ABSTRACT
Cell and Molecular Biology Laboratory, Division of Crop Improvement, Central Potato Research Institute (CPRI), Shimla-171001, Himachal Pradesh, India 1Present address: Biotechnology Unit, Division of Crop Improvement, Central Research Institute for Jute and Allied Fibres (CRIJAF), Barrackpore, Kolkata-700120, West Bengal, India, Telephone: +91 (0) 33 2535 6121 x 236, E-mail: d.sarkar@excite.com; dsarkar@crijaf.org.in; debabrata_s@yahoo.com *Corresponding author
Introduction
Reactive oxygen species (ROS), also known as reactive oxygen intermediates (ROI) or active oxygen species (AOS), are activated or partially reduced derivatives of molecular dioxygen (O2) that was introduced into Earth’s atmosphere by O2-evolving photosynthetic organisms. They include the superoxide radical (O2˙‾), hydrogen peroxide (H2O2) and the hydroxyl radical (OH˙) together with singlet oxygen (
1O2) that represents a physiologically energized form of dioxygen (O2). ROS are produced during reactions of normal aerobic metabolism involving electron transport processes, such as photosynthesis and respiration and/ or during environmental stress responses, such as photorespiration. In living organisms, ROS-producing pathways are balanced by effective ROS-scavenging pathways (mechanisms) that metabolize ROS. However, an imbalance in the generation and metabolism of ROS subjects living organisms to a variety of physiological challenges, which are collectively known as oxidative stress. Amongst all living organisms, with regard to oxidative stress, plants are inherently at a disadvantageous position because they not only generate O2 during photosynthesis but also consume it during respiration, also have the highest concentration of cellular O2 and lead a stationary life style under constantly changing environments (Scandalios 1993). As a result, plants possess effi cient enzymatic and non-enzymatic antioxidant defense systems that protect their cells from oxidative stress. This is executed by a complex network of redox signaling pathways that regulate the spatio-temporal control of ROS production and scavenging mechanisms. Although these signaling pathways are not yet fully resolved, it is well known that ROS at low concentrations induce plant defense responses, but at high concentrations initiate programmed cell death (Vranová et al. 2002). For details, the readers are referred to
the other chapters and few authoritative reviews (Scandalios 1993, Van Breusegem et al. 2001, Neill et al. 2002, Vranová et al. 2002, Apel and Hirt 2004).
