Longan is a nonclimacteric subtropical fruit with high value (Huang, 1995; Paull and Chen, 1987). The edible portion of longan fruit is a fleshy and translucent white aril. However, the fruit deteriorate rapidly after harvest, due to pericarp browning and aril breakdown, resulting in reduced market value (Jiang et al., 2002). Aril breakdown involves loss of turgidity and translucency, and thus the fruit become bland in taste. The disorder starts near the pericarp and appears to be more prevalent at the distal end (Jiang et al., 2002). Some researchers also mentioned aril breakdown in longan fruit as “autolysis” (Jiang et al., 2002; Lin et al., 2007). Storage at low temperature can delay the occurrence of aril breakdown and extend storage life, but when longan fruit were transferred to room temperature after low-temperature storage, the aril breakdown occurred very rapidly (Jiang et al., 2002). Thus, delaying or reducing aril breakdown should be an important approach to extend storage life or maintain quality of longan fruit.
It has been considered that cell-wall components are important for fruit texture (Manganaris et al., 2005, 2006). Cellular-wall modification-related enzymes, including endo-β-1,4-glucanase (EGcase), polygalacturonases (PGcase), pectate lyase, and pectin methyl esterase (PME), are involved in cellular-wall degradation (Rose and Bennett, 1999). Recently, expansin and xyloglucan endotransglucosylase/hydrolase (XET/XTH) have been suggested to be involved in fruit softening (Asha et al., 2007; Hayama et al., 2003; Hiwasa et al., 2003; Lu et al., 2004).
It has been proposed that a function of EXPs loosens the cellular walls by disrupting non-covalent linkages at the cellulose–hemicellulose interface, resulting in relaxing an important constraint to turgor-driven cell expansion (Cosgrove, 2000; McQueen-Mason and Cosgrove, 1994). EXPs are involved in fruit softening in tomato (Solanum lycopersicum L. cv. T5) (Rose et al., 1997), strawberry (Fragaria ananassa Duch. cv. Brighton) (Harrison et al., 2001), and peach [Prunus persica (L.) Batsch] (Hayama et al., 2003). In addition, EXP proteins have been detected in a range of ripe fruit (Rose et al., 2000).
XET/XTH, one of the best-studied enzymes in fruit ripening and softening, is able to disassemble the cellulose–xyloglucan matrix that has been associated with the fruit softening process. XTH genes have been determined to be related to fruit softening in banana (Musa acuminata Colla cv. Grand Nain) (Lu et al., 2004), tomato (cv. 83-G-38) (Maclachlan and Brady, 1994), persimmon (Diospyros kaki L.) (Cutillas-Iturralde et al., 1994), kiwifruit [Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson var. deliciosa cv. Hayward] (Redgwell and Fry, 1993), and pear (Pyrus communis L. cv. La France) (Fonseca et al., 2005).
It has been reported that the aril breakdown of longan fruit resulted from the degradation of cellular-wall components and the decline in aril firmness accompanied with the aril breakdown (Lin et al., 2007). However, the molecular mechanism of longan aril breakdown associated with expressions of Dl-EXP and Dl-XET has not been investigated.
The aim of the present work was to investigate the expression patterns of EXP and XET genes in relation to aril breakdown in harvested longan fruit stored at various temperatures.
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