ABSTRACT
For many years, ROS have been thought of as damaging molecules. The study of their use and roles in biological systems has primarily been centred on host defence and much of the early work was carried out using mammalian systems that involved cells such as neutrophils and macrophages. These phagocytic cells engulf pathogens producing a burst of ROS, which is thought to be instrumental to the killing process (Segal, 2005). However, it is also recognised that ROS, in particular hydrogen peroxide (H2O2), have a major role in the cell signalling mechanisms used by a wide range of organisms, including plants (reviewed by Dröge, 2002; Foyer and Noctor, 2005b; Neill et al., 2002). ROS in plants may be generated via the leakage of electrons from redox pathways in either the mitochondria or chloroplast, but can also be synthesised by a range of more dedicated enzymes such as the NADPH oxidases or the peroxidases (Neill et al., 2002). The ability of chloroplasts to generate H2O2 was reported first by Mehler in 1957 but its effects on photosynthesis were only considered much later (Charles and Halliwell, 1980; Kaiser, 1979). The rationale behind the production of these supposedly harmful compounds is only now coming to light.
