Tomato is one of the most popular and economically important vegetables in the world (Madhavi and Salunkhe, 1998). However, it is susceptible to chilling injury (CI), a physiological disorder that affects mainly tropical and subtropical products when they are exposed to low non-freezing temperatures (Saltveit and Morris, 1990). The minimum safe temperature for tomato fruit storage is about 12 °C, and the main visible CI symptoms are uneven ripening and color development, dark sunken lesions on the peel surface (pitting), and decay (Cheng and Shewfelt, 1988). These visible symptoms generally appear when tomatoes are transferred to non-chilling conditions after being stored at low temperature for longer than 1 week (Lurie and Sabehat, 1997), although cell structure alterations have been observed without a ripening period after prolonged chilling (Gomez et al., 2009).
The mechanisms involved in the CI of plant tissues are poorly understood; the phase transition of the cell membrane lipids during chilling has been considered the detonating event of the CI symptoms (Lyons, 1973), and more recent reports have shown ultrastructural changes of membranes and organelles induced by chilling (Marangoni et al., 1989; Yang et al., 2009). An oxidative stress response has been related to the appearance of the CI symptoms, suggesting that the antioxidant system, including superoxide dismutases (SOD), peroxidases, catalases (CAT), and glutathione reductase (GR), confers protection to the tissue against cold (Hodges et al., 2004; Malacrida et al., 2006). Oxidative damage occurs when this system fails to limit the accumulation of reactive oxygen species such as superoxide and hydrogen peroxide (Kerdnaimongkol and Woodson, 1999), leading to inactivation of enzymes, lipid peroxidation, protein degradation, and DNA damage (Foyer and Noctor, 2005). Ascorbate peroxidase (APX) is one of the main enzymes involved in detoxification of H2O2, and its overexpression in tomato confers tolerance to chilling and salt stress (Wang et al., 2005), although it has been shown that the capacity of the ascorbate-mediated antioxidant system is not enough to protect the cell from photoinhibition under oxidative stress (Shikanai et al., 1998). Malacrida et al. (2006) reported that the antioxidant response of chilled ‘Micro-Tom’ tomato fruit was mediated by CAT and GR but not by SOD or APX, although Gomez et al. (2009) suggested that activation of SOD and APX is an immediate response to chilling stress in this cultivar. The H2O2 that is not detoxified by enzymes such as APX and CAT could also be decomposed by peroxiredoxins (Prx), thiol-dependent peroxidases that can reduce a wide range of active oxygen species (Tripathi et al., 2009).
Chilling-induced changes in the redox state of the cell can trigger a signal transduction pathway that regulates gene expression; the abundance and activities of functional proteins change and might work cooperatively to establish a new redox homeostasis under the stress condition (Foyer and Noctor, 2005). Proteomic studies have identified several proteins associated with plant tissue responses at low temperature; they are involved in several processes, including signal transduction, RNA processing, translation, protein processing, redox homeostasis, photosynthesis, and metabolisms of carbon, nitrogen, sulfur, and energy (Yan et al., 2006). Some of the proteins identified more frequently in these studies include peroxidases, heat shock and RNA-binding proteins, rubisco, and energy metabolism enzymes (Renaut et al., 2006). A recent transcriptomic analysis of chilled ‘Micro-Tom’ tomato fruit identified several housekeeping genes upregulated by cold and a dehydrin homolog, which was proposed as a transcriptional marker of cold stress (Weiss and Egea-Cortines, 2009). Nevertheless, the biochemical bases of CI are still poorly understood and there is little information about cold-responsive proteins in tomato fruit.
To identify proteins related to CI, we compared the protein expression patterns during ripening (21 °C) of ‘Imperial’ tomato fruit previously stored under non-chilling (12 °C) and chilling (5 °C) conditions. This study allowed the identification of some chilling-related proteins and their potential roles in chilling response mechanisms are discussed. To our knowledge, this is the first report about tomato fruit proteome changes under chilling stress conditions.
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