Outlook

Although significant progress has been made in understanding glu-tathione metabolism and regulation chiefly via the introduction of molecular techniques and genetically transformed plants, the results obtained with respect to overall stress responses are still extremely variable. Plants with higher constituent GSH levels or GR activities may or may be not more resistant (Strohm et al. 1995, Aono et al. 1991) or even more susceptible to different oxidative stresses (Creissen et al. 1999) when the whole plant system is observed. It must be taken into account that glutathione is only one, albeit important, constituent of the defence and regulating system in plants.

More crucial than the concentration, redox state, or enzyme activity of specific parts of this system may be the concerted action of all parts. In some cases, under particular conditions, and in certain species, the controlled response to an environmental stress may require direct responses of the glu-tathione systems, whereas in other cases or in other species it may not. The strategies applied to mitigate, avoid or counteract stress effects may be quite different even within the same species, which makes the overall interpretation of data difficult. To enlighten these differences, not only studies in more detail on the molecular level are required, but also approaches that investigate whole plant systems under different environmental conditions, different stress intensities, and include the temporal scale of the response. Multivariate evaluations of multiple constituents of the antioxidant system better represent the concerted action of the whole system (Tausz et al. 1998). Furthermore, the differential response between organs, tissues, cells, and subcel-lular compartments toward stress is currently largely unknown, although the major importance of trans-organ, inter-cellular and intra-cellular signalling is beyond doubt. Only recently, microscopic techniques involving fluorescence microscopy and image analysis (Müller et al. 1999, image on the front cover) or laser-scanning microscopy (Sanchez-Fernandez et al. 1997) allowed the observation of subcellular GSH pools.

The integration of different investigation methods on different organization levels from the gene expression to the whole plant regulation promises further insight in the complex roles of glutathione in plant stress responses.

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