Reactive oxygen species produced in response to oxidative stress can cause permanent damage to the cellular apparatus. Reactive oxygen intermediates (ROI) typically result from the excitation of O2 to form singlet oxygen (O21) or the transfer of one, two, or three electrons to O2 to form superoxide radical (OJ), hydrogen peroxide (H2O2), or a hydroxyl radical (OH-), respectively. The enhanced production of ROIs during stresses can pose a threat to plants because they are unable to detoxify effectively by the ROI scavenging machinery. The unquenched ROIs react spontaneously with organic molecules and cause membrane lipid peroxidation, protein oxidation, enzyme inhibition, and DNA and RNA damage (see Vinocur and Altman, 2005). Oxidative stress arises under environmental stresses, including salinity stress, and may exceed the scavenging capacity of the natural defense system of the plant. The major ROI-scavenging mechanisms of plants include superoxide dismutase, ascorbate peroxidase, catalase, and GSH reductase, which help in the deactivation ofactive oxygen species in multiple redox reactions, thereby contributing to the protective system against oxidative stress. The ROS scavengers can increase the plant resistance to salinity stress. Overexpression of the aldehyde dehydrogenase gene in Arabidopsis has been reported to confer salinity tolerance. The aldehyde dehydrogense catalyzes the oxidation oftoxic aldehydes, which accumulate as a result of side reactions of ROS with lipids and proteins. The enhancement of stress tolerance in transgenic tobacco plants has been shown by overexpressing Chlamydomonas glutathion peroxidase in chloroplast or cytosol (see Vinocur and Altman, 2005).
Because abiotic stresses affect the cellular gene expression machinery, it is possible that molecules involved in nucleic acid metabolism, including helicases, might be involved in stress signaling. Several genes, including genes for helicases, are known to be expressed under the influence of various abiotic stresses, including salinity (reviewed in Owttrim, 2006; Vashisht and Tuteja, 2006). Helicases are ubiquitous enzymes that catalyze the unwinding of energetically stable duplex DNA (DNA helicases) or duplex RNA secondary structures (RNA helicases) (Tuteja and Tuteja, 2004). Most helicases are members of the DEAD-box protein superfamily that play essential roles in basic cellular processes regulating plant growth and development, such as DNA replication, repair, recombination, transcription, ribosome biogenesis, and translation initiation. It seems therefore that the DEAD-box helicase might also be playing an important role in stabilizing growth in plants under stress conditions by regulating some stress-induced pathways. Because RNA molecules are more prone to forming stable nonfunctional secondary structures, their proper functioning requires RNA chaperones. DEAD-box RNA helicases are the best candidates for RNA chaperones because these proteins can use energy derived from ATP hydrolysis to actively disrupt
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