Environmental stress causes considerable losses in productivity of many crops. Among various stresses, low temperature is one of the most crucial signals affecting plant growth and even leading to death (Sung et al., 2003; Veal et al., 2007). Extensive study on oxidative stress has demonstrated that exposure of plants to low temperature always induces the overproduction of reactive oxygen species (ROS), such as superoxide radical (O2 ·−), H2O2, and hydroxyl radical (HO·) in plant cells (Hung et al., 2005). ROS are highly reactive to membrane lipids, protein, and DNA; they are believed to be one of the major contributing factors to chilling injuries (CIs) and to cause rapid cellular damage (Hariyadi and Parkin, 1993; O'Kane et al., 1996; Prasad, 1996). When plants are exposed to low temperature, electron-transport chains tend to form O2 ·−, which dismutates to form H2O2. Furthermore, in chloroplasts, low temperature limits the dark reactions, thus limiting the supply of NADP+ and favoring reduction of O2 by photosystem II. Therefore, exposure to low temperature in combination with high light intensity leads to more serious damage in plants (Allen and Ort, 2001). In mitochondria, inhibition of ATP formation or electron flow through cytochrome b stimulates O2 ·− formation by complex I and by ubiquinone (Elstner, 1991).
Plants have evolved both enzymatic and nonenzymatic mechanisms to scavenge the ROS rapidly evolved under low-temperature stress (Apel and Hirt, 2004; Scandalios, 1993). Among the antioxidant mechanisms, the ascorbic acid (AsA)–GSH cycle is a key component for elimination of ROS, especially H2O2 (Kingston-Smith and Foyer, 2000; Noctor et al., 2002). In the AsA–GSH cycle, AsA reduces both O2 ·− and H2O2. In turn, the crucial antioxidant, GSH, reduces dehydroascorbate to regenerate AsA; meanwhile, GSH itself is oxidized to form GSH disulfide (GSSG). NADPH, catalyzed by glutathione reductase (GR), then reduces GSSG to regenerate GSH (Kocsy et al., 2000a, 2000b, 2001). Therefore, inhibition of GSH synthesis by a specific inhibitor, BSO, could dramatically decrease the chilling tolerance of mung bean seedlings and maize (Zea mays L.) (Kocsy et al., 2000b; Yu et al., 2002, 2003). Experimental evidence also indicates that the level and redox state of GSH might serve as indicators of plant responses to environmental stresses such as chilling (Foyer et al., 1997; May et al., 1998; Tausz et al., 2004). However, how plants sense low temperature and then transmit a precise signal to eventually elevate the cellular GSH levels is still far from clear.
According to our present understanding of signal transduction in plant cells, (Ca2+)cyt plays a pivotal role. The second messenger Ca2+ triggers cellular changes in response to many different signals (e.g., light, hormones, touch, cold, fungal elicitors, and even H2O2) (Knight, 2000; Knight et al., 1996; Sanders et al., 1999). It was reported that transient increases in (Ca2+)cyt levels could be evoked by cold treatment in arabidopsis [Arabidopsis thaliana (L.) Heynh.] (Knight et al., 1996; Lewis et al., 1997; Polisensky and Braam, 1996; Sung et al., 2003). An influx of extracellular Ca2+ seems to play a major role in the low-temperature response, and an intracellular Ca2+ source might also be involved (Polisensky and Braam, 1996; Rentel and Knight, 2004). Evidence also indicated that H2O2-activated Ca2+ channels mediated both the influx of Ca2+ in protoplasts and increases in (Ca2+)cyt in intact guard cells, thus leading to closure of stomata of arabidopsis (Pei et al., 2000). Interestingly, in addition to serving as a link in a signaling cascade, fluctuation of (Ca2+)cyt could be one of the mechanisms that lead plants to memorize what they have suffered (Knight et al., 1996). This inference comes from the observation that arabidopsis treated with either sublethal cold or H2O2 modifies its Ca2+ signal in response to subsequent cold stress as compared with untreated control (Knight et al., 1996). On the basis of these findings, it was therefore proposed that the “calcium memory” is an important mechanism for plants adapt to environmental changes.
In Taiwan, the temperature normally ranges between 17 and 27 °C, but occasionally it may decline to 10 °C or lower and remain for a few days. Hence, it is imperative to develop a simple and reliable method to decrease the agricultural losses due to chilling. In this investigation, we report that multiple H2O2 treatments induce a chilling tolerance comparable to cold acclimation in mung bean seedlings. However, in their response to light, the mechanisms of H2O2- and cold-induced acclimation could be distinguished. Participation of GSH and (Ca2+)cyt in the H2O2-triggered tolerance in mung bean seedlings was also investigated in this study.
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