloglucan endotransglycosylase (EC 18.104.22.168) (Brummell and Harpster, 2001). However, these enzymes may not be individually responsible for disassembly of cell wall polymers because molecular modification of these enzymes has not fully prevented fruit softening in many cases (Goulao and Oliveira, 2008).
Expansins are nonenzymatic cell wall-localized proteins that were first isolated from cucumber hypocotyl walls by analysis of pH-dependent cell-wall extensibility (McQueen-Mason et al., 1992). Expansins are cell wall proteins without hydrolytic activity and function to loosen hydrogen bonds between cellulose microfibrils and matrix polysacchardes in cell walls, particularly xyloglucans. This results in enhanced accessibility of cell wall-modifying enzymes, accelerating cell wall degradation (Brummell and Harpster, 2001; Li et al., 2003; McQueen-Mason and Cosgrove, 1995; Rose and Bennett, 1999). Numerous studies confirmed that expansins play an important role in fruit softening (Goulao and Oliveira, 2008; Payasi et al., 2009; Rose et al., 1997), as has been shown for a variety of fruits such as tomato (Brummell et al., 1999, 2002; Rose et al., 1997, 2000), strawberry (Civello et al., 1999), peach (Hayama et al., 2003), pear (Hiwasa et al., 2003), mango (Sane et al., 2005), kiwifruit (Yang et al., 2007), banana (Asha et al., 2007), loquat (Yang et al., 2008), and papaya (Gaete-Eastman et al., 2009).
In our previous study, two fragments of expansin genes in persimmon (cv. Fupingjianshi) fruit, termed CDK-Exp1 and CDK- Exp2 (not the full length sequences), were cloned during fruit growth and ripening (Tong et al., 2005). The objective of the present study was to identify the cloning for a full-length expansin gene (CDK-Exp3) at the onset of ripening as well as to investigate the hypothesis that the expansin gene might be involved in persimmon fruit softening by interference of gibberellic acid (GA3) on ripening.
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