Oxidative Stress and Signalling in Plants
K.L. Ledeganckstraat 35, B-9000 Gent, Belgium
A wide range of environmental stresses (chilling, ozone, high light, drought, etc.) can damage crop plants, resulting in high yield losses. A common event during all these unrelated adverse conditions, called oxidative stress, is the enhanced production of activated oxygen species (AOS). These AOS, arise in several subcellular compartments and attack very rapidly DNA, lipids and proteins. Under normal growth conditions, both enzymatic and non-enzymatic detoxification mechanisms efficiently scavenge AOS. However during prolonged stress conditions damage becomes inevitable because the detoxification systems gets saturated. The main players within the defence system are superoxide dismutases, ascorbate peroxidases and catalases. These enzymes directly eliminate the harmful AOS. By enhancing the levels of these proteins in transgenic plants, it was attempted to improve the tolerance against oxidative stress. Promising results with transgenic tobacco plants (Bowler et al., 1991; Van Camp et al., 1994) stimulated us to apply this strategy to economical important crops. Transgenic maize, rice and tomato lines with enhanced levels of several antioxidant enzymes are currently produced and/or under evaluation. Transgenic maize lines with enhanced levels of manganese and iron superoxide dismutase respectively, suffer less from oxidative damage in an in vitro assay and exhibit, albeit small, improved growth rates at low temperatures. To extrapolate these observations field trials were initiated with both transgenic lines in order to evaluate stress resistance of these plants in natural conditions.
The long-term goal is to understand how plants sense stress and how this signal is transduced to activate the appropriate defence systems. Therefore we try to identify and eventually modulate several components of the oxidative stress signal transduction pathway. In this way, transgenic plants with a broader set of enhanced defence systems can be generated. A central theme of our research is to clarify the role of H2O2 in signal transduction and in the initiation of cell death pathways. Therefore, transgenic plants that are deficient in catalase (H2O2 (H2O + O2)) have been developed as tool for studying the various roles of H2O2 in plants. Evidence is obtained that H2O2 can activate local and systemic defence mechanisms against pathogens. Furthermore, activation of local defence responses was seen with H2O2 doses that did not provoke tissue damage or cell death. Another aspect of our research is the study of the adaptation responses against oxidative stress with the aim to obtain insight into the complexity of this response and to identify new signal transduction intermediates. From a differential screening of oxidative stress-adapted and non-adapted leaf material, two novel signal proteins were identified: a WRKY binding domain containing protein and a phosphatase (PP2C). Both signal proteins are highly regulated at the mRNA level during various environmental stress conditions.