4 Australian Biochemist S h o w c a s e o n R e s e a r c h What’s a nice plant like you doing in a place like this? Stress, Oxidative Stress and Mitochondria A. Harvey Millar 1,2 and James Whelan 1 1. Department of Biochemistry, Faculty of Medicine and Dentistry, The University of Western Australia, WA 6009 2. Plant Sciences Group, Faculty of Agriculture, The University of Western Australia, WA 6009 Environmental stress and the production of AOS The famous plant biologist John L. Harper is quoted as saying: “Plants stand, they don’t run away when you try to count them” (1). This most remarkable, or some might think unremarkable trait, of plants, underlies a fundamental aspect of plant biology. Plants, by the nature of being rooted to the spot, must cope with whatever their chosen spots bring to bear on them. Plants can sustain growth and development amidst such a wide variety of environmental fluctuations by employ- ing flexible metabolic networks that al- low dynamic changes to the prevailing conditions. When we speak of plant stress we are not simply speaking about adaptation, for all plants adapt both structurally and metabolically in ways inconceivable to the straightjacket of mammalian cellular life. The term plant stress usually refers to conditions in which plant growth and per- formance are adversely affected by the environment to the extent that morpho- logical changes and significant losses in crop yield and/or fertility occur.A key sign of such stress at a molecular level is the increased production of active oxygen species (AOS) and the subsequent accu- mulation of oxidative damage. Investigating the link between plant growth under stressful environments and the endogenous production of AOS has involved a great deal of research at both the whole plant and the molecular level and a variety of mechanisms have been highlighted. AOS are directly produced in plant cells by the chemical interaction of a variety of environmental pollutants with constituents of the intracellular environ- ment. The effects of atmospheric pollut- ants, such as ozone and various nitrogen oxides (NOs) on plants, are well studied and the catalysis of AOS formation by a variety of metal ions is documented in plants (2). Metabolic perturbation can also result in AOS formation, for example, through the initiation of one-electron re- ductions of O2 by electron transport chains of chloroplasts and mitochondria. Such perturbations follow herbicide ap- plication, exposure to low temperature and/or high light conditions, exposure to drought, or to high salinity (3). In addition to these abiotic stresses, initiation of host defence responses to pathogen invasion results in the transient enhanced produc- tion of AOS and NOs during the so-called ‘respiratory burst’ of the hypersensitive response in plants (4). Understanding the interplay of both the beneficial and the detrimental effects of AOS in plant growth and development may lead to the design of strategies for plant improvement. Plant resistance to pathogens has been successfully intro- duced into economically important crop species using both traditional breeding programs and genetic engineering ap- proaches based on the gene-for-gene hy- pothesis of specific host-pathogen inter- actions. The potential for increasing the capac- ity of plant lines to withstand a more di- verse range of unfavourable environmen- tal conditions without dramatic losses in yield, represents a major new initiative and a new set of challenges for the genetic manipulation of plants. Preliminary stud- ies have shown that gains of function that alleviate oxidative stress under one unfa- vourable condition often provide resist- ance to other types of stressful environ- ments.This suggests that induction of fun- damental defence mechanisms against oxidative stress could impart a general stress resistance phenotype to the plant. Strategies for stress tolerance Oxidative stress, defined as the eleva- tion of AOS concentration and the accu- mulation of oxidative damage, could be alleviated by a number of approaches. Firstly, the rate of AOS production could be slowed to allow the existing antioxi- dant defences to prevent the accumula- tion of oxidative damage. Such an ap- proach might involve decreasing the per- meability of plants to AOS-inducing sub- stances such as salts and free metal ions, or increasing the transport of such sub- stances out of the plant or into seques- tered stores that do not metabolically perturb the plant. Alternatively, the rate of AOS destruction could be increased in order to match the higher rate of AOS production induced by the stress condi- tions. Such an approach is exemplified by over-expression of antioxidant defence proteins in plants. What anti-oxidation defences do plant possess? Antioxidant defence systems in plants consist of a range of enzymes and reductants that act to scavenge AOS through the interconversion of partially reduced oxygen molecules, ultimately to produce water. In this manner they avoid the danger of AOS reacting with and func- tionally damaging proteins, lipids or DNA (Fig. 1 next page).The cascade of reactions begins with the superoxide radical, pro- duced by the one electron reduction of O2. A range of superoxide dismutases (SODs)are found in plants which catalyze the dismutation of two superoxide mol- ecules to form O2 and H2O2.These SODs can be classified into three classes on the basis of their metal cofactor: copper/zinc (Cu/Zn), manganese (Mn) and iron (Fe). Plants contain all of their FeSOD and a specific Cu/ZnSOD isoform in the chloroplast; they also contain Cu/ZnSODs in the cytosol and the peroxisomes. In the mitochondrion they contain all of their MnSOD in the matrix space and also a Cu/ZnSOD in the intermembrane space. H2O2 produced from superoxide is not a radical species, is not highly reactive and in itself it poses little danger to the cell. However, the reaction of H2O2 with free Fe 2+ in the cell results in the formation of the hydroxyl radical that is the most re-