No abstract

SH glB-cys-gly Alscher, R. G. 1989. Biosynthesis and antioxidant function of glutathione in plants. -Physiol. Plant. 77: 457-464.Glutathione is widely distributed in plant cells. It appears to be synthesized in both the chloroplast and the cytosol and to occur in both subcellular compartments at relatively high levels. Its function is that of an antioxidant; in concert with ascorbate, it acts to protect labile macromolecules against attack by scavenging free radicals and hydrogen peroxide, which are formed as a consequence of oxidative stresses such as extremes of temperature, drought, herbicides or air pollutants. An integral part of the response of glutathione metabolism to oxidative stress appears to include an increase in the levels of the reduced form of the molecule. The possible role of glutathione as an elicitor of transcriptional events associated with stress responses is briefly discussed.

Reactive O(2) species (ROS) are produced in both unstressed and stressed cells. Plants have well-developed defence systems against ROS, involving both limiting the formation of ROS as well as instituting its removal. Under unstressed conditions, the formation and removal of O(2) are in balance. However, the defence system, when presented with increased ROS formation under stress conditions, can be overwhelmed. Within a cell, the superoxide dismutases (SODs) constitute the first line of defence against ROS. Specialization of function among the SODs may be due to a combination of the influence of subcellular location of the enzyme and upstream sequences in the genomic sequence. The commonality of elements in the upstream sequences of Fe, Mn and Cu/Zn SODs suggests a relatively recent origin for those regulatory regions. The differences in the upstream regions of the three FeSOD genes suggest differing regulatory control which is borne out in the research literature. The finding that the upstream sequences of Mn and peroxisomal Cu/Zn SODs have three common elements suggests a common regulatory pathway. The tools are available to dissect further the molecular basis for antioxidant defence responses in plant cells. SODs are clearly among the most important of those defences, when coupled with the necessary downstream events for full detoxification of ROS.

Differential sensitivity to the oxidant paraquat was observed in pea (Pisum sativum L.) based on cultivar and leaf age. To assess contributions of inductive responses of the antioxidant enzymes in short-term resistance to oxidative damage, activities of glutathione reductase (CR), superoxide dismutase (SOD), and ascorbate peroxidase (APX) and transcript levels for plastidic CR, Cu,Zn SOD, and cytosolic APX were determined. Responses to paraquat exposure from three different leaf age classes of pea were studied. Resistance was correlated with leaf age, photosynthetic rates, enzyme activities, and pretreatment levels of plastid CR and plastid Cu,Zn SOD transcripts. In response to paraquat, small increases in activities of CR and APX were observed in the more resistant leaves. These changes were not reflected at the mRNA leve1 for the plastidic CR or Cu,Zn SOD. Paraquat-mediated increases in cytosolic APX mRNA occurred in all leaf types, irrespective of resistance. Developmentally controlled mechanisms determining basal antioxidant enzyme activities, and not inductive responses, appear to be critical factors mediating short-term oxidative stress resistance.

The imposition of oxidative stress leads to increased production of reactive oxygen species (ROS) in plant cells. Orchestrated defense processes ensue that have much in common between stresses, yet are also particular to the site of action of the stress and its concentration. Possible functional roles of these responses include, but are not restricted to, the protection of the photosynthetic machinery, the preservation of membrane integrity and the protection of DNA and proteins. Superimposed upon our understanding of cellular mechanisms for protection against abiotic stress is a newly discovered role of ROS in signalling and defense response to pathogens (J. L. Dangl, R. A. Dietrich and M. S. Richberg. 1996. Plant Cell 8: 1793–1807). Evidence to date suggests a coordinated response to ROS among different members of the superoxide dismutase (SOD) gene families. A further layer of complexity is afforded by reports of coordination of expression between ascorbate peroxidase and SOD genes. Our understanding of the signalling mechanisms that underlie these coordinated events is in its infancy. An exciting future lies ahead in which the orchestration of successful antioxidant stress responses will be gradually revealed. Current data suggest that complex regulatory mechanisms function at both the gene and protein level to coordinate antioxidant responses and that a critical role is played by organellar localization and inter‐compartment coordination.